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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."

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Articles on Time travel

Displaying 1 - 20 of 25 articles.

Is time travel even possible? An astrophysicist explains the science behind the science fiction

Adi Foord , University of Maryland, Baltimore County

Are black holes time machines? Yes, but there’s a catch

Sam Baron , Australian Catholic University

What are wormholes? An astrophysicist explains these shortcuts through  space-time

Dejan Stojkovic , University at Buffalo

Curious Kids: is it possible to see what is happening in distant solar systems now?

Jacco van Loon , Keele University

Can we time travel? A theoretical physicist provides some answers

Peter Watson , Carleton University

Curious Kids: what would happen if someone moved at twice the speed of light?

Time travel could be possible, but only with parallel timelines

Barak Shoshany , Brock University

Why does gravity pull us down and not up?

Mario Borunda , Oklahoma State University

New warp drive research dashes faster than light travel dreams – but reveals stranger possibilities

Curious Kids: is time travel possible for humans?

Lucy Strang , The University of Melbourne and Jacqueline Bondell , Swinburne University of Technology

Rotating black holes may serve as gentle portals for hyperspace travel

Gaurav Khanna , UMass Dartmouth

The great movie scenes: Back to the Future

Bruce Isaacs , University of Sydney

Time travel is possible – but only if you have an object with infinite mass

Stephen Hawking’s final book suggests time travel may one day be possible – here’s what to make of it

Peter Millington , University of Nottingham

Like a TARDIS in your head, memory helps you travel through time

Alice Mason , The University of Western Australia

Time travel: a conversation between a scientist and a literature professor

Richard Bower , Durham University and Simon John James , Durham University

Star Trek’s version of time travel is more realistic than most sci fi

Lloyd Strickland , Manchester Metropolitan University

Anthill 1: About time

Annabel Bligh , The Conversation and Gemma Ware , The Conversation

How to build a time machine

Steve Humble , Newcastle University

It’s Back to the Future Day today – so what are the next future predictions?

Michael Cowling , CQUniversity Australia ; Hamza Bendemra , Australian National University ; Justin Zobel , The University of Melbourne ; Philip Branch , Swinburne University of Technology ; Robert Merkel , Monash University ; Thas Ampalavanapillai Nirmalathas , The University of Melbourne , and Toby Walsh , Data61

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October 21, 1999

According to current physical theory, is it possible for a human being to travel through time?

As several respondents noted, we constantly travel through time--just forward, and all at the same rate. But seriously, time travel is more than mere fantasy, as noted by Gary T. Horowitz, a professor of physics at the University of California at Santa Barbara:

"Perhaps surprisingly, this turns out to be a subtle question. It is not obviously ruled out by our current laws of nature. Recent investigations into this question have provided some evidence that the answer is no, but it has not yet been proven to be impossible."

Even the slight possibility of time travel exerts such fascination that many physicists continue to study not only whether it may be possible but also how one might do it.

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One of the leading researchers in this area is William A. Hiscock, a professor of physics at Montana State University. Here are his thoughts on the matter:

"Is it possible to travel through time? To answer this question, we must be a bit more specific about what we mean by traveling through time. Discounting the everyday progression of time, the question can be divided into two parts: Is it possible, within a short time (less than a human life span), to travel into the distant future? And is it possible to travel into the past?

"Our current understanding of fundamental physics tells us that the answer to the first question is a definite yes, and to the second, maybe.

"The mechanism for traveling into the distant future is to use the time-dilation effect of Special Relativity, which states that a moving clock appears to tick more slowly the closer it approaches the speed of light. This effect, which has been overwhelmingly supported by experimental tests, applies to all types of clocks, including biological aging.

"If one were to depart from the earth in a spaceship that could accelerate continuously at a comfortable one g (an acceleration that would produce a force equal to the gravity at the earth's surface), one would begin to approach the speed of light relative to the earth within about a year. As the ship continued to accelerate, it would come ever closer to the speed of light, and its clocks would appear to run at an ever slower rate relative to the earth. Under such circumstances, a round trip to the center of our galaxy and back to the earth--a distance of some 60,000 light-years--could be completed in only a little more than 40 years of ship time. Upon arriving back at the earth, the astronaut would be only 40 years older, while 60,000 years would have passed on the earth. (Note that there is no 'twin paradox,' because it is unambiguous that the space traveler has felt the constant acceleration for 40 years, while a hypothetical twin left behind on a spaceship circling the earth has not.)

"Such a trip would pose formidable engineering problems: the amount of energy required, even assuming a perfect conversion of mass into energy, is greater than a planetary mass. But nothing in the known laws of physics would prevent such a trip from occurring.

"Time travel into the past, which is what people usually mean by time travel, is a much more uncertain proposition. There are many solutions to Einstein's equations of General Relativity that allow a person to follow a timeline that would result in her (or him) encountering herself--or her grandmother--at an earlier time. The problem is deciding whether these solutions represent situations that could occur in the real universe, or whether they are mere mathematical oddities incompatible with known physics. No experiment or observation has ever indicated that time travel is occurring in our universe. Much work has been done by theoretical physicists in the past decade to try to determine whether, in a universe that is initially without time travel, one can build a time machine--in other words, if it is possible to manipulate matter and the geometry of space-time in such a way as to create new paths that circle back in time.

"How could one build a time machine? The simplest way currently being discussed is to take a wormhole (a tunnel connecting spatially separated regions of space-time) and give one mouth of the wormhole a substantial velocity with respect to the other. Passage through the wormhole would then allow travel to the past.

"Easily said--but where does one obtain a wormhole? Although the theoretical properties of wormholes have been extensively studied over the past decade, little is known about how to form a macroscopic wormhole, large enough for a human or a spaceship to pass through. Some speculative theories of quantum gravity tell us that space-time has a complicated, foamlike structure of wormholes on the smallest scales--10^-33 centimeter, or a billion billion times smaller than an electron. Some physicists believe it may be possible to grab one of these truly microscopic wormholes and enlarge it to usable size, but at present these ideas are all very hypothetical.

"Even if we had a wormhole, would nature allow us to convert it into a time machine? Stephen Hawking has formulated a "Chronology Protection Conjecture," which states that the laws of nature prevent the creation of a time machine. At the moment, however, this is just a conjecture, not proven.

"Theoretical physicists have studied various aspects of physics to determine whether this law or that might protect chronology and forbid the building of a time machine. In all the searching, however, only one bit of physics has been found that might prohibit using a wormhole to travel through time. In 1982, Deborah A. Konkowski of the U.S. Naval Academy and I showed that the energy in the vacuum state of a massless quantized field (such as the photon) would grow without bound as a time machine is being turned on, effectively preventing it from being used. Later studies by Hawking and Kip S. Thorne of Caltech have shown that it is unclear whether the growing energy would change the geometry of space-time rapidly enough to stop the operation of the time machine. Recent work by Tsunefumi Tanaka of Montana State University and myself, along with independent research by David Boulware of the University of Washington, has shown that the energy in the vacuum state of a field having mass (such as the electron) does not grow to unbounded levels; this finding indicates there may be a way to engineer the particle physics to allow a time machine to work.

