time travel paradox list

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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.

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The Time-Travel Paradoxes

What happens if a time traveler kills his or her grandfather? What is a time loop? How do you stop a time machine from just appearing somewhere in space, millions of kilometers from home? And is there such a thing as free will?

Congratulations! You have a time machine! You can pop over to see the dinosaurs, be in London for the Beatles’ rooftop concert, hear Jesus deliver his Sermon on the Mount, save the books of the Library of Alexandria, or kill Hitler. Past and future are in your hands. All you have to do is step inside and press the red button.

Wait! Don’t do it!

Seriously, if you value your lives, if you want to protect the fabric of reality – run for the hills! Physics and logical paradoxes will be your undoing. From the grandfather paradox to laws of classic mechanics, we have prepared a comprehensive guide to the hazards of time travel. Beware the dangers that lie ahead.

 The Grandfather Paradox

Want to change reality? First think carefully about your grandparents’ contribution to your lives.

The grandfather paradox basically describes the following situation: For some reason or another, you have decided to go back in time and kill your grandfather in his youth. Yeah, sure, of course you love him – but this is a scientific experiment; you don’t have a choice. So your grandmother will never give birth to your parent – and therefore you will never be born, which means that you cannot kill your grandfather. Oh boy! This is quite a contradiction!

The extended version of the paradox touches upon practically every single change that our hypothetical time traveler will make in the past. In a chaotic reality, there is no telling what the consequences of each step will be on the reality you came from. Just as a butterfly flapping its wings in the Amazon could cause a tornado in Texas, there is no way of predicting what one wrong move on your part might do to all of history, let alone a drastic move like killing someone.

There is a possible solution to this paradox – but it cancels out free will: Our time traveler can only do what has already been done. So don’t worry – everything you did in the past has already happened, so it’s impossible for you to kill grandpa, or create any sort of a contradiction in any other way. Another solution is that the time traveler's actions led to a splitting of the universe into two universes – one in which the time traveler was born, and the other in which he murdered his grandfather and was not born.

Information passage from the future to the past causes a similar paradox. Let’s say someone from the future who has my best interests in mind tries to warn me that a grand piano is about to fall on my head in the street, or that I have a type of cancer that is curable if it’s discovered early enough. Because of this warning, I could take steps to prevent the event – but then, there is no reason to send back the information from the future that saves my life. Another contradiction!

Marty finds himself in hot water with the grandfather paradox, from ‘Back to the Future’ 1985

Let’s now assume the information is different: A richer future me builds a time machine to let the late-90s me know that I should buy stock of a small company called “Google”, so that I can make a fortune. If I have free will, that means I can refuse. But future me knows I already did it. Do I have a choice but to do what I ask of myself?

 The Time Loop

In the book All You Zombies by science fiction writer Robert A. Heinlein the Hero is sent back in time in order to impregnate a young woman who is later revealed to be him, following a sex change operation. The offspring of this coupling is the young man himself, who will meet himself at a younger age and take him back to the past to impregnate you know whom.

Confused? This is just one extreme example of a time loop – a situation where a past event is the cause of an event at another time and also the result of it. A simpler example could be a time traveler giving the young William Shakespeare a copy of the complete works of Shakespeare so that he can copy them. If that happens, then who is the genius author of Macbeth?

This phenomenon is also known as the Bootstrap Paradox , based on another story by Heinlein, who likened it to a person trying to pull himself up by his bootstraps (a phrase which, in turn, comes from the classic book The Surprising Adventures of Baron Munchausen). The word ‘paradox’ here is a bit misleading, since there is no contradiction in the loop – it exists in a sequence of events and feeds itself. The only contradiction is in the order of things that we are acquainted with, where cause leads to effect and nothing further, and there is meaning to the question “how did it all begin?”

 Terminator 2 (1991). The shapeshifting android (Arnold Schwarzenegger) destroys himself in order to break the time loop in which his mere presence in the present enabled his production in the future

Time travelers – where have all they gone?

In 1950, over lunch physicist Enrico Fermi famously asked: “If there is intelligent extraterrestrial life in the Universe – then where are they?” indicating that we have never met aliens or came across evidence of their existence, such as radio signals which would be proof of a technological society.  We could pose that same question about time travelers: “If time travel is possible, where are all the time travelers?”

The question, known as the Fermi Paradox, is an important one. After all, if it were possible to travel through time, would we not have bumped into a bunch of observers from the future at critical junctures in history? It is unlikely to assume that they all managed to perfectly disguise themselves, without making any errors in the design of the clothes they wore, their accents, their vocabulary, etc. Another option is that time travel is possible, but it is used with the utmost care and tight control, due to all the dangers we discuss here.

 On June 28, 2009, physicist Stephen Hawking carried out a scientific experiment which was meant to answer this question once and for all. He brought snacks, balloons and champagne and hosted a secret party for time travelers only – but sent out the invitations only on the next day. If no one showed up, he argued, that would be proof that time travel to the past is not possible. The invitees failed to arrive. “I sat and waited for a while, but nobody came,” he reported at the Seattle Science Festival in 2012.

Multiple time travelers also undermine the possibility of a fixed and consistent timeline, assuming that the past can indeed be changed. Imagine, for example, a nail-biting derby between the top clubs, Hapoel Jericho and Maccabi Jericho. Originally Maccabi won, so a Hapoel fan traveled back in time and managed to lead to his team’s victory. Maccabi fans would not give up and did the same. Soon, the whole stadium is filled with time travelers and paradoxes.

 One way or round trip?

When considering travel, it is always continuous – from point A to point B, through all the points in between. Time travel should supposedly be the same: travelers get into their machine, push the button, and go from time A to time B, through all the times in between. But there’s a catch, if we are only travelling through time, then to the casual observer, the time machine continuously exists in the same space between the points in time. The result is that our journey is one-way and the time travelers will stay stuck in the future or the past because the machine itself will block the time-path back. And that is before we even start wondering how to even build this thing in the first place if it already exists in the place where we want to build it.

If that’s the case, then there’s no choice but to assume that there is some way to jump from time to time or place to place and materialize at the destination. How will our machine “know” to jump to an empty area, and to avoid materializing into a wall or a living creature unlucky enough to occupy that same spot? The passengers will undoubtedly need effective navigation and observation equipment to prevent unfortunate accidents at the point of entry.

 Advanced time travel

In addition to the problems that time travel poses for anyone trying to keep the notion of  cause and effect in order, time travelers may also face – or already have faced – other challenges from physics, even classical physics.

One issue you have to consider during time travel, and which science fiction writers usually prefer to ignore for convenience sake, is the question of arrival at the specified time destination and what would happen to us there.

It is usually assumed, with no good reason, that if someone is travelling through time, he or she will land in the same place, but at a different time – past or future. But this is where we hit a snag: the Earth rotates around the sun at a speed of 110,000 kph, and the Solar System itself is moving in its trajectory around the galaxy at a speed of 750,000 kph. If we time-travel for even a few seconds and stay in the same coordinates of space, we will probably find ourselves floating in outer space and perhaps even manage a quick glance around before we die. Our time machine will have to take into account this movement of the heavenly bodies and place us at exactly the right spot in space.

This alone may be resolved, since time travel, in any case, takes place between two points in the four-dimensional space-time continuum. According to the theory of general relativity, the theoretical foundation for time travel, space and time are a single physical entity, known as space-time. This entity can be bent and distorted – in fact gravity itself is an external manifestation of space-time distortion.

The Time Lord ,Doctor Who explains what “time” is exactly (Doctor Who, Season 3, Chapter 10: Blink).

Time travel would be possible if we could create a closed space-time loop, or if we could go from one point to another through a shortcut called a “Wormhole”. This would, in any case, not be just moving from one point in time to another, but would also include moving through space. Thus, from the outset, the journey is not only in time, but necessarily includes a destination point in space, which we will need to pre-program on our machine, of course .

In practice, the situation is more complicated – especially if we want to go into the distant past or distant future. The speed of the celestial bodies, and even the Earth’s shape and the structure of the continents, the seas, and mountains on the face of the Earth, change over the years. And because even a tiny deviation in our knowledge of the past can land us in the core of the Earth, in outer space or somewhere else that immediately reduces life expectancy to zero – time travel becomes a Russian roulette.

 How to travel in time and stay alive

 Let’s assume we coped with this problem and managed to get to the exact point in space-time that can sustain life. Careful – we’re not there yet; we still have to deal with momentum.

Momentum is a conserved quantity, which basically represents the potential of a body to continue moving at the speed and direction in which it is already travelling. If we were to jump out of a moving car (heaven forbid!), conservation of momentum is what would cause us to roll on the ground and probably get injured (in the best-case scenario). And so, if we leap in time – say, a month back – and land at the exact same point on Earth – we would discover, much to our dismay, that even if we started motionless in relation to the ground, now the ground underneath us is moving quickly at one angle or another towards us . Thus, even if we were lucky enough not to crash immediately on impact, we’re likely to hit some obstacle. And if by some miracle we were to survive, we would quickly find ourselves burning up in the atmosphere or gasping for air in space – because we have far exceeded the escape velocity from Earth.

A possible solution to this problem is to plan our landing point ahead, so that the ground speed will be equal in size and direction to our exit speed, but this places many constraints on our journey. We could always leap into space, where there are hardly any moving objects to be bumped into, and only then land again at our point of destination on Earth.

Having said all that, this problem arises chiefly when we assume that time hopping is immediate – that we disappear from one point in time and immediately appear in another, without losing mass, energy, or momentum. But since a “realistic” journey in time is not instantaneous, rather it involves travelling along space-time, it is no different from other types of journeys. This being the case, we can hope that we could adjust our speed to the desired value and direction prior to landing, just like a spacecraft slowing down before landing on a planet.

We should also keep in mind that thankfully, we will have access to a powerful technology that would enable us to cope with such problems: Time-travel technology itself. For example, we might decide to send thousands of tiny probes ahead of us, each to a slightly different point in space-time. Some of them, maybe even most, will be destroyed for one of the reasons already mentioned. The others will wait patiently until the present and then feed their programmed coordinates into the time machine. Thus by definition, the destination entered will be safe for us, except, perhaps for the annoying probe shower hitting the travellers. For the travellers themselves, the entire process will be immediate.

