alternate universe time travel

Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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Booklover Book Reviews

33 Books About Time Travel, Parallel Worlds & Alternate Universes

Books about time travel, and novels involving parallel worlds and alternate universes can offer both wonderful fictional escapes and mentally stimulating reading. Whether an avid fiction time traveler or just dipping your toe in the time pond, you will find a great new read in this time travel books list.

Time Travel Book Terminology

The beauty of books about time travel is that each author writes their own rule set. However, there are some underlying norms in this popular science fiction sub-genre.

Time Travel

Characters being transported to the past or future either by mechanical or scientific means (as in HG Well’s 1895 novel, The Time Machine ) or other other fantastical means, such as ghostly intentional magic (Charles Dickens’ A Christmas Carol , 1843) or less intentional/controllable time slips (Penelope Lively’s 1976 children’s classic, A Stitch in Time ).

A plot device where characters experience a certain span of time repeatedly, either by their own choosing or as some form of lesson or punishment.

Parallel Worlds

Fictional worlds with obvious similarities (or parallels) to our own but also notable differences; like copies of the original where events either have already or will in the future follow a different course. These worlds typically operate on a timeline in parallel with our own. The idea, flowing from quantum physics , is that every time a choice is made new parallel universes are created where each possibility actually happens.

Alternate Universes

The terms ‘alternate’ and ‘parallel’ worlds/universes are often used interchangeably. However, for the purposes of this best time travel books list we will consider ‘alternate universes’ ones with distinct fantastical elements (e.g. Lewis Caroll’s 1865 Alice in Wonderland ), or at least ones where there are more differences than similarities to our own.

Time Travel Book Themes

The time travel and parallel universe story frameworks are most often used to explore what-if scenarios (as in the iconic Sliding Doors movie), the concepts of fate, karma, causation and nature versus nurture, along with the related dramatic notions of hindsight and regret. There’s nothing quite like being transported to a parallel fictional world to hammer home all the things that could go wrong for a character, and how good their current ‘real world’ situation actually is!

Time loops offer protagonists opportunities to learn from past mistakes. Alternate universes can shine a spotlight on closed, narrow-minded thinking or behaviour, force characters to walk-a-day in another’s shoes, and/or reconsider detrimental ‘grass is greener’ attitudes. Some time travel books are more meta in the sense that they overtly interrogate the time travel rule set.

33 Best Books On Time Travel, Parallel Worlds & Alternate Universes

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Iconic Romantic Time Travel Books

The Time Traveler’s Wife by Audrey Niffenegger

One of the best time travel books of all time, this iconic love story is the title that sparked my enduring attraction attraction to the genre. The Time Traveler’s Wife lead characters Henry and Clare are strikingly real despite the very unreal situations they find themselves in. Their struggle to lead normal lives in the face of a force they can neither prevent nor control is intensely moving. A cleverly told heartwarmer and valuable reminder of the fragility of our lives and the virtues of living in the moment. ( Full Review | Amazon | Libro.fm )

To Say Nothing of the Dog by Connie Willis

To Say Nothing of the Dog , a hapless yet charming romp through time and space is one of the most intelligent and enjoyable audiobooks I have ever listened to. Protagonist Ned Henry is your quintessential good-hearted underdog finding his way through a maze of Victorian manners, literary debate, domineering women and animal wrangling with absolutely hilarious consequences. And, Connie Willis’ prose oozes intelligence, wit and class. ( Full Review | Amazon | Libro.fm ) 

Outlander by Diana Gabaldon

Outlander , the chunky, at times raunchy, page-turner that began one of the most epic time travel book series’ and decades later its cinematic TV adaptation has gained countless new fans. This story about time travel plays off the Scottish highland pagan folklore (with a time-slip) and a history scarred by warfare. Strong-willed and highly capable, Claire Randall finds herself torn between duty to her conservative British husband (1945) and the passion and impulsiveness of a Scots warrior she initially clashes with in 1743. ( Amazon | Libro.fm )

Parallel Universe Romance Novels

The Impossible Us (aka Impossible) by Sarah Lotz

This epistolatory, genre-bending novel shimmers with romantic tension, and must rank as one of the best books with parallel universes I have read. The blistering rapid-fire dialogue, and authentically raw and messy pulsating hearts of Lotz’s lead characters Nick and Bee had me glued to its pages.  It is about love in all its mysterious and wonderful forms – the romantic (big tick), but also the love between friends, parents and children, cranky neighbours and animals… even love for a cause and ultimately, for ourselves.( Full Review | Amazon | Libro.fm )

This is How You Lose the Time War by Amal El Mohtar & Max Gladstone

The most ‘poetic’ science fiction I have read. In regards to the time-travelling construct, the nature of the different parallel worlds, warring parties and even the lead characters themselves, readers are constantly being challenged to break out of stereotypical thought patterns and open our minds to new possibilities. Through the exchange of letters, painstakingly composed and hidden where the other will stumble across them years (even millennia) later, they learn the nature and experiences of the other. ( Full Review | Amazon )

The Two Lives of Lydia Bird by Josie Silver

A love story about the what-ifs that arise at life’s crossroads. Freddie and Lydia. They’d been together for more than a decade and thought their love was indestructible. But on Lydia’s twenty-eighth birthday, Freddie died in a car accident. Eventually, knowing he would want her to try to live fully even without him, Lydia takes her first tentative steps into the dating world. But then something inexplicable happens that gives her another chance to live her old life with Freddie, in a world parallel to the new one she’s just begun. ( Amazon | Libro.fm )

Contemporary Romantic Books About Time Travel

What the Wind Knows by Amy Harmon

Anne Gallagher grew up enchanted by her grandfather’s stories of Ireland. On his death, she travels to his childhood home to spread his ashes. There, overcome with memories of the man she adored and consumed by a history she never knew, she is pulled into another time — the Ireland of 1921, teetering on the edge of war. Adopting a missing woman’s identity, she joins their struggle for independence. Ultimately, she must decide whether she’s willing to let go of the life she knew for a love she never thought she’d find. ( Amazon | Libro.fm )

Bonus Read: Ashley Poston’s The Seven Year Slip , is a charming time slip romance novel also.

One Last Stop by Casey McQuiston

In One Last Stop, cynical twenty-three-year-old August believes things like magic and cinematic love stories don’t exist, and the only smart way to go through life is alone. But then, there’s this gorgeous girl on the train. Jane. Dazzling, charming, mysterious. August’s subway crush becomes the best part of her day, but pretty soon, she discovers there’s one big problem: Jane doesn’t just look like an old school punk rocker. She’s literally displaced in time from the 1970s… ( Amazon )

The Rose Garden by Susanna Kearsley

After the death of her sister, Eva Ward leaves Hollywood and all its celebrities behind to return to the only place she feels she truly belongs, the old house on the coast of Cornwall, England. She’s seeking comfort in memories of childhood summers, but instead finds mysterious voices and hidden pathways that sweep her not only into the past, but also into the arms of a man who is not of her time who draws her into a world of intrigue, treason, and love. ( Amazon )

Related Reading List: 30 Best RomCom Books, Smart & Funny Romance Novels

Time Travelling Drama & Adventure Fiction

Faye, Faraway (aka Space Hopper) by Helen Fisher

Faye is happy with her life, but the loss of her mother as a child weighs on her mind even more now that she is a mother herself. So she is amazed when, in an extraordinary turn of events she finds herself back in her childhood home in the 1970s. Faced with the chance to finally seek answers to her questions – but away from her own family – how much is she willing to give up for another moment with her mother? Emotionally engaging and life affirming. ( Full Review | Amazon | Libro.fm )

The Psychology of Time Travel by Kate Mascarenhas

Time travel has largely been a male-dominated zone, and so  The Psychology of Time Travel   featuring a highly intelligent and strikingly independent female character set is refreshing to say the least. Refreshing also is its exploration, and in large part normalisation of societal diversity, e.g. romantic, socio-economic, cultural. Mascarenhas has also re-written the time-travel rule book. A very clever and highly original take on time-travel fiction. ( Full Review | Amazon | Libro.fm )

Here and Now and Then by Mike Chen

I was impressed by the effortless credibility of Mike Chen’s execution of time travel. So many wonderful little details are woven seamlessly into the story fabric — everything from hover cars, food trends and social diversity being the norm. But as impressive as Chen’s deft plotting and world-building is, he never forgets that his characters must remain centre stage. For me, this is a time travel tale a cut above the rest — at times heartrending but ultimately uplifting. A page-turner brimming with heart. ( Full Review | Amazon | Libro.fm )

The Book of Lost Fragrances by M J Rose

A sweeping and suspenseful tale of secrets, intrigue, and lovers separated by time, all connected through the mystical qualities of a perfume created in the days of Cleopatra–and lost for 2,000 years. Jac L’Etoile, heir to a French perfume company, moved to America to flee the pain of her past and her mother’s suicide. Fourteen years later she and her brother have inherited the company and it’s financial problems. But when Robbie suddenly goes missing, leaving a dead body in his wake, after hinting at an earth-shattering discovery in the family archives — Jac is plunged into a world she thought she’d left behind. ( Full Review | Amazon )

The Future of Another Timeline by Annalee Newitz

A highly original and thought-provoking novel about the lengths we’ll go to make history. In a world that’s just a step away from our own, time travel is possible. But war is brewing – a secret group is trying to destroy women’s rights, and their access to the timeline. If they succeed, only a small elite will have the power to shape the past, present, and future. Our only hope lies with an unlikely group of allies, from riot grrls to suffragettes, their lives separated by centuries, battling for a world where anyone can change the future. A final confrontation is coming. ( Amazon | Libro.fm )

The River of No Return by Bee Ridgway

On a battlefield in Spain in 1812, Lord Nicholas Falcott, Marquess of Blackdown, is about to die …. But, the next moment, he inexplicably jumps forward in time, nearly two hundred years – very much alive. Taken under the wing of a mysterious organisation, The Guild, he receives everything he could ever need under the following conditions: He can’t go back. He can’t go home. He must tell no one…. Until a decade later, is forced to confront his nineteenth century past, alongside Julia Percy, who has inherited a closely guarded secret on the death of her enigmatic grandfather the Earl of Dorchester. ( Amazon )

Time Loop Fiction

The Seven Deaths of Evelyn Hardcastle by Stuart Turton

An original high-concept murder mystery described as  Gosford Park  meets  Inception , by way of Agatha Christie’s  Murder on the Orient Express . It is meant to be a celebration but it ends in tragedy. As fireworks explode overhead, Evelyn Hardcastle, the young and beautiful daughter of the house, is killed. But Evelyn will not die just once. Until Aiden – one of the guests summoned to Blackheath for the party – can solve her murder, the day will repeat itself, over and over again. Every time ending with the fateful pistol shot. Read my review of The Seven Deaths of Evelyn Hardcastle >>

The First Fifteen Lives of Harry August by Claire North

A critically acclaimed time loop novels. No matter what he does or the decisions he makes, when death comes, Harry always returns to where he began, a child with all the knowledge of a life he has already lived a dozen times before. Nothing ever changes. But now, as Harry nears the end of his eleventh life, a little girl appears at his bedside. ‘I nearly missed you, Doctor August,’ she says. ‘I need to send a message.’ This is the story of what Harry does next, what he did before, and how he tries to save a past he cannot change and a future he cannot allow. Find out more >>

Life After Life by Kate Atkinson

Global bestseller and TV series… What if you had the chance to live your life again and again, until you finally got it right? Would you eventually be able to save the world from its own inevitable destiny? And would you even want to? This novel follows Ursula Todd as she lives through the turbulent events of the last century again and again. With wit and compassion, the author finds warmth even in life’s bleakest moments. A profound and inventive time loop novel that celebrates the best and worst of ourselves. Find out more >>

Contemporary Parallel World & Alternate Universe Novels

The Eyre Affair by Jasper Fforde

The first title in the author’s Thursday Next Literary Detective Series which achieved an almost cult-status amongst booklovers. Why? The wit, wordplay and satire on display. Time travel is common place and it is possible to travel ‘into a book’, in this alternate world where appreciation of literature is high in the social conscience. And of course, its loveably loyal, flawed and feisty heroine named Thursday Next, a SpecOp-27 (Literary Detective Division of Special Operations) with a genetically flawed dodo as a pet. I found this one a delight. Read my The Eyre Affair review >>

A Wrong Turn at the Office of Unmade Lists by Jane Rawson

One of the more literary books about alternate realities I have read. This 2013 meta-fiction combines a Narnia-like storyline with a dystopian climate future, the latter less fanciful by the day. The fictional ‘climate change disaster’ — floods, fire, drought, energy crises – has led to a drastic widening of the socio-economic divide. Good-natured dry humour and entertaining banter of endearingly flawed characters is the means by which Rawson explores deeper themes — loss and bereavement, resilience and survival. Read my full review >>

The Ten Thousand Doors of January by Alix E Harrow

In a sprawling mansion filled with peculiar treasures, January Scaller is a curiosity herself. As the ward of the wealthy Mr Locke, she feels little different from the artefacts that decorate the halls: carefully maintained, largely ignored and utterly out of place. But her quiet existence is shattered when she stumbles across a strange book. A book that tells a tale of secret doors into new worlds, of love, adventure and danger. Each page reveals more impossible truths, and January discovers a story increasingly entwined with her own. Find out more >>

Time Travel Mystery Thrillers

The Shining Girls by Lauren Beukes

This time travel thriller is not for readers with weak stomachs. Harper Curtis is a serial killer who stepped out of the past, and the stories of his victims are harrowing. Kirby Mazrachi is the girl who was never meant to have a future. Kirby is the last shining girl, one of the bright young women, burning with potential, whose lives Harper is destined to snuff out after he stumbles on a House in Depression-era Chicago that opens on to other times. Read my The Shining Girls review >>

Recursion by Blake Crouch

‘My son has been erased.’  Those are the last words the woman tells Barry Sutton, before she leaps from the Manhattan rooftop. Deeply unnerved, Barry begins to investigate her death, only to learn that this wasn’t an isolated case. All across the country, people are waking up to lives different than the ones they fell asleep to. Are they suffering from False Memory Syndrome, a mysterious, new disease that afflicts people with vivid memories of a life they never lived? Or is something far more sinister behind the fracturing of reality all around him? Find out more >>

Kindred by Octavia E Butler

The Guardian called this title, “The marker you should judge all other time-travelling narratives by”. In 1976, Dana dreams of being a writer. In 1815, she is assumed a slave. When Dana first meets Rufus on a Maryland plantation, he’s drowning. She saves his life – and it will happen again and again. Neither of them understands his power to summon her whenever his life is threatened, nor the significance of the ties that bind them. And each time Dana saves him, the more aware she is that her own life might be over before it’s even begun. Find out more >>

Blackout by Connie Willis

Oxford, England 2060. A trio of time traveling scholars depart for various corners of the Second World War. Their mission is to observe, from a safe vantage point, the day-to-day nature of life during this critical historical moment. As the action ranges from the evacuation of Dunkirk to the manor houses of rural England to the horrors of London during the Blitz, the objective nature of their roles gradually changes. Cut off from the safety net of the future and caught up in the chaos of WWII, they are forced to participate in unexpected ways, in the defining events of the era. Find out more >>

Novels with Timeline Echoes

Cloud Atlas by David Mitchell

The multi-award winning novel that sparked my love of literary fiction, and one of my all-time favourite reads.

Souls cross ages like clouds cross skies . . . Six interlocking lives – one amazing adventure. In a narrative that circles the globe and reaches from the 19th century to a post-apocalyptic future, this story erases the boundaries of time, genre and language to offer an enthralling vision of humanity’s will to power, and where it could lead us. Find out more >>

Sea of Tranquility by Emily St John Mandel

A novel that investigates the idea of parallel worlds and possibilities. This slim epic (272 pages) about art, time travel, love, and plague, takes the reader from Vancouver Island in 1912 to a dark colony on the moon three hundred years later, unfurling a story of humanity across centuries and space. Three people from three different time periods, in a tale that precisely captures the zeitgeist. Find out more >>

The Bone Clocks by David Mitchell

In 1984, teenage runaway Holly Sykes encounters a strange woman who offers a small kindness in exchange for ‘asylum’. Decades will pass before Holly understands exactly what sort of asylum the woman was seeking… This novel follows her life from a scarred adolescence to old age – a life not so far out of the ordinary, yet punctuated by flashes of precognition, visits from people who emerge from thin air and brief lapses in the laws of reality. A metaphysical thriller.  Read my The Bone Clocks review >>

This title’s companion novel Slade House more overtly explores alternate/parallel universe concepts.

Classic Time Travel Books & Alternative Universe Literature

The Lion, the Witch and the Wardrobe by C S Lewis

Narnia must surely rate as one of the most well-known parallel universes in fiction. But did you know this childhood favourite is actually the second book in the author’s 7-book Chronicles of Narnia series?

The four Pevensie children are sent to live in a large house in the country, a house with many rooms, which are filled with many things. But one of the rooms is absolutely empty, except for a single piece of furniture: a large wardrobe. It is a wardrobe, the children discover, which has magical properties. Find out more >>

Time and Again by Jack Finney

Time and Again , first published in 1970, tells the story of Simon Morley, an unfulfilled advertising sketch artist in Manhattan who is enlisted by the US Army to take part in a time travelling experiment. While travelling back in time he takes the opportunity to investigate the origins of a mysterious letter in his girlfriend’s possession dated 1882. But then his, and other operatives’ actions during past time periods begin to result in changed outcomes in the present day. Find out more >>

A Wrinkle in Time by Madeleine L’Engle

In this Newberry award-winning classic, three strange figures, Mrs–Who, Whatsit, and Which, send Meg and Charles Wallace, and their friend Calvin O’Keefe, on a mission (via tesseract, a wrinkle in space-time that allows fast travel between distant points) to find their father Mr. Murry, a missing government physicist. While doing so they fight off a dark force and save our universe. Find out more >>

I reviewed the graphic novel by Hope Larson .

Flatland: A Romance of Many Dimensions by Edwin A Abbott

This classic 1884 satirical novella is ostensibly an exploration of dimension theory. It describes the journeys of A. Square and his adventures in Spaceland (three dimensions), Lineland (one dimension) and Pointland (no dimensions). A. Square also entertains thoughts of visiting a land of four dimensions — a revolutionary idea for which he is banished from Spaceland. But satirically, it delves into the role of women in Victorian society, rigid class structures, the influence of organised religion, the suppression of individuality and even euthanasia. Read my Flatland review >>

How many of these books about time travel, parallel worlds and alternate universes have you read?

I will be routinely updating this list. Pin it for later and check back in over time as more great reads in this popular sub-genre are released.

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The block universe theory, where time travel is possible but time passing is an illusion

Science The block universe theory, where time travel is possible but time passing is an illusion

Your birth is out there in space-time. Your death, too, is in space-time. Every moment of your life is out there, somewhere, in space-time.

So says the block universe model of our world.

According to the block universe theory, the universe is a giant block of all the things that ever happen at any time and at any place. On this view, the past, present and future all exist — and are equally real.

How can this be?

The block has four dimensions: three spatial dimensions — say length, height and width — plus a fourth temporal dimension, or time. Or let's make it easier, by visualising the block model of our world as a three-dimensional rectangle, or cuboid.

Two of that cuboid's dimensions (let's say height and width) represent two of the universe's three spatial dimensions.

The third spatial dimension in the above diagram is left out — the length of the cuboid — and replace it with time. At one end of the cuboid is the big bang. At the other is the very last moment of the universe. Maybe it's a big crunch.

The cuboid is filled with every event that ever happens. Where these events are in the cuboid represents their location in space-time. All events, including your birth and death, and this very moment as you read these words, exist somewhere in the block.

In the block universe, time doesn't pass

It often seems as though where we are "today" is present, and "yesterday" is past, and "tomorrow" is future.

It also seems the present moment changes too — after all, tomorrow it will seem as though tomorrow is present, and yesterday it appeared yesterday was present!

So from our perspective, it appears that time flows or passes. But in the block universe model, time doesn't flow.

In other words, in a block universe, there is no specific present moment, and "past" and "future" moments are relative.

Think about the idea of "here". I am here. You, while reading this, can truly say "I am here", even though your "here" is different to mine.

On the block universe model, talk about the "present" or "now" works just like talk of "here".

Remember last week when you said to your friend, who was late arriving for coffee, "now you're here"; or when, long ago, Caesar said, "I am now crossing the Rubicon"?

These claims are both true. That's because all it means to talk about the present, or now, is to talk about the place in time where you happen to be.

Since we are always located wherever we are (that's trivially true), everyone is located in the present, just as everyone is located at the place they call "here".

According to the block universe view, time or temporal relations of "earlier than" and "later than" exist. These relations hold regardless of where anyone is located.

So, suppose Bert the dinosaur is located earlier than Sally the dog. That relation between Bert and Sally holds, regardless of whether we are located earlier than Bert or later than Sally.

Bearing this in mind, it is possible to see how to make sense of the idea of past and future. Just as on this model "now" picks out whatever time I happen to be located at, "past" picks out any time (or events at those times) that are earlier than my location, and "future" picks out any times or events that are later than my location.

Does that mean we can travel in time?

If time is just another dimension, a lot like the spatial dimensions, does that mean we can travel in time?

The short answer is yes.

Of course, things are way more complicated than that. Travelling in time is clearly much more difficult than travelling in space. It might be very technologically costly to time travel, so perhaps it's not really something that, practically speaking, we can do.

But it's certainly possible.

We already know that travelling very fast will result in time dilation, so we know it's possible to travel into the future just by travelling very fast.

We can travel quite a way into the future if we can travel at some reasonable percentage of the speed of light. We also know how to travel into the past. We can do that by using wormholes, which are short cuts through space-time.

So, if I can travel in time, can I change the past?

No. That would create a contradiction, and there are no contradictions.

Remember, on the block universe model, the past is no different than the future or the present.

Everything is relative: what is past to you, will be future to someone else.

So if I travel back to the past I'm travelling to what is someone else's future. That means the past won't be any different, in kind, to the present.

What will happen if I travel to the past? I'll get out of my time machine and start walking around. I'll breathe the air and chat to people.

Obviously, this will have effects on the time I travel to. I'll tread on ants; I'll talk to people from that time; I'll pat horses, and feed donkeys and so on.

I'll act, in the past, in the sorts of ways I act in the present. But I won't be changing the past. Just as when I eat cornflakes instead of toast tomorrow I am not changing the future, I'm just making the future the way it is, when I travel to the past I don't change it, I just make it the way it is, and always has been.

Confused yet?

Let's recap.

What I do tomorrow makes tomorrow the way it is, and the way it always has been. What I do in the past makes the past time the way it is, and always has been.

If I travel to the past, I am part of the past. Importantly, I was always part of the past.

The events in the block are there for all time: they do not change. So, as a time traveller, it's not as though I suddenly appear at a past time. It's always been the case that I am located at that past time.

Nothing a time traveller does changes anything in the block. Instead, what the traveller does at any time makes that time, and later times, the way they are.

That means that we know that some things we attempt to do in the past, fail. We know that Hitler rose to power in the 1930s, so we know that if our time travelling future selves try to prevent this from happening, they fail.

But that doesn't show that our time travelling selves don't succeed in doing lots of things in the past. For all we know, the reason the past is the way it is, is in part due to the presence of time travellers.

Associate Professor Kristie Miller is the joint director for the Centre for Time at the University of Sydney. Watch her explain the block universe theory on Catalyst on iview .

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How Does Multiverse Theory Relate to Time Travel?

The theoretical physics and paradoxes of time travel often brush up against multiverse theory and the idea of alternate universes..

While many sci-fi writers have wondered if time travel into the past can be allowed as long as you slip into an alternate universe, you should be warned: Nature doesn’t like a cheater.

Time travel into the past is a real pain in the neck. For one, as far as we can tell it’s forbidden in our universe. You can travel into the future as slowly or as quickly as you like, but you can’t stop time or go in reverse.

Is It Possible To Travel Back in Time?

Technically speaking, physicists are not exactly sure why time travel is forbidden. We don’t have a single law of physics that clearly rejects the possibility.

But every time we try to concoct a time travel concept — like wormholes or twisting paths around infinitely long cylinders — we find some reason that it’s not allowed. Wormholes, it turns out, require negative mass to stabilize themselves, and infinitely long cylinders are rather hard to come by. 

Read More: Why Do Humans Perceive Time The Way We Do?

The Trouble With Changing the Past

Even within any glimmer of a possibility that time travel might be possible, traveling into our own pasts opens up all sorts of noxious paradoxes, as plenty of films and comic book storylines have demonstrated.

Beyond the fact that the past is over and done with, we must reckon with the reality that what happened in the past created the present that we experience.

To put the paradox succinctly: If you change the past, you change the present, but the present already exists.

The Grandfather Paradox

One of the most common examples of this head-scratching scenario is known as the grandfather paradox.

Suppose you travel back in time and kill your own grandfather before he met your grandmother (no need to dissect your motivations behind this hypothetical act here). That means one of your parents was never born, which means you never exist. But if you never existed, how did you go back in time to kill your grandfather in the first place?

Read More: What Is the Grandfather Paradox of Time Travel?

Are There Multiple Universes?

To address this dilemma, we could invoke the idea of parallel or alternate universes, a concept under the general umbrella of the multiverse.

Perhaps when you go back and kill your grandfather, you create a new universe where you don’t exist. You can then return to your original universe where everything is hunky-dory (or, at least you’re still alive). Essentially, you only monkeyed around with some other, disconnected cosmos.

There are, in fact, two places in physics where the multiverse concept pops up. Unfortunately for any wannabe time travelers, neither of them allows for this type of parallel-futures hopping. 

Splitting-Off Worlds in Quantum Mechanics

The first place where alternate universe theories have gained traction is in quantum mechanics.

Subatomic processes are inherently random. When we run an experiment we never know what result we’re going to get. One explanation for this is that all experimental results happen, just in separate universes.

If someone shoves an electron through a strong magnetic field, for example, and it has a fifty-fifty chance of deflecting either up or down, then every time they run an electron through the device, they get two universes: one where the electron went up, and one where the electron went down.

As the theory goes, this kind of splitting isn’t just limited to arcane physics experiments. It happens with every single quantum interaction throughout the cosmos, which is a lot of interactions, thus generating countless parallel worlds.

In this view, there is a universe out there that already exists where your grandfather is dead and you were never born. But you could never see it.

Read More: Advanced Quantum Material Curves the Fabric of Space

No Travel Between Worlds in Quantum Mechanics

Even if the theory above is actually happening, the rules of quantum physics are absolutely clear: There is no travel or communication between these universes.

Granted, “universe” is probably the wrong word here. That’s because the splitting is really separate partitions of a single quantum wave function. And that separation is really permanent.

If there was any kind of connection allowed, then the many branching possibilities of quantum processes simply wouldn’t happen.

Bubble Universe Theory

The second place where the multiverse pops up in physics is in studies related to the extremely early universe.

From this lens, our cosmos could be just one of many — like a bubble in an infinite foam of universes. These universes too could contain all the many interesting and varied possibilities that make life so fun.

In this (extremely hypothetical) view, there are many universes out there that contain alternate realities from the one you know. There’s one where your grandfather died. There’s another where no stars ever formed. And there’s a universe where all your matter is replaced with antimatter, and so on.

Because all these bubble universes exist in the same expanding foam that is reality, those alternate universes really are out there , an unimaginable but still-finite distance away.

Time Still Marches Forward in the Multiverse

But even in this foam-like collection of bubble universes, time still does its thing. That is, it moves forward, always. 

Even if you could concoct some scheme to travel among the alternate universes, those universes still share the same spacetime fabric, marching forward along with us. Thus they all appear to forbid time travel, as the theory goes.

They also, in theory, share the reality that nature doesn’t like a cheater. So, if you’re theoretically going to visit a universe where you never existed, maybe nix the time travel method.

Read More: Scientists Attempt to Map the Multiverse

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Time Travel, Alternate Histories, & Parallel Universes

Travel to another time, another world, another reality, or all of the above with one of these speculative fiction selections that explore the boundaries of time and place.

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Time Travel | Alternate Histories | Parallel Universes & Alternate Realities

Time Travel

Atomic Anna

Barenbaum (A Bend in the Stars) burnishes her reputation as an up-and-coming talent with this audacious time travel story. Anna Berkova, a Soviet nuclear scientist who works at Chernobyl, is asleep on the morning of the meltdown in 1986. The catastrophe transports her to 1992 Mount Aragats, Armenia, where she finds a trail of blood leading to her dying daughter, Manya, who’s been shot. Before dying, Manya reveals that the amplifier her mother was working on had pulled her through a ripple in space-time, and that Berkova must travel farther into the future to save a teenage granddaughter, Raisa, she’s never known. Barenbaum then unfolds the three women’s stories, each of which is laced with tragedy and unfulfilled aspirations. The threads build toward a deeply satisfying denouement, and the author uses the sci-fi plot device to explore parent-child relationships and questions about the morality of changing the past. Barenbaum dares greatly, and succeeds.

The Shining Girls

In Depression-era Chicago, Harper Curtis finds a key to a house that opens on to other times. But it comes at a cost. He has to kill the shining girls: bright young women, burning with potential. He stalks them through their lives across different eras until, in 1989, one of his victims, Kirby Mazrachi, survives and starts hunting him back. Available to download: Audio

Dana, a modern black woman, is celebrating her twenty-sixth birthday with her new husband when she is snatched abruptly from her home in California and transported to the antebellum South. Rufus, the white son of a plantation owner, is drowning, and Dana has been summoned to save him. Dana is drawn back repeatedly through time to the slave quarters, and each time the stay grows longer, more arduous, and more dangerous until it is uncertain whether or not Dana's life will end, long before it has a chance to begin. Available to download: eBook Audio

Here and Now and Then

Stranded for eighteen years since the 1990s, time-traveling agent Kin Stewart, suffering from memory loss, has started a new life, but when rescuers from the year 2142 finally arrive, he must choose between his current family and the one he left behind in the future. Available to download: eBook

An Impossible Promise (sequel)

Liam O'Connor has one purpose in this life, to push the woman he loves into the arms of another man. The Irish rogue unknowingly changed the course of destiny when he fell in love with Cora McLeod over a century ago. Their passion was intense, brief and tragic. And the angels have been trying to restore the balance of fate ever since. Available to download: eBook

Three More Months

Chloe Howard's devotion to her job has come at a cost: spending time with the most important person in her life--her mother. Vowing to change, she plans a trip home. Sadly, hours before she arrives, her mother passes away, leaving Chloe without a goodbye and riddled with grief and regret. But maybe...maybe it's not too late. Just days before the funeral, Chloe finds her mother unaccountably alive and well. And it's no longer May; she's been transported back in time to March. No one--not Chloe's brother, friends, or colleagues--understands why Chloe is so confused. How can she make sense of this? It's impossible. But Chloe is going to make the most of it. She's going to do everything differently: repair family rifts, forge new bonds, tell her mother every day how much she loves her, and possibly prevent the inevitable. This is a second chance Chloe never saw coming. She's not wasting a minute of it.

This is How You Lose the Time War

Among the ashes of a dying world, an agent of the Commandant finds a letter. It reads: Burn before reading. Thus begins an unlikely correspondence between two rival agents hellbent on securing the best possible future for their warring factions. Now, what began as a taunt, a battlefield boast, grows into something more. Something epic. Something romantic. Something that could change the past and the future. Except the discovery of their bond would mean death for each of them. Available to download: eBook Audio

When Grantville, West Virginia, was suddenly hurled from 2000 back to 1632, they landed in the middle of the Thirty Years War. But they brought American Freedom and Justice -- and modern guns -- along with them. 

Hurtled back through time more than two hundred years to Scotland in 1743, Claire Randall finds herself caught in the midst of an unfamiliar world torn apart by violence, pestilence, and revolution and haunted by her growing feelings for James Fraser, a young soldier. Available to download: eBook

The Paradox Hotel

Hart follows up his highly praised The Warehouse (2019) with this nifty murder mystery set at a hotel. But this is not your typical hotel. The Paradox Hotel  caters to the clientele of a government-run time-travel facility. January Cole, the hotel’s security chief, has a lot on her plate, what with a bunch of trillionaires coming to the Paradox, each with his or her own special demands, and the last thing she needs is a murder. At least, she thinks there’s been a murder. She’s pretty sure there’s a body, even though she’s the only person who can see it. There’s a distinct possibility she’s imagining it, but there’s also the possibility that the corpse exists in a different time, and that January’s getting glimpses of the body because she is not, shall we say, entirely anchored in the now. This wildly ambitious, well-executed genre-bender is suspenseful, clever, and funny. Available to download: eBook

Tales from the Cafe (sequel)

In a back alley in Tokyo, there is a café that has been serving carefully brewed coffee for more than one hundred years. Local legend says that this shop offers something else besides coffee -- the chance to travel back in time. With faces both familiar and new, Tales from the Cafe follows the story of four patrons who visit to take advantage of café Funiculi Funicula's time-traveling offer and revisit moments with family, friends and lovers. Each one must face up to the past to move on with their lives. Kawaguchi's wistful and heartwarming new novel once again invites the reader to ask themselves, 'What would you do if you could travel back in time?' Available to download: Audio

An Ocean of Minutes

America is in the grip of a deadly flu pandemic. When Frank catches the virus, his girlfriend Polly will do whatever it takes to save him, even if it means risking everything. She agrees to a radical plan--time travel has been invented in the future to thwart the virus. If she signs up for a one-way-trip into the future to work as a bonded laborer, the company will pay for the life-saving treatment Frank needs. But when she arrives in a few weeks to find him twelve years in the future, will Frank still be waiting?

The Future of Another Timeline

1992: Seventeen-year-old Beth finds herself in a car with her friend's abusive boyfriend dead in the backseat, agreeing to help her friends hide the body. 2022: Determined to use time travel to create a safer future, Tess has dedicated her life to visiting key moments in history and fighting for change. But a group of dangerous travelers is bent on stopping her at any cost. Tess and Beth's lives intertwine as war breaks out across the timeline--a war that threatens to destroy time travel and leave only a small group of elites with the power to shape the past, present, and future.

The Heavens

A young man, Ben, meets a young woman, Kate--and they begin to fall in love. From their first meeting, Ben knows Kate is unworldly and fanciful, so at first he isn't that concerned when she tells him about the recurring dream she's had since childhood. In the dream, she's transported to the past, where she lives a second life as Emilia, the mistress of a nobleman in Elizabethan England. But for Kate, the dream becomes increasingly real and compelling until it threatens to overwhelm her life. Available to download: eBook

The Time Traveler’s Wife

The love story of Henry and Claire whose lives are punctuated by Henry's disappearance to different points in time--sometimes even back to visit Claire as a young woman. When Henry meets Claire, he is twenty-eight, and she is twenty. He's a hip, handsome librarian; she is an art student with Botticelli hair. Henry has never met Claire before; Claire has known Henry since she was six... Available to download: eBook Audio

The First Fifteen Lives of Harry August

Harry August is on his deathbed. Again. No matter what he does or the decisions he makes, when death comes, Harry always returns to where he began, a child with all the knowledge of a life he has already lived a dozen times before. Nothing ever changes. Until now. 

Hazards of Time Travel

A recklessly idealistic girl dares to test the perimeters of her tightly controlled (future) world and is punished by being sent back in time to a region of North America -- "Wainscotia, Wisconsin"--that existed eighty years before. Cast adrift in time in this idyllic Midwestern town she is set upon a course of "rehabilitation"--but cannot resist falling in love with a fellow exile and questioning the constraints of the Wainscotia world with results that are both devastating and liberating. Available to download: eBook Audio

Version Control

A woman deals with a strange and persistent sense of everything being slightly off, which may or may not be related to her scientist husband's pet project, a "causality violation device" that might actually be working. Available to download: eBook

The Kingdoms

Joe Tournier, suffering from a form of epilepsy that causes amnesia, leaves the French-ruled London for the rebel land of Scotland to search for answers about his identity. Available to download: eBook and Audio

How To Live Safely In a Science Fictional Universe

Charles Yu, time travel technician, helps save people from themselves in Minor Universe 31, a vast story-space on the outskirts of fiction. When he's not taking client calls, Yu visits his mother and searches for his father, who invented time travel and then vanished. Accompanied by TAMMY, an operating system with low self-esteem, and a nonexistent but ontologically valid dog named Ed, and using a book titled "How to Live Safely in a Science Fictional Universe "as his guide, Yu sets out, and back, and beyond, in order to find the one day where he and his father can meet in memory.

Alternate Histories

Jillian Cantor reimagines the pioneering, passionate life of Marie Curie using a parallel structure to create two alternative timelines, one that mirrors her real life, one that explores the consequences for Marie and for science if she'd made a different choice.