"Perhaps the biggest surprise of the work of the past decade is that it is not obvious that the laws of physics forbid time travel. It is increasingly clear that the question may not be settled until scientists develop an adequate theory of quantum gravity."

John L. Friedman of the physics department at the University of Wisconsin at Milwaukee has also given this subject a great deal of consideration:

"Special relativity implies that people or clocks at rest (or not accelerating) age more quickly than partners traveling on round-trips in which one changes direction to return to one's partner. In the world's particle accelerators, this prediction is tested daily: Particles traveling in circles at nearly the speed of light decay more slowly than those at rest, and the decay time agrees with theory to the high precision of the measurements.

"Within the framework of Special Relativity, the fact that particles cannot move faster than light prevents one from returning after a high-speed trip to a time earlier than the time of departure. Once gravity is included, however, spacetime is curved, so there are solutions to the equations of General Relativity in which particles can travel in paths that take them back to earlier times. Other features of the geometries that solve the equations of General Relativity include gravitational lenses, gravitational waves and black holes; the dramatic explosion of discoveries in radio and X-ray astronomy during the past two decades has led to the observation of gravitational lenses and gravitational waves, as well as to compelling evidence for giant black holes in the centers of galaxies and stellar-sized black holes that arise from the collapse of dying stars. But there do not appear to be regions of spacetime that allow time travel, raising the fundamental question of what forbids them--or if they really are forbidden.

"A recent surprise is that one can circumvent the 'grandfather paradox,' the idea that it is logically inconsistent for particle paths to loop back to earlier times, because, for example, a granddaughter could go back in time to do away with her grandfather. For several simple physical systems, solutions to the equations of physics exist for any starting condition. In these model systems, something always intervenes to prevent inconsistency analogous to murdering one's grandfather.

"Then why do there seem to be no time machines? Two different answers are consistent with our knowledge. The first is simply that the classical theory has a much broader set of solutions than the correct theory of quantum gravity. It is not implausible that causal structure enters in a fundamental way in quantum gravity and that classical spacetimes with time loops are spurious--in other words, that they do not approximate any states of the complete theory. A second possible answer is provided by recent results that go by the name chronology protection: One supposes that quantum gravity allows microscopic structures that violate causality, and one shows that the character of macroscopic matter forbids the existence of regions with macroscopically large time loops. To create a time machine would require negative energy, and quantum mechanics appears to allow only extremely small regions of negative energy. And the forces needed to create an ordinary-sized region with time loops appear to be extremely large.

"To summarize: It is very likely that the laws of physics rule out macroscopic time machines, but possible that spacetime is filled with microscopic time loops.

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Time Travel: Probability and Impossibility

Reader in Philosophy

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There are various arguments for the metaphysical impossibility of time travel, e.g. it’s impossible because objects could then be in two places at once, or it’s impossible because some objects could bring about their own existence. This book argues that no such argument is sound and that time travel is metaphysically possible. The main focus is on the Grandfather Paradox: if someone could go back in time, they could (impossibly!) kill their own grandfather before he met their grandmother, thus time travel is impossible. This book argues that, in such a case, the time traveller would have the ability to do the impossible (so they could kill their grandfather) even though those impossibilities will never come about (so they won’t kill their grandfather). The remainder of the book explores the ramifications of this view, discussing issues in probability and decision theory. It ends by laying out the dangers of time travel and why, even though no time machines currently exist, we should pay extra special care to ensure that nothing, no matter how small or microscopic, ever travels in time.

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Discussions of the nature of time, and of various issues related to time, have always featured prominently in philosophy, but they have been especially important since the beginning of the twentieth century. This article contains a brief overview of some of the main topics in the philosophy of time—(1) fatalism; (2) reductionism and Platonism with respect to time; (3) the topology of time; (4) McTaggart’s argument; (5) the A-theory and the B-theory; (6) presentism, eternalism, and the growing block theory; (7) the 3D/4D debate about persistence; (8) the dynamic and the static theory; (9) the moving spotlight theory; (10) time travel; (11) time and physics and (12) time and rationality. We include some suggestions for further reading on each topic and a bibliography.

Note: This entry does not discuss the consciousness, perception, experience, or phenomenology of time. A historical overview and general presentation of the various views is available in the entry on temporal consciousness . Further coverage can be found in the SEP entry on the experience and perception of time . For those interested specifically in phenomenological views, see the entries on Husserl (Section 6), and Heidegger (Section 2: Being and Time).

1. Fatalism

2. reductionism and platonism with respect to time, 3. the topology of time, 4. mctaggart’s argument, 5. the a-theory and the b-theory, 6. presentism, eternalism, and the growing block theory, 7. three-dimensionalism and four-dimensionalism, 8. the dynamic and the static theory, 9. the moving spotlight theory, 10. time travel, 11. time and physics, 12. time and rationality, other internet resources, related entries.

Many logical questions about time historically arose from questions about freedom and determinism—in particular worries about fatalism. Fatalism can be understood as the doctrine that whatever will happen in the future is already unavoidable (where to say that an event is unavoidable is to say that no agent is able to prevent it from occurring). Here is a typical argument for fatalism:

The conclusion appears shocking. Future moral catastrophes are unavoidable. Every weighty decision that now feels up to you is already determined.

The argument for fatalism makes some significant metaphysical assumptions that raise more general questions about logic, time, and agency.

For example, Premise (1) assumes that propositions describing the future do not come into or go out of existence. It assumes that there are propositions now that can accurately represent every future way things might go. This is a non-trivial logical assumption. You might, for instance, think that different times becoming present and actual (like perhaps possible worlds) have different associated sets of propositions that become present and actual.

Premise (2) appears to be a fundamental principle of semantics, sometimes referred to as the Principle of Bivalence.

The rationale for premise (4) is that it appears no one is able to make a true prediction turn out false. (4) assumes that one and the same proposition does not change its truth value over time. The shockingness of the conclusion also depends on identifying meaningful agency with the capacity to make propositions come out true or false.

A proper discussion of fatalism would include a lengthy consideration of premises (1) and (4), which make important assumptions about the nature of propositional content and the nature of agency. That would take us beyond the scope of this article. For our purposes, it is important to note that many writers have been motivated by this kind of fatalist argument to deny (2), the Principle of Bivalence. According to this line, there are many propositions—namely, propositions about events that are both in the future and contingent—that are neither true nor false right now. Consider the proposition that you will have lunch tomorrow. Perhaps that proposition either has no truth value right now, or else has a third truth value: indeterminate. When the relevant time comes, and you either have lunch or don’t, then the proposition will come to be either true or false, and from then on that proposition will forever retain that determinate truth value.

This strategy for rejecting fatalism is sometimes referred to as the “Open Future” response. The Open Future response presupposes that a proposition can have a truth value, but only temporarily—truth values for complete propositions can change as time passes and the world itself changes. This raises further questions about the correct way to link up propositions, temporal passage and truth values. For example, which of the following formulas expresses a genuine proposition about the present?

Tensed Proposition: “Sullivan is eating a burrito”.

Tenseless Proposition: “Sullivan eats a burrito at <insert present time stamp>”.