Time Travelling Grammar

Finally, we come to the question: How do you actually talk about time travel? The three tenses – past, present, and future – are insufficient to discuss a future event that happened some time in the past with someone who is in the present, which is another’s past and yet another’s future. And what is the correct grammatical tense to use when we talk about an alternative future that would have been created after we killed our grandfather? Or how do we express the future-past tense (or past-future, or past-future-past?), when we get stuck in a time loop where what will happen leads to what had already taken place, and so on? And of course the biggest question that Hebrew editors and translators have faced for years – is there really such a thing as present continuous?

It’s complicated.

Arguing about tenses and a time machine, The Big Bang Theory, Season 8, Episode 5, 2014

In his book, The Restaurant at the End of the Universe, science fiction writer Douglas Adams suggests to his readers to consult (by Dr. Dan Streetmentioner) Time Traveler's Handbook of 1001 Tense Formations (by Dr. Dan Streetmentioner) to find the answers to these questions. That’s all very well, but, Adams tells us, “most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up; and in fact in later editions of the book all pages beyond this point have been left blank to save on printing costs.”

If, despite all of the above, you’re still intent on travelling back to Mount Sinai or the Apollo 11 moon landing – let us then wish you bon voyage, and please give our regards to Neil Armstrong!

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Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

• If time travel were ever to occur, we should see extensive uncaused correlations.
• It is extremely unlikely that we should ever see extensive uncaused correlations.
• Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

• Are the past, present and future equally real?
• Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

• the block universe view: eternalism + B-theory
• the moving spotlight view: eternalism + A-theory
• the presentist view: nowism + A-theory
• the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

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Time Travel Paradoxes

As we mentioned before, the concept of traveling into the past becomes a bit murky the second causality rears its head. Cause comes before effect, at least in this universe, which manages to muck up even the best-laid time traveling plans.

For starters, if you traveled back in time 200 years, you'd emerge in a time before you were born. Think about that for a second. In the flow of time, the effect (you) would exist before the cause (your birth).

To better understand what we're dealing with here, consider the famous grandfather paradox . You're a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your father's father. You raise your laser blaster , but just what happens when you pull the trigger?

Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop .

On the other hand, we have to consider the idea of a consistent causal loop . While equally thought-provoking, this theoretical model of time travel is paradox free. According to physicist Paul Davies, such a loop might play out like this: A math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with.

Then there's the post-selected model of time travel, which involves distorted probability close to any paradoxical situation [source: Sanders]. What does this mean? Well, put yourself in the shoes of the time-traveling assassin again. This time travel model would make your grandfather virtually death proof. You can pull the trigger, but the laser will malfunction. Perhaps a bird will poop at just the right moment, but some quantum fluctuation will occur to prevent a paradoxical situation from taking place.

But then there's another possibility: The quantum theory that the future or past you travel into might just be a parallel universe . Think of it as a separate sandbox: You can build or destroy all the castles you want in it, but it doesn't affect your home sandbox in the slightest. So if the past you travel into exists in a separate timeline, killing your grandfather in cold blood is no big whoop. Of course, this might mean that every time jaunt would land you in a new parallel universe and you might never return to your original sandbox.

Confused yet? Welcome to the world of time travel.

Explore the links below for even more mind-blowing cosmology.

Related Articles

• How Time Works
• How Special Relativity Works
• What is relativity?
• Is Time Travel Possible?
• How Black Holes Work
• How would time travel affect life as we know it?

More Great Links

• NOVA Online: Time Travel
• Into the Universe with Stephen Hawking
• Cleland, Andrew. Personal interview. April 2010.
• Davies, Paul. "How to Build a Time Machine." Penguin. March 25, 2003.
• Davies, Paul. Personal interview. April 2010.
• Franknoi, Andrew. "Light as a Cosmic Time Machine." PBS: Seeing in the Dark. March 2008. (March 1, 2011)http://www.pbs.org/seeinginthedark/astronomy-topics/light-as-a-cosmic-time-machine.html
• Hawking, Stephen. "How to build a time machine." Mail Online. May 3, 2010. (March 1, 2011)http://www.dailymail.co.uk/home/moslive/article-1269288/STEPHEN-HAWKING-How-build-time-machine.html
• "Into the Universe with Stephen Hawking." Discovery Channel.
• Kaku, Michio. "Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos." Anchor. Feb. 14, 2006.
• "Kerr Black Holes and time travel." NASA. Dec. 8, 2008. (March 1, 2011)http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/041130a.html
• Sanders, Laura. "Physicists Tame Time Travel by Forbidding You to Kill Your Grandfather." WIRED. July 20, 2010. (Mach 1, 2011)http://www.wired.com/wiredscience/2010/07/time-travel/

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Paradoxes of Time Travel

Ryan Wasserman, Paradoxes of Time Travel , Oxford University Press, 2018, 240pp., $60.00, ISBN 9780198793335.

Reviewed by John W. Carroll, North Carolina State

Wasserman's book fills a gap in the academic literature on time travel. The gap was hidden among the journal articles on time travel written by physicists for physicists, the popular books on time travel by physicists for the curious folk, the books on the history of time travel in science fiction intended for a range of scholarly audiences, and the journal articles on time travel written for and by metaphysicians and philosophers of science. There are metaphysics books on time that give some attention to time travel, but, as far as I know, this is the first book length work devoted to the topic of time travel by a metaphysician homed in on the most important metaphysical issues. Wasserman addresses these issues while still managing to include pertinent scientific discussion and enjoyable time-travel snippets from science fiction. The book is well organized and is suitable for good undergraduate metaphysics students, for philosophy graduate students, and for professional philosophers. It reads like a sophisticated and excellent textbook even though it includes many novel ideas.

The research Wasserman has done is impressive. It reminds the reader that time travel as a topic of metaphysics did not start with David Lewis (1976). Wasserman (p. 2 n 4) identifies Walter B. Pitkin's 1914 journal article as (probably) the first academic discussion of time travel. The article includes a description of what has come to be called the double-occupancy problem, a puzzle about spatial location and time machines that trace a continuous path through space. The same note also includes a lovely passage, which anticipates paradoxes about changing the past, from Enrique Gaspar's 1887 book:

We may unwrap time but we don't know how to nullify it. If today is a consequence of yesterday and we are living examples of the present, we cannot unless we destroy ourselves, wipe out a cause of which we are the actual effects.

These are just two of the many useful bits of Wasserman's research.

Chapter 1 usefully introduces examples of time travel and some examples one might think would involve time travel, but do not (e.g., changing time zones). There is good discussion of Lewis's definition of time travel as a discrepancy between personal and external time, including a brief passage (p. 13) from a previously unpublished letter from Lewis to Jonathan Bennett on whether freezing and thawing is time travel. I had often wonder what Lewis would have said; now I know what he did say!

Chapter 2 dives into temporal paradoxes deriving from discussions of the status of tense and the ontology of time (presentism vs. eternalism vs. growing block vs. . . . ). Here, Wasserman also includes the double-occupancy problem as a problem for eternalism -- though it is not clear that it is only a problem for eternalism. Then he turns to the question of the compatibility of presentism and time travel, the compatibility of time travel and a version of growing block that accepts that there are no future-tensed truths, and finally to a section on relativity and time travel. The section on relativity is solid and seems to me to pull the rug out from under some earlier discussions. For example, Lewis's definition of time travel is shown not to work. It also becomes clear that presentism and the growing block are consistent with both time-dilation-style forward time travel and traveling-in-a-curved-spacetime "backwards" time travel.

Chapters 3 and 4 cover the granddaddies of all the time-travel paradoxes: the freedom paradoxes that include the grandfather paradox, the possibility of changing the past, and the prospects of such changes given models of branching time, models that invoke parallel worlds, and hyper time models. Chapter 4 gets serious about Lewis's treatment of the grandfather paradox and Kadri Vihvelin's treatment of the autoinfanticide paradox (about which I will have more to say).

Chapter 4 also includes discussion of "mechanical" paradoxes that, as stated, do not require modal premises about what something can and cannot do, and no notion of freedom or free will. (See Earman's bilking argument on p. 139 and the Polchinski paradox on p. 141.) Wasserman introduces modality to these paradoxes, but I would have liked them to be addressed on their own terms. As I see it, these paradoxes are introduced to show that backwards time travel or backwards causation in a certain situation validly lead to a contradiction. On their own terms, for these arguments to be valid, the premises of the arguments themselves must be inconsistent. How can one make trouble for backwards time travel if the argument is thus bound to be unsound?

Chapter 5 takes on the paradoxes generated by causal loops or more generally backwards causation including bilking arguments, the boot-strapping paradox (based on a presumption that self-causation is impossible), and the ex nihilo paradox with causal loops and object loops (i.e., jinn) that seem to have no cause or explanation.

Chapter 6 deals with paradoxes that arise from considerations regarding identity, with a focus on the self-visitation paradox from both perdurantist and endurantist perspectives. I was surprised to learn that Wasserman had defended an endurantist-friendly property compatibilism -- similar to my own -- to resolve the self-visitation paradox. I was then delighted to find out that he cleverly extends this sort of compatibilism to the time-travel-free problem of change (i.e., the so-called, temporary-intrinsics argument).

The outstanding scientific issue regarding backwards time travel is whether it is physically possible. There is no question that forwards time travel is actual, or even whether it is ubiquitous. There is also not much question that backwards time travel is consistent with general relativity. Still, we await more scientific progress before we will know whether backwards time travel really is consistent with the actual laws of nature. In the meantime, there is still much to be said about Lewis's treatment of the grandfather paradox and Vihvelin's stated challenge to that treatment in terms of the autoinfanticide paradox.

I will start by being somewhat critical of Lewis's approach. For his part (pp. 108-114), Wasserman does a terrific job of laying out Lewis's position as a metatheoretic discussion of the context sensitivity of 'can' and 'can't'. My concern is that not enough attention is given to the 'can' and 'can't' sentences that turn out true on the semantics. The semantics works only by a contextual restriction of possible worlds based on relevant facts -- the modal base -- associated with a conversational context. In meager contexts, false 'can' sentences will turn out true too easily. For example, suppose two people are having a conversation about Roger. Maybe all the two know about Roger is his name and that he is moving into the neighborhood. So, the proposition that Roger doesn't play the piano is not in the modal base. So, according to Lewis's semantics applied to 'can', 'Roger can play the piano' is true in this context. That seems wrong. This would be an unwarranted assertion for either of the participants in the conversation to make. Notice it is also true relative to the same meager context that Roger can play the harpsichord, the sousaphone, and the nyatiti. Quite a musician that Roger! [1]

Interestingly, though this problem arises for 'can', it does not arise for other "possibility" modals. For example, notice that, with the meager context described above, there is a big difference regarding the assertability of 'Roger could play the piano' and of 'Roger can play the piano'. Similarly, there is also no serious issue with regard to 'Roger might play the piano'. 'Could' and 'might' add tentativeness to the assertion that seems called for. There also seems to be no problem for the semantics insofar as it applies to 'is possible'. 'It is possible that Roger plays the piano' rings true relative to the context. But 'Roger can play the piano'? That shouldn't turn out true, especially if Roger is physically or psychologically unsuited for piano playing.