The Impeachment of Abraham Lincoln

President Abraham Lincoln survives the assassination attempt at Ford's Theatre on April 14, 1865--only to be charged with overstepping his Constitutional authority during the Civil War, and faced with impeachment. A young black woman named Abigail is assigned to assist on the case, but when one of Lincoln's lead lawyers is found brutally murdered, she finds herself plunged into a web of intrigue, politics, and conspiracy more tangled than she could have imagined. Available to download: eBook Audio  

The Yiddish Policemen's Union

A murder mystery based on the premise that a Jewish settlement was created in Alaska following World War II. In the small town of Sitka, Alyeska, Detective Meyer Landsman finds the body of a prominent town figure who has ties to organized crime. As Landsman digs deeper, he discovers that this is only the tip of the iceberg--and all signs point to a greater danger lurking in the shadows. Available to download: eBook

The Man in the High Castle

It's America in 1962. Slavery is legal once again. The few Jews who still survive hide under assumed names. In San Francisco, the I Ching is as common as the Yellow Pages. All because some twenty years earlier the United States lost a war--and is now occupied by Nazi Germany and Japan. Available to download: eBook Audio

On November 22, 1963, three shots rang out in Dallas, President Kennedy died, and the world changed. What if you could change it back? In this brilliantly conceived tour de force, Stephen King takes readers on an incredible journey into the past and the possibility of altering it. Available to download: eBook Audio

The Calculating Stars

On a cold spring night in 1952, a huge meteorite fell to earth and obliterated much of the east coast of the United States, including Washington D.C. The ensuing climate cataclysm will soon render the earth inhospitable for humanity, as the last such meteorite did for the dinosaurs. This looming threat calls for a radically accelerated effort to colonize space, and requires a much larger share of humanity to take part in the process, and Elma York is determined to be the first woman to earn the title Lady Astronaut. Available to download: eBook

Machines Like Me

In an alternative 1980s London, Britain has lost the Falklands War, Margaret Thatcher battles Tony Benn for power, and Alan Turing achieves a breakthrough in artificial intelligence, leading to the creation of a synthetic human. When Charlie buys Adam, one of the first batch ever sold, he ends up with more than he expected--including a love triangle that’s not entirely human. Available to download: eBook

The Years of Rice and Salt

In an alternate history world in which the population of Europe is almost completely wiped out by the Black Death during the fourteenth century, three superpowers--China, India, and the nations of Islam--battle for supremacy in a World War destined to create a new world order. Available to download: Audio

The Plot Against America

When the renowned aviation hero and rabid isolationist Charles A. Lindbergh defeated Franklin Roosevelt by a landslide in the 1940 presidential election, fear invaded every Jewish household in America. Not only had Lindbergh, publicly blamed the Jews for selfishly pushing America toward a pointless war with Nazi Germany, but upon taking office as the thirty-third president of the United States, he negotiated a cordial "understanding" with Adolf Hitler. What then followed in America is the historical setting for this startling new book by Pulitzer Prize-winner Philip Roth. Available to download: eBook

A thrilling novel that dares to imagine Britain under the thumb of Nazi Germany.

A Neo-Victorian alternate history novel that explores the question of what might have come of Belgium's disastrous colonization of the Congo if the native populations had learned about steam technology a bit earlier. Told from a multiplicity of voices: Africans, Europeans, East Asians, and African Americans in complex relationships with one another, in a compelling range of voices that have historically been silenced. Available to download: Audio

Rodham: A Novel

A novel that imagines a deeply compelling what-might-have-been: What if Hillary Rodham hadn't married Bill Clinton?

The Underground Railroad

From prize-winning, bestselling author Colson Whitehead, a magnificent tour de force chronicling a young slave's adventures as she makes a desperate bid for freedom in the antebellum South through the Underground Railroad, which is no mere metaphor--engineers and conductors operate a secret network of tracks and tunnels beneath the Southern soil. Available to download: eBook Audio

Underground Airlines

It is the present-day, and the world is as we know it: smartphones, social networking and Happy Meals. Save for one thing: the Civil War never occurred. Underground Airlines is a ground-breaking novel, a wickedly imaginative thriller, and a story of an America that is more like our own than we'd like to believe. Available to download: eBook

Parallel Universes & Alternate Realities

Life After Life

On a cold and snowy night in 1910, Ursula Todd is born to an English banker and his wife. She dies before she can draw her first breath. On that same cold and snowy night, Ursula Todd is born, lets out a lusty wail, and embarks upon a life that will be, to say the least, unusual. For as she grows, she also dies, repeatedly, in a variety of ways, while the young century marches on towards its second cataclysmic world war. Does Ursula's apparently infinite number of lives give her the power to save the world from its inevitable destiny? And if she can -- will she? Available to download: eBook Audio

Dark Matter

A mind-bending, relentlessly paced science-fiction thriller, in which an ordinary man is kidnapped, knocked unconscious--and awakens in a world inexplicably different from the reality he thought he knew. Available to download: eBook Audio

In the quiet haven of Clearing, Oregon, four neighbors find their lives upended when they begin to see themselves in parallel realities. At first the visions are relatively benign, but they grow increasingly disturbing--and, in some cases, frightening. When a natural disaster threatens Clearing, it becomes obvious that the visions were not what they first seemed and that the town will never be the same. Available to download: eBook

The Ten Thousand Doors of January

In a sprawling mansion filled with peculiar treasures, January Scaller is a curiosity herself. As the ward of the wealthy Mr. Locke, she feels little different from the artifacts that decorate the halls: carefully maintained, largely ignored, and utterly out of place. Then she finds a strange book. A book that carries the scent of other worlds, and tells a tale of secret doors, of love, adventure and danger. Each page turn reveals impossible truths about the world and January discovers a story increasingly entwined with her own. Available to download: eBook Audio

The Gunslinger

In a world that is weirdly related to our own, Roland Deschain of Gilead, of In-World that was, pursues his enigmatic antagonist to the mountains that separate the desert from the Western Sea. At a way station, he meets Jake, a boy from a particular time (1977) and a particular place (New York City), and soon the two are joined-khef, ka, and ka-tet. The mountains lie before them. So does the man in black and, somewhere far beyond...the Dark Tower. Available to download: eBook Audio

All Our Wrong Todays

It's 2016, and in Tom Barren's world, technology has solved all of humanity's problems. Unfortunately, Tom isn't happy. He's lost the girl of his dreams. And what do you do when you're heartbroken and have a time machine? Something stupid. Finding himself stranded in a terrible alternate reality -- which we immediately recognize as our 2016 -- Tom is desperate to fix his mistake and go home. Right up until the moment he discovers wonderfully unexpected versions of his family, his career, and the woman who may just be the love of his life. Available to download: Audio

An ode to George Orwell's 1984 told in alternating male and female voices relates the stories of Aomame, an assassin for a secret organization who discovers that she has been transported to an alternate reality, and Tengo, a mathematics lecturer and novice writer. Available to download: eBook

A Darker Shade of Magic

Kell is one of the last Travelers; magicians with a rare, coveted ability to travel between parallel universes. He can choose where he lands. There's Grey London, dirty and boring, without any magic, ruled by a mad King George. Then there's Red London, where life and magic are revered, and the Maresh Dynasty presides over a flourishing empire. White London, ruled by whoever has murdered their way to the throne. And once upon a time, there was Black London. But no one speaks of that now. Available to download: eBook Audio

My Real Children

It's 2015, and Patricia Cowan is very old. She forgets things she should know-what year it is, major events in the lives of her children. But she remembers things that don't seem possible. She remembers marrying Mark and having four children. And she remembers not marrying Mark and raising three children with Bee instead. Her childhood, her years at Oxford during the Second World War-those were solid things. But after that, did she marry Mark or not? Had she been a housewife who escaped a terrible marriage after her children were grown, or a successful travel writer with homes in Britain and Italy? And the moon outside her window: does it host a benign research station, or a command post bristling with nuclear missiles? Available to download: eBook

To Paradise

In an alternate version of 1893 America, New York is part of the Free States, where people may live and love whomever they please (or so it seems). The fragile young scion of a distinguished family resists betrothal to a worthy suitor, drawn to a charming music teacher of no means. In a 1993 Manhattan besieged by the AIDS epidemic, a young Hawaiian man lives with his much older, wealthier partner, hiding his troubled childhood and the fate of his father. And in 2093, in a world riven by plagues and governed by totalitarian rule, a powerful scientist's damaged granddaughter tries to navigate life without him--and solve the mystery of her husband's disappearances. These three sections are joined in an enthralling and ingenious symphony, as recurring notes and themes deepen and enrich one another: A townhouse in Washington Square Park in Greenwich Village; illness, and treatments that come at a terrible cost; wealth and squalor; the weak and the strong; race; the definition of family, and of nationhood; the dangerous righteousness of the powerful, and of revolutionaries; the longing to find a place in an earthly paradise, and the gradual realization that it can't exist. What unites not just the characters, but these Americas, are their reckonings with the qualities that make us human: Fear. Love. Shame. Need. Loneliness. Available to download: eBook and Audio

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What is the multiverse—and is there any evidence it really exists?

Scientists can only see so far before they run into the edge of the universe. Will we ever know if anything lies beyond?

What lies beyond the edges of the observable universe? Is it possible that our universe is just one of many in a much larger multiverse? Movies can’t get enough of exploring these questions. From Oscar winners like Everything Everywhere All at Once   to superhero blockbusters like Dr. Strange in the Multiverse of Madness , science fiction stories are full of creative interactions between alternate realities. And depending on which cosmologist you ask, the concept of a multiverse is more than pure fantasy or a handy storytelling device.

Humanity’s ideas about alternate realities are ancient and varied—in 1848 Edgar Allan Poe even wrote a prose poem in which he fancied the existence of “a limitless succession of Universes.” But the multiverse concept really took off when modern scientific theories attempting to explain the properties of our universe predicted the existence of other universes   where events take place outside our reality.

“Our understanding of reality is not complete, by far,” says Stanford University physicist Andrei Linde . “Reality exists independently of us.”

If they exist, those universes are separated from ours, unreachable and undetectable by any direct measurement (at least so far). And that makes some experts question whether the search for a multiverse can ever be truly scientific.

Will scientists ever know whether our universe is the only one? We break down the different theories about a possible multiverse—including other universes with their own laws of physics—and whether many versions of you could exist out there.

What is a multiverse?

The multiverse is a term that scientists use to describe the idea that beyond the observable universe, other universes may exist as well. Multiverses are predicted by several scientific theories that describe different possible scenarios—from regions of space in different planes than our universe, to separate bubble universes that are constantly springing into existence.

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The one thing all these theories have in common is that they suggest the space and time we can observe is not the only reality.

Okay … but why do scientists think there could be more than one universe?

“We cannot explain all the features of our universe if there’s only one of them,” says science journalist Tom Siegfried , whose book The Number of the Heavens investigates how conceptions of the multiverse have evolved over millennia.

“Why are the fundamental constants of nature what they are?” Siegfried wonders. “Why is there enough time in our universe to make stars and planets? Why do stars shine the way they do, with just the right amount of energy? All of those things are questions we don’t have answers for in our physical theories.”

Siegfried says two possible explanations exist: First, that we need newer, better theories to explain the properties of our universe. Or, he says, it’s possible that “we’re just one of many universes that are different, and we live in the one that’s nice and comfortable.”

What are some of the more popular multiverse theories?  

Perhaps the most scientifically accepted idea comes from what’s known as inflationary cosmology , which is the idea that in the minuscule moments after the big bang, the universe rapidly and exponentially expanded. Cosmic inflation explains a lot of the observed properties of the universe, such as its structure and the distribution of galaxies.

“This theory at first looked like a piece of science fiction, although a very imaginative one,” says Linde, one of the architects of cosmic inflationary theory. “But it explained so many interesting features of our world that people started taking it seriously.”

One of the theory’s predictions is that inflation could happen over and over again , perhaps infinitely, creating a constellation of bubble universes. Not all of those bubbles will have the same properties as our own—they might be spaces where physics behaves differently. Some of them might be similar to our universe, but they all exist beyond the realm we can directly observe.

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How fast is the universe really expanding? The mystery deepens.

This supermassive black hole was formed when the universe was a toddler

This is what the first stars looked like as they were being born

What are some other ideas.

Another compelling type of multiverse is called the many-worlds interpretation of quantum mechanics , which is the theory that mathematically describes how matter behaves. Proposed by physicist Hugh Everett in 1957, the many-worlds interpretation predicts the presence of branching timelines, or alternate realities in which our decisions play out differently, sometimes producing wildly different outcomes.

“Hugh Everett says, Look, there’s actually an infinite number of parallel Earths, and when you do an experiment and you get the probabilities, basically all that proves is that you live on the Earth where that was the outcome of that experiment,” says physicist James Kakalios of the University of Minnesota, who has written about the physics (or not) of superheroes. “But on other Earths, there’s a different outcome.”

According to this interpretation, versions of you could be off living the many different possible lives you could have led if you’d made different decisions. However, the only reality that’s perceptible to you is the one you inhabit.

So where do all of those other Earths exist?

They’re all overlapping in dimensions we can’t access. MIT’s Max Tegmark refers to this type of multiverse as a Level III multiverse , where multiple scenarios are playing out in branching realities.

“In the many-worlds interpretation, you still have an atomic bomb, you just don’t know exactly when it’s going to go off,” Linde says. And maybe in some of those realities, it won’t.

By contrast, the multiple universes predicted by some theories of cosmic inflation are what Tegmark calls a Level II multiverse, where fundamental physics can be different across the different universes. In an inflationary multiverse, Linde says, “you do not even know if, in some parts of the universe, atomic bombs are even possible in principle.”

So if I want to meet myself, how do I get there? Can we travel between multiverses?  

Unfortunately, no. Scientists don’t think it’s possible to travel between universes, at least not yet.

“Unless a whole lot of physics we know that’s pretty solidly established is wrong, you can’t travel to these multiverses,” Siegfried says. “But who knows? A thousand years from now, I’m not saying somebody can’t figure out something that you would never have imagined.”

Is there any direct evidence suggesting multiverses exist?  

Even though certain features of the universe seem to require the existence of a multiverse, nothing has been directly observed that suggests it actually exists. So far, the evidence supporting the idea of a multiverse is purely theoretical, and in some cases, philosophical.

Some experts argue that it may be a grand cosmic coincidence that the big bang forged a perfectly balanced universe that is just right for our existence. Other scientists think it is more likely that any number of physical universes exist, and that we simply inhabit the one that has the right characteristics for our survival .

An infinite number of alternate little pocket universes, or bubbles universes, some of which have different physics or different fundamental constants, is an attractive idea, Kakalios says. “That’s why some people take these ideas kind of seriously, because it helps address certain philosophical issues,” he says.

Scientists argue about whether the multiverse is even an empirically testable theory ; some would say no, given that by definition a multiverse is independent from our own universe and impossible to access. But perhaps we just haven’t figured out the right test.

Will we ever know if our universe is just one of many?

We might not. But multiverses are among the predictions of various theories that can be tested in other ways, and if those theories pass all of their tests, then maybe the multiverse holds up as well. Or perhaps some new discovery will help scientists figure out if there really is something beyond our observable universe.

“The universe is not constrained by what some blobs of protoplasm on a tiny little planet can figure out, or test,” Siegfried says. “We can say, This is not testable, therefore it can’t be real—but that just means we don’t know how to test it. And maybe someday we’ll figure out how to test it, and maybe we won’t. But the universe can do whatever it wants.”

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How a rotating universe makes time travel possible

by Paul M. Sutter, Universe Today

It turns out that time travel into the past is actually relatively easy. All you need to do is make the universe rotate.

The famous mathematician Kurt Gödel was a friend and neighbor of Albert Einstein at Princeton. He became incredibly curious about Einstein's general theory of relativity , which was and continues to be our modern formulation of the gravitational force. That theory connects the presence of matter and energy to the bending and warping of space and time, and then connects that bending and warping to the behavior of matter and energy.

Gödel was curious as to whether relativity could allow time travel into the past. Einstein's theory purported to be an ultimate framework for the nature of space and time, and as far as we know time travel into the past is forbidden. So Gödel reckoned that general relativity should automatically disallow it.

And Gödel discovered that actually general relativity is perfectly fine with time travel into the past. The trick is to set the universe in motion.

Gödel constructed a relatively simple and artificial model universe to prove his point. This universe is rotating and contains only one ingredient. That ingredient is a negative cosmological constant that resists the centrifugal force of the rotation to keep the universe static.

Gödel found that if you follow a particular path in this rotating universe you can end up in your own past. You would have to travel incredibly far, billions of light years long, to do it, but it can be done. As you travel, you would get caught up in the rotation of the universe. That isn't just a rotation of the stuff in the cosmos, but of both space and time themselves. In essence, the rotation of the universe would so strongly alter your potential paths forward that those paths loop back around to where you started.

You would set off on your journey and never travel faster than the speed of light, and you would find yourself back where you started but in your own past.

The possibility of backwards time travel creates paradoxes and violates our understanding of causality. Thankfully, all observations indicate that the universe is not rotating, so we are protected from Gödel's problem of backwards time travel . But it remains to this day a mystery why general relativity is OK with this seemingly impossible phenomenon. Gödel used the example of the rotating universe to argue that general relativity is incomplete, and he may yet be right.

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Time Travel and Modern Physics

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn’t this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

1. Paradoxes Lost?

2. topology and constraints, 3. the general possibility of time travel in general relativity, 4. two toy models, 5. slightly more realistic models of time travel, 6. the possibility of time travel redux, 7. even if there are constraints, so what, 8. computational models, 9. quantum mechanics to the rescue, 10. conclusions, other internet resources, related entries.

• Supplement: Remarks and Limitations on the Toy Models

Modern physics strips away many aspects of the manifest image of time. Time as it appears in the equations of classical mechanics has no need for a distinguished present moment, for example. Relativity theory leads to even sharper contrasts. It replaces absolute simultaneity, according to which it is possible to unambiguously determine the time order of distant events, with relative simultaneity: extending an “instant of time” throughout space is not unique, but depends on the state of motion of an observer. More dramatically, in general relativity the mathematical properties of time (or better, of spacetime)—its topology and geometry—depend upon how matter is arranged rather than being fixed once and for all. So physics can be, and indeed has to be, formulated without treating time as a universal, fixed background structure. Since general relativity represents gravity through spacetime geometry, the allowed geometries must be as varied as the ways in which matter can be arranged. Alongside geometrical models used to describe the solar system, black holes, and much else, the scope of variation extends to include some exotic structures unlike anything astrophysicists have observed. In particular, there are spacetime geometries with curves that loop back on themselves: closed timelike curves (CTCs), which describe the possible trajectory of an observer who returns exactly back to their earlier state—without any funny business, such as going faster than the speed of light. These geometries satisfy the relevant physical laws, the equations of general relativity, and in that sense time travel is physically possible.

Yet circular time generates paradoxes, familiar from science fiction stories featuring time travel: [ 1 ]

• Consistency: Kurt plans to murder his own grandfather Adolph, by traveling along a CTC to an appropriate moment in the past. He is an able marksman, and waits until he has a clear shot at grandpa. Normally he would not miss. Yet if he succeeds, there is no way that he will then exist to plan and carry out the mission. Kurt pulls the trigger: what can happen?
• Underdetermination: Suppose that Kurt first travels back in order to give his earlier self a copy of How to Build a Time Machine. This is the same book that allows him to build a time machine, which he then carries with him on his journey to the past. Who wrote the book?
• Easy Knowledge: A fan of classical music enhances their computer with a circuit that exploits a CTC. This machine efficiently solves problems at a higher level of computational complexity than conventional computers, leading (among other things) to finding the smallest circuits that can generate Bach’s oeuvre—and to compose new pieces in the same style. Such easy knowledge is at odds with our understanding of our epistemic predicament. (This third paradox has not drawn as much attention.)

The first two paradoxes were once routinely taken to show that solutions with CTCs should be rejected—with charges varying from violating logic, to being “physically unreasonable”, to undermining the notion of free will. Closer analysis of the paradoxes has largely reversed this consensus. Physicists have discovered many solutions with CTCs and have explored their properties in pursuing foundational questions, such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things, causation, modality, free will, and identity (see, e.g., Earman 1972 and Lewis’s seminal 1976 paper).

We begin below with Consistency , turning to the other paradoxes in later sections. A standard, stone-walling response is to insist that the past cannot be changed, as a matter of logic, even by a time traveler (e.g., Gödel 1949, Clarke 1977, Horwich 1987). Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time traveler’s self-defeating journey. The first question is whether there is an analog of the consistent story when it comes to physics in the presence of CTCs. As we will see, there is a remarkable general argument establishing the existence of consistent solutions. Yet a second question persists: why can’t time-traveling Kurt kill his own grandfather? Doesn’t the necessity of failures to change the past put unusual and unexpected constraints on time travelers, or objects that move along CTCs? The same argument shows that there are in fact no constraints imposed by the existence of CTCs, in some cases. After discussing this line of argument, we will turn to the palatability and further implications of such constraints if they are required, and then turn to the implications of quantum mechanics.

Wheeler and Feynman (1949) were the first to claim that the fact that nature is continuous could be used to argue that causal influences from later events to earlier events, as are made possible by time travel, will not lead to paradox without the need for any constraints. Maudlin (1990) showed how to make their argument precise and more general, and argued that nonetheless it was not completely general.

Imagine the following set-up. We start off having a camera with a black and white film ready to take a picture of whatever comes out of the time machine. An object, in fact a developed film, comes out of the time machine. We photograph it, and develop the film. The developed film is subsequently put in the time machine, and set to come out of the time machine at the time the picture is taken. This surely will create a paradox: the developed film will have the opposite distribution of black, white, and shades of gray, from the object that comes out of the time machine. For developed black and white films (i.e., negatives) have the opposite shades of gray from the objects they are pictures of. But since the object that comes out of the time machine is the developed film itself it we surely have a paradox.

However, it does not take much thought to realize that there is no paradox here. What will happen is that a uniformly gray picture will emerge, which produces a developed film that has exactly the same uniform shade of gray. No matter what the sensitivity of the film is, as long as the dependence of the brightness of the developed film depends in a continuous manner on the brightness of the object being photographed, there will be a shade of gray that, when photographed, will produce exactly the same shade of gray on the developed film. This is the essence of Wheeler and Feynman’s idea. Let us first be a bit more precise and then a bit more general.

For simplicity let us suppose that the film is always a uniform shade of gray (i.e., at any time the shade of gray does not vary by location on the film). The possible shades of gray of the film can then be represented by the (real) numbers from 0, representing pure black, to 1, representing pure white.

Let us now distinguish various stages in the chronological order of the life of the film. In stage \(S_1\) the film is young; it has just been placed in the camera and is ready to be exposed. It is then exposed to the object that comes out of the time machine. (That object in fact is a later stage of the film itself). By the time we come to stage \(S_2\) of the life of the film, it has been developed and is about to enter the time machine. Stage \(S_3\) occurs just after it exits the time machine and just before it is photographed. Stage \(S_4\) occurs after it has been photographed and before it starts fading away. Let us assume that the film starts out in stage \(S_1\) in some uniform shade of gray, and that the only significant change in the shade of gray of the film occurs between stages \(S_1\) and \(S_2\). During that period it acquires a shade of gray that depends on the shade of gray of the object that was photographed. In other words, the shade of gray that the film acquires at stage \(S_2\) depends on the shade of gray it has at stage \(S_3\). The influence of the shade of gray of the film at stage \(S_3\), on the shade of gray of the film at stage \(S_2\), can be represented as a mapping, or function, from the real numbers between 0 and 1 (inclusive), to the real numbers between 0 and 1 (inclusive). Let us suppose that the process of photography is such that if one imagines varying the shade of gray of an object in a smooth, continuous manner then the shade of gray of the developed picture of that object will also vary in a smooth, continuous manner. This implies that the function in question will be a continuous function. Now any continuous function from the real numbers between 0 and 1 (inclusive) to the real numbers between 0 and 1 (inclusive) must map at least one number to itself. One can quickly convince oneself of this by graphing such functions. For one will quickly see that any continuous function \(f\) from \([0,1]\) to \([0,1]\) must intersect the line \(x=y\) somewhere, and thus there must be at least one point \(x\) such that \(f(x)=x\). Such points are called fixed points of the function. Now let us think about what such a fixed point represents. It represents a shade of gray such that, when photographed, it will produce a developed film with exactly that same shade of gray. The existence of such a fixed point implies a solution to the apparent paradox.

Let us now be more general and allow color photography. One can represent each possible color of an object (of uniform color) by the proportions of blue, green and red that make up that color. (This is why television screens can produce all possible colors.) Thus one can represent all possible colors of an object by three points on three orthogonal lines \(x, y\) and \(z\), that is to say, by a point in a three-dimensional cube. This cube is also known as the “Cartesian product” of the three line segments. Now, one can also show that any continuous map from such a cube to itself must have at least one fixed point. So color photography can not be used to create time travel paradoxes either!

Even more generally, consider some system \(P\) which, as in the above example, has the following life. It starts in some state \(S_1\), it interacts with an object that comes out of a time machine (which happens to be its older self), it travels back in time, it interacts with some object (which happens to be its younger self), and finally it grows old and dies. Let us assume that the set of possible states of \(P\) can be represented by a Cartesian product of \(n\) closed intervals of the reals, i.e., let us assume that the topology of the state-space of \(P\) is isomorphic to a finite Cartesian product of closed intervals of the reals. Let us further assume that the development of \(P\) in time, and the dependence of that development on the state of objects that it interacts with, is continuous. Then, by a well-known fixed point theorem in topology (see, e.g., Hocking & Young 1961: 273), no matter what the nature of the interaction is, and no matter what the initial state of the object is, there will be at least one state \(S_3\) of the older system (as it emerges from the time travel machine) that will influence the initial state \(S_1\) of the younger system (when it encounters the older system) so that, as the younger system becomes older, it develops exactly into state \(S_3\). Thus without imposing any constraints on the initial state \(S_1\) of the system \(P\), we have shown that there will always be perfectly ordinary, non-paradoxical, solutions, in which everything that happens, happens according to the usual laws of development. Of course, there is looped causation, hence presumably also looped explanation, but what do you expect if there is looped time?

Unfortunately, for the fan of time travel, a little reflection suggests that there are systems for which the needed fixed point theorem does not hold. Imagine, for instance, that we have a dial that can only rotate in a plane. We are going to put the dial in the time machine. Indeed we have decided that if we see the later stage of the dial come out of the time machine set at angle \(x\), then we will set the dial to \(x+90\), and throw it into the time machine. Now it seems we have a paradox, since the mapping that consists of a rotation of all points in a circular state-space by 90 degrees does not have a fixed point. And why wouldn’t some state-spaces have the topology of a circle?

However, we have so far not used another continuity assumption which is also a reasonable assumption. So far we have only made the following demand: the state the dial is in at stage \(S_2\) must be a continuous function of the state of the dial at stage \(S_3\). But, the state of the dial at stage \(S_2\) is arrived at by taking the state of the dial at stage \(S_1\), and rotating it over some angle. It is not merely the case that the effect of the interaction, namely the state of the dial at stage \(S_2\), should be a continuous function of the cause, namely the state of the dial at stage \(S_3\). It is additionally the case that path taken to get there, the way the dial is rotated between stages \(S_1\) and \(S_2\) must be a continuous function of the state at stage \(S_3\). And, rather surprisingly, it turns out that this can not be done. Let us illustrate what the problem is before going to a more general demonstration that there must be a fixed point solution in the dial case.

Forget time travel for the moment. Suppose that you and I each have a watch with a single dial neither of which is running. My watch is set at 12. You are going to announce what your watch is set at. My task is going to be to adjust my watch to yours no matter what announcement you make. And my actions should have a continuous (single valued) dependence on the time that you announce. Surprisingly, this is not possible! For instance, suppose that if you announce “12”, then I achieve that setting on my watch by doing nothing. Now imagine slowly and continuously increasing the announced times, starting at 12. By continuity, I must achieve each of those settings by rotating my dial to the right. If at some point I switch and achieve the announced goal by a rotation of my dial to the left, I will have introduced a discontinuity in my actions, a discontinuity in the actions that I take as a function of the announced angle. So I will be forced, by continuity, to achieve every announcement by rotating the dial to the right. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. So, there will be a discontinuity at 12 at the latest. In general, continuity of my actions as a function of announced times can not be maintained throughout if I am to be able to replicate all possible settings. Another way to see the problem is that one can similarly reason that, as one starts with 12, and imagines continuously making the announced times earlier, one will be forced, by continuity, to achieve the announced times by rotating the dial to the left. But the conclusions drawn from the assumption of continuous increases and the assumption of continuous decreases are inconsistent. So we have an inconsistency following from the assumption of continuity and the assumption that I always manage to set my watch to your watch. So, a dial developing according to a continuous dynamics from a given initial state, can not be set up so as to react to a second dial, with which it interacts, in such a way that it is guaranteed to always end up set at the same angle as the second dial. Similarly, it can not be set up so that it is guaranteed to always end up set at 90 degrees to the setting of the second dial. All of this has nothing to do with time travel. However, the impossibility of such set ups is what prevents us from enacting the rotation by 90 degrees that would create paradox in the time travel setting.

Let us now give the positive result that with such dials there will always be fixed point solutions, as long as the dynamics is continuous. Let us call the state of the dial before it interacts with its older self the initial state of the dial. And let us call the state of the dial after it emerges from the time machine the final state of the dial. There is also an intermediate state of the dial, after it interacts with its older self and before it is put into the time machine. We can represent the initial or intermediate states of the dial, before it goes into the time machine, as an angle \(x\) in the horizontal plane and the final state of the dial, after it comes out of the time machine, as an angle \(y\) in the vertical plane. All possible \(\langle x,y\rangle\) pairs can thus be visualized as a torus with each \(x\) value picking out a vertical circular cross-section and each \(y\) picking out a point on that cross-section. See figure 1 .

Figure 1 [An extended description of figure 1 is in the supplement.]

Suppose that the dial starts at angle \(i\) which picks out vertical circle \(I\) on the torus. The initial angle \(i\) that the dial is at before it encounters its older self, and the set of all possible final angles that the dial can have when it emerges from the time machine is represented by the circle \(I\) on the torus (see figure 1 ). Given any possible angle of the emerging dial, the dial initially at angle \(i\) will develop to some other angle. One can picture this development by rotating each point on \(I\) in the horizontal direction by the relevant amount. Since the rotation has to depend continuously on the angle of the emerging dial, circle \(I\) during this development will deform into some loop \(L\) on the torus. Loop \(L\) thus represents all possible intermediate angles \(x\) that the dial is at when it is thrown into the time machine, given that it started at angle \(i\) and then encountered a dial (its older self) which was at angle \(y\) when it emerged from the time machine. We therefore have consistency if \(x=y\) for some \(x\) and \(y\) on loop \(L\). Now, let loop \(C\) be the loop which consists of all the points on the torus for which \(x=y\). Ring \(I\) intersects \(C\) at point \(\langle i,i\rangle\). Obviously any continuous deformation of \(I\) must still intersect \(C\) somewhere. So \(L\) must intersect \(C\) somewhere, say at \(\langle j,j\rangle\). But that means that no matter how the development of the dial starting at \(I\) depends on the angle of the emerging dial, there will be some angle for the emerging dial such that the dial will develop exactly into that angle (by the time it enters the time machine) under the influence of that emerging dial. This is so no matter what angle one starts with, and no matter how the development depends on the angle of the emerging dial. Thus even for a circular state-space there are no constraints needed other than continuity.

Unfortunately there are state-spaces that escape even this argument. Consider for instance a pointer that can be set to all values between 0 and 1, where 0 and 1 are not possible values. That is, suppose that we have a state-space that is isomorphic to an open set of real numbers. Now suppose that we have a machine that sets the pointer to half the value that the pointer is set at when it emerges from the time machine.

Figure 2 [An extended description of figure 2 is in the supplement.]

Suppose the pointer starts at value \(I\). As before we can represent the combination of this initial position and all possible final positions by the line \(I\). Under the influence of the pointer coming out of the time machine the pointer value will develop to a value that equals half the value of the final value that it encountered. We can represent this development as the continuous deformation of line \(I\) into line \(L\), which is indicated by the arrows in figure 2 . This development is fully continuous. Points \(\langle x,y\rangle\) on line \(I\) represent the initial position \(x=I\) of the (young) pointer, and the position \(y\) of the older pointer as it emerges from the time machine. Points \(\langle x,y\rangle\) on line \(L\) represent the position \(x\) that the younger pointer should develop into, given that it encountered the older pointer emerging from the time machine set at position \(y\). Since the pointer is designed to develop to half the value of the pointer that it encounters, the line \(L\) corresponds to \(x=1/2 y\). We have consistency if there is some point such that it develops into that point, if it encounters that point. Thus, we have consistency if there is some point \(\langle x,y\rangle\) on line \(L\) such that \(x=y\). However, there is no such point: lines \(L\) and \(C\) do not intersect. Thus there is no consistent solution, despite the fact that the dynamics is fully continuous.

Of course if 0 were a possible value, \(L\) and \(C\) would intersect at 0. This is surprising and strange: adding one point to the set of possible values of a quantity here makes the difference between paradox and peace. One might be tempted to just add the extra point to the state-space in order to avoid problems. After all, one might say, surely no measurements could ever tell us whether the set of possible values includes that exact point or not. Unfortunately there can be good theoretical reasons for supposing that some quantity has a state-space that is open: the set of all possible speeds of massive objects in special relativity surely is an open set, since it includes all speeds up to, but not including, the speed of light. Quantities that have possible values that are not bounded also lead to counter examples to the presented fixed point argument. And it is not obvious to us why one should exclude such possibilities. So the argument that no constraints are needed is not fully general.

An interesting question of course is: exactly for which state-spaces must there be such fixed points? The arguments above depend on a well-known fixed point theorem (due to Schauder) that guarantees the existence of a fixed point for compact, convex state spaces. We do not know what subsequent extensions of this result imply regarding fixed points for a wider variety of systems, or whether there are other general results along these lines. (See Kutach 2003 for more on this issue.)

A further interesting question is whether this line of argument is sufficient to resolve Consistency (see also Dowe 2007). When they apply, these results establish the existence of a solution, such as the shade of uniform gray in the first example. But physicists routinely demand more than merely the existence of a solution, namely that solutions to the equations are stable—such that “small” changes of the initial state lead to “small” changes of the resulting trajectory. (Clarifying the two senses of “small” in this statement requires further work, specifying the relevant topology.) Stability in this sense underwrites the possibility of applying equations to real systems given our inability to fix initial states with indefinite precision. (See Fletcher 2020 for further discussion.) The fixed point theorems guarantee that for an initial state \(S_1\) there is a solution, but this solution may not be “close” to the solution for a nearby initial state, \(S'\). We are not aware of any proofs that the solutions guaranteed to exist by the fixed point theorems are also stable in this sense.

Time travel has recently been discussed quite extensively in the context of general relativity. General relativity places few constraints on the global structure of space and time. This flexibility leads to a possibility first described in print by Hermann Weyl:

Every world-point is the origin of the double-cone of the active future and the passive past [i.e., the two lobes of the light cone]. Whereas in the special theory of relativity these two portions are separated by an intervening region, it is certainly possible in the present case [i.e., general relativity] for the cone of the active future to overlap with that of the passive past; so that, in principle, it is possible to experience events now that will in part be an effect of my future resolves and actions. Moreover, it is not impossible for a world-line (in particular, that of my body), although it has a timelike direction at every point, to return to the neighborhood of a point which it has already once passed through. (Weyl 1918/1920 [1952: 274])

A time-like curve is simply a space-time trajectory such that the speed of light is never equaled or exceeded along this trajectory. Time-like curves represent possible trajectories of ordinary objects. In general relativity a curve that is everywhere timelike locally can nonetheless loop back on itself, forming a CTC. Weyl makes the point vividly in terms of the light cones: along such a curve, the future lobe of the light cone (the “active future”) intersects the past lobe of the light cone (the “passive past”). Traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited. Or, by staying close to such a CTC, one could come arbitrarily close to a point in space-time that one previously visited. General relativity, in a straightforward sense, allows time travel: there appear to be many space-times compatible with the fundamental equations of general relativity in which there are CTC’s. Space-time, for instance, could have a Minkowski metric everywhere, and yet have CTC’s everywhere by having the temporal dimension (topologically) rolled up as a circle. Or, one can have wormhole connections between different parts of space-time which allow one to enter “mouth \(A\)” of such a wormhole connection, travel through the wormhole, exit the wormhole at “mouth \(B\)” and re-enter “mouth \(A\)” again. CTCs can even arise when the spacetime is topologically \(\mathbb{R}^4\), due to the “tilting” of light cones produced by rotating matter (as in Gödel 1949’s spacetime).