The tensed proposition will no longer be true when Sullivan finishes her lunch. So it has, at best, a temporary truth value. The tenseless proposition expresses something like “Sullivan eats a burrito at 3pm on July 20th 2019”. That proposition is always true.

Some philosophers argue that only the latter, eternally true kind of proposition could make sense of how we use propositions to reason over time. We need propositions to have stable truth values if we are to use them as the contents of thoughts and communication. Other philosophers—particularly those who believe that reality itself changes over time—think that tensed propositions are needed to accurately reason about the world. We’ll return to these issues in Section 4 and Section 5 .

Suggestions for Further Reading: Aristotle, De Interpretatione , Ch. 9; Barnes and Cameron 2009; Boethius, The Consolation of Philosophy , Book V; Crisp 2007; Evans 1985; Lewis 1986; Markosian 1995; McCall 1994; Miller 2005; Richard 1981; Sullivan 2014; Taylor 1992; Torre 2011; Van Inwagen 1983.

What if one day things everywhere ground to a halt? What if birds froze in mid-flight, people froze in mid-sentence, and planets and subatomic particles alike froze in mid-orbit? What if all change, throughout the entire universe, completely ceased for a period of, say, one year? Is such a thing possible?

If the answer to this last question is “yes”—if it is possible for there to be time without change—then time is in some important sense independent of the events within time. Other ways of investigating whether time is independent of the events within time include asking whether all of the physical processes that happen in time could happen at a faster or slower rate, and asking whether all events could have happened slightly earlier or later in time. After all, if every physical process could suddenly happen twice as fast, or if every event could take place slightly earlier or later in time, then it follows that in some important sense time can remain the same even if the way that events are distributed in time changes wholesale.

Aristotle and Leibniz, among others, have argued that time is not independent of the events that occur in time. This view is typically called either “reductionism with respect to time” or “relationism with respect to time”, since according to this view, all talk that appears to be about time can somehow be reduced to talk about temporal relations among things and events. The opposing view, normally referred to either as “Platonism with respect to time” or “substantivalism with respect to time” or “absolutism with respect to time”, has been defended by Plato, Newton, and others. On this view, time is like an empty container into which things and events may be placed; but it is a container that is independent of what (if anything) is placed in it.

Another way to present this distinction is to say that those like Plato and Newton who think that time is independent of the events that occur in time believe in “absolute time”. Those like Aristotle and Leibniz, who think that time is not independent of the events that occur in time, deny the existence of absolute time, though they still endorse “relative time”, where relative time is nothing over and above the temporal relations between events.

These views about time are closely connected to views about space and about motion. Most obviously, these views about time have straightforward spatial analogues—one may be a substantivalist about space (and thus endorse the existence of absolute space in addition to spatial relations between things), or one may be a relationist about space (and thus deny the existence of absolute space). Substantivalism and relationism about time have traditionally been taken to stand or fall with their spatial counterparts. In addition, the choice between substantivalism and relationism about space and time has consequences for your theory of motion. If you are a relationist about space and time then you must also be a relationist about motion: all motion is motion relative to something. If you are a substantivalist about space and time, you will endorse, in addition to relative motion, the notion of absolute motion, where absolute motion is motion relative to absolute space and time. If you are a substantivalist, in addition to facts about whether and how fast a train car is moving relative to the track, whether and how fast it is moving relative to the cars, and so on, there will also be a fact about whether and how fast the train car is really moving—whether and how fast it is moving relative to absolute space and time.

Why would someone endorse the existence of absolute time? One reason is that the empty container metaphor has a lot of intuitive appeal. Another reason is that some philosophers have thought that there must be such a thing as absolute motion—as opposed to merely relative motion—in order to explain certain physical phenomena, like the forces felt during acceleration. Newton had an especially famous argument along these lines involving a spinning bucket of water—the entry on Newton’s views on space, time , and motion has a careful discussion of this argument.

Why would someone deny the existence of absolute time? Some relationists have put forward arguments that are supposed to show that absolute space and time are philosophically problematic in some important way. Perhaps most famously, Leibniz argued that the existence of absolute space or time would lead to violations of the principle of sufficient reason and violations of the identity of indiscernibles.

In order to see why, consider two ways of describing the way things could be. On the one hand, everything is as it actually is. On the other, every event happens one second later than it actually does, but is otherwise exactly the same. If there is such a thing as absolute time then these two descriptions would pick out distinct possible worlds. But this, Leibniz claimed, would violate the principle of sufficient reason. For given that the actual world and the one-second-late world are exactly the same except for where things are located in absolute time, there could not (at least according to Leibniz) be any reason why one exists rather than the other. Moreover, Leibniz claimed, the actual world and the one-second-late world are indistinguishable; so if they were in fact distinct possible worlds, that would violate the principle that if two things are indistinguishable, then they are identical.

Leibniz’s arguments are examples of arguments that attempt to identify something philosophically problematic with absolute time and space. Perhaps more generally, many philosophers have been moved by the idea that even if absolute time and space are not problematic in a way that makes them unacceptable, they are still the kinds of things that we should do without if we can. This kind of attitude can be motivated by a straightforward kind of parsimony—we should always make do with the fewest types of entities possible. Or it can be motivated by a more specific worry about the nature of absolute space and time. You might, for instance, be especially loath to admit unobservable entities into your ontology—you are willing to admit them if you must, but you would rather eliminate them wherever possible. As absolute space and time are unobservable, someone who endorses this attitude will be inclined to think there are no such things.

Suggestions for Further Reading : Alexander 1956; Ariew 2000; Arntzenius 2012; Coope 2001; Mitchell 1993; Newton, Philosophical Writings ; Newton-Smith 1980; Shoemaker 1969.

It’s natural to think that time can be represented by a line. But a line has a shape. What shape should we give to the line that represents time? This is a question about the topology, or structure, of time.

One natural way to answer our question is to say that time should be represented by a single, straight, non-branching, continuous line that extends without end in each of its two directions. This is the “standard topology” for time. But for each of the features attributed to time in the standard topology, two interesting questions arise: (a) does time in fact have that feature? and (b) if time does have the feature in question, is this a necessary or a contingent fact about time?

Questions about the topology of time appear to be closely connected to the issue of Platonism versus relationism with respect to time. For if relationism is true, then it seems likely that time’s topological features will depend on contingent facts about the relations among things and events in the world, whereas if Platonism is true, so that time exists independently of whatever is in time, then time will presumably have its topological properties as a matter of necessity. But even if we assume that Platonism is true, it’s not clear exactly what topological properties should be attributed to time.

Consider the question of whether time should be represented by a line without a beginning (so a line, rather than a line segment). Aristotle has argued (roughly) that time cannot have a beginning on the grounds that in order for time to have a beginning, there must be a first moment of time, but that in order to count as a moment of time, that allegedly first moment would have to come between an earlier period of time and a later period of time, which is inconsistent with its being the first moment of time. (Aristotle argues in the same way that time cannot have an end.)