This issue has been frustrating for me, but Wasserman's book has me leaning toward the idea that what is needed is a contextual semantics that includes a distinguishing conditional treatment of 'can' of the sort Wasserman suggests:

(P1**) Necessarily, if someone would fail to do something no matter what she tried, then she cannot do it (p. 122).

This is a suggestion made by Wasserman on behalf of Vihvelin. I find (P1**) as a promising place to start in terms of the conditional treatment.

Speaking of Vihvelin, her thesis is "that no time traveler can kill the baby that in fact is her younger self, given what we ordinarily mean by 'can'" (1996, pp. 316-317). Vihvelin cites Paul Horwich as a defender of a can-kill solution, what she calls the standard reply :

The standard reply . . . goes something like this: Of course the time traveler . . . will not kill the baby who is her younger self . . . But that doesn't mean she can't . (Vihvelin 1996, p. 315)

Vihvelin's doing so is appropriate given what Horwich says about Charles attending the Battle of Hastings: "From the fact that someone did not do something it does not follow that he was not free to do it" (1975, 435). In contrast, it strikes me as odd that Vihvelin (1996, p. 329, fn. 1) also attributes the standard reply to Lewis. I presume that she does so based on some comments by Lewis. He says, "By any ordinary standards of ability , Tim can kill Grandfather," (1976, p. 150, my emphasis) and especially "what, in an ordinary sense , I can do" (1976, p. 151, my emphasis). So, admittedly, Vihvelin fairly highlights an aspect of Lewis's view as holding that, in the ordinary sense of 'can', Tim can kill Gramps. And I can see how this is a useful presentation of Lewis's position for her argumentative purposes.

Nevertheless, I take Lewis's talk of ordinary standards or an ordinary sense to just be a way to identify the ordinary contexts that arise with uses of 'can' in day-to-day dealings, where the possibility of time travel is not even on the table. Simple stuff like:

Hey, can you reach the pencil that fell on the floor?

Sure I can; here it is.

More importantly, we have to keep in mind that the basic semantics only has consequences about the truth of 'can' sentences once a modal base is in place. To me, the fact that Baby Suzy grows up to be Suzy is exactly the kind of fact that we do not ordinarily hold fixed. Lewis's commitment to the semantics does not make him either a can-kill guy or a can't-kill guy.

What is the upshot of this? There is a bit of underappreciation of Lewis's approach in Wasserman's discussion of Vihvelin's views. The pinching case on p. 119 provides a way to make the point. Consider:

(a) If Suzy were to try to kill Baby Suzy, then she would fail.

(b) If Suzy were to try to pinch Baby Suzy, then she would fail.

According to Wasserman, Vihvelin thinks that even in ordinary contexts (a) and (b) come apart (p. 119, note 32) -- (a) is true and (b) is false. As I see it, a natural context for (a) includes the fact that Baby Suzy grows up normally to be Suzy. That is a supposition that is crucial to the description of the scenario and so is likely to be part of the modal base. No canonical story or suppositions are tied to (b), though Vihvelin stipulates that Suzy travels back in time in both cases. We are not, however, told a story of Baby Suzy living a pinch-free life all the way to adulthood. We are not told whether Suzy decided go back in time because Baby Suzy deserved a pinch for some past transgression. My point is that the stories affect the context. So, with parallel background stories, (a) and (b) need not come apart.

I am not sure whether Wasserman was speaking for himself or for Vihvelin when he says about (a) and (b), "Self-defeating acts are paradoxical in a way other past-altering acts are not" (p. 120). Either way, I disagree. Lewis gives a more general way to resolve the past-alteration paradoxes that is not obviously in any serious conflict with Vihvelin's many utterances that turn out true relative to the contexts in which she asserts them. Wasserman also says, "The only disagreement between Lewis and Vihvelin is over whether Suzy's killing Baby Suzy is compatible with the kinds of facts we normally take as relevant in determining what someone can do" (p. 117). That is an odd thing for him to say. Lewis sketches a semantic theory that provides a framework for the truth conditions of 'can' and 'can't' sentences. He is not in disagreement with Vihvelin. For Lewis, there is one specification of truth conditions for 'can' that gives rise to both 'can kill' and 'can't kill' sentences turning out true relative to different contexts. Indeed, it is tempting to think that Vihvelin takes the fact that Baby Suzy grows up to be Adult Suzy as part of the modal base of the contexts from which she asserts the compelling 'can't-kill' sentences.

That all said, Wasserman's book is a significant contribution. There are those of us who focus a good chunk of our research on the paradoxes of time travel for their intrinsic interest, and especially because they are fun to teach. That is contribution enough for me. But, ultimately, from this somewhat esoteric, fun puzzle solving, we also learn more about the rest of metaphysics. The traditional issues of metaphysics: identity-over-time, freedom and determinism, causation, time and space, counterfactuals, personhood, mereology, and so on, all take on a new look when framed by the questions of whether time travel is possible and what time travel is or would be like. Wasserman's book is a wonderful source that spotlights these connections between the paradoxes of time travel and more traditional metaphysical issues.

Cargile, J., 1996. "Some Comments on Fatalism" The Philosophical Quarterly 46, No. 182 January 1996, 1-11.

Gaspar, E., 1887/2012. The Time-Ship: A Chronological Journey . Wesleyan University Press.

Horwich, P., 1975. "On Some Alleged Paradoxes of Time Travel" The Journal of Philosophy 72, 432-444.

Lewis, D., 1976 "The Paradoxes of Time Travel" American Philosophical Quarterly 13, 145-152.

Pitkin, W., 1914. "Time and Pure Activity" Journal of Philosophy, Psychology and Scientific Methods 11, 521-526.

Vihvelin, K., 1996. "What a Time Traveler Cannot Do" Philosophical Studies 81, 315-330.

[1] This criticism was first presented to me by Natalja Deng in the question-and-answer period for a presentation at the 2014 Philosophy of Time Society Conference. Later on, I found a parallel challenge in work by James Cargile (1996, 10-11) about Lewis's iconic, 'The ape can't speak Finnish, but I can'.

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1 Introduction

• Published: November 2017
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Chapter 1 explains the concept of time travel, clarifies the main question to be addressed, and previews the paradoxes to come. Section 1 explains the traditional view of time travel as involving a discrepancy between “personal” and “external” time. Section 2 contrasts this kind of time travel with other, purported examples of time travel. Section 3 distinguishes a number of different questions about time travel, including the question of whether or not time travel is compatible with the laws of metaphysics—particularly those having to do with the nature of time, freedom, causation, and identity. Finally, section 4 provides an outline of the rest of the book by introducing some of the key paradoxes to be addressed. Other topics in this chapter include time, causation, and metaphysical grounding.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• grandfather paradox
• time travel

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Classic Time Travel Paradoxes (And How To Avoid Them)

[Movie still from  Time Machine , Warner Bros. and Dreamworks]

Editor's Note: We're bringing back one of our most loved posts because hey, time travel is always a relevant topic of discussion. Originally published 11/30/12.

Author's Note: I assume that some day, this article will serve as an invaluable guide and warning for our time traveling ancestors-to-be (who will of course be unable to read books and learn these lessons for themselves, either because [a] all the books will have been burned, or [b] kids will have stopped reading books entirely, because grumble grumble, god damn kids, when I was your age, video games, blah blah, detriment to society, buncha hooligans, kids these days, no respect, etc). In the meantime, just enjoy it for all of its delightfully entertaining/convoluted/paradoxical pleasures.

As anyone who’s anyone who’s read any time travel story ever could easily tell you, time travel is a tricky subject. Temporal paradoxes might seem simple and straightforward at the start (no they don’t), but they always devolve quite quickly (linear time-wise) into some sort of trippy, philosophically complicated, timey-wimey conundrum that makes even the most convoluted middle school relationship make sense by comparison. Come to think of it, maybe the reason that all those cool kids in middle school suffer from impossibly complicated and melodramatic romances to begin with is because they’re all too “cool” to read time travel stories in the first place, which would obviously teach them the benefits of temporally linear dating, if nothing else. 

I’m looking at you, River Song.

For the most part, any paradox related to time travel can generally be resolved or avoided by the Novikov self-consistency principle, which essentially asserts that for any scenario in which a paradox might arise, the probability of that event actually occurring is zero — or, to quote from LOST, “whatever happened, happened,” meaning that no matter what anyone does, they can’t actually create a paradox, because the laws of quantum physics will self-correct to avoid such a situation. Still, I’m wary of such a loose explanation for things, and so below, I’ve compiled a list of a few of the more popular time travel paradoxes — and what to do to avoid them. 

ONTOLOGICAL PARADOX : Also known as the “Bootstraps Paradox,” an ontological paradox arises when a person or object is sent through time and recovered by another person, whose actions then lead to the original person or object back to the time from when it came in the first place, thus creating an endless loop with no discernible point of origin. Thus, the original person or object is essentially “pulling itself up by its own bootstraps,” hence the nickname (thanks in no small part to the Robert Heinlein story “By His Bootstraps”).

Example : The Terminator films are a prime and popular example of the Ontological Paradox. In the future, a Terminator is sent back in time to kill the mother of resistance leader John Connor before he is born. While the original T-800 is ultimately destroyed, the leftover pieces are found by scientists who use the technological to…develop and create Skynet, and the Terminator-series robots. Skynet would have never been created if Skynet hadn’t taken over the world and then sent a Terminator back in time to get destroyed and ultimately lead to the creation of Skynet. Trippy, right?

There's also the fact that Future John Connor sends his buddy Kyle Reese back in time to protect his mother from the T-800, only Kyle ends up totally bangin' John's mom (dude high five! I mean, not cool, man) and impregnates her with his buddy John Connor. So to top it all off, if John hadn't sent his friend back in time, his friend would never have had sex with John's mom, and John would never have been born (meaning that Kyle Reese is either the best or worst friend, ever).