General relativity thus appears to provide ample opportunity for time travel. Note that just because there are CTC’s in a space-time, this does not mean that one can get from any point in the space-time to any other point by following some future directed timelike curve—there may be insurmountable practical obstacles. In Gödel’s spacetime, it is the case that there are CTCs passing through every point in the spacetime. Yet these CTCs are not geodesics, so traversing them requires acceleration. Calculations of the minimal fuel required to travel along the appropriate curve should discourage any would-be time travelers (Malament 1984, 1985; Manchak 2011). But more generally CTCs may be confined to smaller regions; some parts of space-time can have CTC’s while other parts do not. Let us call the part of a space-time that has CTC’s the “time travel region” of that space-time, while calling the rest of that space-time the “normal region”. More precisely, the “time travel region” consists of all the space-time points \(p\) such that there exists a (non-zero length) timelike curve that starts at \(p\) and returns to \(p\). Now let us turn to examining space-times with CTC’s a bit more closely for potential problems.

In order to get a feeling for the sorts of implications that closed timelike curves can have, it may be useful to consider two simple models. In space-times with closed timelike curves the traditional initial value problem cannot be framed in the usual way. For it presupposes the existence of Cauchy surfaces, and if there are CTCs then no Cauchy surface exists. (A Cauchy surface is a spacelike surface such that every inextendable timelike curve crosses it exactly once. One normally specifies initial conditions by giving the conditions on such a surface.) Nonetheless, if the topological complexities of the manifold are appropriately localized, we can come quite close. Let us call an edgeless spacelike surface \(S\) a quasi-Cauchy surface if it divides the rest of the manifold into two parts such that

• every point in the manifold can be connected by a timelike curve to \(S\), and
• any timelike curve which connects a point in one region to a point in the other region intersects \(S\) exactly once.

It is obvious that a quasi-Cauchy surface must entirely inhabit the normal region of the space-time; if any point \(p\) of \(S\) is in the time travel region, then any timelike curve which intersects \(p\) can be extended to a timelike curve which intersects \(S\) near \(p\) again. In extreme cases of time travel, a model may have no normal region at all (e.g., Minkowski space-time rolled up like a cylinder in a time-like direction), in which case our usual notions of temporal precedence will not apply. But temporal anomalies like wormholes (and time machines) can be sufficiently localized to permit the existence of quasi-Cauchy surfaces.

Given a timelike orientation, a quasi-Cauchy surface unproblematically divides the manifold into its past (i.e., all points that can be reached by past-directed timelike curves from \(S)\) and its future (ditto mutatis mutandis ). If the whole past of \(S\) is in the normal region of the manifold, then \(S\) is a partial Cauchy surface : every inextendable timelike curve which exists to the past of \(S\) intersects \(S\) exactly once, but (if there is time travel in the future) not every inextendable timelike curve which exists to the future of \(S\) intersects \(S\). Now we can ask a particularly clear question: consider a manifold which contains a time travel region, but also has a partial Cauchy surface \(S\), such that all of the temporal funny business is to the future of \(S\). If all you could see were \(S\) and its past, you would not know that the space-time had any time travel at all. The question is: are there any constraints on the sort of data which can be put on \(S\) and continued to a global solution of the dynamics which are different from the constraints (if any) on the data which can be put on a Cauchy surface in a simply connected manifold and continued to a global solution? If there is time travel to our future, might we we able to tell this now, because of some implied oddity in the arrangement of present things?

It is not at all surprising that there might be constraints on the data which can be put on a locally space-like surface which passes through the time travel region: after all, we never think we can freely specify what happens on a space-like surface and on another such surface to its future, but in this case the surface at issue lies to its own future. But if there were particular constraints for data on a partial Cauchy surface then we would apparently need to have to rule out some sorts of otherwise acceptable states on \(S\) if there is to be time travel to the future of \(S\). We then might be able to establish that there will be no time travel in the future by simple inspection of the present state of the universe. As we will see, there is reason to suspect that such constraints on the partial Cauchy surface are non-generic. But we are getting ahead of ourselves: first let’s consider the effect of time travel on a very simple dynamics.

The simplest possible example is the Newtonian theory of perfectly elastic collisions among equally massive particles in one spatial dimension. The space-time is two-dimensional, so we can represent it initially as the Euclidean plane, and the dynamics is completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an “\(X\)” in space-time, with each particle changing its momentum at the impact. [ 2 ] The dynamics is purely local, in that one can check that a set of world-lines constitutes a model of the dynamics by checking that the dynamics is obeyed in every arbitrarily small region. It is also trivial to generate solutions from arbitrary initial data if there are no CTCs: given the initial positions and momenta of a set of particles, one simply draws a straight line from each particle in the appropriate direction and continues it indefinitely. Once all the lines are drawn, the worldline of each particle can be traced from collision to collision. The boundary value problem for this dynamics is obviously well-posed: any set of data at an instant yields a unique global solution, constructed by the method sketched above.

What happens if we change the topology of the space-time by hand to produce CTCs? The simplest way to do this is depicted in figure 3 : we cut and paste the space-time so it is no longer simply connected by identifying the line \(L-\) with the line \(L+\). Particles “going in” to \(L+\) from below “emerge” from \(L-\) , and particles “going in” to \(L-\) from below “emerge” from \(L+\).

Figure 3: Inserting CTCs by Cut and Paste. [An extended description of figure 3 is in the supplement.]

How is the boundary-value problem changed by this alteration in the space-time? Before the cut and paste, we can put arbitrary data on the simultaneity slice \(S\) and continue it to a unique solution. After the change in topology, \(S\) is no longer a Cauchy surface, since a CTC will never intersect it, but it is a partial Cauchy surface. So we can ask two questions. First, can arbitrary data on \(S\) always be continued to a global solution? Second, is that solution unique? If the answer to the first question is \(no\), then we have a backward-temporal constraint: the existence of the region with CTCs places constraints on what can happen on \(S\) even though that region lies completely to the future of \(S\). If the answer to the second question is \(no\), then we have an odd sort of indeterminism, analogous to the unwritten book: the complete physical state on \(S\) does not determine the physical state in the future, even though the local dynamics is perfectly deterministic and even though there is no other past edge to the space-time region in \(S\)’s future (i.e., there is nowhere else for boundary values to come from which could influence the state of the region).

In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on \(S\), but those data are always consistent with an infinitude of different global solutions. The easy way to see that there always is a solution is to construct the minimal solution in the following way. Start drawing straight lines from \(S\) as required by the initial data. If a line hits \(L-\) from the bottom, just continue it coming out of the top of \(L+\) in the appropriate place, and if a line hits \(L+\) from the bottom, continue it emerging from \(L-\) at the appropriate place. Figure 4 represents the minimal solution for a single particle which enters the time-travel region from the left:

Figure 4: The Minimal Solution. [An extended description of figure 4 is in the supplement.]

The particle “travels back in time” three times. It is obvious that this minimal solution is a global solution, since the particle always travels inertially.

But the same initial state on \(S\) is also consistent with other global solutions. The new requirement imposed by the topology is just that the data going into \(L+\) from the bottom match the data coming out of \(L-\) from the top, and the data going into \(L-\) from the bottom match the data coming out of \(L+\) from the top. So we can add any number of vertical lines connecting \(L-\) and \(L+\) to a solution and still have a solution. For example, adding a few such lines to the minimal solution yields:

Figure 5: A Non-Minimal Solution. [An extended description of figure 5 is in the supplement.]

The particle now collides with itself twice: first before it reaches \(L+\) for the first time, and again shortly before it exits the CTC region. From the particle’s point of view, it is traveling to the right at a constant speed until it hits an older version of itself and comes to rest. It remains at rest until it is hit from the right by a younger version of itself, and then continues moving off, and the same process repeats later. It is clear that this is a global model of the dynamics, and that any number of distinct models could be generating by varying the number and placement of vertical lines.

Knowing the data on \(S\), then, gives us only incomplete information about how things will go for the particle. We know that the particle will enter the CTC region, and will reach \(L+\), we know that it will be the only particle in the universe, we know exactly where and with what speed it will exit the CTC region. But we cannot determine how many collisions the particle will undergo (if any), nor how long (in proper time) it will stay in the CTC region. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Furthermore, the dynamics gives us no handle on what to think of the various possibilities: there are no probabilities assigned to the various distinct possible outcomes.

Changing the topology has changed the mathematics of the situation in two ways, which tend to pull in opposite directions. On the one hand, \(S\) is no longer a Cauchy surface, so it is perhaps not surprising that data on \(S\) do not suffice to fix a unique global solution. But on the other hand, there is an added constraint: data “coming out” of \(L-\) must exactly match data “going in” to \(L+\), even though what comes out of \(L-\) helps to determine what goes into \(L+\). This added consistency constraint tends to cut down on solutions, although in this case the additional constraint is more than outweighed by the freedom to consider various sorts of data on \({L+}/{L-}\).

The fact that the extra freedom outweighs the extra constraint also points up one unexpected way that the supposed paradoxes of time travel may be overcome. Let’s try to set up a paradoxical situation using the little closed time loop above. If we send a single particle into the loop from the left and do nothing else, we know exactly where it will exit the right side of the time travel region. Now suppose we station someone at the other side of the region with the following charge: if the particle should come out on the right side, the person is to do something to prevent the particle from going in on the left in the first place. In fact, this is quite easy to do: if we send a particle in from the right, it seems that it can exit on the left and deflect the incoming left-hand particle.

Carrying on our reflection in this way, we further realize that if the particle comes out on the right, we might as well send it back in order to deflect itself from entering in the first place. So all we really need to do is the following: set up a perfectly reflecting particle mirror on the right-hand side of the time travel region, and launch the particle from the left so that— if nothing interferes with it —it will just barely hit \(L+\). Our paradox is now apparently complete. If, on the one hand, nothing interferes with the particle it will enter the time-travel region on the left, exit on the right, be reflected from the mirror, re-enter from the right, and come out on the left to prevent itself from ever entering. So if it enters, it gets deflected and never enters. On the other hand, if it never enters then nothing goes in on the left, so nothing comes out on the right, so nothing is reflected back, and there is nothing to deflect it from entering. So if it doesn’t enter, then there is nothing to deflect it and it enters. If it enters, then it is deflected and doesn’t enter; if it doesn’t enter then there is nothing to deflect it and it enters: paradox complete.

But at least one solution to the supposed paradox is easy to construct: just follow the recipe for constructing the minimal solution, continuing the initial trajectory of the particle (reflecting it the mirror in the obvious way) and then read of the number and trajectories of the particles from the resulting diagram. We get the result of figure 6 :

Figure 6: Resolving the “Paradox”. [An extended description of figure 6 is in the supplement.]

As we can see, the particle approaching from the left never reaches \(L+\): it is deflected first by a particle which emerges from \(L-\). But it is not deflected by itself , as the paradox suggests, it is deflected by another particle. Indeed, there are now four particles in the diagram: the original particle and three particles which are confined to closed time-like curves. It is not the leftmost particle which is reflected by the mirror, nor even the particle which deflects the leftmost particle; it is another particle altogether.

The paradox gets it traction from an incorrect presupposition. If there is only one particle in the world at \(S\) then there is only one particle which could participate in an interaction in the time travel region: the single particle would have to interact with its earlier (or later) self. But there is no telling what might come out of \(L-\): the only requirement is that whatever comes out must match what goes in at \(L+\). So if you go to the trouble of constructing a working time machine, you should be prepared for a different kind of disappointment when you attempt to go back and kill yourself: you may be prevented from entering the machine in the first place by some completely unpredictable entity which emerges from it. And once again a peculiar sort of indeterminism appears: if there are many self-consistent things which could prevent you from entering, there is no telling which is even likely to materialize. This is just like the case of the unwritten book: the book is never written, so nothing determines what fills its pages.

So when the freedom to put data on \(L-\) outweighs the constraint that the same data go into \(L+\), instead of paradox we get an embarrassment of riches: many solution consistent with the data on \(S\), or many possible books. To see a case where the constraint “outweighs” the freedom, we need to construct a very particular, and frankly artificial, dynamics and topology. Consider the space of all linear dynamics for a scalar field on a lattice. (The lattice can be though of as a simple discrete space-time.) We will depict the space-time lattice as a directed graph. There is to be a scalar field defined at every node of the graph, whose value at a given node depends linearly on the values of the field at nodes which have arrows which lead to it. Each edge of the graph can be assigned a weighting factor which determines how much the field at the input node contributes to the field at the output node. If we name the nodes by the letters a , b , c , etc., and the edges by their endpoints in the obvious way, then we can label the weighting factors by the edges they are associated with in an equally obvious way.

Suppose that the graph of the space-time lattice is acyclic , as in figure 7 . (A graph is Acyclic if one can not travel in the direction of the arrows and go in a loop.)

Figure 7: An Acyclic Lattice. [An extended description of figure 7 is in the supplement.]

It is easy to regard a set of nodes as the analog of a Cauchy surface, e.g., the set \(\{a, b, c\}\), and it is obvious if arbitrary data are put on those nodes the data will generate a unique solution in the future. [ 3 ] If the value of the field at node \(a\) is 3 and at node \(b\) is 7, then its value at node \(d\) will be \(3W_{ad}\) and its value at node \(e\) will be \(3W_{ae} + 7W_{be}\). By varying the weighting factors we can adjust the dynamics, but in an acyclic graph the future evolution of the field will always be unique.

Let us now again artificially alter the topology of the lattice to admit CTCs, so that the graph now is cyclic. One of the simplest such graphs is depicted in figure 8 : there are now paths which lead from \(z\) back to itself, e.g., \(z\) to \(y\) to \(z\).

Figure 8: Time Travel on a Lattice. [An extended description of figure 8 is in the supplement.]

Can we now put arbitrary data on \(v\) and \(w\), and continue that data to a global solution? Will the solution be unique?

In the generic case, there will be a solution and the solution will be unique. The equations for the value of the field at \(x, y\), and \(z\) are:

Solving these equations for \(z\) yields

which gives a unique value for \(z\) in the generic case. But looking at the space of all possible dynamics for this lattice (i.e., the space of all possible weighting factors), we find a singularity in the case where \(1-W_{zx}W_{xz} - W_{zy}W_{yz} = 0\). If we choose weighting factors in just this way, then arbitrary data at \(v\) and \(w\) cannot be continued to a global solution. Indeed, if the scalar field is everywhere non-negative, then this particular choice of dynamics puts ironclad constraints on the value of the field at \(v\) and \(w\): the field there must be zero (assuming \(W_{vx}\) and \(W_{wy}\) to be non-zero), and similarly all nodes in their past must have field value zero. If the field can take negative values, then the values at \(v\) and \(w\) must be so chosen that \(vW_{vx}W_{xz} = -wW_{wy}W_{yz}\). In either case, the field values at \(v\) and \(w\) are severely constrained by the existence of the CTC region even though these nodes lie completely to the past of that region. It is this sort of constraint which we find to be unlike anything which appears in standard physics.

Our toy models suggest three things. The first is that it may be impossible to prove in complete generality that arbitrary data on a partial Cauchy surface can always be continued to a global solution: our artificial case provides an example where it cannot. The second is that such odd constraints are not likely to be generic: we had to delicately fine-tune the dynamics to get a problem. The third is that the opposite problem, namely data on a partial Cauchy surface being consistent with many different global solutions, is likely to be generic: we did not have to do any fine-tuning to get this result.

This third point leads to a peculiar sort of indeterminism, illustrated by the case of the unwritten book: the entire state on \(S\) does not determine what will happen in the future even though the local dynamics is deterministic and there are no other “edges” to space-time from which data could influence the result. What happens in the time travel region is constrained but not determined by what happens on \(S\), and the dynamics does not even supply any probabilities for the various possibilities. The example of the photographic negative discussed in section 2, then, seems likely to be unusual, for in that case there is a unique fixed point for the dynamics, and the set-up plus the dynamical laws determine the outcome. In the generic case one would rather expect multiple fixed points, with no room for anything to influence, even probabilistically, which would be realized. (See the supplement on

Remarks and Limitations on the Toy Models .

It is ironic that time travel should lead generically not to contradictions or to constraints (in the normal region) but to underdetermination of what happens in the time travel region by what happens everywhere else (an underdetermination tied neither to a probabilistic dynamics nor to a free edge to space-time). The traditional objection to time travel is that it leads to contradictions: there is no consistent way to complete an arbitrarily constructed story about how the time traveler intends to act. Instead, though, it appears that the more significant problem is underdetermination: the story can be consistently completed in many different ways.

Echeverria, Klinkhammer, and Thorne (1991) considered the case of 3-dimensional single hard spherical ball that can go through a single time travel wormhole so as to collide with its younger self.

Figure 9 [An extended description of figure 9 is in the supplement.]

The threat of paradox in this case arises in the following form. Consider the initial trajectory of a ball as it approaches the time travel region. For some initial trajectories, the ball does not undergo a collision before reaching mouth 1, but upon exiting mouth 2 it will collide with its earlier self. This leads to a contradiction if the collision is strong enough to knock the ball off its trajectory and deflect it from entering mouth 1. Of course, the Wheeler-Feynman strategy is to look for a “glancing blow” solution: a collision which will produce exactly the (small) deviation in trajectory of the earlier ball that produces exactly that collision. Are there always such solutions? [ 4 ]

Echeverria, Klinkhammer & Thorne found a large class of initial trajectories that have consistent “glancing blow” continuations, and found none that do not (but their search was not completely general). They did not produce a rigorous proof that every initial trajectory has a consistent continuation, but suggested that it is very plausible that every initial trajectory has a consistent continuation. That is to say, they have made it very plausible that, in the billiard ball wormhole case, the time travel structure of such a wormhole space-time does not result in constraints on states on spacelike surfaces in the non-time travel region.

In fact, as one might expect from our discussion in the previous section, they found the opposite problem from that of inconsistency: they found underdetermination. For a large class of initial trajectories there are multiple different consistent “glancing blow” continuations of that trajectory (many of which involve multiple wormhole traversals). For example, if one initially has a ball that is traveling on a trajectory aimed straight between the two mouths, then one obvious solution is that the ball passes between the two mouths and never time travels. But another solution is that the younger ball gets knocked into mouth 1 exactly so as to come out of mouth 2 and produce that collision. Echeverria et al. do not note the possibility (which we pointed out in the previous section) of the existence of additional balls in the time travel region. We conjecture (but have no proof) that for every initial trajectory of \(A\) there are some, and generically many, multiple-ball continuations.

Friedman, Morris, et al. (1990) examined the case of source-free non-self-interacting scalar fields traveling through such a time travel wormhole and found that no constraints on initial conditions in the non-time travel region are imposed by the existence of such time travel wormholes. In general there appear to be no known counter examples to the claim that in “somewhat realistic” time-travel space-times with a partial Cauchy surface there are no constraints imposed on the state on such a partial Cauchy surface by the existence of CTC’s. (See, e.g., Friedman & Morris 1991; Thorne 1994; Earman 1995; Earman, Smeenk, & Wüthrich 2009; and Dowe 2007.)

How about the issue of constraints in the time travel region \(T\)? Prima facie , constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface “small enough”. In the physics literature the following question has been asked: for any point \(p\) in \(T\), and any space-like surface \(S\) that includes \(p\) is there a neighborhood \(E\) of \(p\) in \(S\) such that any solution on \(E\) can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendability of local solutions to global ones, and some simple models in which one does not have such extendability, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. (See, e.g., Yurtsever 1990; Friedman, Morris, et al. 1990; Novikov 1992; Earman 1995; and Earman, Smeenk, & Wüthrich 2009). What are we to think of all of this?

The toy models above all treat billiard balls, fields, and other objects propagating through a background spacetime with CTCs. Even if we can show that a consistent solution exists, there is a further question: what kind of matter and dynamics could generate CTCs to begin with? There are various solutions of Einstein’s equations with CTCs, but how do these exotic spacetimes relate to the models actually used in describing the world? In other words, what positive reasons might we have to take CTCs seriously as a feature of the actual universe, rather than an exotic possibility of primarily mathematical interest?

We should distinguish two different kinds of “possibility” that we might have in mind in posing such questions (following Stein 1970). First, we can consider a solution as a candidate cosmological model, describing the (large-scale gravitational degrees of freedom of the) entire universe. The case for ruling out spacetimes with CTCs as potential cosmological models strikes us as, surprisingly, fairly weak. Physicists used to simply rule out solutions with CTCs as unreasonable by fiat, due to the threat of paradoxes, which we have dismantled above. But it is also challenging to make an observational case. Observations tell us very little about global features, such as the existence of CTCs, because signals can only reach an observer from a limited region of spacetime, called the past light cone. Our past light cone—and indeed the collection of all the past light cones for possible observers in a given spacetime—can be embedded in spacetimes with quite different global features (Malament 1977, Manchak 2009). This undercuts the possibility of using observations to constrain global topology, including (among other things) ruling out the existence of CTCs.

Yet the case in favor of taking cosmological models with CTCs seriously is also not particularly strong. Some solutions used to describe black holes, which are clearly relevant in a variety of astrophysical contexts, include CTCs. But the question of whether the CTCs themselves play an essential representational role is subtle: the CTCs arise in the maximal extensions of these solutions, and can plausibly be regarded as extraneous to successful applications. Furthermore, many of the known solutions with CTCs have symmetries, raising the possibility that CTCs are not a stable or robust feature. Slight departures from symmetry may lead to a solution without CTCs, suggesting that the CTCs may be an artifact of an idealized model.

The second sense of possibility regards whether “reasonable” initial conditions can be shown to lead to, or not to lead to, the formation of CTCs. As with the toy models above, suppose that we have a partial Cauchy surface \(S\), such that all the temporal funny business lies to the future. Rather than simply assuming that there is a region with CTCs to the future, we can ask instead whether it is possible to create CTCs by manipulating matter in the initial, well-behaved region—that is, whether it is possible to build a time machine. Several physicists have pursued “chronology protection theorems” aiming to show that the dynamics of general relativity (or some other aspects of physics) rules this out, and to clarify why this is the case. The proof of such a theorem would justify neglecting solutions with CTCs as a source of insight into the nature of time in the actual world. But as of yet there are several partial results that do not fully settle the question. One further intriguing possibility is that even if general relativity by itself does protect chronology, it may not be possible to formulate a sensible theory describing matter and fields in solutions with CTCs. (See SEP entry on Time Machines; Smeenk and Wüthrich 2011 for more.)

There is a different question regarding the limitations of these toy models. The toy models and related examples show that there are consistent solutions for simple systems in the presence of CTCs. As usual we have made the analysis tractable by building toy models, selecting only a few dynamical degrees of freedom and tracking their evolution. But there is a large gap between the systems we have described and the time travel stories they evoke, with Kurt traveling along a CTC with murderous intentions. In particular, many features of the manifest image of time are tied to the thermodynamical properties of macroscopic systems. Rovelli (unpublished) considers a extremely simple system to illustrate the problem: can a clock move along a CTC? A clock consists of something in periodic motion, such as a pendulum bob, and something that counts the oscillations, such as an escapement mechanism. The escapement mechanism cannot work without friction; this requires dissipation and increasing entropy. For a clock that counts oscillations as it moves along a time-like trajectory, the entropy must be a monotonically increasing function. But that is obviously incompatible with the clock returning to precisely the same state at some future time as it completes a loop. The point generalizes, obviously, to imply that anything like a human, with memory and agency, cannot move along a CTC.

Since it is not obvious that one can rid oneself of all constraints in realistic models, let us examine the argument that time travel is implausible, and we should think it unlikely to exist in our world, in so far as it implies such constraints. The argument goes something like the following. In order to satisfy such constraints one needs some pre-established divine harmony between the global (time travel) structure of space-time and the distribution of particles and fields on space-like surfaces in it. But it is not plausible that the actual world, or any world even remotely like ours, is constructed with divine harmony as part of the plan. In fact, one might argue, we have empirical evidence that conditions in any spatial region can vary quite arbitrarily. So we have evidence that such constraints, whatever they are, do not in fact exist in our world. So we have evidence that there are no closed time-like lines in our world or one remotely like it. We will now examine this argument in more detail by presenting four possible responses, with counterresponses, to this argument.

Response 1. There is nothing implausible or new about such constraints. For instance, if the universe is spatially closed, there has to be enough matter to produce the needed curvature, and this puts constraints on the matter distribution on a space-like hypersurface. Thus global space-time structure can quite unproblematically constrain matter distributions on space-like hypersurfaces in it. Moreover we have no realistic idea what these constraints look like, so we hardly can be said to have evidence that they do not obtain.

Counterresponse 1. Of course there are constraining relations between the global structure of space-time and the matter in it. The Einstein equations relate curvature of the manifold to the matter distribution in it. But what is so strange and implausible about the constraints imposed by the existence of closed time-like curves is that these constraints in essence have nothing to do with the Einstein equations. When investigating such constraints one typically treats the particles and/or field in question as test particles and/or fields in a given space-time, i.e., they are assumed not to affect the metric of space-time in any way. In typical space-times without closed time-like curves this means that one has, in essence, complete freedom of matter distribution on a space-like hypersurface. (See response 2 for some more discussion of this issue). The constraints imposed by the possibility of time travel have a quite different origin and are implausible. In the ordinary case there is a causal interaction between matter and space-time that results in relations between global structure of space-time and the matter distribution in it. In the time travel case there is no such causal story to be told: there simply has to be some pre-established harmony between the global space-time structure and the matter distribution on some space-like surfaces. This is implausible.

Response 2. Constraints upon matter distributions are nothing new. For instance, Maxwell’s equations constrain electric fields \(\boldsymbol{E}\) on an initial surface to be related to the (simultaneous) charge density distribution \(\varrho\) by the equation \(\varrho = \text{div}(\boldsymbol{E})\). (If we assume that the \(E\) field is generated solely by the charge distribution, this conditions amounts to requiring that the \(E\) field at any point in space simply be the one generated by the charge distribution according to Coulomb’s inverse square law of electrostatics.) This is not implausible divine harmony. Such constraints can hold as a matter of physical law. Moreover, if we had inferred from the apparent free variation of conditions on spatial regions that there could be no such constraints we would have mistakenly inferred that \(\varrho = \text{div}(\boldsymbol{E})\) could not be a law of nature.

Counterresponse 2. The constraints imposed by the existence of closed time-like lines are of quite a different character from the constraint imposed by \(\varrho = \text{div}(\boldsymbol{E})\). The constraints imposed by \(\varrho = \text{div}(\boldsymbol{E})\) on the state on a space-like hypersurface are:

• local constraints (i.e., to check whether the constraint holds in a region you just need to see whether it holds at each point in the region),
• quite independent of the global space-time structure,
• quite independent of how the space-like surface in question is embedded in a given space-time, and
• very simply and generally stateable.

On the other hand, the consistency constraints imposed by the existence of closed time-like curves (i) are not local, (ii) are dependent on the global structure of space-time, (iii) depend on the location of the space-like surface in question in a given space-time, and (iv) appear not to be simply stateable other than as the demand that the state on that space-like surface embedded in such and such a way in a given space-time, do not lead to inconsistency. On some views of laws (e.g., David Lewis’ view) this plausibly implies that such constraints, even if they hold, could not possibly be laws. But even if one does not accept such a view of laws, one could claim that the bizarre features of such constraints imply that it is implausible that such constraints hold in our world or in any world remotely like ours.

Response 3. It would be strange if there are constraints in the non-time travel region. It is not strange if there are constraints in the time travel region. They should be explained in terms of the strange, self-interactive, character of time travel regions. In this region there are time-like trajectories from points to themselves. Thus the state at such a point, in such a region, will, in a sense, interact with itself. It is a well-known fact that systems that interact with themselves will develop into an equilibrium state, if there is such an equilibrium state, or else will develop towards some singularity. Normally, of course, self-interaction isn’t true instantaneous self-interaction, but consists of a feed-back mechanism that takes time. But in time travel regions something like true instantaneous self-interaction occurs. This explains why constraints on states occur in such time travel regions: the states “ ab initio ” have to be “equilibrium states”. Indeed in a way this also provides some picture of why indeterminism occurs in time travel regions: at the onset of self-interaction states can fork into different equi-possible equilibrium states.

Counterresponse 3. This is explanation by woolly analogy. It all goes to show that time travel leads to such bizarre consequences that it is unlikely that it occurs in a world remotely like ours.

Response 4. All of the previous discussion completely misses the point. So far we have been taking the space-time structure as given, and asked the question whether a given time travel space-time structure imposes constraints on states on (parts of) space-like surfaces. However, space-time and matter interact. Suppose that one is in a space-time with closed time-like lines, such that certain counterfactual distributions of matter on some neighborhood of a point \(p\) are ruled out if one holds that space-time structure fixed. One might then ask

Why does the actual state near \(p\) in fact satisfy these constraints? By what divine luck or plan is this local state compatible with the global space-time structure? What if conditions near \(p\) had been slightly different?

And one might take it that the lack of normal answers to these questions indicates that it is very implausible that our world, or any remotely like it, is such a time travel universe. However the proper response to these question is the following. There are no constraints in any significant sense. If they hold they hold as a matter of accidental fact, not of law. There is no more explanation of them possible than there is of any contingent fact. Had conditions in a neighborhood of \(p\) been otherwise, the global structure of space-time would have been different. So what? The only question relevant to the issue of constraints is whether an arbitrary state on an arbitrary spatial surface \(S\) can always be embedded into a space-time such that that state on \(S\) consistently extends to a solution on the entire space-time.

But we know the answer to that question. A well-known theorem in general relativity says the following: any initial data set on a three dimensional manifold \(S\) with positive definite metric has a unique embedding into a maximal space-time in which \(S\) is a Cauchy surface (see, e.g., Geroch & Horowitz 1979: 284 for more detail), i.e., there is a unique largest space-time which has \(S\) as a Cauchy surface and contains a consistent evolution of the initial value data on \(S\). Now since \(S\) is a Cauchy surface this space-time does not have closed time like curves. But it may have extensions (in which \(S\) is not a Cauchy surface) which include closed timelike curves, indeed it may be that any maximal extension of it would include closed timelike curves. (This appears to be the case for extensions of states on certain surfaces of Taub-NUT space-times. See Earman, Smeenk, & Wüthrich 2009). But these extensions, of course, will be consistent. So properly speaking, there are no constraints on states on space-like surfaces. Nonetheless the space-time in which these are embedded may or may not include closed time-like curves.

Counterresponse 4. This, in essence, is the stonewalling answer which we indicated in section 1. However, whether or not you call the constraints imposed by a given space-time on distributions of matter on certain space-like surfaces “genuine constraints”, whether or not they can be considered lawlike, and whether or not they need to be explained, the existence of such constraints can still be used to argue that time travel worlds are so bizarre that it is implausible that our world or any world remotely like ours is a time travel world.

Suppose that one is in a time travel world. Suppose that given the global space-time structure of this world, there are constraints imposed upon, say, the state of motion of a ball on some space-like surface when it is treated as a test particle, i.e., when it is assumed that the ball does not affect the metric properties of the space-time it is in. (There is lots of other matter that, via the Einstein equation, corresponds exactly to the curvature that there is everywhere in this time travel worlds.) Now a real ball of course does have some effect on the metric of the space-time it is in. But let us consider a ball that is so small that its effect on the metric is negligible. Presumably it will still be the case that certain states of this ball on that space-like surface are not compatible with the global time travel structure of this universe.

This means that the actual distribution of matter on such a space-like surface can be extended into a space-time with closed time-like lines, but that certain counterfactual distributions of matter on this space-like surface can not be extended into the same space-time. But note that the changes made in the matter distribution (when going from the actual to the counterfactual distribution) do not in any non-negligible way affect the metric properties of the space-time. (Recall that the changes only effect test particles.) Thus the reason why the global time travel properties of the counterfactual space-time have to be significantly different from the actual space-time is not that there are problems with metric singularities or alterations in the metric that force significant global changes when we go to the counterfactual matter distribution. The reason that the counterfactual space-time has to be different is that in the counterfactual world the ball’s initial state of motion starting on the space-like surface, could not “meet up” in a consistent way with its earlier self (could not be consistently extended) if we were to let the global structure of the counterfactual space-time be the same as that of the actual space-time. Now, it is not bizarre or implausible that there is a counterfactual dependence of manifold structure, even of its topology, on matter distributions on spacelike surfaces. For instance, certain matter distributions may lead to singularities, others may not. We may indeed in some sense have causal power over the topology of the space-time we live in. But this power normally comes via the Einstein equations. But it is bizarre to think that there could be a counterfactual dependence of global space-time structure on the arrangement of certain tiny bits of matter on some space-like surface, where changes in that arrangement by assumption do not affect the metric anywhere in space-time in any significant way . It is implausible that we live in such a world, or that a world even remotely like ours is like that.

Let us illustrate this argument in a different way by assuming that wormhole time travel imposes constraints upon the states of people prior to such time travel, where the people have so little mass/energy that they have negligible effect, via the Einstein equation, on the local metric properties of space-time. Do you think it more plausible that we live in a world where wormhole time travel occurs but it only occurs when people’s states are such that these local states happen to combine with time travel in such a way that nobody ever succeeds in killing their younger self, or do you think it more plausible that we are not in a wormhole time travel world? [ 5 ]

An alternative approach to time travel (initiated by Deutsch 1991) abstracts away from the idealized toy models described above. [ 6 ] This computational approach considers instead the evolution of bits (simple physical systems with two discrete states) through a network of interactions, which can be represented by a circuit diagram with gates corresponding to the interactions. Motivated by the possibility of CTCs, Deutsch proposed adding a new kind of channel that connects the output of a given gate back to its input —in essence, a backwards-time step. More concretely, given a gate that takes \(n\) bits as input, we can imagine taking some number \(i \lt n\) of these bits through a channel that loops back and then do double-duty as inputs. Consistency requires that the state of these \(i\) bits is the same for output and input. (We will consider an illustration of this kind of system in the next section.) Working through examples of circuit diagrams with a CTC channel leads to similar treatments of Consistency and Underdetermination as the discussion above (see, e.g., Wallace 2012: § 10.6). But the approach offers two new insights (both originally due to Deutsch): the Easy Knowledge paradox, and a particularly clear extension to time travel in quantum mechanics.

A computer equipped with a CTC channel can exploit the need to find consistent evolution to solve remarkably hard problems. (This is quite different than the first idea that comes to mind to enhance computational power: namely to just devote more time to a computation, and then send the result back on the CTC to an earlier state.) The gate in a circuit incorporating a CTC implements a function from the input bits to the output bits, under the constraint that the output and input match the i bits going through the CTC channel. This requires, in effect, finding the fixed point of the relevant function. Given the generality of the model, there are few limits on the functions that could be implemented on the CTC circuit. Nature has to solve a hard computational problem just to ensure consistent evolution. This can then be extended to other complex computational problems—leading, more precisely, to solutions of NP -complete problems in polynomial time (see Aaronson 2013: Chapter 20 for an overview and further references). The limits imposed by computational complexity are an essential part of our epistemic situation, and computers with CTCs would radically change this.

We now turn to the application of the computational approach to the quantum physics of time travel (see Deutsch 1991; Deutsch & Lockwood 1994). By contrast with the earlier discussions of constraints in classical systems, they claim to show that time travel never imposes any constraints on the pre-time travel state of quantum systems. The essence of this account is as follows. [ 7 ]

A quantum system starts in state \(S_1\), interacts with its older self, after the interaction is in state \(S_2\), time travels while developing into state \(S_3\), then interacts with its younger self, and ends in state \(S_4\) (see figure 10 ).

Figure 10 [An extended description of figure 10 is in the supplement.]

Deutsch assumes that the set of possible states of this system are the mixed states, i.e., are represented by the density matrices over the Hilbert space of that system. Deutsch then shows that for any initial state \(S_1\), any unitary interaction between the older and younger self, and any unitary development during time travel, there is a consistent solution, i.e., there is at least one pair of states \(S_2\) and \(S_3\) such that when \(S_1\) interacts with \(S_3\) it will change to state \(S_2\) and \(S_2\) will then develop into \(S_3\). The states \(S_2, S_3\) and \(S_4\) will typically be not be pure states, i.e., will be non-trivial mixed states, even if \(S_1\) is pure. In order to understand how this leads to interpretational problems let us give an example. Consider a system that has a two dimensional Hilbert space with as a basis the states \(\vc{+}\) and \(\vc{-}\). Let us suppose that when state \(\vc{+}\) of the young system encounters state \(\vc{+}\) of the older system, they interact and the young system develops into state \(\vc{-}\) and the old system remains in state \(\vc{+}\). In obvious notation:

Similarly, suppose that:

Let us furthermore assume that there is no development of the state of the system during time travel, i.e., that \(\vc{+}_2\) develops into \(\vc{+}_3\), and that \(\vc{-}_2\) develops into \(\vc{-}_3\).

Now, if the only possible states of the system were \(\vc{+}\) and \(\vc{-}\) (i.e., if there were no superpositions or mixtures of these states), then there is a constraint on initial states: initial state \(\vc{+}_1\) is impossible. For if \(\vc{+}_1\) interacts with \(\vc{+}_3\) then it will develop into \(\vc{-}_2\), which, during time travel, will develop into \(\vc{-}_3\), which inconsistent with the assumed state \(\vc{+}_3\). Similarly if \(\vc{+}_1\) interacts with \(\vc{-}_3\) it will develop into \(\vc{+}_2\), which will then develop into \(\vc{+}_3\) which is also inconsistent. Thus the system can not start in state \(\vc{+}_1\).