Aristotle’s argument may or may not be a good one, but even if it is unsound, many people will feel, purely on intuitive grounds, that the idea of time having a beginning (or an end) just does not make sense. And here we have an excellent illustration of what is at stake in the controversy over whether time has its topological properties as a contingent matter or as a matter of necessity. For suppose we come to have excellent evidence that the universe itself had a beginning in time. (This seems like the kind of thing that could be supported by empirical evidence in cosmology.) This would still leave open the question of whether the beginning of the universe occurred after an infinitely long period of “empty” time, or, instead, coincided with the beginning of time itself. There are interesting and plausible arguments for each of these positions.

It is also worth asking whether time must be represented by a single line. Perhaps we should take seriously the possibility of time’s consisting of multiple time streams, each one of which is isolated from each other, so that every moment of time stands in temporal relations to other moments in its own time stream, but does not bear any temporal relations to any moment from another time stream. Likewise we can ask whether time could correspond to a branching line (perhaps to allow for the possibility of time travel or to model an open future), or to a closed loop, or to a discontinuous line. And we can also wonder whether one of the two directions of time is in some way privileged, in a way that makes time itself asymmetrical. (We say more about this last option in particular in the section on time and physics.)

Suggestions for Further Reading: (1) On the beginning and end of time: Aristotle, Physics , Bk. VIII; Kant, The Critique of Pure Reason (especially pp. 75ff); Newton-Smith 1980, Ch. V. (2) On the linearity of time: Newton-Smith 1980, Ch. III; Swinburne 1966, 1968. (3) On the direction of time: Price 1994, 1996; Savitt 1995; and Sklar 1974. (4) On all of these topics: Newton-Smith 1980.

In a famous paper published in 1908, J.M.E. McTaggart argued that there is in fact no such thing as time, and that the appearance of a temporal order to the world is a mere appearance. Other philosophers before and since (including, especially, F.H. Bradley) have argued for the same conclusion. We will focus here only on McTaggart’s argument against the reality of time, which has been by far the most influential.

McTaggart begins his argument by distinguishing two ways in which positions in time can be ordered. First, he says, positions in time can be ordered according to their possession of properties like being two days future, being one day future, being present, being one day past, etc. These properties are often referred to now as “ A properties” because McTaggart calls the series of times ordered by these properties “the A series”. But he says that positions in time can also be ordered by two-place relations like two days earlier than, one day earlier than, simultaneous with, etc. These relations are now often called “ B relations” because McTaggart calls the series of times ordered by these relations “the B series”.

McTaggart argues that the B series alone does not constitute a proper time series; the A series is essential to time. His reason for this is that he assumes change is essential to time, and the B series without the A series does not involve genuine change (since B series positions are forever “fixed”, whereas A series positions are constantly changing).

McTaggart also argues that the A series is inherently contradictory. For, he says, the different A properties are incompatible with one another. No time can be both future and past, for example. Nevertheless, he insists, each time in the A series must possess all of the different A properties, since a time that is future will be present and then will be past. McTaggart concludes that, since neither the A-series nor the B-series can order the time series, time is unreal.

One response to this argument that McTaggart anticipates involves claiming that it’s not true of any time, t , that t is both future and past. Rather, the objection goes, we must say that it was future at some moment of past time and will be past at some moment of future time. But this objection fails, according to McTaggart, because the additional times that are invoked in order to explain t ’s possession of the incompatible A properties must themselves possess all of the same A properties (as must any further times invoked on account of these additional times, and so on ad infinitum ). Thus, according to McTaggart, we never resolve the original contradiction inherent in the A series, but, instead, merely generate an infinite regress of more and more contradictions.

McTaggart’s argument has had staying power because it organizes crucial debates about the metaphysics of temporal passage, because it hints at how those debates connect to further debates about where evidence for the time series and the nature of change come from, and because the difference between A-theoretic and B-theoretic approaches to the debate has continued in the intervening century.

Suggestions for Further Reading: Bradley 1893; Dyke 2002; McTaggart 1908; Mellor 1998; Prior 1967, 1968.

In Section 1 , we introduced the distinction between a tensed proposition and a tenseless proposition. Tensed propositions can fully and accurately describe the world, but nevertheless change truth value over time. Tenseless propositions, on the other hand, are always true or always false—they reference a particular time in the proposition and never change. Propositions represent ways reality could be. So, which view of propositions we adopt depends on what we think it means for reality itself to undergo change.

In section 4 , we discussed McTaggart’s distinction between time conceived of as a B-series (events ordered by which come before and which come after) and time conceived of as an A-series (events ordered by which are present, which are past, and which are future). Though not particularly creative as names, the A/B distinction has stuck around as a way of classifying theories of change.

B-theorists think all change can be described in before-after terms. They typically portray spacetime as a spread-out manifold with events occurring at different locations in the manifold (often assuming a substantivalist picture). Living in a world of change means living in a world with variation in this manifold. To say that a certain autumn leaf changed color is just to say that the leaf is green in an earlier location of the manifold and red in a later location. The locations, in these cases, are specific times in the manifold. And all of the metaphysically important facts about change can be captured by tenseless propositions like “The leaf is red at October 7, 2019”. “The leaf is not red at September 7, 2019”.

A-theorists, on the other hand, believe that at least some important forms of change require classifying events as past, present or future. And accurately describing this kind of change requires some tensed propositions—there is a way reality is (now, presently) which is complete but was different in the past and also will be different in the future. These tensed propositions also explain why we tend to attribute significance to the past-present-future distinction. For example, you might think the A-theorist is in a better position to explain why we care whether a horrible event is already in the past versus still in the future. Some A-theorists will argue that we aren’t concerned with location—we care that the event is over with in reality.

Note, also, there is a significant range of views within the A-theory camp about whether there is a spacetime manifold (Moving Spotlighters think there is), or whether only present events are real (the presentist view), or whether only present and past events are real (the Growing Block view). We say more about all of these views below. A-theorists also debate whether objects themselves undergo A-theoretic change or whether it is only entire regions of spacetime that change this way.

A-theorists and B-theorists appeal to different sources of evidence for their different views of passage. A-theorists typically emphasize how psychologically we seem to perceive a world of robust passage or “flow” of time. In physics, the laws of thermodynamics seem to imply a strong past-to-future direction to time. And quantum mechanics seems to identify an important sense of simultaneity, which could be identified with presentness (see section 11 below). Finally many commonsense ways of thinking of change seem to rely on A-theory descriptions of passage. For instance, they will use the fact that we care so much about whether bad events are past as evidence that there are ineliminable tensed propositions and those propositions represent ineliminable A-properties.

B-theorists typically emphasize how special relativity eliminates the past/present/future distinction from physical models of space and time. Thus what seems like an awkward way to express facts about time in ordinary English is actually much closer to the way we express facts about time in physics. Moreover, thinking of change in tenseless terms makes it easier to describe in a logically consistent way how objects survive change—objects have properties only relative to particular times, so there is no worry about attributing absolutely inconsistent properties to anything. We’ll consider some of these arguments in more detail in the remaining sections of this entry, as we consider more specific variations on A-theories and B-theories of time.