How to Avoid : No one’s really sure if a real-life ontological paradox would lead to some massive hemorrhaging of spacetime, or if the closed loop is kind of automatically self-corrected since it all works itself out evenly in the end anyway. Still, better to avoid these kind of complicated situations, and the best way to do that would simply be to stop taking candy from strangers — “candy” in this case being mysterious or alien artifacts with questionable origins, possibly given to you by mysterious people who may or may not come from the future. See? Maybe all those warnings that your Mom gave you when you were a little kid still mean something today. Or maybe all along she was just trying to prevent you from sending your friends back in time to sleep with her. Or perhaps encourage it…

PREDESTINATION PARADOX : The predestination paradox is similar to the ontological paradox in that the Cause leads to an Effect which then leads back to the initial Cause. The basic tenant of the predestination paradox is similar to that of a self-fulfilling prophecy: the motivation for the time traveler to travel in time is ultimately realized to have been the time traveler’s fault, due to his or her decision to time travel in the first place, or else otherwise unavoidable. Stories involving predestination paradoxes often involve a heavy sense of irony — the time traveler might go back in time in order to change something, for example, but his or her actions inadvertently lead to the exact situation that inspired the time traveler to have gone back and changed things. Thus, nothing ultimately changes. Determinism is a bleak friend. 

Example : In Twelve Monkeys, James Cole is sent back in time to prevent a mysterious disaster involving the “Army of the Twelve Monkeys.” His wild rantings in the past about the terrible future from which he came are overheard by Jeffrey Goines, a mental patient who is remembered in the future as the leader of Army of the Twelve Monkeys. Ultimately, Cole’s efforts to prevent his future from happening inspire the actions that lead to his future coming to be. And in a cruel twist of irony, James Cole’s childhood memory of a man in a airport being shot and falling into the arms of a beautiful blonde — the memory that haunts him for the rest of his life — turns out that the guy who was shot was actually him, in the future, dooming young James Cole to grow up and repeat the cycle all over again.

How to Avoid : This one’s tricky, because philosophically, it’s all about free will (or lack thereof). So in fact, by trying to teach you to how to avoid falling victim to the tenants of the predestination paradox, I’m probably going to inspire you to go back in time and create the French film La jetée, which in turn inspires Terry Gilliam to make Twelve Monkeys, which in turn inspires me to use it as an example in this article, et cetera et cetera. Basically we’re all screwed, unless we avoid time travel and time travelers all together. Even a many worlds theory/alternate timeline thing can’t prevent this, because your actions wouldn’t even create a divergent timeline — they would just result in your present situation. So, sorry dude, nothing you can do is going to change anything. Again, unless you don’t do anything at all, although that still doesn’t guarantee anything. 

GRANDFATHER PARADOX : This one perfectly demonstrates the aforementioned Novikov self-consistency principle. The basic idea is that, no matter how hard you try, you can’t go back in time and kill your grandfather, because if you did, your mother or father would never have been born, which means that you would never have been born, which means you couldn’t have gone back in time and killed your grandfather, which means that you didn’t go back in time and kill your grandfather, because you can’t go back in time and kill your grandfather, because if you did, you wouldn’t be born, which you obviously have already been born because if you were never born then you couldn’t have gone back in time and tried (and failed) to kill your grandfather in the first place.

That’s just a simple and straightforward summary though. You know, in Layman’s terms.

Basically, the Grandfather paradox conveys the idea of a self-correcting universe and/or fixed points in time. Even if you were able to go back in time and, I don’t know, shoot your Grandpa in the head before he ever meets your Grandma (jeez, you must really hate that guy, huh?), your Grandfather would turn out to be an early sperm donor or something, who would still manage even posthumously to impregnate your Grandmother, because you would have to exist in order to have shot him in the head in the first place. So you might be able to fudge a few temporal details here and there, but no matter what you do, the end result stays the same.

Example : Let’s just say that when you're LOST on a magical tropical island somewhere in the Pacific Ocean (ish?) and you end up skipping through time and decide to try to kill that evil guy while he’s still a kid and/or stop a nuclear bomb you've so affectionately nicknamed “The Jughead” from exploding and causing all kinds of electromagnetic problems and inconsistencies on your already-mystical island home, the best that’s going to happen is you get some kind of weird Hindu sideways limbo reality that works as a parallel narrative to the entire last season of your television show. Oh, and that little kid you shot still turns out to be pretty evil, and it’s all your fault.

How to Avoid : Uhh, don’t try to kill your grandfather in the past before the birth of your father? Take that as a metaphor all you’d like.

HITLER'S MURDER PARADOX : This is similar to the Grandfather Paradox, in that the time traveller goes back in time to change something significant that has already happened. Unlike the Grandfather Paradox (which we assume would self-correct despite our best efforts), the change that one wishes to affect in the Hitler’s Murder Paradox is one that is more technically feasible — as in not intrinsically paradoxical — but still ultimately problematic.

The name comes from the idea that one could theoretically go back in time and kill Adolf Hitler before the Holocaust happened, thus preventing the systematic annihilation of some six million Jews and other minorities. Which, ya know, all sounds good and well, except that it tends to lead to some kind of downward spiraling domino effect with plenty of other consequences that the well-intentioned time traveler probably didn’t consider, and which ultimately might lead to a worse situation than that which the time traveler had hoped to prevent.

Example : This kind of stuff is rampant in comic books, especially X-Men, but the best example of it was the early 90s Age of Apocalypse storyline, in which Professor Xavier’s schizophrenic mutant son, Legion, decides to make daddy proud by helping his dream of mutant-human co-existence come true. Legion concludes that the best way to do this is to go back in time and kill Magneto before he becomes, ya know, Magneto. The only problem is, Magneto and Xavier were like totally BFF back then, so Xavier ends up taking the bullet for Magneto and dies (so yes, Legion does technically end up killing his own father, but that’s not the point).

As a result of there being no Charles Xavier, the psycho evil Darwinist uber-mutant Apocalypse ends up taking over the world before Magneto’s team of X-Men (named in honor of his deceased friend) are able to stop him, which leads to all kinds of crazy situations like evil Hank McCoy aka Dark Beast, who works alongside the evil versions of Cyclops and Havok, or a Sabretooth who is actually a pretty likeable superhero and a member of the X-Men. Oh, also, Magneto and Rogue totally have the sex, and humans are being systematically slaughtered in concentration camps by Apocalypse and his cronies. So basically, in his attempt to kill a perceived “Hitler” in the form of Magneto, Legion caused a real and even more twisted Holocaust to happen. WHOOPS.

How to Avoid : In addition to the whole alternate-reality-that-is-ironically-worse-than-the-world-as-it-used-to-be problem, there’s also the moral compromise of killing an innocent child, even though you know that child is going to grow up to become pretty much the worst (greatest?) mass murderer in history. The best way to avoid it is simply and sadly to accept that you cannot change the past and shouldn’t even try. That is, unless you’re smart enough to have eliminated any possibility of negative domino effect resulting out of your actions.

For example, if you went back in time and eliminated M. Night Shyamalan shortly before the release of Signs, there would be nothing but positive results; the world would mourn the tragic and mysterious loss of a gifted young filmmaker taken before his time, we would all be so blinded by the shock of his death that we’d be able to ignore how bad the aliens looked in that movie (and the fact that seeing them at all was completely unnecessary), and the rest of us wouldn’t have been forced to endure such awful schlock as The Happening or Lady in the Water. See? That way everyone wins!

BUTTERFLY EFFECT : Similar to the cascading domino effect of the Hitler’s Murder Paradox, but on a different level. Whereas killing Hitler would obviously be a landmark event with quite a significant historical impact, something like, say, accidentally stepping on a bug in the past probably wouldn’t have as big of an effect, right?

Have you even been paying attention? Of course it will! That’s the whole point of a time travel paradox! Just like the way that a butterfly flapping its wings in Brazil can affect a weather system in Texas, one tiny change in the past can lead to all kinds of Rube Goldbergian complications that can subtly — or seriously — affect the present. The term “Butterfly Effect” is actually derived from “A Sound of Thunder,” a short story by Ray Bradbury, in which a character accidentally steps on a butterfly in prehistoric times and causes catastrophic changes in the future from which he came.

Example : In Orpheus With Clay Feet by Philip K. Dick, the main character, Jesse Slade, enlists in the services of a time travel tourism agency, who set him up with a trip that allows him to go back in time and act as a muse for some significant historical figure. Slade chooses to go back and inspire his favorite science fiction writer Jack Dowland (which was also Dick’s pen name). Unfortunately, in his efforts to inspire Dowland’s monumental science fiction work, Slade directly reveals to Dowland that he is a time traveler hoping to inspire his work. Dowland takes this as an insulting ruse, and as a result, never becomes the great science fiction writer that he is meant to be. He does, however, publish a single science short story, under the pen name Philip K. Dick: a story called Orpheus With Clay Feet, about a time traveler that goes back in time to inspire his favorite science fiction writer, a man named Jack Dowland.

How to Avoid : Watch your step

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Time Travel Paradox Examples

Cristina Boros March 26, 2017 8 Comments

Hey, there 🙂 As I told you in my last post about  Time Loop , today I will offer you a few time travel paradox examples and we will try to understand them together.

Which are time travel paradoxes?

First, we will start with asking – what exactly is a paradox? A time travel paradox is a logical contradiction or apparent contradiction which is associated with time travel generally. They are classified into two categories: Consistency Paradoxes and Closed Causal Loop Paradoxes. A Consistency Paradox is also more well-known as a Grandfather Paradox and other variants such as the Hitler Paradox.

A Closed Causal Loop Paradox is the Predestination Paradox and the Bootstrap Paradox. Here we have to deal with a self-existing loop. In this case, cause and effect create a circle.

What is the grandfather paradox?

So, let’s see first what the Grandfather Paradox  is, and I think even from the movie ( Paradox ), most of you know about it. What about if you found out that your grandfather did something very bad, he was a criminal or he killed your mom later…or anything like this?

You would go back in time to kill him before he has the chance to hurt people wouldn’t you?. But what happens in this case? So, if you kill your grandfather before he met your grandmother, you will never have been born and of course, you will not be around at this time to travel into the past. If you don’t travel into the past, nobody will kill your grandfather so….he will still do the crime he did. If he is not dead you will be born so…you will be able to go back in time to kill him, and so on:). It never ends:). Did you understand the idea?