But, says Deutsch, in quantum mechanics such a system can also be in any mixture of the states \(\vc{+}\) and \(\vc{-}\). Suppose that the older system, prior to the interaction, is in a state \(S_3\) which is an equal mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\). Then the younger system during the interaction will develop into a mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), which will then develop into a mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\), which is consistent! More generally Deutsch uses a fixed point theorem to show that no matter what the unitary development during interaction is, and no matter what the unitary development during time travel is, for any state \(S_1\) there is always a state \(S_3\) (which typically is not a pure state) which causes \(S_1\) to develop into a state \(S_2\) which develops into that state \(S_3\). Thus quantum mechanics comes to the rescue: it shows in all generality that no constraints on initial states are needed!

One might wonder why Deutsch appeals to mixed states: will superpositions of states \(\vc{+}\) and \(\vc{-}\) not suffice? Unfortunately such an idea does not work. Suppose again that the initial state is \(\vc{+}_1\). One might suggest that that if state \(S_3\) is

one will obtain a consistent development. For one might think that when initial state \(\vc{+}_1\) encounters the superposition

it will develop into superposition

and that this in turn will develop into

as desired. However this is not correct. For initial state \(\vc{+}_1\) when it encounters

will develop into the entangled state

In so far as one can speak of the state of the young system after this interaction, it is in the mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), not in the superposition

So Deutsch does need his recourse to mixed states.

This clarification of why Deutsch needs his mixtures does however indicate a serious worry about the simplifications that are part of Deutsch’s account. After the interaction the old and young system will (typically) be in an entangled state. Although for purposes of a measurement on one of the two systems one can say that this system is in a mixed state, one can not represent the full state of the two systems by specifying the mixed state of each separate part, as there are correlations between observables of the two systems that are not represented by these two mixed states, but are represented in the joint entangled state. But if there really is an entangled state of the old and young systems directly after the interaction, how is one to represent the subsequent development of this entangled state? Will the state of the younger system remain entangled with the state of the older system as the younger system time travels and the older system moves on into the future? On what space-like surfaces are we to imagine this total entangled state to be? At this point it becomes clear that there is no obvious and simple way to extend elementary non-relativistic quantum mechanics to space-times with closed time-like curves: we apparently need to characterize not just the entanglement between two systems, but entanglement relative to specific spacetime descriptions.

How does Deutsch avoid these complications? Deutsch assumes a mixed state \(S_3\) of the older system prior to the interaction with the younger system. He lets it interact with an arbitrary pure state \(S_1\) younger system. After this interaction there is an entangled state \(S'\) of the two systems. Deutsch computes the mixed state \(S_2\) of the younger system which is implied by this entangled state \(S'\). His demand for consistency then is just that this mixed state \(S_2\) develops into the mixed state \(S_3\). Now it is not at all clear that this is a legitimate way to simplify the problem of time travel in quantum mechanics. But even if we grant him this simplification there is a problem: how are we to understand these mixtures?

If we take an ignorance interpretation of mixtures we run into trouble. For suppose that we assume that in each individual case each older system is either in state \(\vc{+}_3\) or in state \(\vc{-}_3\) prior to the interaction. Then we regain our paradox. Deutsch instead recommends the following, many worlds, picture of mixtures. Suppose we start with state \(\vc{+}_1\) in all worlds. In some of the many worlds the older system will be in the \(\vc{+}_3\) state, let us call them A -worlds, and in some worlds, B -worlds, it will be in the \(\vc{-}_3\) state. Thus in A -worlds after interaction we will have state \(\vc{-}_2\) , and in B -worlds we will have state \(\vc{+}_2\). During time travel the \(\vc{-}_2\) state will remain the same, i.e., turn into state \(\vc{-}_3\), but the systems in question will travel from A -worlds to B -worlds. Similarly the \(\vc{+}\) \(_2\) states will travel from the B -worlds to the A -worlds, thus preserving consistency.

Now whatever one thinks of the merits of many worlds interpretations, and of this understanding of it applied to mixtures, in the end one does not obtain genuine time travel in Deutsch’s account. The systems in question travel from one time in one world to another time in another world, but no system travels to an earlier time in the same world. (This is so at least in the normal sense of the word “world”, the sense that one means when, for instance, one says “there was, and will be, only one Elvis Presley in this world.”) Thus, even if it were a reasonable view, it is not quite as interesting as it may have initially seemed. (See Wallace 2012 for a more sympathetic treatment, that explores several further implications of accepting time travel in conjunction with the many worlds interpretation.)

We close by acknowledging that Deutsch’s starting point—the claim that this computational model captures the essential features of quantum systems in a spacetime with CTCs—has been the subject of some debate. Several physicists have pursued a quite different treatment of evolution of quantum systems through CTC’s, based on considering the “post-selected” state (see Lloyd et al. 2011). Their motivations for implementing the consistency condition in terms of the post-selected state reflects a different stance towards quantum foundations. A different line of argument aims to determine whether Deutsch’s treatment holds as an appropriate limiting case of a more rigorous treatment, such as quantum field theory in curved spacetimes. For example, Verch (2020) establishes several results challenging the assumption that Deutsch’s treatment is tied to the presence of CTC’s, or that it is compatible with the entanglement structure of quantum fields.

What remains of the grandfather paradox in general relativistic time travel worlds is the fact that in some cases the states on edgeless spacelike surfaces are “overconstrained”, so that one has less than the usual freedom in specifying conditions on such a surface, given the time-travel structure, and in some cases such states are “underconstrained”, so that states on edgeless space-like surfaces do not determine what happens elsewhere in the way that they usually do, given the time travel structure. There can also be mixtures of those two types of cases. The extent to which states are overconstrained and/or underconstrained in realistic models is as yet unclear, though it would be very surprising if neither obtained. The extant literature has primarily focused on the problem of overconstraint, since that, often, either is regarded as a metaphysical obstacle to the possibility time travel, or as an epistemological obstacle to the plausibility of time travel in our world. While it is true that our world would be quite different from the way we normally think it is if states were overconstrained, underconstraint seems at least as bizarre as overconstraint. Nonetheless, neither directly rules out the possibility of time travel.

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent. For example, it involves no logical contradiction to suppose that water is not molecular, but if both chemistry and Kripke are right it is a metaphysical impossibility. We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle’s view contains no logical contradiction. It was certainly consistent with Aristotle’s conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can’t be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle’s theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

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• Lloyd, Seth, Lorenzo Maccone, Raul Garcia-Patron, Vittorio Giovannetti, and Yutaka Shikano, 2011, “Quantum Mechanics of Time Travel through Post-Selected Teleportation”, Physical Review D , 84(2): 025007. doi:10.1103/PhysRevD.84.025007
• Malament, David B., 1977, “Observationally Indistinguishable Spacetimes: Comments on Glymour’s Paper”, in Foundations of Space-Time Theories , John Earman, Clark N. Glymour, and John J. Stachel (eds.), (Minnesota Studies in the Philosophy of Science 8), Minneapolis, MN: University of Minnesota Press, 61–80.
• –––, 1984, “‘Time Travel’ in the Gödel Universe”, PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association , 1984(2): 91–100. doi:10.1086/psaprocbienmeetp.1984.2.192497
• –––, 1985, “Minimal Acceleration Requirements for ‘Time Travel’, in Gödel Space‐time”, Journal of Mathematical Physics , 26(4): 774–777. doi:10.1063/1.526566
• Manchak, John Byron, 2009, “Can We Know the Global Structure of Spacetime?”, Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics , 40(1): 53–56. doi:10.1016/j.shpsb.2008.07.004
• –––, 2011, “On Efficient ‘Time Travel’ in Gödel Spacetime”, General Relativity and Gravitation , 43(1): 51–60. doi:10.1007/s10714-010-1068-3
• Maudlin, Tim, 1990, “Time-Travel and Topology”, PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association , 1990(1): 303–315. doi:10.1086/psaprocbienmeetp.1990.1.192712
• Novikov, I. D., 1992, “Time Machine and Self-Consistent Evolution in Problems with Self-Interaction”, Physical Review D , 45(6): 1989–1994. doi:10.1103/PhysRevD.45.1989
• Smeenk, Chris and Christian Wüthrich, 2011, “Time Travel and Time Machines”, in the Oxford Handbook on Time , Craig Callender (ed.), Oxford: Oxford University Press, 577–630..
• Stein, Howard, 1970, “On the Paradoxical Time-Structures of Gödel”, Philosophy of Science , 37(4): 589–601. doi:10.1086/288328
• Thorne, Kip S., 1994, Black Holes and Time Warps: Einstein’s Outrageous Legacy , (Commonwealth Fund Book Program), New York: W.W. Norton.
• Verch, Rainer, 2020, “The D-CTC Condition in Quantum Field Theory”, in Progress and Visions in Quantum Theory in View of Gravity , Felix Finster, Domenico Giulini, Johannes Kleiner, and Jürgen Tolksdorf (eds.), Cham: Springer International Publishing, 221–232. doi:10.1007/978-3-030-38941-3_9
• Wallace, David, 2012, The Emergent Multiverse: Quantum Theory According to the Everett Interpretation , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780199546961.001.0001
• Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press. doi:10.1093/oso/9780198793335.001.0001
• Weyl, Hermann, 1918/1920 [1922/1952], Raum, Zeit, Materie , Berlin: Springer; fourth edition 1920. Translated as Space—Time—Matter , Henry Leopold Brose (trans.), New York: Dutton, 1922. Reprinted 1952, New York: Dover Publications.
• Wheeler, John Archibald and Richard Phillips Feynman, 1949, “Classical Electrodynamics in Terms of Direct Interparticle Action”, Reviews of Modern Physics , 21(3): 425–433. doi:10.1103/RevModPhys.21.425
• Yurtsever, Ulvi, 1990, “Test Fields on Compact Space‐times”, Journal of Mathematical Physics , 31(12): 3064–3078. doi:10.1063/1.528960

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Adlam, Emily, unpublished, “ Is There Causation in Fundamental Physics? New Insights from Process Matrices and Quantum Causal Modelling ”, 2022, arXiv: 2208.02721. doi:10.48550/ARXIV.2208.02721
• Rovelli, Carlo, unpublished, “ Can We Travel to the Past? Irreversible Physics along Closed Timelike Curves ”, arXiv: 1912.04702. doi:10.48550/ARXIV.1912.04702

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5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

• Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
• Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

• Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
• Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

• Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
• Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

• Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
• Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

• Books:  Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

• The Solution: time travel is impossible because of the very paradox it creates.
• Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
• The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
• Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

Related Posts

© Copyright 2023 Astronomy Trek

Do parallel universes exist? We might live in a multiverse.

Sci-fi loves parallel universes. But could we really be in one?

• Eternal inflation & the Big Bang theory
• Quantum mechanics
• Infinite space

The mirror-image universe

• Multiverse: For and against

Parallel universes in fiction

Parallel universes are no longer just a feature of a good sci-fi story. There are now some scientific theories that support the idea of parallel universes beyond our own. However, the multiverse theory remains one of the most controversial theories in science. 

Our universe is unimaginably big. Hundreds of billions, if not trillions, of galaxies spin through space, each containing billions or trillions of stars . Some researchers studying models of the universe speculate that the universe's diameter could be 7 billion light-years across. Others think it could be infinite.

But is it all that's out there? Science fiction loves the idea of a parallel universe, and the thought that we might be living just one of an infinite number of possible lives. Multiverses aren't reserved for "Star Trek," "Spiderman" and "Doctor Who," though. Real scientific theory explores, and in some cases supports, the case for universes outside, parallel to, or distant from but mirroring our own. 

Multiverses and parallel worlds are often argued in the context of other major scientific concepts like the Big Bang , string theory and quantum mechanics . 

Related: How big is the universe?

Eternal inflation, the Big Bang theory and parallel universes

Around 13.7 billion years ago, everything we know of was an infinitesimal singularity. Then, according to the Big Bang theory, it burst into action, inflating faster than the speed of light in all directions for a tiny fraction of a second. Before 10^-32 seconds had passed, the universe had exploded outward to 10^26 times its original size in a process called cosmic inflation . And that's all before the actual expansion of matter that we usually think of as the Big Bang itself, which was a consequence of all this inflation: As the inflation slowed, a flood of matter and radiation appeared, creating the classic Big Bang fireball, and began to form the atoms, molecules, stars and galaxies that populate the vastness of space that surrounds us. 

Related: How an inflating universe could create a multiverse  

That mysterious process of inflation and the Big Bang have convinced some researchers that multiple universes are possible, or even very likely. According to theoretical physicist Alexander Vilenkin of Tufts University in Massachusetts, inflation didn't end everywhere at the same time. While it ended for everything that we can detect from Earth 13.8 billion years ago, cosmic inflation in fact continues in other places. This is called the theory of eternal inflation. And as inflation ends in a particular place, a new bubble universe forms, Vilenkin wrote for Scientific American in 2011. 

Those bubble universes can't contact each other because they continue to expand indefinitely. If we were to set off for the edge of our bubble, where it might butt up against the next bubble universe over, we'd never reach it because the edge is zipping away from us faster than the speed of light, and faster than we could ever travel.

Related: How many stars are in the universe?  

But even if we could reach the next bubble, according to eternal inflation (combined with string theory), our familiar universe with its physical constants and habitable conditions could be totally different from the hypothetical bubble universe next to our own. 

"This picture of the universe, or multiverse, as it is called, explains the long-standing mystery of why the constants of nature appear to be fine-tuned for the emergence of life," Vilenkin wrote. "The reason is that intelligent observers exist only in those rare bubbles in which, by pure chance, the constants happen to be just right for life to evolve. The rest of the multiverse remains barren, but no one is there to complain about that."

Vilenkin's explanation implies that in some of the infinite bubble universes outside our own, there could be other intelligent observers. But in every instant that passes, we get farther away from them, and we will never intersect.

Quantum mechanics and parallel universes

Some researchers base their ideas of parallel universes on quantum mechanics, the mathematical description of subatomic particles. In quantum mechanics, multiple states of existence for tiny particles are all possible at the same time — a "wave function" encapsulates all of those possibilities. However, when we actually look, we only ever observe one of the possibilities. According to the Copenhagen interpretation of quantum mechanics as described by the Stanford Encyclopedia of Philosophy , we observe an outcome when the wave function "collapses" into a single reality. 

But the many-worlds theory proposes instead that every time one state, or outcome, is observed, there is another "world" in which a different quantum outcome becomes reality. This is a branching arrangement, in which instant by instant, our perceived universe branches into near-infinite alternatives. Those alternate universes are completely separate and unable to intersect, so while there may be uncountable versions of you living a life that's slightly — or wildly — different from your life in this world, you'd never know it. 

Related: Black holes, aliens, multiverse & Mars: Space TED talks you need to watch  

The many-worlds theory is the most "courageous" take on the quandary of quantum mechanics, physicist Sean Carroll wrote in his book, " Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime " (Dutton, 2019). He also argued that it is the most straightforward theory, although not without wrinkles. 

One of those wrinkles is that the many-worlds idea is not really falsifiable. This is an important component of scientific thought and is the way the scientific community develops ideas that can be explored with observation and experimentation. If there's no opportunity to find evidence against a theory, that's bad for science as a whole, science journalist John Horgan argued in a blog post for Scientific American .

Infinite space, infinite universes?

Some physicists believe in a flatter version of multiple universes. That is, if the universe that we live in goes on forever, there are only so many ways that the building blocks of matter can arrange themselves as they assemble across infinite space. Eventually, any finite number of particle types must repeat a particular arrangement. Hypothetically, in a big enough space, those particles must repeat arrangements as large as entire solar systems and galaxies.

So, your entire life might be repeated elsewhere in the universe, down to what you ate for breakfast yesterday. At least, that's the theory.

But if the universe began at a finite point, as nearly every physicist agrees that it did, an alternate version of you likely doesn't exist, according to astrophysicist Ethan Siegel's 2015 Medium article. 

According to Siegel, "the number of possible outcomes from particles in any Universe interacting with one another tends towards infinity faster than the number of possible Universes increases due to inflation." 

"So what does this mean for you?" Siegel wrote. "It means it's up to you to make this Universe count."

In a relatively recent addition to the pantheon of multiverse theories, researchers from the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, have proposed that the universe began at the Big Bang — and on the opposite side of the Big Bang timeline, stretching backwards in time, a universe once existed that was the exact mirror image of our own. 

"Instead of saying there was a different universe before the bang, we're saying that the universe before the bang is actually, in some sense, an image of the universe after the bang," Neil Turok, a Perimeter Institute researcher, told Space.com sister site Live Science .

That means everything — protons, electrons, even actions like cracking an egg — would be reversed. Antiprotons and positively charged electrons would make up atoms, while eggs would un-crack and make their way back inside chickens. Eventually, that universe would shrink down, presumably to a singularity, before expanding out into our own universe.

Seen another way, both universes were created at the Big Bang and exploded simultaneously backward and forward in time.

Multiverse: Arguments for and against

Arguments for the multiverse theory.

Cosmic inflation 

Our universe grew exponentially in the first moments of its existence, but was this expansion uniform? If not, it suggests different regions of space grew at different rates — and may be isolated from one another.

Mathematical constants 

How are the laws of the universe so exact? Some propose that this happened only by chance — we are the one universe out of many that happened to get the numbers right. 

The observable universe

What is beyond the edge of the observable space around us? No one knows for sure, and until we do (which could be never), the thought that ou universe extends indefinitely is an interesting one. 

Arguments against the multiverse theory

Falsifiability 

There is no way for us to ever test theories of the multiverse. We will never see beyond the observable universe, so if there is no way to disprove the theories, should they even be given credence? 

Occam's razor 

Sometimes, the simplest ideas are the best. Some physicists argue that we don't need the multiverse theory at all. It doesn't solve any paradoxes, and only creates complications. 

No evidence 

Not only can we not disprove any multiverse theory, we can't prove them either. We currently have no evidence that multiverses exists, and everything we can see suggests there is just one universe — our own. 

Countless works of myth and fiction draw from ideas of parallel universes and the multiverse. Overlapping worlds make appearances in Norse mythology as well as Buddhist and Hindu cosmology. The idea of multiple universes coming into contact showed up in print as early as Edwin A. Abbott's novella "Flatland: A Romance of Many Dimensions" (Seeley & Co., 1884), and can still be seen in recent movies such as the 2016 Marvel film "Doctor Strange." An entire genre of Japanese graphic novels, called isekai, deals with characters transported to parallel worlds, as described by the New York Public Library . 

Nearly every "Star Trek" series incorporates some form of mirror universe, and the 2009 reboot film starring Chris Pine and Zachary Quinto took subsequent "Star Trek" movies into an entirely new timeline that explicitly branches off from the original series.

And comics, as well as their corresponding movies, delve deeply into the idea of parallel worlds. Recent Marvel Comics' storylines (both film and in print), DC's Flashpoint arc and 2018's "Into the Spider-Verse" all explore multiple universes and the intersections between them.

This is an incomplete list of some appearances of multiverses, split-timeline universes and parallel universes in fiction:

• Into the Spider-Verse (2018)
• Terminator Genisys (2015) 
• Phineas and Ferb the Movie: Across the Second Dimension (2011) 
• Star Trek (2009) 
• Donnie Darko (2001) 
• Run Lola Run (1998) 
• Sliding Doors (1998) 
• Back to the Future 1-3 (1985, 1989, 1990) 
• The Adventures of Buckaroo Banzai Across the 8th Dimension (1984)
• Star Trek: Discovery, multiple episodes 
• Star Trek: Enterprise, multiple episodes 
• Star Trek: Deep Space Nine, multiple episodes 
• Star Trek: The Next Generation, "Parallels" (Episode 11, Season 7) (1993) 
• Star Trek: The Original Series, "Mirror, Mirror", (Episode 4, Season 2) (1967) 
• Doctor Who, multiple episodes  
• Sliders, entire series 
• Community, "Remedial Chaos Theory" (Episode 4, Season 3) (2011) 
• Rick and Morty, multiple episodes 
• Futurama, multiple episodes 
• Eureka, multiple episodes 
• Agents of Shield, multiple episodes
• "The Chronicles of Narnia" series (Geoffrey Bles, 1950-56) by C. S. Lewis
• "His Dark Materials" series (Scholastic, 1995-2000) by Phillip Pullman
• The "Discworld" series (HarperCollins, 1983-2015) by Terry Pratchett
• "Men Like Gods" (Macmillan, 1923) by H. G. Wells
• "The Dark Tower" series (Donald M. Grant, 1982-2012) by Stephen King

Video games

• BioShock Infinite, 2013
• Kingdom Hearts, 2002-2020
• Chrono Cross, 1999
• Half-Life, 1998-2020
• Metroid Prime 2: Echoes, 2004
• Zero Escape, 2009-2016

This article was adapted in part from previous work by Space.com contributor Elizabeth Howell. 

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Fanfic AU Ideas

In the world of fan fiction, creativity knows no bounds. This article introduces a variety of exciting Alternate Universe (AU) ideas, where beloved characters are reimagined in different settings.

Whether you’re a seasoned fanfic enthusiast or new to the world of AUs, these ideas promise to ignite your imagination and take you on a thrilling journey beyond the confines of canon storytelling, offering diverse and transformative narratives. Related post: 120+ Fanfic Title Ideas

Slice of life doesn’t have to be boring! Here are Alternate Universe ideas that take place in contemporary settings and amidst seemingly mundane daily life.

• Artistic Community AU : Characters are part of a vibrant artistic community, such as actors, writers, musicians, or dancers, exploring their creative passions, collaborations, and the pursuit of artistic fulfillment.
• Band AU : Imagine your characters forming a band or being part of the music industry, going on tour, and dealing with the challenges and triumphs of a musical career.
• Celebrity AU : Place your characters in the glamorous and chaotic world of fame and fortune, where they navigate the challenges of stardom, paparazzi, and intense media scrutiny.
• Celebrity Bodyguard AU : Pair your characters as a famous celebrity and their loyal bodyguard, exploring the complexities of their relationship as they face danger and navigate the spotlight.
• Coffee Shop AU: The characters work or frequent a cozy coffee shop where they navigate friendships, romances, and everyday life.
• Chef AU: The story revolves around a competitive cooking show, where characters engage in culinary battles, showcasing their skills and creativity in the kitchen.
• College/University AU: Follow your characters as they navigate the ups and downs of college life, from new friendships and relationships to academic challenges and self-discovery.
• Dark Romance AU: Two characters with tumultuous pasts and complex personalities are drawn to each other in a passionate, yet dangerous, relationship. As they navigate their intense connection, they delve into the depths of their darkest desires, challenging societal norms and their own moral compasses.
• Fake Dating AU: Two characters, forced by circumstances, pretend to be in a romantic relationship to achieve their individual goals. Then, conflicting emotions, humorous mishaps, and growing chemistry start to blur the lines between what’s real and what’s not. RELATED: Not Your Usual Fake Dating Prompts (2023) Pregnancy Prompts from Cute to Angsty (2023)
• High School AU : Explore the drama, friendships, and romance of teenage life by reimagining your characters as high school students navigating classes, cliques, and extracurricular activities. RELATED : High School Romance Story Ideas (2023)
• Office AU : Characters work together in a particular profession or industry, such as a law firm, tech start-up, hospital, or media company, navigating office politics, professional rivalries, and office romances.
• Roommates AU : Characters share a living space, whether it’s a college dorm, an apartment, or a house, experiencing the joys and challenges of cohabitation, friendships, and unexpected romances.
• Modern-Day Royalty AU : Transform your characters into modern day royalty, exploring the dynamics of power, duty, and love in a kingdom filled with intrigue and royal scandals.
• Musical AU : Integrate music and song into your story, with your characters involved in a band, musical theater, or even a singing competition, exploring their passions and dreams.
• Political Campaign AU : Characters become involved in a political campaign, working as campaign managers, journalists, or activists, navigating the complexities of elections, public opinion, and personal ideals.
• Reality TV Show AU : Characters participate in a reality TV show, where they must compete, form alliances, and face challenges, all while cameras capture their every move.
• Small Town AU : Characters reside in a quaint, close-knit small town, where everyone knows each other’s business, forming deep connections and facing the unique challenges of rural life.

• Sports AU: Place your characters in the world of competitive sports, whether it’s soccer, basketball, swimming, or any other athletic pursuit, and delve into their triumphs and struggles.
• Travel AU : Characters embark on a journey together, whether it’s a road trip, backpacking adventure, or exploring different countries, discovering new cultures, and forging deep connections along the way. Related posts: Dramatic Survival Story Ideas (2023) Family Drama Story Ideas (2023) Decadently Surreal Story Ideas (2023)

From ancient civilizations to more recent eras, the historical genre provides the perfect backdrop for romance, adventure, and intrigue for your favorite characters. Here are a few ideas:

• Ancient Egypt AU : Transport characters to the mystical world of ancient Egypt, where they become pharaohs, priestesses, or adventurers, unraveling ancient secrets and encountering gods and goddesses.
• Ancient Greece Olympics AU : Set in ancient Greece, characters compete in the Olympic Games, showcasing their athletic prowess, engaging in rivalries, and navigating the pantheon of gods.
• Ancient Rome Gladiator AU : Characters become gladiators, fighting for their lives in the grand arenas of ancient Rome, facing fierce battles, political maneuvering, and the spectacles of the Colosseum.
• Aztec Empire AU : Characters immerse themselves in the vibrant world of the Aztec civilization, exploring rituals, mythology, and the conquest of the New World.
• Cowboy AU: Place your characters in the Old West, where they ride horses, face outlaws, and experience the rugged frontier life. RELATED : 25+ Cowboy Writing Prompts (2023)
• Feudal Japan Samurai AU : Immerse characters in feudal Japan as samurais, ronin, or ninjas, exploring the samurai code, honor, and the conflicts of a war-torn era.
• Jazz Age AU : Characters find themselves in the glamorous and decadent world of the 1920s, experiencing the exuberance of flapper culture, speakeasies, and the jazz scene.
• Medieval AU: The characters embark on an epic adventure in a realm filled with knights, castles, and mystical creatures. They must navigate political intrigue, ancient prophecies, and the clash of kingdoms as they fight to protect their loved ones and restore balance to the realm. RELATED : The Best Medieval Story Starters (2023)
• Pirates AU: Plunge your characters into a world of swashbuckling pirates, treacherous seas, and hidden treasures. RELATED : Pirate Writing Prompts and Story Ideas (2023)
• Regency Romance AU : Transport characters to the elegant world of Jane Austen’s novels, where they experience ballroom etiquette, societal expectations, and the pursuit of love and marriage.
• Renaissance Art Studio AU : Characters become renowned artists, painters, or sculptors during the Renaissance, navigating the artistic rivalries, patronage, and the creation of masterpieces.
• Steampunk AU : The story is set in a Victorian-era world where advanced steam-powered technology and gadgets dominate, adding a sense of adventure and a touch of the industrial revolution. RELATED : The Most Creative Steampunk Story Ideas (2023)
• Tudor Court Intrigue AU : Set in the Tudor era, characters become courtiers, navigating the treacherous world of Henry VIII’s court, with its political schemes, betrayals, and royal romances.
• Victorian Era AU: Transport your characters to the elegant and romantic Victorian era, with its societal rules, lavish balls, and intricate courtship rituals.
• Viking AU : Characters become fierce Viking warriors, embarking on raids, exploring uncharted territories, and encountering mythical creatures in the Viking Age.
• World War I Nurse AU : Characters become nurses or doctors in the backdrop of the First World War, tending to wounded soldiers, experiencing the horrors of war, and finding love amidst chaos.
• World War II Resistance AU : Characters become part of the resistance movement during World War II, fighting against oppressive forces, forging alliances, and undertaking dangerous missions to protect freedom and justice. Related post: Historical Romance Story Ideas (2023)

More into werewolves and fairies? Here are some ideas:

• Animal Shapeshifter AU: Transform your characters into beings capable of shifting into animal forms, exploring themes of identity, acceptance, and the bond between humans and animals.
• Elemental Powers AU : Characters possess unique elemental powers (fire, water, air, earth) and must navigate a world where these abilities shape society and conflict.
• Fairy Court AU : Create a realm of fairies, elves, and mythical creatures, where your characters navigate the intricate politics and power struggles of the fairy court. Related post: Enchanted Forest Story Ideas (2023)
• Fairy Godparent AU : Give your characters magical godparents who grant wishes and guide them through life’s challenges, adding a touch of whimsy and magic to their journeys.
• Fairytale Retelling AU: Give a fresh twist to classic fairytales by placing your characters in a modern or futuristic setting, while still incorporating elements of magic and enchantment. RELATED : 13 Underrated Folktale & Folklore Story Ideas (2023) Bone-Chilling Urban Legend Story Ideas 2023
• Ghost Whisperer AU : The main character has the ability to communicate with spirits and helps solve mysteries and unresolved issues of the deceased, adding a supernatural element to the story. RELATED: The Eeriest Gothic Story Ideas (2023) The Most Unsettling Ghost Story Ideas (2023) Scary Story Starters that Will Unnerve You (2023) Horror Setting Ideas (2023)
• High Fantasy AU: Mythical creatures roam vast and majestic realms while heroes embark on epic quests to save the world from darkness. Magic, adventure, and ancient prophecies intertwine as ordinary individuals are thrust into extraordinary destinies. RELATED : Dragon Story Ideas (2023)
• Magical Bookshop AU : Characters work or frequent a mystical bookshop where the books have magical properties, transporting readers to different worlds or influencing their destinies.
• Mermaid/Merman AU : Dive into an underwater world, where your characters are merfolk exploring the depths of the ocean, encountering underwater kingdoms and mystical creatures. RELATED : Alluring Mermaid Story Ideas (2023)
• Mythology AU : Incorporate elements from various mythologies and folklore, giving your characters powers, abilities, and challenges inspired by gods, goddesses, and mythical creatures.
• Soulmate AU: Two characters are destined to be with each other, their soulmate, based on a predetermined bond or connection. It explores the concept of finding one’s perfect match, often with elements of destiny, fate, or supernatural forces at play. In these stories, the characters may have unique marks, symbols, or abilities that help them identify their soulmate, and the narrative revolves around their journey of discovery, challenges, and ultimately finding true love and companionship. RELATED : Enticing Soulmate Story Ideas (2023)
• Superhero AU : Imagine your favorite characters with superpowers, fighting crime and saving the world in an alternate universe where they are superheroes.
• Urban Fantasy AU : Set in modern day setting, populate your story with monsters, demons, vampires, werewolves, witches, or other supernatural beings, exploring their unique abilities, conflicts, and forbidden romances. RELATED: Urban Fantasy Story Ideas & Writing Prompts (2023) . Deep Dark Fantasy Story Ideas (2023) The Most Disturbing Demon Story Ideas (2023) The Most Unnerving Monster Story Ideas (2023) Vampire Romance Story Ideas & Writing Prompts (2023) Fantasy Romance Story Ideas (2023) Werewolves Story Ideas (2023) Paranormal Romance Story Ideas (2023)
• Time Travel AU: Send your characters on an exciting journey through time, allowing them to witness historical events, interact with famous figures, and potentially alter the course of history. RELATED: Not Your Usual Time Travel Story Ideas (2023)
• Witch/Wizard Academy AU: Enroll your characters in a magical academy where they learn spells, potions, and battle dark forces in a world filled with mystical creatures and hidden secrets. Related post: Spellbinding Witch Story Ideas (2023)
• Zombie Apocalypse AU: Explore a post-apocalyptic world infested with zombies, where your characters must fight for survival and forge unlikely alliances. RELATED : Unique Zombie Story Ideas (2023) “

The possibilities within the realm of Alternate Universes are limitless. Here are a few more ideas:

• Arranged Marriage AU: Two characters from different backgrounds are forced into a marital union for political or societal reasons. As they navigate their new relationship, they struggle with conflicting emotions, cultural clashes, and unexpected bonds that form amidst the pressures of duty and personal desires. RELATED : Not Your Usual Arranged Marriage Prompts (2023)
• Artificial Intelligence AU : Imagine a world where advanced AI technology is a prominent part of society, with your characters forming unique relationships with sentient machines. Related posts: The Most Intriguing Futuristic Story Ideas (2023)
• Canon Divergence AUs : In these AUs, the story diverges from canon at a specific point, creating an alternate timeline and exploring the consequences of that change.
• Criminal AU: a group of unlikely allies, each with their own illicit skills and pasts, come together to navigate the treacherous landscape of organized crime. Amidst personal agendas, the constant threat of law enforcement and rival factions, they form a bond that challenges their notions of right and wrong, blurring the line between hero and villain. RELATED: Cozy Mystery Story Ideas (2023) Crime Thriller Story Ideas (2023) Hero Story Ideas (2023) Murder Mystery Story Ideas (2023) Psychological Thriller Story Ideas (2023) Vigilante Writing Prompts (2023)
• Detective Noir AU: Dive into a gritty and atmospheric world of crime, mystery, and intrigue, where your characters become detectives, criminals, or femme fatales. RELATED : Detective Noir Writing Prompts & Story Ideas (2023)
• Dystopian AU: Characters are thrust into a bleak and oppressive world ruled by a tyrannical regime, navigating dangerous landscape, concealing their identities while developing covert strategies to resist the regime’s control.
• Galactic Academy AU : Characters attend an intergalactic academy where they learn to become space explorers, navigating alien cultures, and discovering the secrets of the universe. Related posts: Imaginative Alien Writing Prompts (2023) Underrated Sci-Fi Tropes (2023)
• Genderbend AU : Swap the genders of your characters, presenting new dynamics and exploring how their relationships and personalities change in this alternate reality.
• Harem AU (or Reverse Harem AU): In a harem AU, a central character finds himself surrounded by a group of romantic interests vying for his affections, navigating jealousy, conflicting emotions, and the challenge of making genuine connections amidst the chaotic dynamics of the harem. A reverse harem usually refers to the same situation, but with a female central character. Reverse Harem Story Ideas & Writing Prompts (2023)
• Mafia AU: In a gripping mafia AU, a reluctant outsider finds themselves entangled in the dangerous underworld of organized crime, where loyalties are tested and alliances are forged in blood. Intriguing Mafia Romance Story Ideas (2023)
• Parallel Universe AU : Explore the concept of parallel dimensions, where your characters encounter alternate versions of themselves and must navigate the consequences of crossing paths.
• Post-Apocalyptic AU : Set your story in a world devastated by a cataclysmic event, where your characters must adapt, survive, and rebuild society. RELATED: Not Your Usual Post Apocalypse Story Ideas (2023)
• Reincarnation AU : Explore the concept of reincarnation, where your characters are destined to meet and fall in love across different lifetimes, overcoming obstacles and discovering their shared past.
• Reverse Age AU : Characters are aged in reverse, starting as elderly individuals and gradually becoming younger as the story progresses.
• Role Reversal AU : In this AU, characters’ roles and relationships are altered. For instance, a hero becomes a villain, or allies become adversaries. This category also includes gender-swapped AUs, where characters change genders.
• Space Opera AU : Set your story in a futuristic universe where characters explore space, encounter alien civilizations, and embark on thrilling intergalactic adventures. RELATED: Space Opera Story Ideas and Writing Prompts 2023
• Spy/Secret Agent AU : Transform your characters into secret agents, spies, or detectives, embarking on dangerous missions, solving mysteries, and navigating a world of espionage. RELATED: Spy Writing Prompts & Story Ideas (2023)
• Time Loop AU : Characters find themselves trapped in a time loop, reliving the same events over and over again until they discover a way to break free and unravel the mystery.
• Utopian AU: Set in a utopian society, the characters uncover the hidden flaws and dark secrets that challenge the perfect facade to confront the ethical compromises and strive for true equality. It’s a thought-provoking narrative that explores the complexities of a seemingly flawless world and the consequences of extreme idealism. RELATED: Not Your Usual Utopian Story Ideas (2023)
• Virtual Reality AU : Characters enter a virtual reality world where they can create their own avatars and live out extraordinary adventures, blurring the lines between reality and the digital realm.
• What-If AU : This AU explores hypothetical scenarios by altering key events or decisions in the original story. They often ask “what if” questions, presenting alternative outcomes and exploring the consequences.

Now that we’ve seen some awesome ideas, let’s dive a little deeper into the subject!

Fanfic or fanfiction is a dynamic and diverse realm where devoted fans breathe fresh life into cherished stories. It provides a distinct platform for enthusiasts to immerse themselves in their favorite fictional worlds and engage with the source material in a personal and imaginative manner .

These stories are typically shared within fan communities, where authors can receive feedback and support from fellow enthusiasts. With numerous stories available in each fandom, choosing the right AU becomes essential to stand out and capture attention.

A well-chosen AU sets the stage for unique scenarios, character dynamics, and plot developments, offering creative freedom and engaging readers with fresh and intriguing concepts. By selecting the right AU, authors can delve into character development, showcase their writing skills, and connect with the fandom, creating a fanfic that resonates and captivates its audience.

If you need more story ideas and prompts, please browse our Story Ideas & Writing Prompts category!