Suggestions for Further Reading: For general discussion of The A theory and The B theory: Emery 2017; Le Poidevin 1998; Le Poidevin and McBeath 1993; Markosian 1993; Maudlin 2007 (especially Chapter 4); Mellor 1998; Paul 2010; Prior 1959 [1976], 1962 [1968], 1967, 1968, 1970, 1996; Sider 2001; Skow 2009; Smart 1963, 1949; Smith 1993; Sullivan 2012a; Williams 1951; Zimmerman 2005; Zwart 1976.

A further question that you might ask about time is an ontological question. Does whether something is past, present, or future make a difference to whether it exists? And how do these ontological theses connect to debates about the A-theory and the B-theory?

According to presentism, only present objects exist. More precisely, presentism is the view that, necessarily, it is always true that only present objects exist. Even more precisely, no objects exist in time without being present (abstract objects might exist outside of time). (Note that some writers have used the name differently, and unless otherwise indicated, what is meant here by “present” is temporally present, as opposed to spatially present.) According to presentism, if we were to make an accurate list of all the things that exist—i.e., a list of all the things that our most unrestricted quantifiers range over—there would be not a single merely past or merely future object on the list. Thus, you and the Taj Mahal would be on the list, but neither Socrates nor any future Martian outposts would be included. (Assuming, that is, both (i) that each person is identical to his or her body, and (ii) that Socrates’s body ceased to be present—thereby going out of existence, according to presentism—shortly after he died. Those who reject the first of these assumptions should simply replace the examples in this article involving allegedly non-present people with appropriate examples involving the non-present bodies of those people.) And it is not just Socrates and future Martian outposts, either—the same goes for any other putative object that lacks the property of being present. No such objects exist, according to presentism.

There are different ways to oppose presentism—that is, to defend the view that at least some non-present objects exist. One version of non-presentism is eternalism, which says that objects from both the past and the future exist. According to eternalism, non-present objects like Socrates and future Martian outposts exist now, even though they are not currently present. We may not be able to see them at the moment, on this view, and they may not be in the same space-time vicinity that we find ourselves in right now, but they should nevertheless be on the list of all existing things.

It might be objected that there is something odd about attributing to a non-presentist the claim that Socrates exists now, since there is a sense in which that claim is clearly false. In order to forestall this objection, let us distinguish between two senses of “ x exists now”. In one sense, which we can call the temporal location sense, this expression is synonymous with “ x is present”. The non-presentist will admit that, in the temporal location sense of “ x exists now”, it is true that no non-present objects exist now. But in the other sense of “ x exists now”, which we can call the ontological sense, to say that “ x exists now” is just to say that x is now in the domain of our most unrestricted quantifiers. Using the ontological sense of “exists”, we can talk about something existing in a perfectly general sense, without presupposing anything about its temporal location. When we attribute to non-presentists the claim that non-present objects like Socrates exist right now, we commit non-presentists only to the claim that these non-present objects exist now in the ontological sense (the one involving the most unrestricted quantifiers).

According to the eternalist, temporal location does not affect ontology. But according to a somewhat less popular version of non-presentism, temporal location does matter when it comes to ontology, because only objects that are either past or present exist. On this view, which is often called the growing block theory, the correct ontology is always increasing in size, as more and more things are added on to the leading “present” edge (temporally speaking). (Note, however, that the growing block theory does not involve any commitment to four-dimensionalism as discussed in section 7 . In this way, the name “growing block” is somewhat misleading and the view is probably better described as the growing universe theory.) Both presentism and the growing block theory are versions of the A-theory.

Despite the claim by some presentists that theirs is the commonsense view, it is pretty clear that there are some major problems facing presentism (and, to a lesser extent, the growing block theory; but in what follows we will focus on the problems facing presentism). One problem has to do with what appears to be perfectly meaningful talk about non-present objects, such as Socrates and the year 3000. If there really are no non-present objects, then it is hard to see what we are referring to when we use expressions such as “Socrates” and “the year 3000”.

Another problem for the presentist has to do with relations involving non-present objects. It is natural to say, for example, that Abraham Lincoln was taller than Napoleon Bonaparte, and that World War II was a cause of the end of The Depression. But how can we make sense of such talk, if there are no non-present objects to be the relata of those relations?

A third problem for the presentist has to do with the very plausible principle that for every truth, there is a truth-maker—something whose existence suffices for the truth of the proposition or statement. If you are a presentist, it is hard to see what the truth-makers could be for truths such as that there were dinosaurs and that there will be Martian outposts.

Finally, the presentist, in virtue of being an A-theorist, must deal with the arguments against the A-theory that were mentioned above, including especially the worry that the A-theory is incompatible with special relativity. We will discuss these physics-based objections below.

Suggestions for Further Reading: Adams 1986; Bourne 2006; Bigelow 1996; Emery 2020; Hinchliff 1996; Ingram 2016; Keller and Nelson 2001; Markosian 2004, 2013; McCall 1994; Rini and Cresswell 2012; Sider 1999, 2001; Sullivan 2012b; Tooley 1997; Zimmerman 1996, 1998.

In Section 4 and Section 5 we saw that there have been two main theories developed in response to McTaggart’s Argument: The A-theory and The B-theory. Then, in Section 6 we saw that there are two main ways of thinking about the relation between ontology and time: presentism and eternalism. (There was also a third way, The Growing Block Theory, which we will mainly set aside for the sake of simplicity in this section.) Two main ways of thinking about time emerge from these discussions. On the one hand, A-theorists and presentists think that our pre-theoretical idea of time as flowing or passing, and thus being very different from the dimensions of space, corresponds to something objective and real. B-theorists and eternalists, on the other hand, reject the idea of time’s passage and instead embrace the idea of time as being a dimension like space. There is another important way in which philosophers in the second camp (the B-theory/eternalist camp) think time is like space, and it has to do with how objects and events persist over time. The debate typically centers around the doctrine of “temporal parts”, which those in the B-theory/eternalist camp tend to accept while those in the A-theory/presentist camp tend to reject.

To get an intuitive idea of what temporal parts are supposed to be, think of a film strip depicting you as you walk across a room. It is made up of many frames, and each frame shows you at a moment of time. Now picture cutting the frames, and stacking them, one on top of another. Finally, imagine turning the stack sideways, so that the two-dimensional images of you are all right-side-up. Each image of you in one of these frames represents a temporal part of you, in a specific position, at a particular location in space, at a single moment of time. And what you are, on this way of thinking, is the fusion of all these temporal parts. You are a “spacetime worm” that curves through the four-dimensional manifold known as spacetime . Moreover, on this view, what it is to have a momentary property at a time is to have a temporal part at the time that has the property in question. So you are sitting right now in virtue of the fact that your current temporal part is sitting.

The doctrine of temporal parts that B-theorists and eternalists tend to like can be stated like this:

Four-Dimensionalism: Any physical object that is located at different times has a different temporal part for each moment at which it is located.

On this view you have a temporal part right now, which is a three-dimensional “time slice” of you. And you have a different temporal part at noon yesterday, but no temporal parts in the year 1900 (since you are not located at any time in 1900). Also on this view, the physical object that is you is a fusion of all of your many temporal parts. (Note: there is a variation on the standard four-dimensional view, which is sometimes called “the worm view”. The variation, known as “the stage view”, holds that names and personal pronouns normally refer, not to entire fusions of temporal parts but, rather, to the individual person-stages, each of which is located at just an instant of time, and each of which counts as a person, rather than a mere part of a person).