This idea of going back in time and killing your grandfather, parents or any other ancestor before you are born presents a real problem for time travelers. When you go back in time and you try to kill your grandfather, we have two possibilities.

First, we have the Timeline Protection Hypothesis . In this situation, you try to kill him and every time something stops you, and you fail: Or your gun blocks or something in your way or…anything else but YOU JUST CAN’ T KILL HIM. The universe protects him and you at the same time, keeping safe the timeline you are in.

And again we have the situation when you succeed. You kill him and you just go back to your time. What happens there? You never existed, also your family and everything related to your life you knew until now would not exist. You would create a new timeline.

Scientists tried to find the answers to this problem and a new theory suggests that quantum teleportation can help to avoid the grandfather paradox; this theory was proposed by physicists at MIT. First, this is why they say it is important the post selection. Scientists claim that what a traveler can do and what he cannot do has to be post selected and such an action as killing your ancestors should be strictly forbidden during a travel into the past.

Other alternatives of grandfather paradox

Talking about changing history and timelines, the Grandfather Paradox is only on a personal level. But what about if you want to go back in time to kill Hitler (Killing Hitler paradox).

Of course, the whole world can change. The effect will not be only for you but you will create a totally different world. Even the history books are different 🙂

But, again in this situation, if you would succeed and kill Hitler, then the world would be different and you wouldn’t have any reason to kill him. It is a circle.

This idea was accepted by scientists because it was based upon the laws of motion. Polchinski claims that while time travel is possible, paradoxes are forbidden. This is the solution he gave and it was called ‘timeline protection hypothesis’. It states that something will occur to stop you killing your grandfather, in order to avoid the paradox.

Scientists’ results

Seth Lloyd and other scientists have a new theory. They combine quantum teleportation with post-selection. So, in this case, they had a theory of post selected closed timelike curves (CTC). CTC is a path of the space-time which returns to the point where it started.

Talking about quantum teleportation, quantum states can be transmitted one to the other in different locations and with the post selection applied, scientists have the possibility to accept only certain results and only a certain type of state can be teleported. The states which are post selected to be self-consistent are the ones that will be teleported and only then, after being selected they can travel in the past. In this way, paradoxes will be avoided.

What do you think about this? Is it mind blowing?

Quantum conclusions:)

The post selected closed timelike curves theory has an important advantage. It doesn’t require the distortion of space-time to travel in the past as in other theories. The distortion of space-time can be found only in black holes and other extreme environments which make time travel in the past impossible to realize as we can see in those theories.

Post selected closed timelike curve is still a theory but scientists hope that soon will be proved and they say that it will maybe allow time travel in the past without general relativistic closed timelike curves and if nature provides the nonlinear dynamics than particles and humans can tunnel from the future into the past.

What can we do? Just let the time and the great minds to do research and give us more details? Can we travel in the past without creating a mess in the present? What do you think?

I will show you here a short video, helping you to understand everything easily. Thanks for visiting my site and today and I would really like to know your opinion 🙂 Feel free to leave your comments below.

So if you are still here and read the post until the end I suppose you may like the posts below too :). Have a nice day.

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Very interesting article! I never heard of the grandfather paradox before. It seems like its similar to the butterfly effect (but backwards maybe). The idea of a small action changing the course of the future. It’s amazing to find that scientists are looking into ideas like these. Great post. Thanks for sharing!

thanks for reading and for visiting my site Liz. Yes, and is not the only one paradox:)The scientists are trying to explain them and to understand them. I hope soon we ll have all the answers:)

Grandfather paradox, it is similar with the butterfly effect but has to do with the past,you are right. i hope I made it as simple as i could:)

Im siting here and Im amazed. Timetravel was always something that draws my attention, but after I read about this paradoxes, everything have more sense. So if someone goes to kill something, they wouldnt have to kill him couse there is no reason to kill him. So messy to me haha

It is really confusing:) That’s why the scientists try to avoid it . I understand sometimes is difficult to understand but it is true, everything is a circle.

Thanks for visiting my site and reading my post 🙂 For more paradoxes,i will be back soon 🙂

Have a nice day.

This is one of he most insightful Paradox theories I’ve seen in a while. You explained it very clearly and digestible.

Hello there:)I am happy that you could simply understand it. this my goal to explain a few things in a way that everybody can read an understand :)) Ok I know sometimes is difficult to make it simple because the subject is enough Difficult:))if there is something else you would like to talk about , just tell me 🙂

When I was a teenager I experienced several trips in a “time bend” of sorts that I’ve never been able to explain. For a period of 3 days I would “zone out” and end up approx 5 hours ahead from the last time I looked at the clock. I didn’t fall asleep or day dream. I don’t know where I was during those missing hours,all I remember was being aware that it was happening and when I was getting ready to time jump through time. I would suddenly jump ahead 5 hours with no recollection of the hours that were missing.I would always jump ahead and end up sitting in the same place I was hours earlier very confused and disorientated. I have never found an explanation nor have I forgotten over 20+ years later.

Hello venus, Amazing and strange experience you had there. you never tryed to find an explanation? Happened just for three days? Than never?Every time was for the same amount of time, 5 hours? Thanks for sharing your experience with me. can you tell me more details?

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Engineers Garage

Time Travel: Theories, Possibilities, and Paradoxes Explained

By Neha Rastogi

Time Travel has been a matter of great interest for Science fiction since ages. Whether it’s the movies like Planet of the Apes (1968) or modern franchises like “Doctor Who” and “Star Trek” ; the concept is grabbing a lot of eyeballs. Not only movies and shows but even some mythological tales like Mahabharata and the Japanese story of Urashima Taro support the evidence that time travel exists. We often see stories where characters use time machines to jaunt through the years but the reality is far more complex and inexplicable.

Understanding the Concept of Time Travel

Time Travel is defined as the phenomenon of moving between different points in time through a hypothetical device called “Time Machine”. Despite being predominantly related to the field of philosophy and fiction, it’s somehow supported to a small extent by physics in conjunction with quantum mechanics. However, before getting into the argument of how real it is, let’s comprehend the fundamental meaning of time.

Basically, the whole idea of Time Travel is administered by the concept of time. Usually, people believe that time is constant but the famous Physicist Albert Einstein introduced the “Theory of Relativity” as per which, time is relative. In other words, time slows down or speeds up depending on how fast the observer moves relative to something else. According to him, a person traveling inside a spaceship at the speed of light would age much slower than his/her twin back at home.

Time is Relative

After Einstein’s Theory of Relativity, his teacher Herman Minkowski emphasized on space-time, a mathematical model that joins both space and time in a continuum. This implies that time and space cannot exist without each other. Space is a 3-dimensional arena consisting of length, width, and height. This is joined by Time with the fourth dimension called direction. So anything that happens in the universe takes place in this space-time continuum. Although this validates that space travelers are slightly younger than their twins when they return to earth, yet a huge leap in the past or future is not possible with the current technology.

Time Machines

It is believed that in order to travel back or forward in time, one would require a device called Time Machine . The research on such a device would involve bending space-time to such an extent that time lines turn back on themselves to form a loop, which is termed as “closed time-like curve.” Such an action demands the use of an exotic form of matter with “negative energy density” that has a unique property of moving in the opposite direction of the normal matter when pushed. Even if it exists, the quantity would be too small to construct a machine.

Pictorial Representation of Time Travel through closed time-like curve

However, some another research suggests that time machines can also be constructed by building a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped hole filled with vacuum, gravitational force can be used to bend the space-time so as to form a closed time-like curve. After racing around inside this doughnut a traveler would be able to go back in time with each lap. But in reality, it’s quite complex because the gravitational fields have to be very strong and would demand precise manipulation.

Time Travel Approaches in Physics

After studying and researching about Time Travel, various physicists have come up with approaches that may support its possibility, at least theoretically. Let’s take a look at these concepts so as to understand how Time Travel could actually work someday.

Time Dilation

Time Dilation Explanation

An important aspect of Einstein’s relativity theory is the term “time dilation” , which is defined as the difference of elapsed time between two events as measured by observers who are either moving relative to each other or are situated at different locations from the gravitational mass. As per the theory, time dilation can be summarized as a phenomenon which occurs due to the difference in either gravity or relative velocity.

In special relativity the time dilation effect is reciprocal i.e. when two clocks are in motion with respect to each other, for both the observers, the other one will be time dilated or the other clock will move slower. However, in general relativity, an observer at the top of the tower will find the clock closer to the ground to be slower and the other observer would agree about the direction and magnitude of this difference.

Due to the concept of time dilation, the current human time travel record is held by Russian cosmonaut Sergei Krikalev . Owing to the high-speed (7.66 km/s) of ISS and the length of time spent in space, it is believed that the cosmonaut actually arrived 0.02 seconds in the future while returning to the earth.

Cosmic String

Diagram Depicting Cosmic Strings

In 1991 J Richard Gott gave the idea of Cosmic Strings , which are believed to be left over from the early cosmos. These are defined as string-like objects or narrow tubes of energy that are stretched across the entire length of the universe. Owing to the huge amount of mass and massive gravitational pull, it would allow objects attached to the Cosmic Strings to travel at the speed of light.

So if two strings are pulled close to each other or one of them is stretched near the black hole, it might warp space-time to such an extent that would lead to creating a closed time-like curve and hence leading to the possibility of time travel. Theoretically, the gravity generated by these two Cosmic strings would help in propelling a spaceship into the past.

However, coming to the reality, the loop of strings is required to contain half the mass-energy of an entire galaxy so as to travel one year back in time. This implies that powering a time machine would require splitting half the atoms present in the whole galaxy.

Black holes

Illustration of Kerr Hole

When stars (having a mass of more than four times our sun) reach their end of life and all their fuel is burned up, they collapse under the pressure of their own weight creating “Black Holes” . The boundary of a Black Hole, called Event Horizon , has such a strong gravitational pull that it doesn’t even allow light to pass through it. Since light travels at the fastest speed, everything else traveling through a black hole is also dragged back. Such a non-rotating black hole is named as Schwarzschild black hole .

However, traveling to a parallel universe is possible through a rotating black hole named Kerr Hole . It was proposed in 1963 by a mathematician named Roy Kerr . As per his theory, if dying stars collapse into a rotating ring of neutron stars, that would produce enough centrifugal force to prevent the formation of singularity.

Note: Singularity can be perceived as the point into which the black hole tapers much like an ice-cream cone. At this point, the laws of Physics cease to exist and all the matter is crushed beyond recognition.