Have any question or feedback? Feel free to contact me here . Until next time!

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Parallel Universe & Time Travel Movies

Parallel Universe Movies Time Travel Movies Parallel Universe & Time Travel Movies Parallel Universe and Time Travel Movies

• Movies or TV
• IMDb Rating
• In Theaters
• Release Year

1. Interstellar (2014)

PG-13 | 169 min | Adventure, Drama, Sci-Fi

When Earth becomes uninhabitable in the future, a farmer and ex-NASA pilot, Joseph Cooper, is tasked to pilot a spacecraft, along with a team of researchers, to find a new planet for humans.

Director: Christopher Nolan | Stars: Matthew McConaughey , Anne Hathaway , Jessica Chastain , Mackenzie Foy

Votes: 2,096,231 | Gross: $188.02M

2. Inception (2010)

PG-13 | 148 min | Action, Adventure, Sci-Fi

A thief who steals corporate secrets through the use of dream-sharing technology is given the inverse task of planting an idea into the mind of a C.E.O., but his tragic past may doom the project and his team to disaster.

Director: Christopher Nolan | Stars: Leonardo DiCaprio , Joseph Gordon-Levitt , Elliot Page , Ken Watanabe

Votes: 2,548,121 | Gross: $292.58M

3. Source Code (2011)

PG-13 | 93 min | Action, Drama, Mystery

A soldier wakes up in someone else's body and discovers he's part of an experimental government program to find the bomber of a commuter train within 8 minutes.

Director: Duncan Jones | Stars: Jake Gyllenhaal , Michelle Monaghan , Vera Farmiga , Jeffrey Wright

Votes: 549,395 | Gross: $54.71M

4. Edge of Tomorrow (2014)

PG-13 | 113 min | Action, Adventure, Sci-Fi

A soldier fighting aliens gets to relive the same day over and over again, the day restarting every time he dies.

Director: Doug Liman | Stars: Tom Cruise , Emily Blunt , Bill Paxton , Brendan Gleeson

Votes: 736,089 | Gross: $100.21M

5. Tenet (2020)

PG-13 | 150 min | Action, Sci-Fi, Thriller

Armed with only the word "Tenet," and fighting for the survival of the entire world, CIA operative, The Protagonist, journeys through a twilight world of international espionage on a global mission that unfolds beyond real time.

Director: Christopher Nolan | Stars: John David Washington , Robert Pattinson , Elizabeth Debicki , Juhan Ulfsak

Votes: 591,835 | Gross: $58.46M

6. Donnie Darko (2001)

R | 113 min | Drama, Mystery, Sci-Fi

After narrowly escaping a bizarre accident, a troubled teenager is plagued by visions of a man in a large rabbit suit who manipulates him to commit a series of crimes.

Director: Richard Kelly | Stars: Jake Gyllenhaal , Jena Malone , Mary McDonnell , Holmes Osborne

Votes: 849,449 | Gross: $1.48M

7. Coherence (2013)

Not Rated | 89 min | Drama, Mystery, Sci-Fi

Strange things begin to happen when a group of friends gather for a dinner party on an evening when a comet is passing overhead.

Director: James Ward Byrkit | Stars: Emily Baldoni , Maury Sterling , Nicholas Brendon , Elizabeth Gracen

Votes: 145,696 | Gross: $0.07M

8. The Butterfly Effect (2004)

R | 113 min | Drama, Sci-Fi, Thriller

Evan Treborn suffers blackouts during significant events of his life. As he grows up, he finds a way to remember these lost memories and a supernatural way to alter his life by reading his journal.

Directors: Eric Bress , J. Mackye Gruber | Stars: Ashton Kutcher , Amy Smart , Melora Walters , Elden Henson

Votes: 520,784 | Gross: $57.94M

9. Predestination (I) (2014)

R | 97 min | Action, Drama, Sci-Fi

As his last assignment, a temporal agent is tasked to travel back in time and prevent a bomb attack in New York in 1975. The hunt, however, turns out to be beyond the bounds of possibility.

Directors: Michael Spierig , Peter Spierig | Stars: Ethan Hawke , Sarah Snook , Noah Taylor , Madeleine West

Votes: 304,453 | Gross: $0.07M

10. Looper (2012)

R | 119 min | Action, Drama, Sci-Fi

In 2074, when the mob wants to get rid of someone, the target is sent into the past, where a hired gun awaits - someone like Joe - who one day learns the mob wants to 'close the loop' by sending back Joe's future self for assassination.

Director: Rian Johnson | Stars: Joseph Gordon-Levitt , Bruce Willis , Emily Blunt , Paul Dano

Votes: 602,646 | Gross: $66.49M

11. Groundhog Day (1993)

PG | 101 min | Comedy, Drama, Fantasy

A narcissistic, self-centered weatherman finds himself in a time loop on Groundhog Day.

Director: Harold Ramis | Stars: Bill Murray , Andie MacDowell , Chris Elliott , Stephen Tobolowsky

Votes: 684,360 | Gross: $70.91M

12. Mr. Nobody (2009)

R | 141 min | Drama, Fantasy, Romance

A boy stands on a station platform as a train is about to leave. Should he go with his mother or stay with his father? Infinite possibilities arise from this decision. As long as he doesn't choose, anything is possible.

Director: Jaco Van Dormael | Stars: Jared Leto , Sarah Polley , Diane Kruger , Linh-Dan Pham

Votes: 245,714 | Gross: $0.00M

13. Frequency (2000)

PG-13 | 118 min | Crime, Drama, Mystery

An accidental cross-time radio link connects father and son across 30 years. The son tries to save his father's life, but then must fix the consequences.

Director: Gregory Hoblit | Stars: Dennis Quaid , Jim Caviezel , Shawn Doyle , Elizabeth Mitchell

Votes: 115,683 | Gross: $45.01M

14. The Time Traveler's Wife (2009)

PG-13 | 107 min | Comedy, Drama, Fantasy

Henry DeTamble, a librarian, possesses a unique gene that lets him involuntarily travel through time. His wife, Claire Abshire, finds it difficult to cope with it.

Director: Robert Schwentke | Stars: Eric Bana , Rachel McAdams , Ron Livingston , Michelle Nolden

Votes: 157,840 | Gross: $63.41M

15. Deja Vu (2006)

PG-13 | 126 min | Action, Crime, Sci-Fi

After a ferry is bombed in New Orleans, an A.T.F. agent joins a unique investigation using experimental surveillance technology to find the bomber, but soon finds himself becoming obsessed with one of the victims.

Director: Tony Scott | Stars: Denzel Washington , Paula Patton , Jim Caviezel , Val Kilmer

Votes: 327,389 | Gross: $64.04M

16. Midnight in Paris (2011)

PG-13 | 94 min | Comedy, Fantasy, Romance

While on a trip to Paris with his fiancée's family, a nostalgic screenwriter finds himself mysteriously going back to the 1920s every day at midnight.

Director: Woody Allen | Stars: Owen Wilson , Rachel McAdams , Kathy Bates , Kurt Fuller

Votes: 449,662 | Gross: $56.82M

17. 12 Monkeys (1995)

R | 129 min | Mystery, Sci-Fi, Thriller

In a future world devastated by disease, a convict is sent back in time to gather information about the man-made virus that wiped out most of the human population on the planet.

Director: Terry Gilliam | Stars: Bruce Willis , Madeleine Stowe , Brad Pitt , Joseph Melito

Votes: 646,333 | Gross: $57.14M

18. Primer (2004)

PG-13 | 77 min | Drama, Sci-Fi, Thriller

Four friends/fledgling entrepreneurs, knowing that there's something bigger and more innovative than the different error-checking devices they've built, wrestle over their new invention.

Director: Shane Carruth | Stars: Shane Carruth , David Sullivan , Casey Gooden , Anand Upadhyaya

Votes: 114,068 | Gross: $0.42M

19. Time Lapse (2014)

Not Rated | 104 min | Mystery, Sci-Fi, Thriller

Three friends discover a mysterious machine that takes pictures twenty-four hours into the future, and conspire to use it for personal gain, until disturbing and dangerous images begin to develop.

Director: Bradley King | Stars: Danielle Panabaker , Matt O'Leary , George Finn , John Rhys-Davies

Votes: 49,107

20. Sliding Doors (1998)

PG-13 | 99 min | Comedy, Drama, Fantasy

After personal and professional setbacks, a woman experiences an alternate reality.

Director: Peter Howitt | Stars: Gwyneth Paltrow , John Hannah , John Lynch , Jeanne Tripplehorn

Votes: 71,458 | Gross: $11.88M

21. 2001: A Space Odyssey (1968)

G | 149 min | Adventure, Sci-Fi

After uncovering a mysterious artifact buried beneath the Lunar surface, a spacecraft is sent to Jupiter to find its origins: a spacecraft manned by two men and the supercomputer HAL 9000.

Director: Stanley Kubrick | Stars: Keir Dullea , Gary Lockwood , William Sylvester , Daniel Richter

Votes: 718,850 | Gross: $56.95M

22. Men in Black³ (2012)

PG-13 | 106 min | Action, Adventure, Comedy

Agent J travels in time to M.I.B.'s early days in 1969 to stop an alien from assassinating his friend Agent K and changing history.

Director: Barry Sonnenfeld | Stars: Will Smith , Tommy Lee Jones , Josh Brolin , Jemaine Clement

Votes: 386,752 | Gross: $179.02M

23. Back to the Future (1985)

PG | 116 min | Adventure, Comedy, Sci-Fi

Marty McFly, a 17-year-old high school student, is accidentally sent 30 years into the past in a time-traveling DeLorean invented by his close friend, the maverick scientist Doc Brown.

Director: Robert Zemeckis | Stars: Michael J. Fox , Christopher Lloyd , Lea Thompson , Crispin Glover

Votes: 1,305,487 | Gross: $210.61M

24. Back to the Future Part II (1989)

PG | 108 min | Adventure, Comedy, Sci-Fi

After visiting 2015, Marty McFly must repeat his visit to 1955 to prevent disastrous changes to 1985...without interfering with his first trip.

Director: Robert Zemeckis | Stars: Michael J. Fox , Christopher Lloyd , Lea Thompson , Tom Wilson

Votes: 572,050 | Gross: $118.50M

25. Back to the Future Part III (1990)

PG | 118 min | Adventure, Comedy, Sci-Fi

Stranded in 1955, Marty McFly learns about the death of Doc Brown in 1885 and must travel back in time to save him. With no fuel readily available for the DeLorean, the two must figure how to escape the Old West before Emmett is murdered.

Director: Robert Zemeckis | Stars: Michael J. Fox , Christopher Lloyd , Mary Steenburgen , Tom Wilson

Votes: 480,390 | Gross: $87.73M

26. Hot Tub Time Machine (2010)

R | 99 min | Comedy, Sci-Fi

A malfunctioning time machine at a ski resort takes a man back to 1986 with his two friends and nephew, where they must relive a fateful night and not change anything to make sure the nephew is born.

Director: Steve Pink | Stars: John Cusack , Rob Corddry , Craig Robinson , Clark Duke

Votes: 186,352 | Gross: $50.29M

27. Hot Tub Time Machine 2 (2015)

R | 93 min | Comedy, Mystery, Sci-Fi

When Lou's shot in the groin, Nick and Jacob drag him in the Hot Tub Time Machine to go back in time and save Lou. The three end up 10 years in the future, where they need to go to find the shooter.

Director: Steve Pink | Stars: Rob Corddry , Craig Robinson , Clark Duke , Adam Scott

Votes: 41,625 | Gross: $12.28M

28. The Terminator (1984)

R | 107 min | Action, Adventure, Sci-Fi

A human soldier is sent from 2029 to 1984 to stop an almost indestructible cyborg killing machine, sent from the same year, which has been programmed to execute a young woman whose unborn son is the key to humanity's future salvation.

Director: James Cameron | Stars: Arnold Schwarzenegger , Linda Hamilton , Michael Biehn , Paul Winfield

Votes: 923,373 | Gross: $38.40M

29. Terminator 2: Judgment Day (1991)

R | 137 min | Action, Adventure, Sci-Fi

A cyborg, identical to the one who failed to kill Sarah Connor, must now protect her ten year old son John from an even more advanced and powerful cyborg.

Director: James Cameron | Stars: Arnold Schwarzenegger , Linda Hamilton , Edward Furlong , Robert Patrick

Votes: 1,173,020 | Gross: $204.84M

30. Terminator 3: Rise of the Machines (2003)

R | 109 min | Action, Sci-Fi

A machine from a post-apocalyptic future travels back in time to protect a man and a woman from an advanced robotic assassin to ensure they both survive a nuclear attack.

Director: Jonathan Mostow | Stars: Arnold Schwarzenegger , Nick Stahl , Kristanna Loken , Claire Danes

Votes: 418,317 | Gross: $150.37M

31. Terminator Salvation (2009)

PG-13 | 115 min | Action, Adventure, Sci-Fi

In 2018, a mysterious new weapon in the war against the machines, half-human and half-machine, comes to John Connor on the eve of a resistance attack on Skynet. But whose side is he on, and can he be trusted?

Director: McG | Stars: Christian Bale , Sam Worthington , Anton Yelchin , Moon Bloodgood

Votes: 377,114 | Gross: $125.32M

32. Terminator Genisys (2015)

PG-13 | 126 min | Action, Adventure, Sci-Fi

When John Connor, leader of the human resistance, sends Sgt. Kyle Reese back to 1984 to protect Sarah Connor and safeguard the future, an unexpected turn of events creates a fractured timeline.

Director: Alan Taylor | Stars: Arnold Schwarzenegger , Jason Clarke , Emilia Clarke , Jai Courtney

Votes: 292,400 | Gross: $89.76M

33. Next (2007)

PG-13 | 96 min | Action, Sci-Fi, Thriller

A Las Vegas magician who can see into the future is pursued by FBI agents seeking to use his abilities to prevent a nuclear terrorist attack.

Director: Lee Tamahori | Stars: Nicolas Cage , Julianne Moore , Jessica Biel , Thomas Kretschmann

Votes: 166,308 | Gross: $18.21M

34. The One (2001)

PG-13 | 87 min | Action, Sci-Fi, Thriller

A rogue Multiverse agent goes on a manhunt for alternate versions of himself, getting stronger with each kill. Only the last version of himself, an LASD cop, can stop his crusade before he becomes "The One".

Director: James Wong | Stars: Jet Li , Carla Gugino , Delroy Lindo , Jason Statham

Votes: 96,041 | Gross: $43.91M

35. Star Trek (2009)

PG-13 | 127 min | Action, Adventure, Sci-Fi

The brash James T. Kirk tries to live up to his father's legacy with Mr. Spock keeping him in check as a vengeful Romulan from the future creates black holes to destroy the Federation one planet at a time.

Director: J.J. Abrams | Stars: Chris Pine , Zachary Quinto , Simon Pegg , Leonard Nimoy

Votes: 620,052 | Gross: $257.73M

36. Star Trek Into Darkness (2013)

PG-13 | 132 min | Action, Adventure, Sci-Fi

After the crew of the Enterprise find an unstoppable force of terror from within their own organization, Captain Kirk leads a manhunt to a war-zone world to capture a one-man weapon of mass destruction.

Director: J.J. Abrams | Stars: Chris Pine , Zachary Quinto , Zoe Saldana , Benedict Cumberbatch

Votes: 496,923 | Gross: $228.78M

37. Star Trek Beyond (2016)

PG-13 | 122 min | Action, Adventure, Sci-Fi

The crew of the USS Enterprise explores the furthest reaches of uncharted space, where they encounter a new ruthless enemy, who puts them, and everything the Federation stands for, to the test.

Director: Justin Lin | Stars: Chris Pine , Zachary Quinto , Karl Urban , Zoe Saldana

Votes: 258,367 | Gross: $158.85M

38. Cloverfield (2008)

PG-13 | 85 min | Horror, Sci-Fi, Thriller

A group of friends venture deep into the streets of New York on a rescue mission during a rampaging monster attack.

Director: Matt Reeves | Stars: Mike Vogel , Jessica Lucas , Lizzy Caplan , T.J. Miller

Votes: 420,380 | Gross: $80.05M

39. 10 Cloverfield Lane (2016)

PG-13 | 103 min | Drama, Horror, Mystery

A young woman is held in an underground bunker by a man who insists that a hostile event has left the surface of the Earth uninhabitable.

Director: Dan Trachtenberg | Stars: John Goodman , Mary Elizabeth Winstead , John Gallagher Jr. , Douglas M. Griffin

Votes: 354,397 | Gross: $72.08M

40. The Cloverfield Paradox (2018)

PG-13 | 102 min | Action, Horror, Sci-Fi

Orbiting a planet on the brink of war, scientists test a device to solve an energy crisis, and end up face-to-face with a dark alternate reality.

Director: Julius Onah | Stars: Gugu Mbatha-Raw , David Oyelowo , Daniel Brühl , John Ortiz

Votes: 112,936

41. The Golden Compass (2007)

PG-13 | 113 min | Adventure, Family, Fantasy

In a parallel universe, young Lyra Belacqua journeys to the far North to save her best friend and other kidnapped children from terrible experiments by a mysterious organization.

Director: Chris Weitz | Stars: Nicole Kidman , Daniel Craig , Dakota Blue Richards , Ben Walker

Votes: 196,769 | Gross: $70.11M

42. Journey to the Far Side of the Sun (1969)

G | 101 min | Drama, Sci-Fi

The European Space Exploration Council sends two astronauts to explore a planet similar to the Earth but located on the opposite side of the sun.

Director: Robert Parrish | Stars: Roy Thinnes , Ian Hendry , Patrick Wymark , Lynn Loring

Votes: 4,042 | Gross: $0.33M

43. Another Earth (2011)

PG-13 | 92 min | Drama, Mystery, Romance

On the night of the discovery of a duplicate Earth in the Solar system, an ambitious young student and an accomplished composer cross paths in a tragic accident.

Director: Mike Cahill | Stars: Brit Marling , William Mapother , Matthew-Lee Erlbach , DJ Flava

Votes: 99,964 | Gross: $1.32M

44. Annihilation (I) (2018)

R | 115 min | Adventure, Drama, Horror

A biologist signs up for a dangerous, secret expedition into a mysterious zone where the laws of nature don't apply.

Director: Alex Garland | Stars: Natalie Portman , Jennifer Jason Leigh , Tessa Thompson , Benedict Wong

Votes: 360,855 | Gross: $32.73M

45. Parallels (I) (2015)

Not Rated | 83 min | Action, Sci-Fi

A mysterious building is the gateway to infinite parallel Earths.

Director: Christopher Leone | Stars: Mark Hapka , Jessica Rothe , Eric Jungmann , Constance Wu

Votes: 15,046

46. Shrek Forever After (2010)

PG | 93 min | Animation, Adventure, Comedy

Rumpelstiltskin tricks a mid-life crisis burdened Shrek into allowing himself to be erased from existence and cast in a dark alternate timeline where Rumpelstiltskin rules supreme.

Director: Mike Mitchell | Stars: Mike Myers , Cameron Diaz , Eddie Murphy , Antonio Banderas

Votes: 224,338 | Gross: $238.37M

47. The Mist (2007)

R | 126 min | Horror, Sci-Fi, Thriller

A freak storm unleashes a species of bloodthirsty creatures on a small town, where a small band of citizens hole up in a supermarket and fight for their lives.

Director: Frank Darabont | Stars: Thomas Jane , Marcia Gay Harden , Laurie Holden , Andre Braugher

Votes: 339,972 | Gross: $25.59M

48. Run Lola Run (1998)

R | 80 min | Action, Crime, Thriller

After a botched money delivery, Lola has 20 minutes to come up with 100,000 Deutschmarks.

Director: Tom Tykwer | Stars: Franka Potente , Moritz Bleibtreu , Herbert Knaup , Nina Petri

Votes: 206,879 | Gross: $7.27M

49. Fatherland (1994 TV Movie)

106 min | Drama, Romance, Sci-Fi

In April 1964, more than twenty years after the Nazis won World War II, S.S. officer Xavier March uncovers a plot to eliminate the attendees of the Wannsee Conference so that Germany can establish better relations with the U.S.

Director: Christopher Menaul | Stars: Rutger Hauer , Miranda Richardson , Peter Vaughan , Michael Kitchen

Votes: 6,249

50. The Family Man (2000)

PG-13 | 125 min | Comedy, Drama, Fantasy

A fast-lane investment broker, offered the opportunity to see how the other half lives, wakes up to find that his sports car and girlfriend have become a mini-van and wife.

Director: Brett Ratner | Stars: Nicolas Cage , Téa Leoni , Don Cheadle , Jeremy Piven

Votes: 119,325 | Gross: $75.79M

51. Escaflowne: The Movie (2000)

PG-13 | 98 min | Adventure, Animation, Drama

A grim retelling of the television series "The Vision of Escaflowne".

Directors: Kazuki Akane , Yoshiyuki Takei | Stars: Shin'ichirô Miki , Jôji Nakata , Maaya Sakamoto , Tomokazu Seki

Votes: 3,811

52. Eternal Sunshine of the Spotless Mind (2004)

R | 108 min | Drama, Romance, Sci-Fi

When their relationship turns sour, a couple undergoes a medical procedure to have each other erased from their memories forever.

Director: Michel Gondry | Stars: Jim Carrey , Kate Winslet , Tom Wilkinson , Gerry Robert Byrne

Votes: 1,077,085 | Gross: $34.40M

53. The Dark Tower (2017)

PG-13 | 95 min | Action, Adventure, Fantasy

A boy haunted by visions of a dark tower from a parallel reality teams up with the tower's disillusioned guardian to stop an evil warlock known as the Man in Black who plans to use the boy to destroy the tower and open the gates of Hell.

Director: Nikolaj Arcel | Stars: Idris Elba , Matthew McConaughey , Tom Taylor , Dennis Haysbert

Votes: 145,760 | Gross: $50.70M

54. Being John Malkovich (1999)

R | 113 min | Comedy, Drama, Fantasy

A puppeteer discovers a portal that leads literally into the head of movie star John Malkovich .

Director: Spike Jonze | Stars: John Cusack , Cameron Diaz , Catherine Keener , John Malkovich

Votes: 353,195 | Gross: $22.86M

55. The Matrix (1999)

R | 136 min | Action, Sci-Fi

When a beautiful stranger leads computer hacker Neo to a forbidding underworld, he discovers the shocking truth--the life he knows is the elaborate deception of an evil cyber-intelligence.

Directors: Lana Wachowski , Lilly Wachowski | Stars: Keanu Reeves , Laurence Fishburne , Carrie-Anne Moss , Hugo Weaving

Votes: 2,050,817 | Gross: $171.48M

56. The Matrix Reloaded (2003)

R | 138 min | Action, Sci-Fi

Freedom fighters Neo, Trinity and Morpheus continue to lead the revolt against the Machine Army, unleashing their arsenal of extraordinary skills and weaponry against the systematic forces of repression and exploitation.

Votes: 630,037 | Gross: $281.49M

57. The Matrix Revolutions (2003)

R | 129 min | Action, Sci-Fi

The human city of Zion defends itself against the massive invasion of the machines as Neo fights to end the war at another front while also opposing the rogue Agent Smith.

Votes: 543,061 | Gross: $139.31M

58. Open Your Eyes (1997)

R | 119 min | Drama, Mystery, Sci-Fi

A very handsome man finds the love of his life, but he suffers an accident and needs to have his face rebuilt by surgery after it is severely disfigured.

Director: Alejandro Amenábar | Stars: Eduardo Noriega , Penélope Cruz , Chete Lera , Fele Martínez

Votes: 73,201 | Gross: $0.37M

59. The Thirteenth Floor (1999)

R | 100 min | Mystery, Sci-Fi, Thriller

A computer scientist running a virtual reality simulation of 1937 becomes the primary suspect when his colleague and mentor is murdered.

Director: Josef Rusnak | Stars: Craig Bierko , Gretchen Mol , Armin Mueller-Stahl , Vincent D'Onofrio

Votes: 77,221 | Gross: $15.50M

60. Dark City (1998)

R | 100 min | Fantasy, Mystery, Sci-Fi

A man struggles with memories of his past, which include a wife he cannot remember and a nightmarish world no one else ever seems to wake up from.

Director: Alex Proyas | Stars: Rufus Sewell , Kiefer Sutherland , Jennifer Connelly , William Hurt

Votes: 212,142 | Gross: $14.38M

61. Project Almanac (2015)

PG-13 | 106 min | Drama, Mystery, Sci-Fi

A group of teens discovers secret plans for a time machine, and construct one. However, things start to get out of control.

Director: Dean Israelite | Stars: Amy Landecker , Sofia Black-D'Elia , Virginia Gardner , Jonny Weston

Votes: 84,201 | Gross: $22.35M

62. Prince of Persia: The Sands of Time (2010)

PG-13 | 116 min | Action, Adventure, Fantasy

A young fugitive prince and princess must stop a villain who unknowingly threatens to destroy the world with a special dagger that enables the magic sand inside to reverse time.

Director: Mike Newell | Stars: Jake Gyllenhaal , Gemma Arterton , Ben Kingsley , Alfred Molina

Votes: 303,735 | Gross: $90.76M

63. Gamer (2009)

R | 95 min | Action, Sci-Fi, Thriller

In a future mind-controlling game, death row convicts are forced to battle in a 'Doom'-type environment. Convict Kable, controlled by Simon, a skilled teenage gamer, must survive thirty sessions in order to be set free.

Directors: Mark Neveldine , Brian Taylor | Stars: Gerard Butler , Michael C. Hall , Ludacris , Amber Valletta

Votes: 139,561 | Gross: $20.49M

64. The Life Before Her Eyes (2007)

R | 90 min | Drama, Fantasy, Mystery

A woman's survivor's guilt from a Columbine-like event 15 years ago causes her present-day idyllic life to fall apart.

Director: Vadim Perelman | Stars: Uma Thurman , Evan Rachel Wood , Eva Amurri , Gabrielle Brennan

Votes: 14,888 | Gross: $0.30M

65. The Jacket (2005)

R | 103 min | Drama, Fantasy, Mystery

A Gulf war veteran is wrongly sent to a mental institution for insane criminals, where he becomes the object of a doctor's experiments, and his life is completely affected by them.

Director: John Maybury | Stars: Adrien Brody , Keira Knightley , Daniel Craig , Kris Kristofferson

Votes: 119,229 | Gross: $6.30M

66. Timecrimes (2007)

R | 92 min | Horror, Mystery, Sci-Fi

A man accidentally gets into a time machine and travels back in time nearly an hour. Finding himself will be the first of a series of disasters of unforeseeable consequences.

Director: Nacho Vigalondo | Stars: Karra Elejalde , Candela Fernández , Bárbara Goenaga , Nacho Vigalondo

Votes: 68,726 | Gross: $0.04M

67. Cloud Atlas (2012)

R | 172 min | Drama, Mystery, Sci-Fi

An exploration of how the actions of individual lives impact one another in the past, present and future, as one soul is shaped from a killer into a hero, and an act of kindness ripples across centuries to inspire a revolution.

Directors: Tom Tykwer , Lana Wachowski , Lilly Wachowski | Stars: Tom Hanks , Halle Berry , Hugh Grant , Hugo Weaving

Votes: 374,236 | Gross: $27.11M

68. The Lake House (2006)

PG | 99 min | Drama, Fantasy, Romance

A lonely doctor who once occupied an unusual lakeside house begins to exchange love letters with its former resident, a frustrated architect. They must try to unravel the mystery behind their extraordinary romance before it's too late.

Director: Alejandro Agresti | Stars: Keanu Reeves , Sandra Bullock , Christopher Plummer , Ebon Moss-Bachrach

Votes: 157,592 | Gross: $52.33M

69. Happy Accidents (2000)

R | 110 min | Comedy, Romance

New Yorker Ruby Weaver believes she has found the man of her dreams in Sam Deed, who is her best catch in some time--except that he assures her that he came from the future.

Director: Brad Anderson | Stars: Marisa Tomei , Vincent D'Onofrio , Holland Taylor , Mick Weber

Votes: 10,223 | Gross: $0.69M

70. Comet (2014)

R | 91 min | Comedy, Drama, Romance

Set in a parallel universe, Comet bounces back and forth over the course of an unlikely but perfectly paired couple's six-year relationship.

Director: Sam Esmail | Stars: Justin Long , Emmy Rossum , Kayla Servi , Eric Winter

Votes: 19,646

71. Tomorrowland (2015)

PG | 130 min | Action, Adventure, Family

Bound by a shared destiny, a teen bursting with scientific curiosity and a former boy-genius inventor embark on a mission to unearth the secrets of a place somewhere in time and space that exists in their collective memory.

Director: Brad Bird | Stars: George Clooney , Britt Robertson , Hugh Laurie , Raffey Cassidy

Votes: 191,285 | Gross: $93.44M

72. The Incident (2014)

100 min | Horror, Sci-Fi, Thriller

Two parallel stories about characters trapped in illogical endless spaces: two brothers and a detective locked on an infinite staircase, and a family locked on an infinite road - for a very long time.

Director: Isaac Ezban | Stars: Raúl Méndez , Magda Brugengheim , Humberto Busto , Erick Trinidad Camacho

Votes: 4,308

73. Mulholland Drive (2001)

R | 147 min | Drama, Mystery, Thriller

After a car wreck on Mulholland Drive renders a woman amnesiac, she and a Hollywood-hopeful search for clues and answers across Los Angeles in a twisting venture beyond dreams and reality.

Director: David Lynch | Stars: Naomi Watts , Laura Harring , Justin Theroux , Jeanne Bates

Votes: 383,499 | Gross: $7.22M

74. Click (2006)

A workaholic architect finds a universal remote that allows him to fast-forward and rewind to different parts of his life. Complications arise when the remote starts to overrule his choices.

Director: Frank Coraci | Stars: Adam Sandler , Kate Beckinsale , Christopher Walken , David Hasselhoff

Votes: 356,432 | Gross: $137.36M

75. I Love You, I Love You (1968)

91 min | Drama, Sci-Fi

After attempting suicide, Claude is recruited for a time travel experiment, but, when the machine goes haywire, he may be trapped hurtling through his memories.

Director: Alain Resnais | Stars: Claude Rich , Olga Georges-Picot , Anouk Ferjac , Alain MacMoy

Votes: 3,268 | Gross: $0.06M

76. The Lost Room (2006)

TV-PG | 90 min | Action, Fantasy, Mystery

A detective investigates a mysterious motel room which acts as a portal to anywhere in the world.

Stars: Peter Krause , Julianna Margulies , Peter Jacobson , Dennis Christopher

Votes: 33,052

77. Premonition (I) (2007)

PG-13 | 96 min | Drama, Fantasy, Mystery

A depressed woman learns that her husband was killed in a car accident the previous day, then awakens the next morning to find him alive and well at home; then awakens the day after that to find that he's dead.

Director: Mennan Yapo | Stars: Sandra Bullock , Julian McMahon , Amber Valletta , Shyann McClure

Votes: 81,673 | Gross: $47.85M

78. The I Inside (2004)

R | 90 min | Mystery, Sci-Fi, Thriller

An amnesiac discovers himself leaping through time between 2000 and 2002 as his past returns to him.

Director: Roland Suso Richter | Stars: Ryan Phillippe , Sarah Polley , Piper Perabo , Robert Sean Leonard

Votes: 11,381

79. Frequently Asked Questions About Time Travel (2009)

PG-13 | 83 min | Comedy, Sci-Fi

While drinking at their local pub, three social outcasts attempt to navigate a time-travel conundrum.

Director: Gareth Carrivick | Stars: Chris O'Dowd , Marc Wootton , Dean Lennox Kelly , Anna Faris

Votes: 36,545

80. Bill & Ted's Excellent Adventure (1989)

PG | 90 min | Adventure, Comedy, Music

Two rock-'n-rolling teens, on the verge of failing their class, set out on a quest to make the ultimate school history report after being presented with a time machine.

Director: Stephen Herek | Stars: Keanu Reeves , Alex Winter , George Carlin , Terry Camilleri

Votes: 141,553 | Gross: $40.49M

81. Bill & Ted's Bogus Journey (1991)

PG | 93 min | Adventure, Comedy, Fantasy

A tyrant from the future creates evil android doubles of Bill and Ted and sends them back to eliminate the originals.

Director: Peter Hewitt | Stars: Keanu Reeves , Alex Winter , William Sadler , Joss Ackland

Votes: 81,004 | Gross: $38.04M

82. About Time (I) (2013)

R | 123 min | Comedy, Drama, Fantasy

At the age of 21, Tim discovers he can travel in time and change what happens and has happened in his own life. His decision to make his world a better place by getting a girlfriend turns out not to be as easy as you might think.

Director: Richard Curtis | Stars: Domhnall Gleeson , Rachel McAdams , Bill Nighy , Lydia Wilson

Votes: 385,484 | Gross: $15.32M

83. Minority Report (2002)

PG-13 | 145 min | Action, Crime, Mystery

John works with the PreCrime police which stop crimes before they take place, with the help of three 'PreCogs' who can foresee crimes. Events ensue when John finds himself framed for a future murder.

Director: Steven Spielberg | Stars: Tom Cruise , Colin Farrell , Samantha Morton , Max von Sydow

Votes: 583,932 | Gross: $132.07M

84. Total Recall (I) (2012)

PG-13 | 118 min | Action, Adventure, Sci-Fi

A factory worker, Douglas Quaid, begins to suspect that he is a spy after visiting Rekall - a company that provides its clients with implanted fake memories of a life they would like to have led - goes wrong and he finds himself on the run.

Director: Len Wiseman | Stars: Colin Farrell , Bokeem Woodbine , Bryan Cranston , Kate Beckinsale

Votes: 266,239 | Gross: $58.88M

85. District 9 (2009)

R | 112 min | Action, Sci-Fi, Thriller

Violence ensues after an extraterrestrial race forced to live in slum-like conditions on Earth finds a kindred spirit in a government agent exposed to their biotechnology.

Director: Neill Blomkamp | Stars: Sharlto Copley , David James , Jason Cope , Vanessa Haywood

Votes: 716,390 | Gross: $115.65M

86. The Adjustment Bureau (2011)

PG-13 | 106 min | Crime, Drama, Mystery

The affair between a politician and a contemporary dancer is affected by mysterious forces keeping the lovers apart.

Director: George Nolfi | Stars: Matt Damon , Emily Blunt , Lisa Thoreson , Florence Kastriner

Votes: 270,032 | Gross: $62.50M

87. Event Horizon (1997)

R | 96 min | Horror, Sci-Fi, Thriller

A rescue crew is tasked with investigating the mysterious reappearance of a spaceship that had been lost for seven years.

Director: Paul W.S. Anderson | Stars: Laurence Fishburne , Sam Neill , Kathleen Quinlan , Joely Richardson

Votes: 196,998 | Gross: $26.67M

88. Deep Impact (1998)

PG-13 | 120 min | Action, Drama, Sci-Fi

A comet is discovered to be on a collision course with Earth. As doomsday nears, the human race prepares for the worst.

Director: Mimi Leder | Stars: Robert Duvall , Téa Leoni , Elijah Wood , Morgan Freeman

Votes: 190,245 | Gross: $140.46M

89. Sunshine (2007)

R | 107 min | Sci-Fi, Thriller

A team of international astronauts is sent on a dangerous mission to reignite the dying Sun with a nuclear fission bomb in 2057.

Director: Danny Boyle | Stars: Cillian Murphy , Rose Byrne , Chris Evans , Cliff Curtis

Votes: 267,204 | Gross: $3.68M

90. Paycheck (2003)

PG-13 | 119 min | Action, Mystery, Sci-Fi

What seemed like a breezy idea that would net an engineer millions of dollars ends up leaving him on the run for his life and trying to piece together why he's being chased.

Director: John Woo | Stars: Ben Affleck , Aaron Eckhart , Uma Thurman , Michael C. Hall

Votes: 113,242 | Gross: $53.79M

91. Solaris (2002)

PG-13 | 99 min | Drama, Mystery, Romance

A troubled psychologist is sent to investigate the crew of an isolated research station orbiting a bizarre planet.

Director: Steven Soderbergh | Stars: George Clooney , Natascha McElhone , Ulrich Tukur , Viola Davis

Votes: 86,248 | Gross: $14.97M

92. One Point O (2004)

R | 95 min | Drama, Horror, Mystery

After receiving mysterious empty packages inside his apartment, a young computer-programmer begins a personal investigation into their origins.

Directors: Jeff Renfroe , Marteinn Thorsson | Stars: Richard Rees , Jeremy Sisto , Udo Kier , Deborah Kara Unger

Votes: 5,985

94. Contact (1997)

PG | 150 min | Drama, Mystery, Sci-Fi

Dr. Ellie Arroway, after years of searching, finds conclusive radio proof of extraterrestrial intelligence, sending plans for a mysterious machine.