The opposing view is three-dimensionalism, which is just the denial of the claim that temporally extended physical objects must have temporal parts. Here is a formulation of the view:

Three-Dimensionalism: Any physical object that is located at different times is wholly present at each moment at which it is located.

According to three-dimensionalism, the thing that was doing whatever you were doing at noon yesterday was you. It was you who was doing that, and now you are doing something different (namely, reading this sentence). So the relation between “you then” and “you now” is identity . According to four-dimensionalism, on the other hand, the thing that was doing whatever you were doing at noon yesterday was an earlier temporal part of the thing that is you, and the thing that is doing what you are doing now is the present temporal part of you. The relation between “you then” and “you now” is the temporal counterpart relation. (This is similar to the relation between your left hand and your right hand, which is the spatial counterpart relation. Your two hands are distinct parts of a bigger thing that contains them both.)

David Lewis, one of the main proponents of four-dimensionalism, suggests that the principal reason to accept the view is to solve what he calls “the problem of temporary intrinsics”. How can a single thing—Lewis, for example—have different intrinsic properties—like being straight, while he is standing, and then being bent, while seated—at different times? Not by standing in different relations—the being straight at and being bent at relations—to different times, he argues. (Since, he says, being straight and being bent are genuine properties rather than disguised relations.) And not in virtue of there being only one reality—such as the time when Lewis is bent—so that reality consists of Lewis, and every other thing, being the way it is now and not any other way. (For Lewis points out that we all believe we have a past and a future, in addition to a present.) So Lewis suggests that the best answer to the question about how a single thing can have different intrinsic properties at different times is that such an object has different temporal parts which themselves have the different intrinsic properties.

There is, however, a natural three-dimensionalist response to this argument. It involves appealing to a certain way of thinking about time, truth, and propositions that we touched on briefly in Section 1 , namely, the idea that propositions are in some way “tensed” as opposed to “tenseless”. Here is a way to formulate the relevant semantic thesis:

The Tensed Conception of Semantics

• Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time.
• We cannot eliminate verbal tenses like is , was , and will be from an ideal language.

On this view, a sentence like “Sullivan is eating a burrito” expresses a proposition that used to be true, but is false now.

The alternative to the tensed conception of semantics is the tenseless conception of semantics . On the latter view, an utterance of a sentence like “Sullivan is eating a burrito” expresses a proposition about a B-relation between events—it says that Sullivan’s eating a burrito is simultaneous with the utterance itself (or perhaps with the time of the utterance). Here is a way of stating this view:

The Tenseless Conception of Semantics

• Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time.
• We can in principle eliminate verbal tenses like is , was , and will be from an ideal language.

Consideration of Lewis’s argument from temporary intrinsics has shown that a three-dimensionalist should probably endorse the tensed conception of semantics, in order to account for changing truths about the world and its objects. And once we have seen this, it also becomes clear that A-theorists, presentists, and proponents of the growing block theory all have similar reasons for adopting the tensed conception of semantics. For the A-theorist is committed to there being changing truths about which times and events are future, which are present, and which are past; and presentists and growing block theorists are both committed to there being changing truths about what exists.

Suggestions for Further Reading: Hawley 2004 [2020]; Lewis 1986; Sider 2001; Thomson 1983; van Inwagen 1990

Many of the above considerations—especially those about McTaggart’s Argument; the A-theory and the B-theory; presentism, eternalism, and the growing block theory; and the dispute between three-dimensionalism and four-dimensionalism—suggest that there are, generally speaking, two very distinct ways of thinking about the nature of time. The first is the Static Theory of Time, according to which time is like space, and there is no such thing as the passage of time; and the second is the Dynamic Theory of Time, according to which time is very different from space, and the passage of time is a real phenomenon. These two ways of thinking about time are not the only such ways, but they correspond to the two most popular combinations of views about time to be found in the literature, which are arguably the most natural combinations of views on these issues. In this section we will spell out these two popular combinations, mainly as a way to synthesize much of the preceding material, and also to allow the reader to appreciate in a big-picture way how the different disputes about the nature of time are normally taken to be interrelated.

The guiding thought behind the Static Theory of Time is that time is like space. Here are six ways in which this thought is typically spelled out. (Note: The particular combination of these six theses is a natural and popular combination of related claims. But it is not inevitable. It is also possible to mix and match from among the tenets of the Static Theory and its rival, the Dynamic Theory.)

The Static Theory of Time

• The universe is spread out in four similar dimensions, which together make up a unified, four-dimensional manifold, appropriately called spacetime .
• Any physical object that is located at different times has a different temporal part for each moment at which it is located.
• There are no genuine and irreducible A-properties; all talk that appears to be about A-properties can be correctly analyzed in terms of B-relations. Likewise, the temporal facts about the world include facts about B-relations, but they do not include any facts about A-properties.
• The correct ontology does not change over time, and it always includes objects from every region of spacetime.
• Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time. Also, we can in principle eliminate verbal tenses like is , was , and will be from an ideal language.
• There is no dynamic aspect to time; time does not pass.

Static Theorists of course admit that time seems special to us, and that it seems to pass. But they insist that this is just a feature of consciousness—of how we perceive the world—and not a feature of reality that is independent of us.

The second of the main ways of thinking about time is the Dynamic Theory of Time. The guiding thought behind this way of thinking is that time is very different from space. Here are six ways in which this thought is typically spelled out. (Note: The particular combination of these six theses is a natural and popular combination of related claims. But, like the Static Theory, it is not inevitable. It is also possible to mix and match from among the tenets of the Dynamic Theory and the Static Theory.)

The Dynamic Theory of Time

• The universe is spread out in the three dimensions of physical space, and time, like modality, is a completely different kind of dimension from the spatial dimensions.
• Any physical object that is located at different times is wholly present at each moment at which it is located.
• There are genuine and irreducible A-properties, which cannot be correctly analyzed in terms of B-relations. The temporal facts about the world include ever-changing facts involving A-properties, including facts about which times are past, which time is present, and which times are future.
• The correct ontology changes over time, and it is always true that only present objects exist.
• Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time. Also, we cannot eliminate verbal tenses like is , was , and will be from an ideal language.
• The passage of time is a real and mind-independent phenomenon.

Opponents of the Dynamic Theory (and sometimes proponents as well) like to characterize the theory using the metaphor of a moving spotlight that slides along the temporal dimension, brightly illuminating just one moment of time, the present, while the future is a foggy region of potential and the past is a shadowy realm of what has been. The moving spotlight is an intuitively appealing way to capture the central idea behind the Dynamic Theory, but in the end, it is just a metaphor. What the metaphor represents is the idea that A-properties like being future , being present , and being past are objective and metaphysically significant properties of times, events, and things. Also, the metaphor of the moving spotlight represents the fact that, according to the Dynamic Theory, each time undergoes a somewhat peculiar but inexorable process, sometimes called temporal becoming . It goes from being in the distant future to the near future, has a brief moment of glory in the present, and then recedes forever further and further into the past.