Since there will be no singularity, it would be safe to pass through a black hole without being crushed and exit out of a “White Hole” . A white hole is believed to be the exhaust end of a black hole which pushes everything away from it. Hence we may travel into another time or even another universe.

Although Kerr Holes are just theoretical, if they exist then we may find our way to a one-way trip to the past or future. However, physicist Kip Thorne believes that such a black hole doesn’t exist and it would suck everything before someone even reaches the Singularity.

Diagrammatic Representation of Wormhole 

Wormholes, also known as Einstein-Rosen Bridges , are believed to be the most potential means for time travel. It could allow us to travel several light years from earth and in much less time as compared to the conventional space travel methods. The possibility of wormholes is based on Einstein’s theory of relativity which says that any mass curves space-time. The following example is used to explain this curvature.

If two persons are holding a bed sheet stretching it tight and a baseball is placed on the sheet, its weight will make it roll to the middle of the sheet creating a curve at that point. Now if a marble is placed on the sheet, it would travel towards the baseball because of the curve. Here space is depicted as a two-dimensional plane than the four dimensions that actually makes up space-time.

Now if this sheet is folded over leaving a space at the top and bottom, placing the baseball on the top would form a curvature. If an equal mass is placed at the bottom part at a point corresponding to the location of the baseball, the second mass would eventually meet with the baseball. Similarly, wormholes might develop.

In space, masses that place pressure on different parts of the universe combine together to form a tunnel. Theoretically, this tunnel joins two separate times and allows passage between them. However, it’s possible that certain unforeseen physical properties may prevent the occurrence of wormholes and even if they exist, these might be really unstable.

Possibly someday human may learn to capture, stabilize and enlarge these tunnels but according to Dr. Hawking, prolonging the life of a tunnel through folded space-time may lead to a radiation feedback loop destroying the time tunnel.

Time Travel Paradoxes

If we ever work out a theory for time travel, we would give way to certain complexities known as paradoxes. A paradox is something that contradicts itself. In other words, time travel is not believed to be a practical concept because of certain situations that are likely to arise as the after-effects. These are broadly classified as -:

1. Closed Casual Loops: The cause and effect run in a circle causing a loop and is also internally consistent with the timeline’s history.

Diagram depicting time loop

• Predestination Paradox

It is defined as a situation when a traveler going back in time causes the event which he is trying to prevent from happening. It implies that any attempt to stop any event from occurring in the past would simply lead to the cause itself. The paradox suggests that things are destined to turn out the way they have happened and anyone attempting to change the past would find himself trapped in the repeating loop of time. For example, if you travel in the past to prevent your lover from dying in a road accident, you will find out that you were the one who accidentally ran over her.

• Bootstrap Paradox

A bootstrap paradox, also known as an Ontological Paradox where an object, person, or piece of information sent back in time leads to an infinite loop where the object has no discernible origin and is believed to exist without ever being created. It implies that the past, present and future and not defined, thus making it complicated to pinpoint the origin of anything. It raises questions like how were the objects created and by whom.

2. Consistency Paradox: It generates a number of timeline inconsistencies related to the possibility of altering the past. It can be further divided into the following categories.

• The Grandfather Paradox

Grandfather Paradox

This paradox talks about a hypothetical situation where a person travels back in time and kills his paternal grandfather at the time when his grandfather didn’t even meet his grandmother. In such a situation, his father would never have been born and neither would the traveler himself. So if he was never born, how would he travel to the past to kill his grandfather?

The paradox also talks about auto-infanticide where a time traveler goes into the past to kill himself when he was an infant. Now if he killed himself when he was a kid, how would he exist in the future to come back in time? Some physicists say that you would be able to go back in time but you won’t be able to change it, while others suggest that you would be born in one universe but unborn in another universe.

• The Hitler Paradox

Similar to the grandfather paradox, the killing Hitler paradox erases the reason for which you would want to go into the past and kill Hitler. Moreover, killing grandfather might have a “butterfly effect” but killing Hitler would have a far-reaching impact on the History as it would change the whole course of events. If you were successful in killing Hitler, there’d be no reason that would make you want to go back in time and kill him.

This paradox has been explained very well in a Twilight Zone episode called “Cradle of Darkness” as well as an episode “Let’s Kill Her” from Dr. Who.

• Polchinski’s Paradox

American physicist Joseph Polchinski proposed a paradox where a billiard ball enters a wormhole and emerges out of the other end in the past just in time to collide with its younger version and prevents it from entering the wormhole in the first place. While proposing this scenario, Joseph had Novikov’s Self Consistency Principle in his mind which states that time travel is possible but time paradoxes are forbidden.

A number of solutions have been suggested to avoid these inconsistencies like the billiard ball will deliver a blow which changes the course of the younger version of the ball but it would not stop it from entering the wormhole. This also explains that if you go back in time to kill your grandfather then something or the other will happen to prevent you from making it happen thus preserving the consistency of the History.

Solutions for the Paradoxes

In order to come up with a solution for these above-mentioned paradoxes, scientists have proposed some explanations which are enlisted below

The Solution: Time Travel is impossible because of the paradoxes that it creates.

Self-Healing Hypothesis: If we succeed to change the events in the past, it will set off another set of events that will keep the present unchanged.

The Multiverse: Every time an event in the past is altered, an alternate parallel universe or timeline is created.

Erased Timeline Hypothesis: A person traveling to the past would exist in the new timeline but their own timeline would be erased.

Is Time Travel Possible?

Nobody seems to have a definite answer in support or against the existence of Time Travel. On one hand, Einstein suggested to traveling at the speed of light in order to jaunt through the future but this would mean an unimaginable amount of energy would be required. Moreover, the centrifugal force on the body would prove to be fatal. Although it has been observed that space travelers age a little slower as compared to their identical twin on earth but some believe that there is no definite answer to travel back in space.

Theoretical physicist Brian Greene of Columbia University says that “No one has given a definite proof that you can’t travel to the past. But every time we look at the proposals and detail it seems kind of clear that they’re right at the edge of the known laws of physics.” Besides, Prof. Hawking feels that “Today’s science fiction is tomorrow’s science fact.”

However, the paradoxes, especially the grandfather paradox, have imposed a big question mark on the possibility of Time Travel. Basically, with the present laws and knowledge of Physics, the human won’t be able to survive in the process of Time Travel. So, we need certain developments in the quantum theories till we are sure as to how the paradoxes can be solved.

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Time Travel Paradoxes: Logical Contradictions of Time Travel

Logical Contradictions of Time Travel are also known as time travel paradox, temporal paradox, or time paradox. These are apparent contradictions related to time and time travel. In physics, Time Travel Paradoxes are classified into consistency paradoxes, such as the grandfather paradox and causal loops. Other paradoxes related to time travel include a variant of the Fermi paradox and free will paradoxes arising from causal loops, such as Newcomb’s paradox.

• 1 What is the Time Travel Paradox?
• 2 Causal loop
• 3 Grandfather paradox
• 4 Fermi Paradox
• 5 Newcomb’s Paradox

What is the Time Travel Paradox?

The time travel paradox is an apparent or logical contradiction associated with the idea of time and time travel. The time travel paradox is also called a temporary paradox or time paradox.

There are two broad groups of time travel paradoxes in physics. First, consistency paradoxes are imitated by grandfather paradoxes and causal loops. Other paradoxes associated with time travel are the Fermi paradox, a form of the paradox of free will, and Newcomb’s paradox. All are explained below.

Causal loop

A Causal loop is a paradox of time travel that occurs when a future event causes an event in the past, which in turn causes a future event when both events are present in spacetime, but their origin Cannot be determined. A Causal loop may include an event, person, object, or information. The terms bootstrap, foreseeable, or ontological paradox are sometimes used in fiction to refer to a causal loop.

Grandfather paradox

Grandfather paradox occurs when the past is altered, thus creating a paradox. A time traveler can do anything that has happened but cannot do anything that has not happened to do something with no contradiction. Therefore, whenever it is possible to change the past, a Grandfather paradox Consistency Paradox arises. Know this in more detail.

Fermi Paradox

As such, the Fermi paradox refers to the apparent contradiction between various evidence of the existence of extraterrestrial civilizations elsewhere in our galaxy and the lack of probability estimates. But here, the Fermi paradox can be adapted to time travel in some way – if time travel is possible, where are all future visitors? The answer to this question may vary – “From time to time, travel is not possible, future visitors may not reach any arbitrary point in the past, or they may avoid identifying themselves Live in secret.”

Newcomb’s Paradox

Newcomb’s paradox is a thought experiment that contradicts the expected utility theory and strategic dominance theory. Thought experimentation often promotes work-causal reasoning and free will by allowing for “right prophets”: if the suitable predictors of the future exist, for example, if time travel is a mechanism for making correct predictions As present, then the absolute predictions would seem to be independently contradictory because decisions made explicitly for free would already know the true prophet.

• Lewis, D. (1976). The paradoxes of time travel .  American Philosophical Quarterly ,  13 (2), 145-152.
• Wasserman, R. (2017).  Paradoxes of time travel . Oxford University Press.
• Horwich, P. (1975). On some alleged paradoxes of time travel .  The journal of philosophy ,  72 (14), 432-444.
• Eldridge-Smith, P. (2007). Paradoxes and hypodoxes of time travel .
• Wolpert, D. H.; Benford, G. (June 2013). (The lesson of Newcomb’s paradox)
• Divine Foreknowledge and Newcomb’s Paradox
• Leora Morgenstern (2010),  Foundations of a Formal Theory of Time Travel

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Den of Geek

Five Greatest Films with Time Travel Paradoxes

Our list of the Top 5 films with time travel paradoxes and a discussion of how each one makes those paradoxes work.

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Time travel is a sticky subject for a plot device in that it comes with a set of built-in contradictions. Could a time traveler bring back objects or information from the future if those objects or information only exist because they were brought back from the future? (The “Bootstrap Paradox .”) Could a time traveler truly influence time if their presence in the past means they’ve already time traveled? (The “Predestination Paradox.”) Could a time traveler change something in the past that would prevent them from time traveling in the first place? (The “Temporal Paradox.”)

Some films get around this metaphysical grey area by simply ignoring these messy repercussions, but the most interesting films are those that hinge on them, exploring time travel not as a device to spur conflict but rather as the conflict itself. Here are five of cinema’s best time travel paradoxes, in all their bewildering, inscrutable glory. 

5. Donnie Darko

Paradoxes: bootstrap, predestination.