Director: Robert Zemeckis | Stars: Jodie Foster , Matthew McConaughey , Tom Skerritt , John Hurt

Votes: 292,901 | Gross: $100.92M

95. Prometheus (I) (2012)

R | 124 min | Adventure, Mystery, Sci-Fi

Following clues to the origin of mankind, a team finds a structure on a distant moon, but they soon realize they are not alone.

Director: Ridley Scott | Stars: Noomi Rapace , Logan Marshall-Green , Michael Fassbender , Charlize Theron

Votes: 643,751 | Gross: $126.48M

96. Total Recall (1990)

R | 113 min | Action, Adventure, Sci-Fi

When a man goes in to have virtual vacation memories of the planet Mars implanted in his mind, an unexpected and harrowing series of events forces him to go to the planet for real - or is he?

Director: Paul Verhoeven | Stars: Arnold Schwarzenegger , Sharon Stone , Michael Ironside , Rachel Ticotin

Votes: 354,202 | Gross: $119.39M

97. The Wizard of Oz (1939)

PG | 102 min | Adventure, Family, Fantasy

Young Dorothy Gale and her dog Toto are swept away by a tornado from their Kansas farm to the magical Land of Oz, and embark on a quest with three new friends to see the Wizard, who can return her to her home and fulfill the others' wishes.

Directors: Victor Fleming , King Vidor | Stars: Judy Garland , Frank Morgan , Ray Bolger , Bert Lahr

Votes: 428,310 | Gross: $2.08M

98. Harry Potter and the Prisoner of Azkaban (2004)

PG | 142 min | Adventure, Family, Fantasy

Harry Potter, Ron and Hermione return to Hogwarts School of Witchcraft and Wizardry for their third year of study, where they delve into the mystery surrounding an escaped prisoner who poses a dangerous threat to the young wizard.

Director: Alfonso Cuarón | Stars: Daniel Radcliffe , Emma Watson , Rupert Grint , Richard Griffiths

Votes: 690,989 | Gross: $249.36M

99. It's a Wonderful Life (1946)

PG | 130 min | Drama, Family, Fantasy

An angel is sent from Heaven to help a desperately frustrated businessman by showing him what life would have been like if he had never existed.

Director: Frank Capra | Stars: James Stewart , Donna Reed , Lionel Barrymore , Thomas Mitchell

Votes: 498,811

100. Time After Time (1979)

PG | 112 min | Adventure, Drama, Sci-Fi

H.G. Wells pursues Jack the Ripper to the 20th Century when the serial murderer uses the future writer's time machine to escape his time period.

Director: Nicholas Meyer | Stars: Malcolm McDowell , Mary Steenburgen , David Warner , Charles Cioffi

Votes: 20,618

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We may have spotted a parallel universe going backwards in time

Strange particles observed by an experiment in Antarctica could be evidence of an alternative reality where everything is upside down

By Jon Cartwright

8 April 2020

Graham Carter

IN THE Antarctic , things happen at a glacial pace. Just ask Peter Gorham. For a month at a time, he and his colleagues would watch a giant balloon carrying a collection of antennas float high above the ice, scanning over a million square kilometres of the frozen landscape for evidence of high-energy particles arriving from space.

When the experiment returned to the ground after its first flight, it had nothing to show for itself, bar the odd flash of background noise. It was the same story after the second flight more than a year later.

While the balloon was in the sky for the third time, the researchers decided to go over the past data again, particularly those signals dismissed as noise. It was lucky they did. Examined more carefully, one signal seemed to be the signature of a high-energy particle. But it wasn’t what they were looking for. Moreover, it seemed impossible. Rather than bearing down from above, this particle was exploding out of the ground.

That strange finding was made in 2016. Since then, all sorts of suggestions rooted in known physics have been put forward to account for the perplexing signal, and all have been ruled out . What’s left is shocking in its implications. Explaining this signal requires the existence of a topsy-turvy universe created in the same big bang as our own and existing in parallel with it. In this mirror world, positive is negative, left is right and time runs backwards. It is perhaps the most mind-melting idea ever to have emerged from the Antarctic ice ­­– but it might just be true.

The ambitions…

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The Curious Case of Time Travel, Parallel Universes, and Alternate Dimensions

Why is time travel so complicated, and is it even possible? Are there other universes in "space" apart from ours, and what exactly is the meaning of alternate dimensions?

The truth is, we are still not entirely sure how our reality functions. Although complicated, physics is doing its best to make things more understandable to the public, but nonetheless remains complicated for most to understand. The late Professor  Hawking was excellent at explaining complicated scientific subjects to the public, and many of his books have made science understandable to the masses.

In this article, we will discuss, among other things, the subject concerning parallel universes and dimensions beyond the three familiar to us in a perhaps equally understandable way.

How did we come up with the idea of higher dimensions?

In modern physics, there is a great deal of trouble with  unifying  the theory of general relativity and the theory of quantum mechanics. General relativity explains how the force of gravity, large-scale stars, and galaxies behave, while quantum mechanics describes how small atoms, subatomic particles, and three fundamental forces interact.

However, it’s hard to bring these theories together because each seems to have very different rules that govern the level they observe. Einstein made one of the primary efforts to solve this dilemma in the 1940s by postulating  String theory , which claims everything is made up of 1-dimensional strings in a universe with extra dimensions.

This was the first step in better understanding why large-scale objects and gravity behave differently than small particles. In the meantime, scientists developed string theory into superstring theory, which arose from the M-theory . These theories have in common that they propose a reality based on 10 or 11 dimensions. Well-known physicist  Michio Kaku believes this to be precisely the reality we live in.

What’s hidden in these higher dimensions?

In the early 20th century, scientists formulated the theory of  dark matter because galaxies’ motions didn’t match the calculations based on known gravitational theories. According to previous calculations, some galaxies would have floated apart or couldn’t form in the first place. They seem tied together by some unseen matter, which makes up 85% of our universe. Some theories suggest this invisible matter exists in the extra dimensions.

Are there parallel universes?

According to the theory of the  multiverse , a parallel universe is created where this decision plays out each time we make a personal decision. In another parallel universe, we might have made a different decision with different consequences.

This also seems to be the case with  quantum particles making arbitrary decisions under scientific observation. Not only is it possible that there’s another almost identical universe to ours where the particle acted differently than here but it’s also possible such parallel universes are interacting with our own.

Where are these extra dimensions and parallel universes?

Understanding the interactions of a 4-dimensional hypercube will be difficult for a layman. It might be easier to explain the 11 dimensions in the following way; As humans, we are trapped in 3 spatial dimensions, plus the fourth “time “dimension. We call “time “a “line “in the 4th dimension. We move from dot to dot on this line in a single forward direction, from one moment to the next.

We can move through 3-D spaces more or less than we wish, but we can’t do so through the 4th dimension. If we could hypothetically be in the fifth dimension, then time (“line“) and all its moments (“dots“) would become as freely movable space to us – we could go forward into the future and backward in the past as we wish.

In the  5th dimension , we might see our entire life’s timeline from birth to passing away, all at once in front of us, just as if it were a spatial object. If we hypothetically looked at things from the  6th dimension , we would see all different possible timelines of events in our universe as a single space across which we could travel.

In the seventh dimension, we could go to a  universe with a different set of physical laws; in the 8th, we travel between all such possible universes, and in the 9th, we navigate between all possible timelines in all possible universes. Finally, in the 10th, all possibilities would exist, even things seemingly impossible  in our universes.

So the question is, will we ever be able to travel to these extra dimensions?

One of the most exciting things in the  universe  is our  brain .  It’s a tool that helps us keep track of past events to calculate future happenings and support our logical decision-making in the present. Through our imagination, our brain tries to visualize possible timelines in case we make different decisions.  We may even imagine a universe with different physical constants. In a way, our brain helps us imagine higher dimensions.

Perhaps developing technology will make navigating through parallel universes a bit easier?

According to John D. Barrow’s revised  Kardashev scale measuring, the developmental phases of civilizations are as follows: type 1 – capable of building structures, harnessing the energy of its planet; types 2 and 3: manipulating genes and molecules, uses the energy of stars and galaxies; Type 4, 5 and 6: manipulating atoms, quarks, creating complex artificial life and harnessing the energy of the entire universe; and finally type Omega : capable of manipulating the multiverse , space and time.

Is there any civilization in our universe that managed to become a type Omega?

The strange thing is that over  40 billion planets  in our galaxy alone could support life, as per astrophysicists. For the last 50 years, we have made serious efforts to try to pick up any radio or communication signal from an advanced civilization, but to no avail.  So far, there seems to be no trace of any highly technological species whatsoever. As scientist  Enrico Fermi  famously asked, “Where is everybody? “

Ancient civilizations often told stories about immortal intelligent deities such as gods and angels who seemed outside the dimension of time , traveling through space instantaneously and assuming shapes according to their wishes.  They are even said to have created humans. Could this be a written record of type Omega civilization that mastered the multiverse and the creation of complex artificial life?  We may never know.

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13 Great Movies That Explore Parallel Universes & Alternate Realities

Lila Sommers

Diving into the realm of parallel universes has always been a cinematic treat. There's something undeniably captivating about the idea of an alternate reality, a 'what if' scenario that challenges our perception of life.

Over the years, I've been drawn to films that masterfully weave this concept into their narratives, offering both mind-bending plots and profound reflections on existence.

From hidden gems to blockbuster hits, I've curated a list of  13 must-watch parallel universe movies  that promise to transport you to worlds both eerily familiar and astonishingly different.

Whether you're a sci-fi aficionado or just someone looking for a film escape, this list is your portal to cinematic dimensions waiting to be explored.

1. "Donnie Darko" (2001)

Donnie darko (2001).

Jake Gyllenhaal

Jena Malone

James Duval

Drew Barrymore

Maggie Gyllenhaal

Mary McDonnell

Holmes Osborne

Katharine Ross

In the quiet suburban world of Middlesex, young Donnie Darko's life takes an eerie turn when he narrowly escapes a bizarre accident. Plagued by visions of a menacing, oversized rabbit named Frank, Donnie is led down a rabbit hole of time travel, existential dilemmas, and alternate realities.

As he grapples with these supernatural occurrences, the lines between reality and imagination blur. The film masterfully intertwines dark themes with teen angst, creating a haunting tapestry of the human psyche, destiny, and the universe's intricate design.

2. "Pan's Labyrinth" (2006)

Pan's labyrinth (2006).

Ivana Baquero

Maribel Verdú

Sergi López

Ariadna Gil

Álex Angulo

Roger Casamajor

Manolo Solo

Iván Massagué

Set against the grim backdrop of post-Civil War Spain, young Ofelia discovers a mysterious labyrinth near her new home. Guided by a curious faun, she's thrust into a world of mythical creatures, ancient tasks, and magical realms.

As she navigates this enchanting underworld, the harsh realities of the world above, marked by her cruel stepfather and the ongoing war, converge. Guillermo del Toro's masterpiece is a poignant blend of fantasy and reality, exploring innocence, bravery, and the transformative power of storytelling.

3. "Another Earth" (2011)

Another earth (2011).

Brit Marling

William Mapother

Matthew-Lee Erlbach

Meggan Lennon

Kumar Pallana

Bruce Colbert

In a world much like ours, the discovery of a mirror Earth in the sky sets the stage for a tale of redemption, identity, and cosmic wonder. Rhoda, a brilliant young woman, finds her life intertwined with that of a composer named John after a tragic accident.

As the two form an unexpected bond, the looming presence of the duplicate Earth forces them to confront their past, their choices, and the tantalizing possibility of another version of oneself.

This evocative drama delves deep into the human soul, questioning our place in the universe and the profound nature of second chances.

4. "The Chronicles of Narnia: The Lion, the Witch and the Wardrobe" (2005)

The chronicles of narnia: the lion, the witch and the wardrobe (2005).

Georgie Henley

Skandar Keynes

William Moseley

Anna Popplewell

Tilda Swinton

James McAvoy

Jim Broadbent

James Cosmo

Judy McIntosh

In the heart of war-torn England, four siblings - Peter, Susan, Edmund, and Lucy - stumble upon a magical wardrobe that serves as a gateway to the enchanting realm of Narnia.

This once-peaceful land, now under the icy grip of the malevolent White Witch, awaits salvation. As prophecies unfold and allegiances are tested, the children find themselves intertwined in an epic battle of good versus evil.

Guided by the majestic lion, Aslan, the siblings embark on a journey of self-discovery, bravery, and redemption. This tale, rich in fantasy and adventure, beckons viewers to believe in the extraordinary and embrace the magic within.

5. "The Discovery" (2017)

The discovery (2017).

Jason Segel

Rooney Mara

Robert Redford

Jesse Plemons

Riley Keough

Brian McCarthy

Kimleigh Smith

Adam Khaykin

In a world where the afterlife's existence has been scientifically proven, the very fabric of human existence undergoes a seismic shift. Millions opt for the allure of the beyond, leading to a global suicide epidemic.

Amidst this existential chaos, Will, a skeptic, arrives at a mysterious mansion owned by the scientist responsible for the groundbreaking discovery.

As he navigates a complex web of love, loss, and the quest for purpose, Will confronts haunting questions about the nature of life and what lies beyond. "The Discovery" delves deep into the human psyche, challenging perceptions of reality and urging viewers to ponder the profound implications of eternity.

6. "Coherence" (2013)

Coherence (2013).

Emily Baldoni

Maury Sterling

Nicholas Brendon

Elizabeth Gracen

Hugo Armstrong

Alex Manugian

Lauren Maher

Kelly Donovan

Aqueela Zoll

On a seemingly ordinary evening, eight friends gather for a dinner party, unaware that a comet passing overhead is about to unravel the fabric of their reality. As the night progresses, bizarre occurrences hint at a larger, more unsettling truth: they might be trapped in a labyrinth of parallel universes.

As tensions rise and trust wanes, the group grapples with their doppelgängers, fragmented memories, and the chilling realization that any decision could alter their fate.

"Coherence" is a masterclass in psychological suspense, inviting viewers into a mesmerizing maze of possibilities, where the line between reality and illusion blurs, and nothing is as it seems.

7. "The One I Love" (2014)

The one i love (2014).

Mark Duplass

Elisabeth Moss

Kiana Cason

Kaitlyn Dodson

Lori Farrar

Marlene Malin

Tim Peddicord

Brett Bietz

Venturing into the realm of romantic retreats, "The One I Love" offers a twist unlike any other. Ethan and Sophie, a couple on the brink of separation, decide to take a weekend getaway to rekindle their relationship.

However, the picturesque vacation home they arrive at holds an uncanny secret. They encounter alternate versions of each other, leading to a whirlwind of emotions, revelations, and self-discovery.

As they grapple with these surreal doppelgängers, the line between reality and illusion blurs, challenging their perceptions of love, identity, and commitment. This film is a mesmerizing blend of romance, drama, and science fiction, promising a journey that's both heartwarming and mind-bending.

8. "The Butterfly Effect" (2004)

The butterfly effect (2004).

Ashton Kutcher

Eric Stoltz

Melora Walters

Ethan Suplee

William Lee Scott

Elden Henson

Logan Lerman

John Patrick Amedori

Irina Gorovaia

Delving deep into the intricacies of choices and consequences, "The Butterfly Effect" unravels the life of Evan Treborn. Plagued by blackouts and traumatic events from his childhood, Evan discovers an extraordinary ability: to travel back in time through his journal entries.

As he revisits pivotal moments, he makes alterations, hoping to mend the present. However, each change ushers in a cascade of unforeseen outcomes, often more harrowing than the last.

The film masterfully explores the ripple effect of our decisions, reminding us of the delicate balance of fate and free will. A gripping narrative intertwined with emotional depth, this movie is a thought-provoking journey through time and its repercussions.

9. "Sliding Doors" (1998)

Sliding doors (1998).

Gwyneth Paltrow

John Hannah

Jeanne Tripplehorn

Zara Turner

Douglas McFerran

Paul Brightwell

Virginia McKenna

Kevin McNally

Life's moments of happenstance take center stage in "Sliding Doors." Helen, played by the radiant Gwyneth Paltrow, finds her life bifurcating into two parallel paths after a seemingly mundane event: missing a train.

In one reality, she catches her boyfriend in an act of betrayal, and in the other, she remains blissfully unaware. These two timelines unfold simultaneously, showcasing the profound impact of chance on love, career, and personal growth.

The film elegantly dances between the two worlds, offering a poignant reflection on destiny, choices, and the 'what ifs' that haunt our daily lives. A romantic dramedy at its core, "Sliding Doors" is a heartfelt exploration of the roads taken and not taken in the labyrinth of life.

10. "The Thirteenth Floor" (1999)

The thirteenth floor (1999).

Craig Bierko

Armin Mueller-Stahl

Gretchen Mol

Vincent D'Onofrio

Dennis Haysbert

Steven Schub

Jeremy Roberts

Janet MacLachlan

Delving deep into the digital fabric of reality, "The Thirteenth Floor" unravels a tale where the lines between the virtual and the real blur. Douglas Hall finds himself embroiled in a murder mystery, but the clues lie not in our world but within a simulated 1937 Los Angeles.

As he descends into this digital realm, he confronts unsettling truths about his own existence. This cinematic journey challenges our perceptions, asking profound questions about reality, existence, and the nature of consciousness.

11. "The One" (2001)

The one (2001).

Jason Statham

Carla Gugino

Delroy Lindo

James Morrison

Dylan Bruno

Tucker Smallwood

Harriet Sansom Harris

Dean Norris

Ashlyn Gere

In a multiverse teeming with alternate realities, "The One" introduces us to a rogue operative, Gabriel Yulaw, who hunts down his other-dimensional counterparts. With each kill, he absorbs their power, aiming to become an invincible being known as 'The One.'

As he nears his goal, he faces his final challenge: a version of himself from our universe. This high-octane thriller is a collision of action and existential philosophy, exploring the duality of man and the consequences of power unchecked.

12. "Parallel" (2018)

Parallel (2018).

Martin Wallström

Georgia King

Alyssa Diaz

Mark O'Brien

Carrie Genzel

Shannon Chan-Kent

Cassidy Darling

Izabel Pearce

David Harewood

A seemingly ordinary mirror becomes the gateway to endless possibilities in "Parallel." A group of friends stumbles upon this mirror, revealing a multiverse of alternate realities.

As they exploit the knowledge and opportunities from these parallel worlds, they soon realize that every action has consequences. The film is a riveting exploration of ambition, morality, and the ripple effects of choices made in alternate realities.

13. "Source Code" (2011)

Source code (2011).

Michelle Monaghan

Vera Farmiga

Jeffrey Wright

Michael Arden

Russell Peters

Brent Skagford

Craig Thomas

Gordon Masten

Time, memory, and reality intertwine in the gripping narrative of "Source Code." Captain Colter Stevens, on a mission to prevent a catastrophic train bombing, finds himself trapped in a perplexing loop, reliving the same eight minutes.

Within this brief window, he must identify the bomber and avert disaster. But as he delves deeper, he grapples with questions about his own existence and the nature of reality. A thrilling ride that melds action with introspection, the film challenges our understanding of time and destiny.

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About lila sommers.

Lila is your media maven with a passion for the extraordinary! Spot her at the local bookstore or crafting her own stories on lazy Sunday afternoons. From indie flicks to hidden gem tracks, Lila's knack for weaving words and tunes is unmatched, making her your ultimate companion in the exciting world of media and entertainment.

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Time Travel, Alternate Dimensions, and Parallel Universes: Exploring the Limits of Reality

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http://tvtropes.org/pmwiki/pmwiki.php/Main/AlternateTimeline

Alternate Timeline

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Spock: You are assuming that Nero knows how events are predicted to unfold. The contrary: Nero's very presence has altered the flow of history... thereby creating an entire new chain of incidents that cannot be anticipated by either party. Uhura: An alternate reality? Spock: Precisely. Whatever our lives might have been, if the time continuum was disrupted, our destinies have changed. — Star Trek (2009)

This is often caused by Time Travel and What If? scenarios.

This trope does get a bit confusing on a meta level, when you take an installment and have Alternate Continuities in parallel. To explain, consider two works of speculative fiction: one optimistic and depicts a utopian future, the other is pessimistic and depicts a dystopian future. Obviously both can't happen at once, but until the future arrives they're both equally valid, and both take place in the same world — our world. Basically, the main difference between an Alternate Timeline and an Alternate Continuity is that Alternate Timelines share Backstory and were formed at a point of divergence, with the "new" timeline simply overwriting the "old timeline" as a result of said divergence. In other words: Alternate Continuities do not share the same Canon , whereas Alternate Timelines do .

Unlike plain vanilla Alternate Continuity , these kinds of timelines do not necessarily happen due to Adaptation Decay or Adaptation Distillation in moving from medium to medium, but were often chosen deliberately by creators to take a franchise in a new direction while preserving the original material. This can oftentimes be done to achieve Canon Discontinuity . Didn't like that last installment? It was in an Alternate Timeline and really has no effect on your main franchise. Sometimes, these forked timelines can run simultaneously, each providing a different take on the franchise, its characters, and its events. The Alternate Timeline can also be employed as a kind of "soft" Continuity Reboot , creating a new universe while keeping the original in- Canon .

If what it diverged from is Real Life , then it's Alternate History . In that case, the Nazis may inexplicably win the war . If this doesn't happen, particularly in Video Games with Multiple Endings , it's called " Cutting Off the Branches ". In some stories involving alternate timelines, you can Flash Sideways or meet your Alternate Self . Alternately, if a single diverging event causes the story to switch between two or more different timelines, it's a Split Timelines Plot . If one timeline is treated as "truer" or more important than the others, that's a Prime Timeline .