Despite its being intuitively appealing (especially for Static Theorists, who see it as a caricature of the Dynamic Theory), the moving spotlight metaphor has a major drawback, according to some proponents of the Dynamic Theory: it encourages us to think of time as a fourth dimension, akin to the dimensions of space. For many proponents of the Dynamic Theory, this way of thinking—“spatializing time”—is a mistake. Instead, we should take seriously the ways that time seems completely different from the dimensions of space—for instance, time’s apparent directionality, and the distinctive ways that time governs experience.

Suggestions for Further Reading: Hawley 2001; Lewis 1986; Markosian 1993; Markosian 2004; Markosian (forthcoming); Moss 2012; Price 1977; Prior 1967; Prior 1968; Sider 2001; Smart 1949; Sullivan 2012a; Thomson 1983; and Williams 1951.

Above we mentioned that a metaphor sometimes used to characterize the Dynamic Theory is that of a moving spotlight that slides along the temporal dimension and that is such that only objects within the spotlight exist. A similar sort of metaphor can also be used to characterize the Moving Spotlight Theory, which is an interesting hybrid of the Static Theory and the Dynamic Theory. Like the Static Theory, the Moving Spotlight Theory incorporates the idea of spacetime as a unified manifold, with objects spread out along the temporal dimension in virtue of having different temporal parts at different times, and with past, present, and future parts of the manifold all equally real. But like the Dynamic Theory, it incorporates the thesis that A-properties are objective and irreducible properties, as well as the idea that time genuinely passes. The metaphor that characterizes the Moving Spotlight Theory is one on which there is a moving spotlight that slides along the temporal dimension and that is such that only things that are within the spotlight are present (but things that are outside the spotlight still exist).

Thus the Moving Spotlight Theory is an example of an eternalist A-theory that subscribes to the dynamic thesis. Unlike presentist or growing block theories, spotlighters deny that any objects come into or out of existence. Unlike the B-theories, however, spotlighters think that there is an important kind of change that cannot be described just as mere variation in a spacetime manifold. Spotlighters think instead that there is a spacetime manifold, but one particular region of the manifold is objectively distinguished—the present. And this distinction is only temporary—facts about which region of spacetime count as the present change over time. For example, right now a region of 2019 is distinguished as present. But in a year, a region of 2020 will enjoy this honor. The term “moving spotlight theory” was coined by C.D. Broad—himself a growing blocker—because he thought this view of time treated passage on the metaphor of a policeman’s “bull’s eye” scanning regions in sequence and focusing attention on their contents.

Just as there are different understandings of presentism and eternalism, there are different versions of the moving spotlight theory. Some versions think that even though the present is distinguished, there is still an important sense in which the past and future are concrete. Other versions (like Cameron 2015) treat the spotlight theory more like a variant of presentism—past and future objects still exist, but their intrinsic properties are radically unlike those of present objects. Fragmentalists (see Fine 2005) think that there is a spacetime manifold but that every point in the manifold has its own type of objective presentness, which defines a past and future relative to the point.

Why be a spotlighter? Advocates think it combines some of the best features of eternalism while still making sense of how we seem to perceive a world of substantive passage. It also inherits some of the counterintuitive consequences of eternalism (i.e., believing dinosaurs still exist) and the more complicated logic of the A-theories (i.e., it requires rules for reasoning about tensed propositions involving the spotlight).

Suggestions for Further Reading: Broad 1923; Cameron 2015; Fine 2005; Hawley 2004 [2020]; Lewis 1986 (especially Chapter 4.2); Sider 2001; Skow 2015; Thomson 1983; Van Inwagen 1990; Zimmerman 1998.

We are all familiar with time travel stories, and there are few among us who have not imagined traveling back in time to experience some particular period or meet some notable person from the past. But is time travel even possible?

One question that is relevant here is whether time travel is permitted by the prevailing laws of nature. This is presumably a matter of empirical science (or perhaps the correct philosophical interpretation of our best theories from the empirical sciences). But a further question, and one that falls squarely under the heading of philosophy, is whether time travel is permitted by the laws of logic and metaphysics. For it has been argued that various absurdities follow from the supposition that time travel is (logically and metaphysically) possible. Here is an example of such an argument:

Another argument that might be raised against the possibility of time travel depends on the claim that presentism is true. For if presentism is true, then neither past nor future objects exist. And in that case, it is hard to see how anyone could travel to the past or the future.

A third argument, against the possibility of time travel to the past, has to do with the claim that backward causation is impossible. For if there can be no backward causation, then it is not possible that, for example, your pushing the button in your time machine in 2020 can cause your appearance, seemingly out of nowhere, in, say, 1900. And yet it seems that any story about time travel to the past would have to include such backward causation, or else it would not really be a story about time travel.

Despite the existence of these and other arguments against the possibility of time travel, there may also be problems associated with the claim that time travel is not possible. For one thing, many scientists and philosophers believe that the actual laws of physics are in fact compatible with time travel. And for another thing, as we mentioned at the beginning of this section, we often think about time travel stories; but when we do so, those thoughts do not have the characteristic, glitchy feeling that is normally associated with considering an impossible story. To get a sense of the relevant glitchy feeling, consider this story: Once upon a time there was a young girl, and two plus two was equal to five . When you try to consider that literary gem, you mainly have a feeling that something has gone wrong (you immediately want to respond, “No, it wasn’t”), and the source of that feeling seems to be the metaphysical impossibility of the story being told. But nothing like this happens when you consider a story about time travel (especially if it is one of the logically consistent stories about time travel, such as the one depicted in the movie Los Cronocrímenes (Timecrimes) ). One task facing the philosopher who claims that time travel is impossible, then, is to explain the existence of a large number of well-known stories that appear to be specifically about time travel, and that do not cause any particular cognitive dissonance.

Suggestions for Further Reading: Bernstein 2015, 2017; Dyke 2005; Earman 1995; Markosian (forthcoming); Meiland 1974; Miller 2017; Sider 2001; Thorne 1994; Vihvelin 1996; Yourgrau 1999.

Our best physical theories have often had implications for the nature of time, and by and large, it is assumed that philosophers working on time need to be sensitive to the claims of contemporary physics. One example of the interaction between physics and philosophy of time that was mentioned in Section 2 was Newton’s bucket argument, which used the observed effects of acceleration to argue for absolute motion (and thus absolute space and time). Another example mentioned above was the worry that the A-theory conflicted with special relativity. The latter has proved especially influential in contemporary metaphysics of time and so deserves some further discussion.

According to standard presentations of special relativity, there is no fact of the matter as to whether two spatially separated events happen at the same time. This principle, which is known as the relativity of simultaneity , creates serious difficulty for the A-theory in general and for presentism in particular. After all, it follows from the relativity of simultaneity that there is no fact of the matter as to what is present, and according to any A-theory there is an important distinction between what is present and what is merely past or future. According to presentism, that distinction is one of existence—only what is present exists.

A different way of describing the relativity of simultaneity involves the combination of two claims:

• the claim that whether two spatially separated events happen at the same time depends on the reference frame you use to describe them, and
• the claim that no reference frame is privileged.