Writer-director Richard Kelly has made so many missteps in recent years (a losing streak consisting of the triple threat of Domino , Southland Tales and The Box ) it’s easy to forget his debut feature was one of the most original, satisfying and atmospheric puzzles ever put to film.

Though Donnie Darko ‘ s circular plot revolves around time travel, there’s no time machine and the eponymous hero never goes back in time himself. Instead, Donnie (played with striking vulnerability by Jake Gyllenhaal) learns over the course of the film that he can manipulate time, altering events that have already occurred. But its a skill he realizes only because his future self has already used it, setting into motion a spiral of destruction that must be prevented – by learning to manipulate time and setting it in motion all over again.

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It’s a paradox of a plot, but one more concerned with the intricate workings of a generic small town than the mechanics behind time travel. Backed by a nostalgic period setting (and a fantastic soundtrack to match) as well as a host of great actors, including Gyllenhaal’s on and off-screen sister Maggie, a young Seth Rogen and Patrick Swayze in perhaps his greatest, most ironic role, Donnie Darko is a code worth the multiple-viewings needed to decipher it. Hopefully, Kelly begins to show that initial ingenuity again. 

4. 12 Monkeys

Based on Chris Marker’s short La Jetée , but expanding that film’s lyrical meditation on premonition to a sci-fi saga involving bio-terrorists, a humanity destroying virus and a post-apocalyptic future spent underground, Terry Gilliam’s 12 Monkeys centers around a time traveler, played by Bruce Willis, sent to the past to prevent a catastrophe from nearly destroying the entire human race.

Things don’t go as planned, however, as things so rarely do, and Willis’ mission to save the world is hindered by the general perception that he’s a paranoid schizophrenic, an explanation so much more plausible than the reality, Willis begins to believe it himself.

In the end, even with his knowledge of future events, Willis was never truly able to keep them from happening. The paradox of a time traveler sent back to change the event leading him to become a time traveler is inherent, but the real stumbling block is memory itself, which blurs and distorts the past enough to make the same mistakes an inevitability.

3. Back to the Future

Paradoxes: bootstrap, temporal  .

Easily the lightest, most popcorn-friendly film on this list, Back to the Future is, nevertheless, serious about the implications of time travel. When he’s accidentally transported to 1955, Marty McFly (Michael J. Fox) inadvertently interferes with his own parents meeting, an event which leads to Marty’s own mother falling for him and, potentially, his ceasing to exist entirely.

Luckily, Marty and his scientist pal Doc Brown (Christopher Lloyd) devise a plan to get her to fall for the senior McFly, though their adventures in the past have interesting effects on the future. Yet unlike most other time travel films, Marty’s interferences almost universally make things better, saving his parents’ relationship, making his father more successful and putting the bully Biff (Thomas F. Wilson) in his place. In fact, the biggest paradox in Back to the Future isn’t that Marty nearly prevented his own birth, it’s that his parents seem to have forgotten that the man who introduced them looked exactly like their teenaged son.

2. Los Cronocrímenes

Paradoxes: predestination, temporal.

Writer-director Nacho Vigalondo’s Los Cronocrímenes ( Timecrimes ) opens with Héctor, a seemingly average middle-age husband, peering into the forest behind his new home with binoculars and finding not birds or foxes but a young woman undressing. Naturally, he walks into the woods to investigate, only to be stabbed by a man whose face is menacingly obscured by apparently blood soaked bandages.

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Describing what happens next, and how time travel becomes involved, would risk spoiling Vigalondo’s meticulously constructed script, which adds a few new mysteries for each one it solves and piles overlapping timelines upon overlapping timelines. It’s a testament to Vigalondo that he manages to keep so many plates spinning, and in such a precisely choreographed fashion, transforming a taut thriller into an expertly crafted exercise in metaphysics.

Paradoxes: Bootstrap, Predestination,Temporal

Primer is a decidedly small film – it has essentially no special effects, no big action set pieces and it isn’t interested in holding the audience’s hand through its increasingly knotty structure – yet it seems to offer the most fully realized look at the problems inherent in time travel despite of (or, more likely, because of) its impossibly low budget.

Written, directed and starring Mathematics major and engineer Shane Carruth for only $7,000, Primer isn’t a flashy film, but it uses its nuances to great effect, creating a time machine that seems real and plausible and populating its minuscule world with telling clues that hint at the consequences such a device would cause.

Carruth and co-star David Sullivan speak like real engineers, act like real people and react plausibly to an increasingly implausible scenario. Their time machine has rigidly defined rules that, rather than simplify its use, instead make its application that much more complex, leading to a hilariously impenetrable infographic that doesn’t explain the film as much as it complicates it even further.

But Primer ‘s difficulty isn’t a demerit, it’s the reason the film works as well as it does. It rewards and practically demands repeat viewings, each revealing a new thread the viewer hadn’t noticed before. Ostensibly, Carruth and Sullivan are merely experimenting with their incredible discovery (and trying to make some money on the side), but the machine quickly leads to repercussions the protagonists (and the audience) don’t understand and each attempt to set things right only creates an ever more baffling mess to clean up. 

It’s Primer ‘s low-key (and low-budget) charm that sells the fantastic premise as a credible reality, but that credibility ultimately makes Primer ‘s paradoxes even more terrifying to consider. It’s a film that eschews the larger question of “How would time travel change the world?” in favor of one even grimmer: “How would time travel change you?”

Kyle Phaneuf

Kyle Phaneuf likes art rock, genre films and magical realism. He lives in Brooklyn, New York. Music: Can, Jason Molina, Sufjan Stevens Film: Eternal Sunshine of…

Screen Rant

10 best movies about time travel paradoxes.

Time travel movies are as confusing as cinema gets, but when paradoxes enter the stage, it is even easier to get lost in the plot's complexities.

• Time travel movies often contain paradoxes, creating confusion for viewers. Each film's unique rules and characters' reactions to those paradoxes shape the plot.
• Different types of paradoxes exist, such as bootstrap, predestination, and temporal paradoxes, which add depth and complexity to time travel movies.
• Despite the presence of paradoxes, time travel movies can still be entertaining and thought-provoking, providing great storytelling and exploration of love, fate, and the concept of free will.

Time travel can't exist without paradoxes, and neither can science fiction movies about time travel. The way time travel works in each individual film is the first thing a director needs to think through to make sure their movie is consistent and viewers aren't left scratching their heads afterward. Unfortunately, time travel is a tricky subject, and specific rules do not guarantee the absence of paradoxes. The only difference between all time travel movies is that some characters acknowledge the paradox and try to do something about it, and others just ignore its existence and proceed with their goal no matter what.

There are quite a few types of time paradoxes. For instance, a bootstrap paradox is about information or objects that seemingly have no starting point in their timeline; a predestination paradox centers on the cause of someone's time travel being of their own doing in the past; and a temporal paradox revolves around someone's actions in the past that remove the necessity to time travel in the first place. From Interstellar to About Time , time travel movies are riddled with paradoxes , sometimes for the better, providing a great story, and sometimes for the worse, confusing anyone who tries to follow the plot.

The One Problem Sci-Fi Time Travel Movies NEVER Resolve

10 interstellar, cooper gives himself the idea of contacting murth, interstellar.

Christopher Nolan's movies are largely regarded as sci-fi masterpieces, and Interstellar is no exception. The movie's main mystery, the identity of the ghost, was based on a time paradox. At the beginning of the movie, a book fell out of a shelf on its own, and Interstellar 's surprising ending revealed that it was Cooper who made the book fall out via the Tesseract mechanism to send his past self a message . However, Cooper just did what he'd already seen happen, so the concept raises the question of who originally thought of sending the message in this way. Still, this mind-bending time loop worked against all odds.

9 The Terminator

Kyle reese is john connor's father.

James Cameron's epic sci-fi tale is a classic example of a predestination paradox. In The Terminator , Kyle Reese arrived from the future to stop the Terminator from killing Sarah Connor , the mother of his colleague John. Unknowingly, Kyle ended up fathering John when he developed a romantic relationship with Sarah. If the Terminator hadn't been sent to kill Sarah, and Kyle hadn't followed him, John wouldn't have been born, since his father wouldn't have traveled to the past and met his mother. The Terminator 's paradox ending was controversial, and yet the movie managed to make the story entertaining enough to look past it.

8 The Time Traveler's Wife

Henry & clare meet out of order.

The Time Traveler's Wife explores a beautiful notion that love can transcend any boundaries — apparently, that includes the boundaries of time. The movie didn't pretend to be a serious sci-fi title, but it was essentially based on a paradox, specifically, the incorrect order, in which Henry met Clare. Henry first saw Clare when he time traveled to 1991 , but she already knew him because she had met Henry when she was but a child. That is a confusing concept that raises two questions: when their first meeting took place and how they ended up together at all. The Time Traveler's Wife' s paradoxical love story is endearing nevertheless.

7 The Butterfly Effect

Evan causes his own blackouts, the butterfly effect.

The Butterfly Effect is one of the most mind-blowing time travel movies out there, partly because the rules are very specific, and yet they make no sense whatsoever. The movie featured quite a few time paradoxes, but the biggest one was probably the existence of Evan's blackouts. Young Evan experienced blackouts, caused by his adult self's time travel ; adult Evan had to travel to his past because he knew that he was supposed to cause these blackouts. It is unclear how blackouts appeared in the first place. This plot detail makes The Butterfly Effect 's understanding of time circular rather than linear, but the paradox is still there.

6 About Time

Tim prevents the car crash.

About Time 's central point was Tim going back to the past to prevent the car crash, as this resulted in the erasure of his daughter Posy from existence. Although the film is full of inconsistencies, this event in particular showcases the classic paradox of time travel movies — if the car crash motivated Tim to go back in time to prevent it , then in doing so, he erased the very reason for him to travel to the past. Still, Domhnall Gleeson and accidental time-travel expert Rachel McAdams make up a dynamic duo, and it is impossible not to feel for Tim's struggle to help everyone through his gift.

The Protagonist Founds Tenet

The ending of Tenet , Christopher Nolan's sci-fi follow-up to Interstellar , turned out to be even more confusing than that of its predecessor, and not just because of the inverted entropy concept. The entire plot wouldn't exist if the Protagonist hadn't founded the mysterious organization Tenet that helped him in the first place and led him to create it in the future. Tenet explores the notion of a person's future and past intertwining and being part of the same time loop, with no one able to tell what the original cause of the event was. The Protagonist's survival in the film depended solely on himself from the future, who apparently knew that his past self once needed saving.