    open/close all folders 

• Timeline A is the base timeline. A resurrected Frieza and his father King Cold arrive on Earth and are defeated by Goku with his Instant Transmission, but Goku dies of a heart virus sometime after. A year later, two Androids appear and kill off everyone except Gohan, creating a dystopia where they run rampant. Bulma eventually creates a time machine for her teenage son Trunks to use to change history.
• Timeline B is created when Trunks arrives in the past via the time machine. Trunks defeats Frieza and King Cold before Goku does, then gives him the cure for the heart virus along with a warning about the Androids, allowing them to prepare for their arrival. Trunks returns later to help the Z Warriors defeat the Androids, and eventually recovers their blueprints. This allows him to shut down the Androids both here and back in Timeline 1, meaning they aren't defeated through training and combat (this is the speculative "fourth" timeline mentioned above).
• Here's where things get confusing. After Trunks returns to Timeline A and shuts down the Androids there, he's killed by a creature called Cell just before he uses the time machine to tell his friends the good news. Cell takes the time machine and arrives in the past, two years before Frieza and King Cold do, creating Timeline C (the one depicted in full via the manga and the anime). Cell won't emerge in earnest until soon after the Androids show up, and usurps them as the main threat to defeat by absorbing them and their power. While Trunks trains to try and defeat him, Cell would ultimately be killed by Gohan. Trunks also kills an embryonic Cell before he can become fully grown, which prevents another future from being ruined. After Cell's defeat, Shenron revives everyone who was killed by him. This includes the Androids, who undergo a Heel–Face Turn .
• In terms of how most of the fandom and the guidebooks list the timelines above, it's a different order. Timeline C is considered to be Timeline 1 as that's the one viewers and readers are most familiar with, while Timeline A is Timeline 2 (the Future Trunks timeline) and Timeline D is Timeline 3. Timeline B is the speculative fourth timeline that isn't directly shown outside of Cell's exposition flashback.
• Dragon Ball Super makes it even more confusing with the introduction of Time Rings, which Gods use to time travel without creating alternate timelines. Goku Black used a Time Ring to travel from Timeline 1 to Timeline 2 (Future Trunks' timeline) and caused havoc there, leading Future Trunks to go back to the Timeline 1 to ask for help. At the end of that storyline, Timeline 2 is destroyed by Timeline 2's Zeno and, thanks to a lot of string-pulling by the other gods, Timeline 5 is created where Goku Black and Zamasu, the one who kicked off these events, never ran havoc in the timeline.
• In the spin-off manga, Dragon Ball: That Time I Got Reincarnated as Yamcha! , there is one in which Yamcha manages to stay on par with the heroes in the early parts of the series and avoid his embarrassing death that befell him in the Saiyan Saga. This causes the timeline to diverge further because of this, such as causing all the Z Fighters that fought Nappa and Vegeta to survive that fight and accompany Krillin, Gohan, and Goku to Namek. It should be noted that this scenario, contrary to the other timelines, isn't canon, and wasn't created by Time Travel .
• The Spin-Off Paradox takes place in one. It actually turns out the be very important to the main timeline in the last chapter, however.
• Near the end of the main series, it's revealed that the timeline the series takes place in is actually one of these. In the original timeline, Yuno was the winner of the game and became the new God of Time and Space, but since she couldn't resurrect her world's Yuki, she instead used her powers to go back in time, creating the actual timeline or "second world", killed this timeline's version of herself and took her place in the game, all in order to be with Yuki once again. In the finale, a third timeline is created when Yuno tries to go back in time once again and here all the characters manage to get a happy ending .
• HuGtto! Pretty Cure only has two timelines, with the major divergence point occurring before the first episode even starts: The main timeline where Hana switches schools after being bullied out of her previous one and decides to change herself for the better, and a Bad Future where Hana stays at the school she was bullied at and ends up dying through undisclosed means in her adult years (which is where the Big Bad and his corporation originates from). Of course, this doesn't begin to cover the countless other timelines that implies to co-exist with the main Hugtto story...
• The Magical Girl Lyrical Nanoha A's Portable game series splits off from the main Lyrical Nanoha continuity at the end of Magical Girl Lyrical Nanoha A's . Here, Reinforce Eins does not perform a Heroic Sacrifice , causing Humanoid Abomination Evil Twins of Nanoha, Fate, and Hayate to be born from the remnants of the Darkness of the Book of Darkness .
• By the end of Negima! Magister Negi Magi , there's at least three different timelines. The first one is the Bad Future Chao Lingshen comes from, the second one in which Asuna goes into a sleep for 130 years and finds all her friends are dead when she finally wakes up , and the third timeline where Chao and Eva bring Asuna back in time for her to be with her friends this time around and at which the epilogue takes place.
• UQ Holder! takes place in a timeline where Asuna went to sleep and couldn't be there to defeat the Big Bad , resulting in Negi being possessed by the latter. It diverges from any timeline seen in the first series because unlike the second timeline mentioned above, Asuna never gets sent back to the past and stays in the same timeline with Negi after both are freed from the Big Bad 's control .
• The finales of Neon Genesis Evangelion and The End of Evangelion (arguably) diverge at Episode 25. ("Arguably", because in the opinions of some fans, they merely present the same events from different points of views.) Rebuild of Evangelion , meanwhile, diverges quite considerably from The Original Series . It's especially notable in the second movie, but it starts a good deal before.
• Puella Magi Madoka Magica : The anime series shows five different timelines where Homura Akemi failed to save Madoka Kaname, having to watch her either die or become a witch . Word of God states Homura has reset the timeline at least 100 times . Spin-Off manga, like Puella Magi Oriko Magica and Puella Magi Madoka Magica: The Different Story , take place in other timelines that came before the final one seen in the anime . The PSP game shows several additional timelines, though at least some aren't canon due to Homura dying or becoming a witch .
• Republica , from BBV Productions , is set in an alternate timeline where Charles II was never restored to the throne after the English Civil War. In the 20th century, England is still a republic (although the last scion of the House of Stuart, who styles himself Charles XIV, is plotting to take back what he considers his).
• Even Archie Comics has ventured into this, with their controversial Archie Marries Veronica / Archie Marries Betty combined mini-series which became a combined regular series.
• One alternate Earth in the DC Multiverse was formed soon after Jason Todd died. Batman , stricken with grief, abandoned his Thou Shalt Not Kill policies and virtually eliminated crime worldwide.
• Then there's Earth-12 and Earth-50. Earth-12 is the DC Animated Universe universe, currently in the era of Batman Beyond . Earth-50's history diverged from the DCAU with the death of the Flash at Lex Luthor's hands, leading the Justice League to become the totalitarian Justice Lords.
• Flashpoint details an altered DC Universe in which only Barry Allen ( The Flash ) seems to be aware of significant differences between the regular timeline and the altered one, including Cyborg's place as the world's quintessential hero, Superman's detention by the government, and a Thomas Wayne version of Batman who spends his days "running Wayne Casinos."
• DC's The Kingdom introduced the concept of Hypertime, "the vast, interconnected web of parallel timelines which comprise all reality", at the end of Issue 2. It also acts as a variation of the multiverse (every story, even the ones just in your head, are canonical somewhere). This was also used to explain away some continuity errors and subtly bringing in elements from other realities , but was later abandoned and instead the anomalies were the result of Superboy punching reality . Hunter: How does 'Hypertime' work? Off the Central Timeline we just left. Events of importance often cause divergent 'tributaries' to branch off the main timestream. ...On occasion, those tributaries return — sometimes feeding back into the Central Timeline, other times overlapping it briefly before charting an entirely new course.
• In the Doctor Who Magazine comic strip, the Eighth Doctor comic stories were initially explicitly depicted as taking place in an alternate timeline to most of the Seventh Doctor comic stories (Ace dies in a Heroic Sacrifice at a point that is clearly before her character development in the Seventh Doctor stories, and the Seventh Doctor very quickly regenerates into the Eighth, with his first major story arc being his pursuit of the Threshold — all this was because a new editor strongly disliked the Doctor Who New Adventures which the Seventh Doctor comics had shared a continuity with, and wanted to demonstrate that ). However, the post-2005 stories have occasionally made continuity references to stories from both allegedly separate timelines.
• The Spider-Girl series takes place in the MC2 universe, which diverged from the main continuity after the Clone Saga. In this continuity, Spider-Man 's daughter survives and becomes Spider-Girl in the future. The point of divergence may be even further back, as later on it's revealed that Simon Williams (a.k.a. Wonder Man) died in issue 9 and was never resurrected.
• Marvel tends to mess in and out of clones and alternate timelines, to the point where several characters have a modus operandi off of it.
• It was an early rule of Marvel Comics that any attempt to change history would just create an alternate timeline, which among other things is how the X-Men have so many bad futures to choose from. Writers have tended to forget this when it's convenient.
• Exiles is all about exploring alternate timelines with a group of original characters (for the most part). One interesting commonality about them is that typically Thor wasn't involved in them.
• Mighty Morphin' Power Rangers (Boom! Studios) features two timelines: a Setting Update of the original show, and another (revealed much later) where Tommy kept working for Rita even after her spell was lifted . The second one is a few years ahead of the first, and has gone full Bad Future . The comic's story arc is a result of evil!Tommy invading the regular timeline for his own reasons.
• Dr. Ivo "Eggman" Robotnik as he appeared in Sonic the Hedgehog (Archie Comics) actually came from an alternate timeline where he roboticized himself to defeat Sonic. After wiping out the Freedom Fighters in his home dimension, this Robotnik became the successor of the original Robotnik, who was killed in the 50th issue. He was eventually turned back into an organic being in Issue 118.
• Star Trek: Early Voyages : In the story arc "Futures", Mia Colt accidentally activates the tachyon energy suspended in an Algolian keepsake when she scans it with her tricorder. As a result, she is sent 39 years forward in time to 2293 and thereby creates an alternate timeline. After her disappearance, James T. Kirk, newly graduated from Starfleet Academy, was assigned to replace her as Captain Pike's yeoman. However, the two men had a severe personality clash and Kirk resigned from Starfleet after a year. He went into the commercial sector and eventually became the captain of the freighter Bounty . Captain Pike remained in command of the Enterprise until it was decommissioned and became an exhibit at the San Francisco Smithsonian Museum of Air, Sea and Space. He then assumed command of the Enterprise -A. In 2293, the Federation has a very fragile peace with the Klingon Empire and it is feared that an instance of border violation may result in war.
• Sons Of Star Trek has Q Junior send Jake Sisko, Nog and Alexander Rozenkho to an alternate universe where first contact with the Breen lead to a ten-year war between them and the Federation. The three are on board the USS Avery , captained by Dukat (which really chaffs Jake) with crew members including Morn (who really is talkative), a Commander Beckett Mariner (suggesting that Sito Jaxa never died here ), Tuvix and a seemingly well-adjusted but a Lieutenant Liam Shaw.
• Way of the World : After Thomas Price's death, the story shows an alternate future where Supergirl defeats an alien conqueror and considers to use his time-travelling gadget to save Thomas.
• Superman & Batman: Generations had an alternate timeline play itself out in the third series, only for it to be undone and revert to the ending of the first series, with Batman, Superman, and Lana Lang together wondering what had happened.
• The Transformers Classics continuity follows on from the end of The Transformers (Marvel) , but ignores the events of G2 and some (maybe all) of the UK comics. Don't ask where Earthforce fits in.
• Titan Magazines had a comic based off the 2007 film which diverges from the main timeline when the Decepticons win the Battle of Mission City.
• In Transformers: More than Meets the Eye , Brainstorm's time machine is designed specifically to avoid creating one when he attempts to prevent the Autobot-Decepticon war. It doesn't quite work — while all the time travelling he and the others do follows You Already Changed the Past , Perceptor messes with the machine enough for a second timeline to come into being. In it, Megatron was never born and all the war's collateral damage is averted — at the cost of Cybertron remaining a complete dystopia where your worth is defined by your alt-mode.
• In Wonder Woman: Odyssey , Themyscira was destroyed when Diana was a young child, leading to the death of Hippolyta and Diana being raised in exile with surviving Amazons.
• The fic focuses on a new timeline created when Shadow Moth successfully gets the Ladybug and Black Cat Miraculous and make the wish to undo his wife's coma, which leads him to be in a coma instead and Emilie to become the Butterfly villain Lycaena . As Lycaena, she is a far more dangerous and ruthless villain than Hawk Moth was, forcing Master Fu to select more permanent heroes like Ryoko, Rena Rouge, and Pegasus to join in the fight alongside Ladybug and Chat Noir. In addition, Emilie is shown to be a skilled Manipulative Bitch , using emotional manipulation to keep Adrien from going to public school , getting the staff at Dupont to be lenient towards bullies so more potential akumas can be made from the victims, having Marinette blacklisted as a designer so she couldn't claim the stolen hat design Chloe submitted, and training Lila as an apprentice who caused Marinette, Alya, and Max to be expelled from their school, leading to the latter three to work in exposing the various forms of corruption in Paris.
• In the penultimate chapter, it's revealed that there was a previous timeline before the canon-compliant timeline that Hawk Moth started from . The original timeline involved a British version of the original Miraculous Ladybug preview, with Felix as Black Cat, Marinette's cousin Bridgette as Ladybug, and Felix's father as the supervillain Papillon, who planned on using the wish to bring back his dead wife. After the final battle resulted in Papillon's defeat and Ladybug's death, Felix desperately stole the Rabbit Miraculous to go back in time so he can save his mother and his girlfriend . Just like the wish, his actions also resulted in an Equivalent Exchange that brought back Amelie and Bridgette but caused his aunt to enter a coma from abusing the Peacock Miraculous and his father to be killed by Gabriel for the Butterfly Miraculous .
• Justified in Brother on Brother, Daughter on Mother , which takes the approach of explaining the inconsistent behavior of Star Trek Time Travel with the notion that time is like a rope composed of alternate timelines (or more specifically sequences of probabilistically determined events) that are fundamentally similar overall, but which can fray off dramatically different timelines in the event of major temporal incursions. The Time Police and the Temporal Prime Directive exist to prevent such frays from happening, since they damage time itself.
• An unused idea for a Calvin & Hobbes: The Series Made-for-TV Movie chronicled the eponymous characters visiting one, where Calvin (as an adult) became rich by selling the MTM, only for a war to break out as several nations all try to use it to change history in their favor .
• In Carpe Noctem , while going back in time with the help of the Time Turner, Hermione accidentally lets her past self see her future self, which disrupts the space-time continuum and causes an alternate world's Hermione to see the events of the canon Harry Potter timeline in her dreams .
• In Custody Battle , a My Hero Academia fanfic, the first One For All user traveled forward in time instead of dying. This created a darker AU since he never passed on One For All.
• Down the Karmic Hole takes place in one, being set in the same universe as The Karma of Lies . The main divergence so far is Alix encountering her alternate self's future self, Bunnyx, who informs her of the events that lead to Karma of Lies one week before they happen.
• Empath: The Luckiest Smurf had at least three different alternate timeline stories played out. The first was when Empath was born without his abilities, but instead was lost in the forest for 150-something years before he was discovered and named Wild Smurf. The second was when Smurfette fell in love with and married Papa Smurf, resulting in a child and the new family being exiled from the Smurf Village by the other Smurfs. The third was when Empath was forcibly returned to Psychelia and Smurfette ended up falling in love with and marrying Hefty, having a child through him.
• In Equal & Opposite Attraction , there is a massive divergence where Negi never stops teaching and Fate goes soul-searching . This kicks off the plot when Negi leaves Mahora to get his life in order.
• Fate of the Clans : In this, the multiverse exists. A Quantum Time-Lock occurs every 100 years in order to cull the Dead End timelines so the Solar System doesn't die. A person with the Second Magic, Kaleidoscope, can travel to these different realities.
• In Fear No Evil , a My Hero Academia fanfic, Izuku is forced to face Humarise early when they kidnap him while he is still in middle school.
• In Flashpoint 2: Advent Solaris , the Sonic Universe is brought in not as its own universe but rather an alternate timeline of the DC Animated Movie Universe . Barry Allen is just as confused about it.
• In the Forever Captain series, Steve is uncertain what effect his return to the midcentury will have on the proceeding of the timeline in this version of the multiverse.
• In Forgiveness is the Attribute of the Strong , a My Hero Academia fanfic, eleven-year-old All for One and his younger brother travel to canon time. This story uses the parallel universe version of time travel, so their older selves are also around (as a vestige in the case of the latter).
• The Legend of Zelda Ocarina of Time (DragonRand100) features the creation of multiple timelines, not unlike the original game . The first timeline is the adult timeline where Link defeated Ganon as the world healed; the second timeline is the child timeline where Link was sent back to his childhood where Ganondorf could be stopped before his plan was put in motion. Other timelines, created by some timey-wimey stuff involving the Hero of Time, are hinted at.
• Maris Stella : Chapters 27 and 28 detail a timeline in which Riodinidae overpowers the heroes to steal the Miraculous . Yuu manages to travel back in time to change events, creating the main timeline.
• No Such Luck, No Such Love is a The Loud House fanfic that centers around one of these during the events of "No Such Luck", but instead of having the Loud family believe that Lincoln was bad luck, they knew that he lied about it so he could get out of going to his sisters' events. The things that Lincoln suffered through in the actual episode (not allowed to attend activities that he enjoys, having his room completely removed of his furniture and boarded up, and being forced to sleep outside) were really done to punish him for lying, but the problem is that Lincoln doesn't know this due to no one telling him , so he ends up thinking that his family really thought he was bad luck.
• The main setting of Odaliaverse is an alternate version of the Boiling Isles that, while initially looking very similar to the original timeline, is soon revealed to have a couple of changes, chief among them being Phillip Wittebane having been petrified a long time ago, with someone else, implied to be his brother, taking his place as Emperor Belos .
• Passing Days for Fate/Grand Order itself can be considered an Alternate Timeline to canon due to the main protagonist Vy's actions in regards to the game's events and the Servants she summons. Scathach-Skadi herself in this story is an Alternate Self and Sole Survivor of her Lostbelt who was summoned in Part 1.5 as an example, and it results in her saving Vy and Mash from Kadoc and Anastasia in the Russian Lostbelt. Day 15 of the story even has Chaldea explore a mini-Lostbelt of an Alternate Timeline of Fate/Extra Last Encore where Dan Blackmore and Robin Hood succeeded in killing Deadface!Hakuno and Nero in their first encounter.
• The Pony POV Series is established as being an Alternate Timeline to the main series timeline. While originally tied to the "heart-world", the universe diverged completely and became independent as of "The Last Roundup" .
• In The Sage's Disciple , this appears to be the case for events of the mundane world , as it appears the presence of the supernatural has affected the timeline. Russia remained an absolute monarchy due to Tsar Alexander III surviving to influence the events leading up to World War I, the Yggdmillennia family prevented the outbreak of the Second Balkan War, the Titanic managed to reach New York City before sinking due to the intervention of an unnamed magus, Operation Valkyrie succeeded in 1944, etc. During the sequel The Puppeteer's Bodyguard , Crow spends much of his free time researching other such changes and trying to figure out why it's so different from the world he knew .
• Spirit Lines takes place in an altered timeline of Parts 1 to 6 of JoJo's Bizarre Adventure after Pucci used Made in Heaven to reset the universe in Stone Ocean . While the events of Phantom Blood and Battle Tendency remained the same, many events afterwards are changed due to the death of Joseph Joestar . Namely, this story's version of Stardust Crusaders has Katarou (this timeline's Jotaro) implicitly get sick from Dio's influence, leading to Holly — who requests to go by Seiko — joining Joseph and Avdol on their quest to Egypt. Katarou and his daughter Irene, this timeline's Jolyne, are normal humans without Stands .
• A defining plot point in Split Second (My Little Pony) , which diverges at the moment of the first sonic rainboom, in which Twilight became a White Mage and Sparkle became a Black Mage .
• In Star Wars: Light in Dark Time , the series' real plot begins when Doc and Marty inadvertently get Luke and his friends to create an alternate timeline where Anakin never became Darth Vader, but Order-66 still happened. The result is an evil version of Luke, a more competent Empire, and a potentially worse galaxy.
• Kara , and by extension, Astra and co., were not due to arrive on Earth for another five years. Kal-El is the right age, but he's being raised by Kara and his original adoptive parents, the Kents, were killed in a fire before they could ever meet. To say nothing of the different landing place. This is also implied to be the reason why her supporting cast, and the location of National City, are not a part of the story — most of them are kids/teenagers living in different parts of the country.
• Bruce Wayne's parents were never killed, so he is a well-adjusted individual who never becomes Batman . This presumably has a ripple effect that prevents the rise of most of his Rogues Gallery .
• Princess Diana never leaves Themyscira, so there's no Wonder Woman either.
• Felicity Smoak is incarcerated for the creation of her hacktivist supervirus, rather than moving to Starling City and eventually becoming infatuated with Oliver .
• APEX starts off in a normal version of 2073, when a time-travelling experiment goes wrong sending an A.P.E.X drone to a century ago, in 1973. The hero, Nicholas Sinclair, travels to 1973 to shut down the drone, but then returns to an alternate version of 2073 where the A.P.E.X. drones tutned against humanity, and mankind are on the losing end of a Robot War . Sinclair even meets alternate versions of his wife and commanding officer, and they didn't recognize him at all .
• About half of the plot of Back to the Future Part II is based around this. 2015!Biff Tannen steals the DeLorean time machine, goes back to 1955, and gives his younger self a sports almanac that allows him to win every bet and become fantastically rich. By 1985, his greed and corruption have turned Hill Valley into a Wretched Hive . Marty and Doc find out about this when they try to return home, then figure out the point of divergence and spend the rest of the movie trying to steal the almanac away from 1955!Biff so they can destroy it.
• The IDW comic book did a mini-series titled Biff to the Future , co-authored by series co-creator Bob Gale, that explored the alternate timeline in greater depth and detailed events that were just minor background elements in the movie. Among other things, Biff became good buddies with Richard Nixon , helping the latter avoid Watergate and stay in office for five terms, which also lead to the Vietnam War continuing into the 1980s .
• In the DC Extended Universe , Batman witnesses a horrific Bad Future in a dream in Batman v Superman: Dawn of Justice , with the forces of Apokolips dominating the Earth and Superman having turned to their side. All of this likely happened because Lois Lane died and the Justice League didn't gather. The Flash travels back in time and tries to warn Bruce Wayne, who doesn't fully understand it until Superman's death at the end of the film. By the time of Justice League , Bruce and Diana/Wonder Woman do their best to gather the eponymous Super Team to repel the incoming Alien Invasion by the Apokoliptian war chief Steppenwolf and spare the Earth the aforementioned Bad Future . While the 2017 theatrical film does not delve into it, it becomes more of a key plot point in Zack Snyder's Justice League , where it becomes more clear what happened: Darkseid killed Aquaman and Wonder Woman, corrupted Superman, and Batman, Flash, Cyborg, Mera, and Deathstroke are forced to pull an Enemy Mine with The Joker to stand a greater chance against their fallen comrade (though Batman had promised Harley to kill the clown when the alliance is over).
• The Godzilla series has done this several times to the point where each of the movies from Godzilla 2000 and onward (with the exception of the two with Mechagodzilla) are their own continuity branching off from the original .
• Avengers: Endgame deals with something like this. In order to find and retrieve the Infinity Stones and restore those erased by Thanos, the Avengers do a bit of time travelling, intending on "borrowing" the items before putting them right back. However, things don't go so well. In 2012, during the Battle of New York , Hulk accidentally knocks out present-day Tony, allowing the Loki of that era to temporarily escape with the Tesseract (and lead into his own show in the process). Later, the Thanos of 2014 catches present-day Nebula and learns of what happens in the future, leading him to travel to 2023 in order to take care of everything there, taking all of his forces (including his own Nebula and Gamora) with him. This implies there's at least two of these — one where Loki escaped capture and one where Thanos never arrived on Earth at all, but the Guardians never got Gamora on their side. Additionally, the directors have confirmed that Steve going back in time to live a life with Peggy also created another alternate timeline... a statement which is contradicted by the writers , who have instead suggested that Steve was actually a part of the original timeline all along as Peggy's mystery husband . According to the Ancient One, the Infinity Stones create what is perceived as time, and that removing one creates a branched alternate reality. This can be interpreted to mean that any changes caused by Time Travel result in alternate realities, or that this only applies if an Infinity Stone is directly involved.
• In Spider-Man: No Way Home , Peter tampering with Dr. Strange's magic leads to villains from the Raimi and Webb universes being brought into the MCU from timelines parallel to the original ones where they were defeated by their respective Spider-Men, shortly before it happened.
• It's almost certain that there are two continuities in the original Planet of the Apes franchise. The first is the continuity that led to the society of the original film , possibly as described by Cornelius to the committee in Escape from the Planet of the Apes . The second is the altered continuity that the birth of their son, Caesar, sparked. It appears that the ape revolution was greatly sped up with his arrival. However, fans are split as to whether the continuities ultimately re-merge when history reaches the point where the original film was set, and humans will again end up mute wild animals and the world destroyed by the Alpha-Omega bomb or whether history was changed for good by the end of Battle for the Planet of the Apes and Caesar forged a new future. The editing of Battle didn't help things, the theatrical release had a hopeful tone, but the unedited version on most DVD releases clearly sets up the beginnings of the mutant society of Beneath the Planet of the Apes . The TV series likely exists in one of these continuities somewhere, while the animated series and the 2001 remake don't, and and Rise may or may not. Rise of the Planet of the Apes is a Continuity Reboot , but there have been statements by the creators contradicting that and painting it as a prequel as well.
• In Sliding Doors , the movie shows two alternate timelines. The story starts off with Helen Quilley getting fired, while her boyfriend is cheating on her. She goes home, and either just catches the tube in time, finding her boyfriend in bed with another woman, or misses the tube, causing some other events to happen which means she arrives home after the other woman has left. The movie then alternates between the two story lines.
• Explained in-story in Star Trek (2009) . The two main timelines diverge at the attack on the USS Kelvin . In the Prime Timeline , Kirk is born on Earth in March of 2233 and the timeline proceeds how you remember it. In the new timeline, Kirk is born in space in January during Nero 's attack on the Kelvin , his father is killed pulling a Heroic Sacrifice , and Vulcan is eventually destroyed — so there is no point asking Spock-Prime what's going to happen next, since now he has no idea either.
• Although his knowledge of things that predated the Kelvin attack but became important later or that originated elsewhere wouldn't have necessarily been affected. Things like the Guardian of Forever, the "green space hand", Doomsday Machine, giant space amoeba, V'Ger , Khan's sleeper ship (which is realized in Star Trek Into Darkness , where it is discovered several years earlier than the original timeline), the Nexus , the existence of the Borg and the Dominion, subspace damage caused by warp drives, the need for whales , and so on. Also included is the entirety of Star Trek: Enterprise .
• The IDW comic series reveals that Q knows about this timeline and decides to go mess with it to teach Kirk a lesson, despite Picard begging him not to.
• In 2016, shortly before the release of Star Trek Beyond , this timeline got officially dubbed the "Kelvin Timeline", referring to the USS Kelvin attack as the diverging point.
• In Triangle , Jess gets stuck in a "Groundhog Day" Loop of her co-passengers getting killed by a masked killer. The first Jess tries to break the chain by meeting the Jess from the next loop. This creates a new timeline where the other passengers died differently by getting killed by a different masked killer. Subverted in that the two timelines play off at the same time and each influence each other. For example the new timeline eventually causes the second Jess to have her own story, which we don't see, where she eventually killed some of the passengers as the second masked killer which eventually made the first Jess turn into the first masked killer . However, it is highly, highly implied that Jess is not going through any real kind of "time travel", but that she died IRL in the car accident we see at the end, and everything we're witnessing is her in some kind of purgatory/hell due to her guilt/grief over the way she treated her special needs child while alive , ultimately averting the trope.
• The series entered this territory after X-Men: Days of Future Past . All films set after that point take place in an alternate version of Earth in which the existence of mutants became public knowledge in 1973, nearly three decades earlier than in the original timeline, meaning there are quite a few differences.
• Logan is said to be its own timeline even to the one post- Days of Future Past .
• Eric Flint 's 1632 : One of the many things that the main characters bemoan is that so many great artists and musicians born after the arrival of Grantville in the past will now never be born because the timeline has diverged way too far for probability to allow it. This fact causes several artists and musicians native to the time to investigate what would have been created by those now-erased artists and create it themselves.
• Adam R Brown 's Alterien features a story at a later point in the series in which Oberon and his rival, Theseus, end up in an alternate reality. In this alternate universe, many of the events of Oberon and Theseus' lives occurred similar to the way they did before, but set a century earlier and with new people in place of some of the people they remember.
• The Black Trillium novel was co-written by three authors, who each went on to write a sequel (or two), effectively spawning three mostly incompatible timelines from a single root.
• Gary Paulsen 's Brian's Saga : The first book, Hatchet , ends with Brian being rescued . However, in the third book, Brian's Winter , it takes place in an alternate series of events where Brian has to survive through the winter . Of course, marketing spoiled this on the back of the book .
• Stephen King 's The Dark Tower : In The Waste Lands , alternate timelines created in the previous book, The Drawing of the Three , are explored. The alternate timeline, one where Roland travelled with Jake, versus one where he was alone , slowly drive both Jake and Roland insane as their minds simultaneously believe both two different versions of events to be true.
• Scott Ciencin 's Dinoverse has a vast number of alternate universes, but it's implied that the peaceful utopian Dinoverse, which has a present-day society of, well, dinosaurs, is the result of characters in the Mesozoic era rescuing an unusually smart Deinonychus from certain doom. There's still the heroes' reality with human civilization, and there are human and dinosaur versions of several characters.
• In Lords and Ladies , Granny finds herself wondering if she's going mad when she starts subconsciously setting up her house as if two people are living there. After learning of the Trousers of Time from her ex-boyfriend Mustrum Ridcully, she realizes that her mind has been mingling with an alternate version of herself where she married him instead of remaining single.
• In Jingo , Sam Vimes leads the Watch to rescue a captured member, and then somehow accidentally collects his Disorganizer (which has started adding future events to his schedule) from the leg where he didn't. This results in him learning what could have been had he not gone - his departure caused the Ankh-Morpork fleet to attack Klatch ahead of schedule, which disrupted Klatch's invasion plans, buying time for Vetinari to come up with a political solution; in the other, Klatch attacked first, and the Watch was slaughtered in the city's defense.
• The Empirium Trilogy : When Eliana returns after a failed trip into the past, she learns that her brief interaction with her birth mother altered the pasts of various characters and events, and not for the better. Characters that were alive are now dead or missing and the deadline for the current mission was moved forward by a few days.
• Later Joel Suzuki books reveal the existence of alternate timelines. When a person travels to one, they displace the version of themself that was already there, resulting in a person who combines traits and memories of both, and retconning the history of the new timeline to reflect that. In Dance of the Darkeye , Joel and Felicity travel to different timelines to use different versions of the Songshell to infuse Joel with huge amounts of power so he can defeat Marshall. But it turns out that excessive timeline-hopping can weaken the boundaries between timelines, and their actions cause all the timelines to eventually collapse into one.
• Isaac Asimov 's Living Space : Alec Mishnoff is able to figure out that the German-speaking builders are from an alternate Earth timeline where Nazi Germany won a tribal conflict . Still the same year, ignoring differences of Nach Hitler versus Anno Domini .
• Isaac Asimov and Janet Asimov 's Norby and the Queen's Necklace : The characters call these time-tracks, having created a "false" one when Albany took the diamond necklace that was the focus of Marie-Antoinette 's scandal before it reached Comtesse de la Motte, preventing the scandal from occurring in the first place . Without that scandal, The French Revolution was a peaceful change in government, creating a constitutional monarchy. In order to return to their own time , the characters have to first return the necklace to the jewelers it was taken from.
• In Orson Scott Card 's Pastwatch: The Redemption of Christopher Columbus , the historians of the titular organization try to figure out if it's possible to change the past to avert The End of the World as We Know It , only to discover that it has already been done once. It's heavily implied that our timeline is the alternate to what was supposed to have happened originally. In the original timeline, Columbus never sailed West, instead leading a new, disastrous, Crusade to retake the Holy Land. Without European interference, the Aztec Empire fell under the onslaught of Tlaxcala, a rival nation (in our history, they allied with the Spaniards to destroy the Aztecs). Unlike the Aztecs, the Tlaxcalans were not averse to innovation and were even more bloodthirsty. They forged an even greater empire, by the time the Americas are finally visited by Europeans in the 18th century. The captured Portuguese sailors were forced to give up the secrets of gunpowder and shipbuilding, after which the the Tlaxcalans built a massive armada and sailed East. They fell upon a devastated Europe, crushing all enemies and sacrificing thousands to their god of war. However, that original timeline was also doomed to result in the end of human civilization, so scientists from that timeline sent back a holographic message to Columbus, pretending to be God telling him to sail West, resulting in our timeline. Realizing that both versions of history are dead ends, the Pastwatch historians send three people into the past around the time of Columbus's arrival to the New World. One sacrifices himself to trap the Europeans in the Caribbean, another convinces Columbus to work on unifying the island tribes, and the third starts forming a more benevolent empire on the mainland. Many decades later, Columbus returns to Europe at the head of a massive well-armed fleet of trade ships as both an example of Gunboat Diplomacy and an offer for the Old World to trade fairly with the New World with neither culture dominating the other. Supposedly, this results in a much better timeline.
• Relativity : The entire series apparently takes place in an "alternate timeline." In the short story "Tempest" (which takes place in the distant past, relative to the main narrative), Phanthro alters history, and uses a memory-projector to show a version of history where Matt's son dies in his forties. Since his son dies as a child in the "current" history, the memories Phanthro has must be from an "earlier" version.
• Ginn Hale 's The Rifter : Kahlil, after a lonely youth training in Rathal’pesha, spent years in Nayeshi waiting to bring the Rifter (John) to Basawar; then his key to the gates fell into John’s hands and John crossed through. He landed at an earlier point in time and met Ravishan the trainee-Kahlil; together the two of them changed history radically. Now the future where Ravishan becomes Kahlil will never happen, but his future self is still over in Nayeshi, not knowing that events he remembers have been wiped out of existence. Kahlil crosses to Bashawar and arrives thirty years after John’s arrival. He doesn’t meet himself because by this time Ravishan is dead. However, he does pick up Ravishan’s memories; the two timelines coexist confusingly in his mind. Among other things, the Payshmura church that Kahlil remembers serving all those years doesn't exist here, it was destroyed soon after John's arrival.
• Worm takes place on Earth Bet, which diverged from our timeline in 1982 with the appearance of Scion, the first superhero . Other worlds exist with their own divergent timelines — Earth Aleph being one.
• The 4400 : In the two-part story "Gone", the woman from the future (who posed as Maia's sister Sarah Rutledge) and her accomplices kidnap five 4400 children — Maia, Lindsey Hammond, Tyler Downing and Duncan and Olivia Germaine — and transport them further back in time in order to create an alternate timeline. Their reason for doing so is that the 4400 have failed to change history and prevent the catastrophe of which the messenger in Kyle's body warned Tom in "White Light". Maia is sent to 1832 and eventually dies of smallpox while on the Oregon Trail in October 1847. Tyler is sent to 1893 and invents synthetic petroleum in 1915. Lindsey is sent to 1918, gets adopted by Robert Goddard and invents the composite materials used in the construction of the first lunar colonies .
• The series flashes over to the "Darkest Timeline" every now and then to see how things are going (or at least Abed does in his own mind).
• The episode with the two model U.N.s also used this explanation (at Abed's insistence) to explain the presence of two Earth's each with their own U.N.. Ultimately, it's integral to how the study group wins the competition.
• Dallas : When the show retconned a whole season that had been unpopular with fans as All Just a Dream , the producers of the Spin-Off Knots Landing didn't want to retcon plots that had been influenced by events in that season, causing the show to split off as an Alternate Timeline.
• "Turn Left" shows what would happened if Donna had turned right instead of left at a fork of the road to take a different job offer some time before meeting the Doctor. She never would've gotten involved in the Racnoss plot and not been there to keep the Doctor from going too far and dying under the Thames. Then it goes From Bad to Worse , as the Doctor's absence causes every single crisis he prevented to take place, sometimes even worse (e.g. the Adipose end up killing millions in the US, while the starliner Titanic ends up wiping out London). At the end of the episode, Donna is convinced by Rose to put events back on the right track.
• The events of "The Pandorica Opens" / "The Big Bang" end up triggering a Total Event Collapse, where all of reality aside from the Earth, Moon, and everyone on it are erased from existence across every point in time . Species that were at the heart of the collapse but not from Earth were preserved as statues; small impressions on the universe. Stars no longer exist, including the sun, but the TARDIS manages to keep the Earth warm by containing its explosion in a "Groundhog Day" Loop across all of time as well. This allows humanity to develop, though they see stars as just a myth. Aside from Richard Dawkins, who runs a "star cult".
• Lampshaded when Henry explains to Sheriff Carter exactly why they can't accurately predict the future after hitting the Reset Button on the past five years to undo Henry meddling to keep his wife from dying in the past , since the differing actions alone will cause a Ripple Effect to make the timelines gradually diverge. And then something different happens.
• It should be noted that Eureka has gotten two Alternate Timeline treatments. The first one (mentioned above) only lasted an episode before being reverted; the second one, however, became the permanent timeline for the show.
• Farscape : A few later episodes see John Crichton exploring a few of these, particularly as interdimensional wormholes and the associated wibblie-wobblies become more important to the plot.
• It's eventually revealed that the entirety of the show takes place in an alternate timeline created by the Reverse-Flash going back in time to kill Barry as a child. When Barry saves his past self, the villain kills Barry's mother in a fit of rage. Barry's father is arrested for the murder, and Barry ends up being raised by Detective Joe West. The villain further messes with the timeline after discovering that he is stuck there and does everything possible to try to return to his own time, including re-creating the Flash . By extension, this also means that Arrow also takes place in an alternate timeline.
• In the same season, it's revealed that the villain Savitar hails from one, where he slays Iris West, Barry defeats him, and then slips into a funk over the loss of Iris until his past self arrives and snaps him out of it. Then it's revealed to be a Stable Time Loop that occurred because Savitar is really Barry — or rather a Speed Force duplicate of Barry, who wants to ensure his existence remains. When H.R. Wells takes Iris's place, Savitar fades, altering the timeline.
• It's also worth noting that, according to the time-sensitive Observers, the first three seasons already took place in an Alternate Timeline that was thrown off-course from what was supposed to happen by the actions of an Observer. The Alternate Timeline of Season 4 is probably a lot closer to how the original timeline was supposed to run.
• Kamen Rider Ryuki : Since the overall plot of the series is that Big Bad Shiro Kanzaki is looping time over and over in an attempt to save the life of his sister Yui , the movie Episode Final and the TV special 13 Riders are the events of different timelines rather than simply being Alternate Continuities like most Kamen Rider Non-Serial Movies .
• Loki (2021) elaborates on the concept of time travel introduced in Avengers: Endgame and explains that any change to the timeline results in a branched separate timeline. The Time Variance Authority claims that an uncontrolled branching of timelines would be catastrophic and upholds their goal of maintaining the Sacred Timeline, one where every event happens according to how it's intended to happen, and pruning the unintended branched timelines.
• The Magicians (2016) : There are forty alternate timelines because of the time loops created by Jane Chatwin, in order to defeat the Beast . The timelines seem to be largely the same until the main group gets into Brakebills, at which point they diverge. Timeline 23 features in a couple episodes, once in Season 2 and again in Season 3. In that timeline, Julia got into Brakebills, she and Penny are in love, and Quentin is the Beast (after killing Martin Chatwin), among other changes. They bring Marina 23 and Penny 23 back with them after stopping Quentin. We also see a different timeline's version of Dean Fogg, who has had a Face–Heel Turn in response to everything going to shit in his timeline .
• Malcolm in the Middle : "Bowling" played with this, the divergence being which parent took the kids bowling instead of staying at home. The conclusion? Lois / Hal : Next time, you take 'em.
• The first instance is when Curtis first discovers his Mental Time Travel powers. When the storm turns the probation worker crazy and he attacks Kelly, the rest of the gang doesn't believe her until he comes in the door and kills her in front of them. Curtis travels back to right before he comes in and warns the gang.
• In another episode, he decides to try to use his power to stop himself and his ex-girlfriend from getting caught with cocaine in the first place. He manages to do so but this event causes the deaths of all of the other Misfits except Nathan ( probably because of his immortality ) because he isn't there to see the potential future. He ends up creating a Close-Enough Timeline in which he does end up on community service but his girlfriend doesn't get arrested.
• In a Season 3 episode, an old man buys Curtis' power from Seth in order to go back in time and kill Hitler . He fails, but he leaves behind his 2011 cell phone which leads to a future where Nazi Germany was able to use that technology to win the war and take over the UK.
• In the Series Finale , Jess is taken forward a year by some guy she meets in a bar, to a future in which she has a baby with this man.
• The series explores the same events differing in each timeline, usually because of Joe's different life. For example, Cop Joe manages to stop a shooter from firing a gun at a congressman. In the other realities, Nurse Joe has to tend to that wounded congressman while Music Joe's concert forced that congressman to move his rally to avoid the shooting altogether.
• The series does have some In Spite of a Nail touches as in each timeline, Joe has the same best friend. More dramatically, it appears that Nurse Joe is the only one with a child, Chris, having married college girlfriend Jenny. But at their college reunion, Music Joe is stunned when Jenny reveals she was pregnant on Graduation Day and gave the child up for adoption. Meanwhile, Cop Joe meets Jenny with her son (the same boy, but named Lucas in this world) with no idea he's the father.
• The Orville : In the penultimate episode of Season 2, Kelly's past self from seven years earlier, the morning after her first date with Ed, is transported to the present. When she returns, she decides not to go on a second date. As a result, Ed's career never goes off track and he and Kelly never join The Orville. This has the ripple effect of Claire Finn never joining either — she goes where she needed, and felt that she was needed on a vessel helmed by a washed up first time captain being given a second chance. This means that Isaac never bonded with her sons and therefore never betrayed the Kaylon, resulting in a finale in which they've all but taken over the galaxy.
• In " A Stitch in Time ", numerous alternate timelines are created due to time travel.
• In " Final Appeal ", the sequel to "A Stitch in Time", Ezekiel tells the US Supreme Court justices that time travel has taught him that the future is malleable and, as a result, it is more accurate to talk about futures plural as he has witnessed several different timelines. He cites the example of an alien race launching a devastating retaliatory attack on Earth in the 24th Century in one of these timelines (as seen in "Relativity Theory") as evidence that technology is inherently evil and destructive to humanity. The events of other episodes presumably take place in different alternate timelines.
• Paper Girls : The 2019 that the girls visit is implied to be one: The girls have adult selves, which would be impossible if there was no version of them in the past to grow up. Also, Adult!Dylan mentions that Mac died of brain cancer when she was 16, as opposed to remembering her mysteriously disappearing when she was 12.
• Psych : "Right Turn or Left for Dead" features an alternate timeline that diverges with Shawn giving Juliet Chief Vick's shawl instead of his jacket, thus preventing their break-up in the previous episode. Shawn ends up working the same case in both timelines, but the differences bring clues to light at different times. Near the end of the episode, the alternate timeline is revealed to be All Just a Dream of Shawn's .
• Scrubs : "My Butterfly", where the cast explored what different events resulted from a butterfly landing on an attractive woman's chest or alternatively, an unattractive man's chest. Ultimately subverted in that the patient ends up dying in both timelines.
• The show branches off from the Terminator movie continuity after T2: Judgment Day — Sarah and John Connor leap through time from 1999 to 2007, skipping Sarah's death from cancer in 2002 and the date at which Judgement Day would have happened if Rise of the Machines were Canon . Terminator Salvation , on the other hand, carries on from the Rise of the Machines continuity. Then, Terminator Genisys branches off to form its own continuity, while Terminator: Dark Fate only follows past Judgement Day . For a series with that much Time Travel , it's a wonder it didn't develop alternate continuities sooner.
• Two members of the future human resistance who were sent back to the past at different times (and were a couple in the future) in The Sarah Connor Chronicles find out that their versions of the future don't exactly match when comparing notes, implying that they come from different timelines.
• The show ends on a Cliffhanger and yet another timeline caused by John Connor traveling forward to the Bad Future , which still happens but now he wasn't there to lead the resistance, and it may be even worse because of that.
• Timecop : Naturally, some episodes have this happen. In one episode, a criminal messes up Jack's personal history, causing their roles to reverse (i.e. the criminal is now a timecop, while Jack is a known felon). In another, a neo-Nazi travels back in time to World War II and helps them win. He's smart enough to dress a local as himself and set up events to look like he is accidentally killed by the Nazis. Satisfied, Jack returns home only to find a different timeline. It's not explored, since he immediately jumps back to fix it.
• A major plot point of the pilot episode, where despite attempts to keep from altering history, the Hindenburg explodes a day later, destroyed by a group of "renegade terrorists" (the main characters), with only two victims. After they get back, Lucy finds her mother, who was comatose before, perfectly fine, she's engaged, and her sister never existed.
• Most episodes end up with an alternate timeline. The heroes fix the major damage to the timeline but can't restore it 100% the way it was so the present changes in some ways and only the time travellers remember the original timeline. They have to settle for Close-Enough Timeline .
• The Twilight Zone (1985) : In " Profile in Silver ", a time traveling historian from 2172 named Professor Joseph Fitzgerald prevents the assassination of his Famous Ancestor John F. Kennedy on November 22, 1963. The resulting change to the timeline leads to the creation of significant time distortions and a temporal rift of unprecedented proportions. Tornadoes appear without warning in Texas as part of the initial attempt to counterbalance the temporal damage. The assassination of Nikita Khrushchev results in the new Soviet premier sending in troops to capture West Berlin in an attempt to force the Western powers out of the rest of West Germany. Fitzgerald's wrist computer determines that there is a 77% probability that a nuclear war will break out between the United States and the Soviet Union, resulting in the total annihilation of the biosphere . At 12%, the best case scenario is that Western Europe will surrender within six years. Military costs will cause the Soviet economy to collapse, leading the USSR to blackmail the West for food. The subsequent agro-bacterial war will completely destroy the biosphere within a century. The remaining 11% accounts for all other probabilities in which the biosphere is destroyed. From this, Fitzgerald learns that Kennedy's death is a Necessary Fail and that history must be restored to its proper course if humanity is to survive.
• BIONICLE had at least four different parallel timelines. For instance, in Dark Mirror , Takua was killed by a Toa of Iron, and in The Kingdom universe, as well as the universe briefly seen at the end of Brothers in Arms , he never became a Toa at all. He is a Toa in the main universe.
• Different portions of the Doctor Who Expanded Universe in different media may or may not take place in different timelines — there are various claims in-universe about this, some of them mutually contradictory. Some people will try to place them in the same continuity. However, in the Milestone Celebration of Big Finish Doctor Who for the 40th anniversary "Zagreus", the Doctor claims the events in other continuities happen in other universes, which is the line TV Tropes has taken.
• One is the Last Generation comic. Set in a universe ruled by the Klingon empire after a plot change during The Undiscovered Country (i.e., Kirk wasn't fast enough to save the president]], and it's all Captain Braxton's fault ). In this Dystopian version of the Trekverse, Sulu captains the last remaining Star Fleet vessel as a Mysterious Protector , Tasha lives, Jean-Luc Picard leads a Resistance movement , Wesley took a few levels in badass (well, physical prowess, anyway) and Data is the key to changing history. Either that or he's the key to the most destructive computer weapons system ever created.
• The series Star Trek: Picard revealed that galactic history turned out very differently in the years after the movie Star Trek: Nemesis was released. It was decided that a Continuity Reboot needed to be done for the post- Nemesis years but executives were anxious to avoid making the same mistakes made by Disney when they rebooted Star Wars canon and placed everything not seen on screen in to the Star Wars Legends continuity. They decided that all the Star Trek Novel 'Verse works written after Star Trek: First Contact took place in an alternate timeline created by the Borg when they fired a temporal beam at the Enterprise causing the timelines to branch off.
• In Alpha Team: Mission Deep Freeze RPG , Future Frozeen created an alternate timeline by going back and time and changing the past, with the point of divergence being Mission Deep Freeze when he rescued the rest of Alpha Team from the assassin robot that killed them in the original timeline.
• In Dino Attack RPG , the Future Villains created an alternate timeline by going back in time and changing the past, with the point of divergence being the early Dino Attack when they joined forces with their past selves.
• In the first, they inadvertently change Hermione and Ron's relationship so that the two never marry, causing the erasure of Albus's cousins and drastically altering the personalities of both Hermione and Ron.
• In the second, they technically accomplish their goal by preventing Cedric Diggory's death. Instead, Diggory's humiliation leads to him joining the Death Eaters and killing Neville Longbottom, which ensures Voldemort's survival.
• The surprise antagonist of the story then attempts to create yet a third Alternate Timeline while the protagonists are attempting to undo their own changes. The protagonists stop her and eventually wind up with a Close-Enough Timeline .
• If/Then follows the main character Elizabeth down two separate timelines, depending on which friend and which activity she chooses to go with at the start of the play. Amusingly, the play is then filled to the brim with more choices, much weightier than the one made at the start, that would seem to be fodder for creating yet more timelines, but these are not explored, despite hints that more such timelines do exist.
• The Madness Musical; Our House , has the Lead Character breaking in to a development site, so he can get a view of his Neighbourhood, but then the Police arrive. It then shows two time lines, one where he turns himself in, another where he runs, showing the consequence of both choices, while his father's ghost watches and makes commentary on his choices. Then at the end it Snaps Back to the beginning and shows a third time line, where he doesn’t break in to the development site in the first place .
• Twilight: 2000 invented an alternate setup for World War III after the end of the Cold War which had Germany start the war instead of the Soviet Union. This one is a bit of Fanon Discontinuity .
• The Babylon Project campaign "The Earth-Brakiri War" is set in an alternate 2260 where Babylon 5 had been destroyed and Earth had alienated itself from the other races due to a failure of diplomacy.
• Chrono Cross initially seems to revolve around Dual-World Gameplay where the key difference between Home World and Another World comes down to whether Serge survived his childhood, but as the plot unfolds it turns out that a lot of what is going on is being influenced by the potential timelines created over the course of Chrono Trigger . One faction is from the good future resulting from that game, a high-tech civilization trying to prevent any Temporal Paradoxes that might threaten its future existence, while a rival faction hails from a timeline in which humanity never rose to dominate the planet, and wants to cleanse the world of us filthy usurpers. And then there's a Time Crash where Chrono Trigger 's Bad Future is trying to reassert itself, with dire consequences for everyone.
• Dark Cloud and its sequel . The first game created a different timeline entirely by stopping the Dark Genie from ever coming to existence. The second game was mostly about this, you had to restore Origin Points (events in past which influenced the future), it even creates new innovations in the future such as Aeroharmonic technology and Paznos' Colossus Mode which never existed before.
• Disgaea RPG has an alternate timeline where a staggering number of canon characters have rapidly diverging history from canon , due to multiple changes in history. Hailing from this timeline is the "Xeno" organization, which is chasing down a refugee from their timeline.
• Dragon Ball Xenoverse takes place in an alternate timeline from the main Dragon Ball series, created as a result of Towa and Mira interfering with time and changing the outcomes of key battles (allowing Raditz to evade an otherwise fatal attack from Piccolo, just to name one example). It is up to the player and an alternate future version of Trunks to Set Right What Once Went Wrong and correct the changes being made in history.
• Eternal Darkness , once the player has begun reading the eponymous Tome, details the Roman Centurian Pious Augustus claiming the artifact of an Eldritch Abomination , one of three choices. It turns out that he technically selected all of them across three different timelines, as the player learns by way of New Game Plus : Pious serving each of the three different Ancients was necessary for Mantorok to take care of them individually, and then merge the timelines together such that all three are utterly defeated, leaving him the sole victor.
• Contrary to its title, the remake trilogy of Final Fantasy VII takes place in an alternate universe that is independent of the original one . Throughout Remake , the characters frequently cross paths with interdimensional beings called " whispers " (or " arbiters of fate "), whose role in the plot is to ensure that a specific chain of events (i.e. the plot of the original game ) unfolds as preordained by Destiny itself. Any attempts made by the characters to change history (e.g. Sephiroth trying to kill Barret in the Shinra tower ) are immediately thwarted (or corrected soon afterwards) by the whispers. This comes to a head in the final chapter, wherein the party chooses to fight and defeat the whispers so as to free themselves from the bounds of Destiny, thus creating an Alternate Timeline wherein Biggs is still alive and recovering in an orphanage, Aerith is possibly aware of Zack's connection to Cloud, and Zack himself appears to be alive after defeating the horde of Shinra infantry sent to stop him and Cloud near the city entrance . Rebirth clarifies that the main timeline of the remake trilogy (which slightly deviates from the original), and the one where plot has gone crazy and jumped Off the Rails (Biggs and Zack surviving) are separate from each other .
• In Final Fantasy XIII-2 the protagonists have the ability to navigate to various points in the their world's timeline. In some cases their interactions create alternate realities that they can travel between freely. For example, "Academia 400 AF" and "Academia 4XX AF" are alternate timeline versions of the same location in the same time period.
• Final Fantasy Brave Exvius : Dark Rain comes from an alternate timeline where Rain, as Hyoh, succeeds in becoming the Alpha Star, and he ends up being corrupted by Emperor Vlad .
• Fire Emblem Warriors: Three Hopes takes place in an alternate timeline from Fire Emblem: Three Houses where a mercenary named Shez is saved from death at Byleth's hands by a mysterious being known as Arval and rescues the three main lords from the bandit attack before Byleth can. This causes a ripple effect that ends up quickly and vastly changing the geopolitical situation in 1180 Fodlan from what it was in Houses . Notably, Edelgard is able to rescue Monica from Kronya, those who slither in the dark are excised from Garreg Mach and the Empire before the war even starts, and Dimitri executes his treasonous uncle and ousts Cornelia from the Kingdom before she can launch her coup .
• For Honor takes place following a massive cataclysm around 1000-1100 A.D. that reshaped the Earth (among other things, the Japanese Archipelago sunk beneath the ocean) and enforced Medieval Stasis among the survivors (the game actually takes place around The New '10s , but due to said cataclysm, technology is still at the level of the Middle Ages).
• Ghost Trick : Phantom Detective has an in-game instance: the first timeline is a Bad Future where everybody dies, Missile is unable to save them due to lacking the needed ghost tricks, and Sissel refuses to help Missile due to only being concerned about his own quest for identity. At the end, Missile uses Yomiel's no-longer-immortal body to travel back in time 10 years to the Tesmik Park incident and wait out those ten years until that night returns . Then starts the second timeline, which is the Bad Future again, but diverging when Sissel awakens and Missile from the first timeline guides him under the guise of Ray, making Sissel think he's a blond-haired man in red instead and claiming that he'll cease to exist once morning comes, so that he'll naturally save Lynne and everyone else that could be a lead in his quest for identity due to said man's interactions with them . The third timeline begins after Sissel, Missile from the second timeline, Yomiel, and Detective Jowd go back 10 years again like the original Missile did and Set Right What Once Went Wrong by saving Yomiel's life and thus preventing his Start of Darkness , undoing the first two timelines and the first Missile's existence in the process.
• The main premise of the Half-Life mod series "Timeline." Rogue Black Mesa scientists have discovered how to use the dimensional portals to permit time travel, and have been messing with and creating new timelines. Eventually this technology falls into the hands of Those Wacky Nazis .
• The Halloween Hack takes place after EarthBound (1994) . This time, the Chosen Four didn't return since they were sent to a timeline created when Giygas was killed in the past. Dr. Andonuts went mad with grief and became a Serial Killer because he thought he killed them.
• The Game Boy Advance duology of The King of Fighters EX and EX2 charts a rather significant divergence from the mainline series. The King of Fighters '99 establishes that Kyo Kusanagi was abducted by the NESTS cartel after the conclusion of '97 , with his DNA used to engineer a Clone Army and various other weapons as part of the organization's multiple nefarious plots. Here, he's instead found by newcomer Moe Habana and the KOF tournament's latest mysterious sponsors turn out to be Geese Howard , still interested in the Orochi Power ( as per '96 ), and then remnants of the Orochi clan that was seemingly dealt with throughout the Orochi Saga. The EX games effectively replace '99 and 2000 in this timeline, and while characters sport their NESTS Chronicles attire and appear to be going through some of the same plot beats from that stretch of time (like Kensou mysteriously losing his psychic powers ), only a single character from that Story Arc (Bao) canonically appears in the EX games, with all others relegated to non-canon Striker cameos.
• The Maximum Impact subseries is a nebulous example. While it appears to be a standalone Alternate Continuity wherein prominent characters from the first two arcs interact with a slew of fresh faces, the animated series Another Day incorporated elements of the then-current Tales of Ash saga ( Ash himself , Iori's loss of his cursed flames ) despite being set after the events of the first Maximum Impact . Regulation "A" (the Updated Re-release of Maximum Impact 2 ) would then make Ash a playable character as well as introduce a Canon Foreigner (Xiao Lon) who — unlike the other MI -exclusive fighters — was designed specifically to provide ties to the main installments (being the younger half-sister of Duo Lon, a major supporting character introduced in 2003 ). Unlike the mainline and EX entries (but similar to the original Fatal Fury continuity), Geese is dead , appearing in-series as his "Nightmare Geese" incarnation. (There's also the matter of his son Rock appearing as a teenager, due to the series operating on Comic-Book Time and Rock already appearing as a child in one of Terry's KOF 2001 win poses, but the main games would eventually follow suit starting with XIV .)
• The first timeline focuses on the Adult part of Ocarina of Time , which doesn't cease to exist after Link is sent back in time. Ganon rises again, causing Hyrule to be flooded since Link is gone from the timeline. The Wind Waker , Phantom Hourglass and Spirit Tracks follow this timeline.
• The second timeline, which starts in the Child part of the game, is created after Zelda sends Link back into the past after defeating Ganon in the Adult timeline. Link successfully convinces the king that Ganondorf is after the Triforce, preventing Ganon from taking over Hyrule in the first place and causing him to be banished to the Twilight Realm. Majora's Mask , Twilight Princess and Four Swords Adventures take place in this timeline.
• In the third timeline, Link is killed in battle by Ganon, causing the Imprisoning War. A Link to the Past , Link's Awakening , the Oracle Games , A Link Between Worlds , Tri-Force Heroes , the original Zelda and Zelda II: The Adventure of Link are all set in this timeline.
• The main plot of Hyrule Warriors involves a Crisis Crossover across different time periods thanks to the actions of Cia . The game appears to take place in the Child Timeline, with the three main eras forming a consistent line down ( Skyward Sword , Ocarina of Time and Twilight Princess ), and the Downloadable Content characters consisting of Midna in her Twili form, as well as Young Link and Tingle as seen in Majora's Mask . The Legends Updated Re Release includes a Playable Epilogue that heavily involves the setting and characters of The Wind Waker , which is all but stated to be a different dimension, on top of adding content based on Phantom Hourglass and Spirit Tracks . Even before that, the Ocarina of Time levels appear to take place after the Adult portion of that game, with Princess Ruto having been aged up and several implications that the game's events had already happened. Finally, said DLC adds warriors from the Fallen Hero Timeline, such as Marin , Ravio and Yuga , as well as their corresponding Adventure Maps.
• For a long time there was no official word on which of these timelines Breath of the Wild takes place in, with clues for each one. Some say that it's the result of all three timelines somehow merging back into one. Others say that it takes place so far in the future that it no longer matters which timeline it is . The developers eventually gave a Shrug of God , saying that they prefer for players to decide how they want to interpret it. Its sequel Tears of the Kingdom makes things even more complicated, as it has elements that seem to contradict things that took place even before the split.
• Hyrule Warriors: Age of Calamity creates yet another alternate timeline. As opposed to being a straight retelling of the Great Calamity as advertised, Age of Calamity takes place in a timeline splintered from the events of Breath of the Wild by a time-travelling Guardian, whose efforts to Set Right What Once Went Wrong result in the Champions and the King of Hyrule being saved from their untimely deaths and Calamity Ganon being defeated.
• Love of Magic : Book 3 is set in an alternate timeline called 'R2' where Owyn died at the lake because he never made Molly a Chosen. The Emily of R2 crossed over to bring "The Sword of Mankind" back with her.
• Luminous Avenger iX is set in an alternate history from the main Azure Striker Gunvolt Series . Specifically, it's a Bad Future that occurs a century after the bad ending of Azure Striker Gunvolt where Gunvolt was not revived by Joule, thus allowing Asimov to take over Sumeragi and dominate the world . This leaves Copen , who had to reconfigure himself into a cyborg to give himself a fighting chance, as the only hero left to deal with the threat.
• The Mega Man (Classic) metaseries has one, expanded on in Mega Man Battle Network . A Point of Divergence occurred when both Doctors Light and Wily presented their research to the government: network and robotics, respectively. In the world where Light's network research won out, the Battle Network and Mega Man Star Force games take place. However, when Wily's robotics were chosen instead, it led to the "Classic" series and its various Sequel Series in X , Zero , and ZX (with Legends occurring far off in the future of this timeline).
• The Game Boy Mega Man games ( Rockman World in Japan ) have an alternate case too, starting with Mega Man II : In one specific future, Dr. Wily gave up his machinations and truly reformed, leading to a world where not only were robots and humans living in harmony, but there was no longer a need for Mega Man, who had become peaceful and disarmed. The "present" Dr. Wily happened across a time machine, however, and kidnapped Mega Man from that timeline, reprogramming him, naming him Quint, and giving him a jackhammer as his weapon for some reason. Following the events, it can probably safely be said that timeline no longer exists, though whether it's canon at all is another matter.
• Surprisingly, this is present in the Metal Gear series: Metal Gear: Ghost Babel takes place in an alternate timeline where neither the events of the second game nor the events of the Solid series have taken place.
• The original timeline shows Raiden and Shao Kahn as the final two kombatants in the Battle of Armageddon , with Kahn defeating Raiden . Knowing that he is about to be killed, Raiden sends a message to his past self to hopefully change the future , setting up the events of Mortal Kombat 9 . The current timeline, while still near-identical to the original, finally kills Shao Kahn for good . Unfortunately, the new decisions made and results cost the lives of a lot of kombatants , made both Earthrealm and Outworld more vulnerable for attack , and later on placed Raiden in a much darker state after Shinnok's defeat .
• Mortal Kombat 11 at first explores Dark Raiden's campaign against the Netherrealm that was set up by The Stinger of X , only for Greater-Scope Villain Kronika to enact a Time Crash as part of her plan to restart the flow of history anew , bringing together the present-day cast with various heroes and villains from the Mortal Kombat II era (who were apparently in the middle of Kung Lao's chapter from MK9 ; Raiden also notes that he had been receiving visions of future events for the past few months, indicating things were playing out in that timeline more or less the same as in the 2011 game). Later on, Raiden experiences a vision of multiple other timelines, all of which involve him coming to blows with Liu Kang . Kronika then reveals to Raiden that she engineered these outcomes to prevent the two from teaming up against her, all but saying she's pressed the Reset Button countless times prior to MK11 proper.
• Mortal Kombat 1 takes place in a whole new timeline created following the events of MK11 : Aftermath , which ended with Liu Kang, having become a god, taking the reins of the Sands of Time and creating a new history with the hope of allowing peace to flourish . The trailers for the game suggest a number of alterations to the dynamics between characters that used to be bitter rivals, such as Scorpion and Sub-Zero (and the Shirai Ryu and Lin Kuei clans, by extension) being allies, and Kitana and Mileena being supportive sisters. Despite this, many events in the New Era echo that of past histories , even if the context behind them is different this time around. For example, Sub-Zero and Scorpion's alliance is because the New Era Scorpion is not Hanzo Hasashi , but Kuai Liang (the younger Sub-Zero in previous games), and when Bi-Han betrays Earthrealm and allies the Lin Kuei with Shang Tsung , the Shirai Ryu is formed by Scorpion and Smoke in order to uphold the original principles of their clan .
• On the weakest end is Lostbelt #3, the Sublime Immortal Nation. Here, Chinese Emperor Qin Shi Huang achieved immortality and then conquered the whole world, ushering in a Golden Age where everyone can live in peace. This might sound good if one ignores how Shi Huang enforces mandatory restriction on knowledge, arts, and progress without his approval outside of his inner circle on pain of dropping a meteor on your village , except that one of the Powers That Be judges timelines based upon whether humans have any chance for progression. With there being no need to progress in S.I.N., the timeline was wiped away to ensure mankind could keep striving forward. Its Crypter is Akuta Hinako.
• On the opposite end of the spectrum is Lostbelt #6, Avalon le Fey, with Difference Depth EX. Here, the nail is that Morgan le Fey was selected to be ruler of Camelot, which apparently lead to fairies taking over the world. It turns out that the actual nail was that the six fairies who would have forged Excalibur didn't do so out of laziness, meaning that when Sefar invaded, it succeeded in wiping out everything on Earth . As fairies ended up being the ones who got out of this the best, they became the dominant species instead of humans . This is the reason that the Difference Depth is so high — history as we know it did not occur at all during this timeline . The divergence is so great that Avalon le Fey is no longer a lostbelt, but a lost world , equal to Proper Human History .
• Only slightly below Lostbelt #6 is Lostbelt #7, Nahui Mictlan. The nail here has something to do with the asteroid that destroyed the dinosaurs. As it turns out, it's that the asteroid arrived early and allowed humans to appear earlier in this timeline than they do in Proper Human History. The problem is that this meant there were humans around when ORT , the Ultimate One of the Oort Cloud , showed up. In both timelines, ORT is called Gaia to destroy mankind when it appears they will outlive her, but in PHH, it shows up too early and thus goes into hibernation. Here, mankind was around when ORT landed, so ORT wasted no time in trying to obliterate us. It was only stopped when all of mankind sacrificed itself to empower the current king, Kamazotz, with immortality so that he could defeat ORT, and it still took a million years of non-stop fighting to do so , and ORT still isn't dead. Ironically, despite the massive changes in history, Nahui Mictlan isn't a Lostworld because its dominant species are too lazy and fulfilled to do anything, so they have completely stagnated .
• Subtly implied by the One Game for the Price of Two nature of the franchise, with each version having differences in the Pokémon you can catch. Since Gen III , these divergences expanded to include which evil team you fight or the legendary Pokémon that is the focal point of the story. Gen V took it a step further and had the Unova region be more old-fashioned in White and more modern in Black . note  Opelucid City in particular has an NPC who explicitly references "another Opelucid."
• The news broadcasts in Devil Summoner establish this series (as well as Shin Megami Tensei if... and Persona , by extension) as an alternate timeline from Shin Megami Tensei I and onward. The divergence here is twofold; Gotou is arrested before he can even attempt his military coup d'etat, and Ambassador Thorman dies of a heart attack well before he reveals himself as Thor and orders a nuclear strike on Tokyo .
• It might not end there either. Shin Megami Tensei NINE , while explicitly set thirty years after Thorman's nuclear strike and mostly consistent with the events of Shin Megami Tensei , implies the game may take place in a timeline where certain aspects of SMT I , such as the protagonist and the Cathedral/Basilica, didn't exist, which therefore means Tokyo Millennium might not have been built as a result. Shin Megami Tensei IMAGINE depicts another possible extension of the original, a course of history where the protagonist was present yet doesn't seem to match what's shown in II . Giten Megami Tensei also refers to a Great Cataclysm — the terminology used in SMT II and its associated materials for the first game's calamity — in the year 199X that was caused by nuclear fallout , suggesting it's yet another branch. The first two Majin Tensei games may or may not be connected as well. Majin Tensei mentions a period of extreme international hostility in the 90s, only nuclear weapons weren't used... and Tokyo ends up falling into disarray anyway . Meanwhile, Spiral Nemesis results from a demon infestation and the Japanese government being overthrown in a coup, both in the year 1996. Gotou wasn't involved, but the game is packed with enough Mythology Gags to SMT I that one might wonder otherwise.
• You see two of these in-game: Blasted Tokyo and Infernal Tokyo. Both are horrible Death Worlds ; it's just that they embody what would have ultimately happened if certain events in the past had swayed the balance in favor of one faction or the other.
• The events of SMT IV as a whole appear to be linked to SMT I . Though some backstory details differ ( or could have been unseen previously ), exposition given by recurring NPC Stephen in DLC content refers to the Great Cataclysm by name, once again making mention of demons establishing contact with the human world , an ICBM strike , and the resulting nuclear holocaust . However, perhaps the biggest divergence is that this all happened in 2013, not 199X. Further confusing matters is how Stephen is even around to tell Flynn this in the first place, as the events of the Raidou Kuzunoha duology suggest that the timeline stemming from SMT I was effectively wiped out , which would be explained by the sequel : Stephen has transcended humanity and can freely traverse The Multiverse as a result .
• The spin-off Shin Megami Tensei IV: Apocalypse is itself set in an alternate timeline to the original game's Neutral route. Specifically, this is the result of Nanashi coming back from the dead with the help of the god Dagda, which, long story short, leads to the unsealing of the Divine Powers and some other drastic things happening to and around Tokyo. It's implied that if Nanashi refuses to be revived (which can actually be done at the start of the game for a Non-Standard Game Over ), IV ' s Neutral ending continues on as usual. Additionally, the DLC quest "Messiahs in the Diamond Realm" sees the aforementioned Stephen unite Nanashi with alternate versions of the Hero , Aleph , the Demi-fiend , and Flynn , all of whom were killed at some point during their respective quests and removed from the cycle of reincarnation due to being deemed threats by YHVH .
• The existence of one of these and the desire of certain odd factions to bring it back fuels the plot of Persona 2 : Eternal Punishment .
• Small Town Murders : In Case 6, Nora wishes the case never happened. The Monkey Paw grants the wish and makes the cause of death of Tyler Barnes the same for his three brothers .
• Sonic Forces and official word established that the Classic Era visited in Sonic Generations has become a alternate/split timeline in which Sonic Mania takes place, possibly as a result of the time travel that took place in Generations . The new timeline has seemingly the same history as the main timeline up to Sonic 3 & Knuckles , and is treated as an alternate dimension , with travel between the two being made possible by the combination of the powers of the Phantom Ruby and the Chaos Emeralds.
• The first is that Zasalamel receives a vision from the future, likely from the original timeline, that convinces him to change his plans to be a Death Seeker before he ever begins the plot seen in Soulcalibur III .
• The second is that Cassandra meets her original timelime counterpart in Astral Chaos , who warns Cassandra of the Bad Future that awaits while giving the information of how to stop it from happening . These set the precedent for a massive shift in the status quo later.
• Starcraft II 's co-op modes are set in an (non-canon) alternate timeline where the fallen Xel'Naga, Amon , successfully awakens and unleashes his hybrid army to wipe out all life in the universe, and the Terran, Zerg, and Protoss armies unite to stand against him . It also invokes a Everyone Lives as more co-op commanders were added, so you have someone like Tychus and Mengsk alive for whatever reason.
• Stellaris allows players to forge their own history, but prior to the game's start there are three possible outcomes for the human race. In the main one the United Nations of Earth discovered FTL travel in the year 2200 and began colonizing the stars, with a Lost Colony ship potentially splintering off to form their Evil Counterpart the Commonwealth of Man. Then there's the Earth Custodianship , where humanity surrendered our free will to robots that waited on us hand and foot while conquering the stars on our behalf. When playing as an alien empire, there's a 50% chance that humanity's development was delayed and we're fighting World War II in the 23rd century, making us ripe for invasion .
• In Super Robot Wars Reversal , the Original Generation characters come from a Bad Future where the events of Mobile Suit Gundam: Char's Counterattack and Martian Successor Nadesico : The Prince of Darkness play out. After accidentally traveling back in time , they unintentionally Set Right What Once Went Wrong by keeping the events of Mobile Suit Gundam Wing: Endless Waltz from going bad, which makes everything better for everyone invovled.
• In Tales of Destiny 2 , there are three of them. Nanaly hails from a Bad Future , Kyle and Loni hail from the original timeline, Judas was resurrected from a point 18 years in the past and Harold is from the origin point of two of them, 2000 years in the past.
• Wolfenstein: The New Order is set in a timeline where at some point in 1942, the Nazis got hold of a cache of hyper-advanced technology and consequently reversed-engineered it and used it to curb stomp the Allies into submission and Take Over the World . This becomes extremely ironic considering that the technology was developed by a sect of Jewish scientists who all took a binding vow to God that they would never apply their technology to real life and just use it to help scientists make even more advanced technology with the sole intent to understand the universe better. This means that the Nazis owe their completely ill-begotten victory to a philosophy that they (wrongly) label as inferior .
• The plot for World of Warcraft 's fifth expansion, Warlords of Draenor , is kicked off by Garrosh going back in time 35 years to Draenor (the world the orcs come from) and introducing Azeroth's modern vaguely steampunk technology to the hunter-gatherer orcs . Subverted in that there are already differences before he gets there, like Ner'zhul's wife still being alive when she died in our timeline, and Garrosh himself not existing in this timeline — Grom's wife died before Garrosh was born here. Kairoz (the bronze dragon who helped Garrosh get there) even tells him that it is not his Draenor, just one that is similar enough for their purposes. Garrosh's arrival did still change things more though.
• The premise behind Area X , which deals with Time-Travel Romance without actual time travel, as the protagonist is hopping to Alternate Timelines rather than the strict past. They were created when, back in the Middle Ages, Livan tried to rewrite the world to save her life, thus splitting time into multiple streams.
• This becomes important in later stories in the Demonbane franchise: all of the first game's endings are canon and did happen in one timeline or another. One of the prequel novels also involves Nyarlathotep erasing a timeline from existence when things in it go catastrophically wrong for everyone (including itself). Finally, at the climax of the second game, Demonbane summons alternate versions of itself from an infinite number of alternate timelines to deal the finishing blow against the Clockwork Phantom... some of these variations are only slightly different, some are radically different.
• This forms a core plotpoint in Dies Irae where each of the novels story paths are technically canon due to an Eternal Recurrence created by Mercurius' law. The foreknowledge that Shirou and the main Big Bad Reinhard constantly experiences is in fact the result of echoes from prior and alternate timelines.
• Steins;Gate is built on this trope. Each timeline in the game is represented by a number called the "divergence number", where each 1% difference in divergence number represents a drastic change in the fate of the future. Characters can move timelines by altering the past. The events of the game kick off when the protagonist accidentally jumps from the Beta timeline(s) to the Alpha timeline(s) , and most of the game involve getting out and back to the original timeline. In the end, it's revealed that the future of the Beta timeline(s) is just as bad as the Alpha timeline(s), so the protagonist must find a third, "golden" timeline that avoids both of these Bad Futures — Steins Gate. Additionally, John Titor (see the Web Original folder below) is an actual member of the supporting cast, portrayed as a legitimate time traveler .
• In Zero Escape , this is pretty much the basis for how Junpei in the first game, and Sigma and Phi in the second, are able to know information they shouldn't. The games work on the multiple world theory, in which every single difference in action, human or otherwise, creates another branching universe . The plot of the first two games weaponizes this, especially the latter, in which the Ambidex Game was played to send Sigma and Phi's consciousnesses back through time, to create an alternate history where the world wasn't torn apart by a viral infection .
• In All Night Laundry , there's at least thirteen different timelines. We only see one though.
• The Wraith in Charby the Vampirate is Zeno from an alternate timeline. Several of the other cabinmates fates in this alternate timeline have been briefly explained and illustrated on the artist's tumblr.
• Earth, 2068 : Whenever the STORMaDO is used for time travel into the past, it creates a branching timeline.
• In Homestuck , creating an alternate timeline that contradicts information known in any future time that is also technically the present (effectively, defying a predetermined future event) turns your timeline into a doomed offshoot of the "alpha" timeline, the one that's supposed to happen. Luckily, only the people from the alternate timelines are doomed; the "alpha" characters will be okay. At least, until John gains the power to redefine which timeline becomes the Alpha timeline at the cost of all his other selves .
• With a twist when it comes to Two Evil Scientists : Two super villains from alternate timelines are competing to make their timelines a reality, as well as Quint and the Time Skimmer.
• John Titor, a person claiming to be a time traveler from 2036 on several message boards, had many predictions for the future. These included a second American civil war in 2004, followed by a swift World War III in 2015, where a nuclear exchange between Russia, America, the European Union, and China killed 3 billion people. Obviously, none of this happened, but Titor also stated that the many-worlds theory of quantum mechanics had been proven to be true and our world might not go down the exact same path as the one he came from.
• Dumbledore's Opening Narration implies that aside from the canon series , there are countless other alternate timelines where Harry became something other than "The Boy Who Lived." Welcome Back, Potter just happens to be one of those "What if?" scenarios.
• 1. In the original timeline, Sandy Smith came down with multiple sclerosis and died by suicide because she thought she was a burden to her daughter Debbie. But Debbie was so traumatized by this that when time travel was invented in the 2050s, she used this to change her own history by giving her mother a vaccine in the year 2021 so she would remain able-bodied and thus live much longer.
• 2. Sandy Smith not only lived to a ripe old age, she became mayor of her Town and lives there became so much better through her ethical leadership.
• 3. Ted Wilson originally lived to the year 2060 and then as a old man attempted to use a time machine to murder Carrie Sims in the year 2020, but this was stopped by both his own grandson Tony Wilson and Debbie and Carrie's son Richard Sims. Richard instead was killed in the past, spawning a new timeline in which Debbie and Carrie became superheros by the year 2030 and sought to avenge Richard's death. They were assisted by the 2060 version of Debbie.
• 4. Superhero Carrie tracked down Ted Wilson in the year 2005 and killed him, but not before he murdered Lucy Sims, one of Carrie's mothers (this was an accident, Ted was trying to kill Jessica, Carrie's other mother).
• 5. Superheros Debbie and Carrie, guided by the 2060 version of Debbie, finally sabotaged Ted Wilson's time machine and it killed him in an explosion before he could make any time trips, undoing the events of timelines 3 and 4.
• 6. Debbie of the year 2061 then traveled back in time again to warn her mother of Ted Wilson having murdered his wife Rebekah, the mother of their daughter Janet and then blinding Janet. However, Debbie told Sandy about these events AFTER they had happened; she wanted Ted to be sent to prison for those crimes before he could force Janet to bear his grandson Tony and thus she erased Tony from the timeline completely. Ted Wilson then died in the year 2036 at the Battle of Austin.
• 7. Richard Sims and his wife Diana Hudson sought to undo a timeline in which Suzanne, Diana's aunt, had died of a variant of Covid-19. They were successful in giving Suzanne a vaccine for the virus when she was a teen.
• 8. Debbie and Carrie traveled back from the year 2061 to the year 2029 to stop a homophobic terrorist from murdering a gay couple, Michael Jefferson and Charles Mc Kinsey, who were Richard Sims' fathers. This was successful, but also changed events affecting not only Debbie and Carrie themselves, but others like the Belmont triplets, who were never adopted by Debbie and Carrie.
• 9. Debbie, Carrie, and James, Debbie's brother, went back to the 19th Century to visit the time when the Town was first founded. When they returned to the year 2063, they found that Ted Wilson was not only still alive, but was mayor of their Town and living in James' mansion! To undo this, they went to the year 2020 to entice Carrie and her mothers, Jessica and Lucy, to move to Texas to start the chain reaction of events that would bring down Ted Wilson to begin with.
• The Adventure Time episodes "Finn the Human" and "Jake the Dog" deal with this. Finn makes a wish that the Lich never existed. We then see an alternate world where Marceline never became a vampire, Simon stopped the nuke which irradiated the world, and Finn is living with his poverty stricken family and normal-bulldog Jake. Eventually Finn ends up donning the Ice King's crown, causing the nuke to blow up and has a different Lich be born out of Jake .
• Ben's Ridiculously Successful Future Self Ben 10,000 is eventually revealed to be from a parallel timeline to Ben's own, as evidenced by Kevin still being a villain despite having a Heel–Face Turn in the prime continuity (although this plot point suffered from a severe case of Depending on the Writer ).
• Ben has met other alternate versions as well, like a Gwen who ended up with the Omnitrix instead of Ben, and a Ben who never got the Omnitrix at all and had a normal, dull summer vacation.
• The end of Scooby-Doo! Mystery Incorporated has the Gang force history to reset itself in a much more pleasant direction after Scooby kills the Nibiru Entity. Harlan Ellison tells them that this isn't the first time that they've had their timeline rebooted either.
• In the South Park Episode "Go God Go!", Cartman uses a phone that can call homes in other time periods, even the commercial for it states that it is only for prank phone calls.
• In the Teenage Mutant Ninja Turtles (1987) episode "Shredderville", the Turtles thought they have somehow stepped into an alternate timeline/world/reality where the Turtles never existed, in which Shredder has taken over the world, Rocksteady and Bebop are still humans, April O'Neil and Irma are their servants, and humans and mutants are mutual enemies because of Krang and his Dimension X allies. Oh, and on top of all this, everything in that world is falling apart. Fortunately, the whole thing turns out to be All Just a Dream .
• What If...? gives glimpses at multiple alternate timelines/universes of the Marvel Cinematic Universe 's Multiverse .
• X-Men: The Animated Series deals with these thanks to Bishop and Cable. Long story short, Bishop's attempt to stop his Bad Future leads him to keep hopping back and forth through time to stop other events and sometimes Cable gets dragged along because some of these plots also involve his Arch-Enemy Apocalypse.
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Astrophysicist says he has cracked the code for time travel