4 Back to the Future

Marty mcfly has to bring his parents together, back to the future.

Back to the Future is the movie that started the time travel film craze in the first place, and it features one of the most well-known paradoxes in the genre. When Marty traveled to the past, he saved George's life by preventing a car accident, but in the aftermath of the events, he accidentally jeopardized his own existence and had to make his parents fall in love with each other all over again. However, since there was a possibility that Marty would never be born , he should have disintegrated right then and there in the 1950s before he had a chance to fix his own timeline.

Back to the Future Writer Explains Marty’s Parents Plot Hole

3 donnie darko, the plane comes out of nowhere, donnie darko.

Donnie Darko is a stunning dark tale with a timeless message about a person forging their own fate. In the film, the protagonist sent the engine of the plane that would kill his mother and sister back in time and allowed it to fall on him to prevent the catastrophe. However, Donnie Darko 's timeline created a paradox in the fact that the plane existed in the first place. If the future had rewound and the plane had never started to crash, the engine couldn't have been there in the past to kill Donnie in his bedroom. The movie is incredibly thought-provoking in a way that more lighthearted time travel films never are.

2 12 Monkeys

James cole originates the virus.

12 Monkeys' post-apocalyptic nature paints its deterministic narrative in dark colors, adding to the eerie atmosphere of the story at hand. Bruce Willis' James Cole traveled to the past to prevent humanity's extinction , but his every action just led to the devastating virus scenario taking place in the end. 12 Monkeys' predestination paradox lied in the fact that if Cole hadn't planted the idea of the viral outbreak in the past, it wouldn't have happened at all. The protagonist's desperate attempts to stop the apocalypse explore the notion that there is no such thing as free will and that everything in life is already determined.

Aaron & Abe Create A Causal Loop

Primer is decidedly the time-travel movie that has the most rules on the subject and surprisingly follows them through with the help of extremely complicated tech jargon. Aaron and Abe discovered how to create a causal loop and use it to their own advantage. Unfortunately, their actions unraveled in a heap of consequences, and each attempt to fix the problem just made it a lot worse. Primer requires at least two or three watches to fully understand its core concepts and follow the characters' decisions with ease, but it is worth every minute of the time spent.

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Time travel/paradox

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

1. 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: 114,073 | Gross: $0.42M

2. 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: 591,869 | Gross: $58.46M

3. 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: 602,656 | Gross: $66.49M

4. 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: 68,727 | Gross: $0.04M

5. Triangle (2009)

R | 99 min | Fantasy, Mystery, Sci-Fi

Five friends set sail and their yacht is overturned by a strange and sudden storm. A mysterious ship arrives to rescue them, and what happens next cannot be explained.

Director: Christopher Smith | Stars: Melissa George , Joshua McIvor , Jack Taylor , Michael Dorman

Votes: 129,643

6. 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: 304,462 | Gross: $0.07M

7. The Endless (I) (2017)

Not Rated | 111 min | Drama, Fantasy, Horror

As kids, they escaped a UFO death cult. Now, two adult brothers seek answers after an old videotape surfaces and brings them back to where they began.

Directors: Justin Benson , Aaron Moorhead | Stars: Aaron Moorhead , Justin Benson , Callie Hernandez , Tate Ellington

Votes: 49,330 | Gross: $0.27M

8. Lunopolis (2010 Video)

Not Rated | 98 min | Sci-Fi

Two documentary filmmakers accidentally uncover the greatest moon secret the world has never known, and the powerful organization determined to keep it that way.

Director: Matthew Avant | Stars: Dave Potter , Jed Himel , Matthew Avant , Hal Maynor

Votes: 1,515

9. 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

10. 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: 81,677 | Gross: $47.85M

11. Time Bandits (1981)

PG | 110 min | Adventure, Comedy, Fantasy

A young boy accidentally joins a band of time travelling dwarves, as they jump from era to era looking for treasure to steal.

Director: Terry Gilliam | Stars: Sean Connery , Shelley Duvall , John Cleese , Katherine Helmond

Votes: 68,283 | Gross: $42.37M

12. Safety Not Guaranteed (2012)

R | 86 min | Adventure, Comedy, Drama

Three magazine employees head out on an assignment to interview a guy who placed a classified advertisement seeking a companion for time travel.

Director: Colin Trevorrow | Stars: Aubrey Plaza , Mark Duplass , Jake Johnson , Karan Soni

Votes: 130,823 | Gross: $4.01M

13. 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: 385,499 | Gross: $15.32M

14. 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

15. Time After Time (2017)

TV-14 | 60 min | Drama, Sci-Fi

The adventures of young H.G. Wells and his time machine.

Stars: Freddie Stroma , Josh Bowman , Will Chase , Genesis Rodriguez

Votes: 3,977

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: 449,672 | Gross: $56.82M

17. Before I Fall (2017)

PG-13 | 98 min | Drama, Fantasy, Mystery

February 12 is just another day in Sam's charmed life, until it turns out to be her last. Stuck reliving her last day over and over, Sam untangles the mystery around her death and discovers everything she's losing.

Director: Ry Russo-Young | Stars: Zoey Deutch , Halston Sage , Cynthy Wu , Medalion Rahimi

Votes: 56,778 | Gross: $12.24M

18. Enter Nowhere (2011)

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

Three strangers arrive one by one at a mysterious cabin in the middle of nowhere only to learn they've been brought together for a reason.

Director: Jack Heller | Stars: Katherine Waterston , Scott Eastwood , Sara Paxton , Shaun Sipos

Votes: 12,618

19. The Fare (I) (2018)

16+ | 82 min | Fantasy, Mystery, Romance

A taxi driver finds himself trapped in a time loop. He picks up a pretty woman in a rural area at night. Later she disappears. The loop starts over, when he switches the meter back to vacant.

Director: D.C. Hamilton | Stars: Gino Anthony Pesi , Brinna Kelly , Jason Stuart , Jon Jacobs

Votes: 3,436

20. Time Trap (2017)

Not Rated | 87 min | Action, Adventure, Mystery

A professor enters a cave and goes missing. Some of his students come looking for him and get trapped in the cave as well.

Directors: Mark Dennis , Ben Foster | Stars: Reiley McClendon , Cassidy Gifford , Brianne Howey , Olivia Draguicevich

Votes: 43,437

21. The Map of Tiny Perfect Things (2021)

PG-13 | 98 min | Comedy, Fantasy, Romance

Two teens live the same day repeatedly, enabling them to create a map of things to remember.

Director: Ian Samuels | Stars: Kathryn Newton , Kyle Allen , Jermaine Harris , Anna Mikami

Votes: 27,828

22. Travelers (2016–2018)

TV-MA | 45 min | Drama, Mystery, Sci-Fi

Hundreds of years from now, surviving humans discover how to send consciousness back through time, into people of the 21st century, while attempting to change the path of humanity.

Stars: Eric McCormack , MacKenzie Porter , Nesta Cooper , Jared Abrahamson

Votes: 64,302

23. 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: 646,340 | Gross: $57.14M

24. Boss Level (2020)

TV-MA | 100 min | Action, Adventure, Comedy

Trapped in a time loop that constantly repeats the day of his murder, a former special forces agent must unlock the mystery behind his untimely demise.

Director: Joe Carnahan | Stars: Frank Grillo , Mel Gibson , Naomi Watts , Michelle Yeoh

Votes: 79,075

25. Palm Springs (2020)

R | 90 min | Comedy, Fantasy, Mystery

Stuck in a time loop, two wedding guests develop a budding romance while living the same day over and over again.

Director: Max Barbakow | Stars: Andy Samberg , Cristin Milioti , J.K. Simmons , Peter Gallagher

Votes: 182,427

26. Late for Dinner (1991)

PG | 99 min | Drama, Sci-Fi

Two young men evading the police for a crime they didn't commit are cryogenically frozen in the early 1960s. The next thing they know is that they are in a strange new world (thirty years on).

Director: W.D. Richter | Stars: Peter Berg , Brian Wimmer , Marcia Gay Harden , Cassy Friel

Votes: 1,901 | Gross: $8.91M

27. Sliders (1995–2000)

TV-PG | 60 min | Adventure, Fantasy, Sci-Fi

A boy genius and his comrades travel to different parallel universes, trying to find their way back home.

Stars: Jerry O'Connell , Sabrina Lloyd , John Rhys-Davies , Cleavant Derricks

Votes: 20,379

28. Orphan Black (2013–2017)

TV-MA | 44 min | Drama, Sci-Fi, Thriller

A streetwise hustler is pulled into a compelling conspiracy after witnessing the suicide of a girl who looks just like her.

Stars: Tatiana Maslany , Dylan Bruce , Jordan Gavaris , Kevin Hanchard

Votes: 116,896

29. Timeless (2016–2018)

TV-14 | 60 min | Action, Adventure, Drama

An unlikely trio travels through time to battle unknown criminals and protect history as we know it.

Stars: Abigail Spencer , Matt Lanter , Malcolm Barrett , Paterson Joseph

Votes: 47,067

30. Manifest (2018–2023)

TV-14 | 43 min | Drama, Mystery, Sci-Fi

When a commercial airliner suddenly reappears after being missing for five years, those aboard must reintegrate into society.

Stars: Melissa Roxburgh , Josh Dallas , J.R. Ramirez , Luna Blaise

Votes: 91,170

31. Outlander (2014– )

TV-MA | 60 min | Drama, Fantasy, Romance

Claire Beauchamp Randall, a nurse in World War II, mysteriously goes back in time to Scotland in 1743. There, she meets a dashing Highland warrior and gets drawn into an epic rebellion.

Stars: Caitríona Balfe , Sam Heughan , Sophie Skelton , Richard Rankin

Votes: 178,893

32. The One I Love (2014)

R | 91 min | Comedy, Drama, Fantasy

A troubled couple vacate to a beautiful getaway, but bizarre circumstances further complicate their situation.

Director: Charlie McDowell | Stars: Mark Duplass , Elisabeth Moss , Ted Danson , Kiana Cason

Votes: 43,499 | Gross: $0.51M

33. Dark (2017–2020)

TV-MA | 60 min | Crime, Drama, Mystery

A family saga with a supernatural twist, set in a German town where the disappearance of two young children exposes the relationships among four families.

Stars: Louis Hofmann , Karoline Eichhorn , Lisa Vicari , Maja Schöne

Votes: 443,253

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