C an you imagine going back in time to visit a lost loved one? This heartwrenching desire is what propelled astrophysicist Professor Ron Mallett on a lifelong quest to build a time machine. After years of research, Professor Mallett claims to have finally developed the revolutionary equation for time travel.

The idea of bending time to our will – revisiting the past, altering history, or glimpsing into the future – has been a staple of science fiction for over a century. But could it move from fantasy to reality?

The inspiration: A father's love and a classic novel

Professor Mallett's obsession with time travel and its equation has its roots in a shattering childhood experience. When he was just ten years old, his father, a television repairman who fostered his son's love of science, tragically passed away from a heart attack.

Devastated, the young Mallett sought solace in books. It was H.G. Wells' The Time Machine that sparked a lifelong fascination.

Wells' opening lines became his mantra: "Scientific people know very well that Time is only a kind of Space. And why cannot we move in Time as we move about in the other dimensions of Space?"

This profound question ignited Mallett's scientific journey. He dedicated himself to understanding the nature of time, determined to find a way to revisit the past and see his beloved father once more.

Time travel equation in the hospital

Decades of research into black holes and Einstein's theories of relativity led to the time travel equation.

While hospitalized for a heart condition, Mallett had a revelation. "It turns out that black holes can create a gravitational field that could lead to the creation of time loops that could allow us to go back in time," he explained.

Imagine the fabric of spacetime as a river. While time usually flows in one direction, Mallett theorizes that the immense gravity of a spinning black hole can create whirlpools, where time twists back on itself.

The time machine blueprint

Mallett's vision for a time machine centers on what he calls "an intense and continuous rotating beam of light" to manipulate gravity. His device would use a ring of lasers to mimic the spacetime-distorting effects of a black hole.

"Let's say you have a cup of coffee in front of you. Start stirring the coffee with the spoon. It started to spin, right? That's what a spinning black hole does," explained Mallett.

"In Einstein's theory, space and time are related to each other. That's why it's called space-time. So when the black hole spins, it will actually cause time to shift."

"Eventually, a rotating beam of laser lights can be used as a kind of time machine and cause a time warp that will allow us to go back to the past," said Mallett. Perhaps, what began as a son's wish to see his father one last time might one day transform our understanding of time itself.

Challenges and limitations

The obstacles on the path from time travel equation to machine are immense. Mallett acknowledges the "galactic amounts of energy" needed to power such a device – energy levels far beyond our current capabilities.

The sheer size of a theoretical time machine is also unknown. While Mallett optimistically states, "I figured out how to do it. In theory, it is possible," the reality is that he may not live to see the machine built.

Furthermore, Mallett's theory comes with a significant constraint. "You can send information back, but you can only send it back to the point where you started operating the device."

In this sense, the time machine is like a one-way message service to the past. You can't travel to a point before the machine existed.

Mallet's lifetime of dedication to time travel

Despite the daunting challenges, Mallett's remarkable journey is a testament to the human spirit. Alongside his time travel research, he's led a distinguished academic career, teaching physics at the University of Connecticut .

Now in his seventies, his work has been propelled by an unwavering belief in the possibility of the seemingly impossible.

Equation vs. reality of time travel

Whether Mallett's time machine will ever transcend the realm of theory is uncertain. Skeptics point to the vast technological hurdles and potential paradoxes raised by tampering with time.

Yet, the mere possibility that science might one day unlock the secrets of temporal travel is enough to ignite the imagination. Could we rewrite our regrets, learn from past mistakes, or witness historical events firsthand?

Perhaps Professor Mallett's greatest legacy won't be a time travel equation itself, but the inspiration he provides – a reminder that audacious dreams and unrelenting curiosity have the power to push the boundaries of what we believe is possible.

Read more about Professor Mallett's work here .

More about space-time

As discussed above, space-time, a concept that feels as vast and complex as the universe itself, forms the backbone of our cosmic understanding.

At its core, it blends the dimensions of space and time into a single four-dimensional continuum, challenging our perceptions of reality. This intertwined nature of space and time underpins everything from the motion of planets to the flow of time itself.

Einstein's groundbreaking contribution

Albert Einstein, with his theory of relativity, revolutionized our understanding of space-time. He posited that space and time are not separate entities but are connected in a dynamic relationship affected by mass and energy.

This relationship implies that the presence of a massive object, like a planet or a star, can warp the fabric of space-time around it. It's a concept that turns the notion of a flat, unchanging universe on its head, suggesting that the very structure of the cosmos is malleable.

The warp and weft of the cosmos

Imagine space-time as a vast sheet of fabric. When a heavy object sits on this fabric, it creates a dip or curve. This curvature is what we perceive as gravity.

Planets orbit stars not because they are being "pulled" in a straight line towards them, but because they are following the curved space-time geometry that these massive objects create.

This curvature of space-time is not just a theoretical concept; it's observable and measurable, especially in the presence of extremely massive and dense objects, like black holes.

Gravitational waves: Echoes of cosmic events

One of the most compelling pieces of evidence for the theory of relativity and the dynamic nature of space-time comes from the detection of gravitational waves.

These waves are ripples in the fabric of space-time, generated by some of the most violent and energetic processes in the universe, such as colliding black holes.

Their discovery not only confirmed Einstein's predictions but also opened a new window into observing cosmic events that were previously invisible to us.

Practical impact of space-time

While these concepts might seem distant from daily life, they have real-world applications, particularly in technology. The Global Positioning System (GPS), a technology integral to modern navigation, relies on an understanding of space-time.

Satellites orbiting Earth need to account for the effects of gravitational time dilation -- a consequence of the curvature of space-time -- to provide accurate location data to users on the ground.

In summary, space-time is a framework that shapes our understanding of the universe. From guiding the planets in their orbits to enabling precise navigation on Earth, its effects are both profoundly cosmic and surprisingly practical.

As we continue to explore and understand the intricacies of space-time, we edge closer to unraveling the mysteries of the cosmos, one gravitational wave at a time.

How black holes are linked to time travel

Playing a major role in Dr. Mallett's time machine, black holes exert a gravitational pull so immense that not even light can escape their grasp. This intense gravity fundamentally alters the fabric of space-time around the black hole.

The stronger the gravity, the more pronounced these effects become, leading to what scientists call gravitational time dilation -- a phenomenon where time itself warps, slowing down relative to an observer far from the gravitational pull.

Time dilation explained

At the heart of this phenomenon lies Albert Einstein's theory of General Relativity, which posits that gravity is the result of masses warping the space-time around them.

In the vicinity of a black hole, this warping becomes so extreme that it significantly affects the flow of time.

An observer standing at a safe distance would perceive time to pass much slower for someone closer to the black hole.

This effect intensifies as one approaches the event horizon, the point of no return beyond which the gravitational pull becomes inescapable.

Boundary between time zones

The event horizon of a black hole marks a stark boundary in the universe, where time as we understand it undergoes a dramatic transformation.

To an external observer, objects approaching the event horizon appear to slow down and almost freeze in time, never quite crossing the threshold.

This illusion results from the light from those objects taking longer and longer to reach the observer as the objects move closer to the event horizon, due to the extreme gravitational pull affecting the light's path.

Theoretical implications and observations

This warping of time around black holes is not just a theoretical curiosity. As Dr. Mallett explained previously in this article, it has practical implications for our understanding of the universe. For instance, it plays a crucial role in the behavior of binary systems where one star orbits a black hole.

Moreover, advancements in technology, such as precise atomic clocks and observations from space telescopes, have allowed scientists to measure these effects, further confirming the predictions of General Relativity.

In summary, black holes serve as natural laboratories for testing the limits of our understanding of physics, offering insights into the complex interplay between gravity and the fabric of space-time.

The phenomenon of time dilation near these cosmic behemoths challenges our notions of time and space, inviting us to explore beyond the boundaries of our current knowledge.

As we continue to observe and study these fascinating objects, we inch closer to unraveling the mysteries of the universe.

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