This way of putting the relativity of simultaneity requires a new bit of technical jargon: the notion of a reference frame. For our purposes, a reference frame is nothing more than a coordinate system that is used to identify the same point in space at different times. Someone on a steadily moving train, for instance, will naturally use a reference frame that is different from someone who is standing on the station platform, since it is natural for the person on the train to think of themselves as stationary, while for the person on the platform it seems obvious that they are moving.

The reason why it is worth introducing this bit of jargon is that once you present the relativity of simultaneity as the combination of claims (i) and (ii), you can also note that the motivation for claim (i) is importantly different from the motivation for claim (ii). The motivation for (i) is a series of empirical results at the end of the nineteenth and beginning of the twentieth century, including, most famously, the Michelson-Morley experiment. No one should deny this part of the relativity of simultaneity. The motivation for (ii), by contrast, is less often explicitly discussed, and seems to involve the commitment to some sort of general extra-empirical principle like “eliminate unobservable entities whenever possible”, or “eliminate excess spacetime structure whenever possible”. This means that presentists and other A-theorists have a way of avoiding the worry from relativity without any conflict with empirical results—they can reject whatever extra-empirical principle motivates (ii). Whether you think the costs associated with this move are worth paying will depend on your degree of commitment to the A-theory, what exactly you think of the relevant extra-empirical principle supporting (ii), and whether that principle plays an important role elsewhere in physics.

It is often said that philosophers should defer to physics with respect to what the latter says about time. But the interaction between the A-theory and special relativity illustrates one way in which that claim is more complicated than it first appears. Must philosophers respect both the empirical and the extra-empirical aspects of our best physical theories? Or is it sufficient that they respect the former?

Another way in which this assumption is complicated is that different physical theories often seem to imply different things about the nature of time. Consider, for instance, the fact that in general relativity there is sometimes (though not always!) a preferred way of “foliating” spacetime into instants of time and thus reintroducing a notion of absolute simultaneity, or the fact that on some interpretations of quantum mechanics, the dynamical laws seem to require a notion of absolute simultaneity.

Two additional questions about the nature of time that have been especially influenced by contemporary physics have to do with the arrow of time and the extent to which time itself might be emergent.

To motivate the first question, start from the observation that the order in which events happen in time seems to matter a great deal. There seems to be an important difference, for instance between a train traveling from Boston to Providence and a train traveling from Providence to Boston. This is because, even though both of these sequences may be constituted by the very same events, those events are in a different order in each sequence. In the former sequence the train being in Boston happens earlier than the train being in Providence. In the latter, the train being in Boston happens later.

These straightforward observations show that we experience time as having a direction. This is what philosophers call “the arrow of time”. But is the arrow of time a fundamental feature of the world? Or can it be reduced to some other feature, thus simplifying our metaphysics as a whole?

One way to try to eliminate the arrow of time at the fundamental level is to make use of certain interpretations of statistical mechanics inspired by Ludwig Boltzmann’s work. Imagine the history of the universe as a long timeline, but with no indication of which end of the time line represents the first moment of time and which end represents the last moment. It follows from certain interpretations of statistical mechanics that there is a physical quantity, the entropy of the universe, that will be relatively low at one end of the timeline and relatively high at the other end and will always increase as you move from the former end of the timeline to the latter. (More carefully, the entropy will almost always increase or at least stay constant.) The thought, then, is that we might be able to reduce the arrow of time to this entropy gradient. Earlier moments of time are just moments of time when the entropy of the universe is lower.

This way of eliminating the arrow of time from the fundamental level is promising, but has at least some unintuitive consequences. For instance, it seems natural to think that entropy could have decreased over time, instead of increasing over time as it actually does. But given the reduction described above, it is not in fact possible for entropy to decrease over time.

The second question mentioned above is a question about whether time itself—as opposed to just some particular feature of time, like time’s arrow—might merely be an emergent feature of the world. This question has become especially pressing as philosophers of physics have turned their attention to theories of quantum gravity in which there does not seem to be anything like temporal structure at the fundamental level. Work in this area is nascent, but some of the questions of interest include: Does quantum gravity eliminate time entirely or does it merely make time a non-fundamental feature of the world? What would it mean for something temporal to be grounded in something atemporal and what sort of grounding relation would be involved? What is the distinction between causal structure (especially the causal structure in causal set theory—one approach to quantum gravity) and temporal structure? And how can a theory that eliminates time entirely be empirically confirmed or disconfirmed?

Suggestions for Further Reading: Albert 2000; Emery 2019 & forthcoming; Godfrey-Smith 1979; Healey 2002; Huggett and Wüthrich 2013; Knox 2013; Markosian 2004; Maxwell 1985; Monton 2006; Price 1996; Putnam 1967; Rovelli 2017; Savitt 2000; Stein 1968, 1970; Weingard 1972; Wüthrich and Callender 2017.

A final important question concerns how considerations about the nature of time ought to impact the ways that we reason about time. For example, if it turns out that a B-theory is true, and there is no metaphysically important difference between the past and future, then should we adopt a more neutral attitude about events in our personal past and future? Epicurean philosopher Lucretius famously suggested that if there is no substantive difference between the times in the past before we came to exist and the times in the future after we die, we should care much less about the deprivation that death will bring. But we may think that even if the B-theory can describe everything that is metaphysically important without positing an important difference between the past and future, there is still an indispensable psychological importance to the past/future distinction that rational agents honor. Still other A-theorists argue that while there is an important metaphysical distinction between the past and future, the distinction has no normative importance.

If we deny three-dimensionalism and instead view ourselves as objects that persist through time by having temporal parts, then does that justify caring less about temporal parts in the distant future that are less strongly linked with our present part? Derek Parfit famously argued that a proper understanding of what we care about when we care about our own future persistence should motivate us to be less self-interested and more interested in redistributing resources to others. Endurantists have argued that facts about how we persist through time underwrite a strong distinction between moral principles (which concern what we owe to others now) and prudential rationality (which concerns what we owe to our future selves).

Another interesting line of research uses empirical work in psychology to better understand what is happening cognitively when we judge time as passing. This is especially pressing for B-theorists, who must explain why time seems to pass in psychologically or rationally significant ways, even though all passage is really just variation in an eternal manifold. Some B-theorists explain the apparent passage of time as an illusion of flow caused by perceptual processes that attribute apparent motion to events that happen in sequence. Another, compatible approach considers the way that evolutionary pressures might have shaped emotions and cognitive heuristics to give us a strong past/future distinction in our reasoning.

Suggestions for Further Reading: Brink 2003; Suhler and Callender 2012; Parfit 1971; Paul 2010; Prosser 2016; Sullivan 2018.

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• A.N. Prior: The Founding Father of Temporal Logic , a web site devoted to the study of Arthur Norman Prior (the founder of tense logic), at the Danish Centre for Philosophy and Science Studies
• The Centre for Time at the Department of Philosophy, University of Sydney.
• Eternalism , entry in Wikipedia
• Philosophy of space and time , entry in Wikipedia
• Philosophical presentism , entry in Wikipedia
• Time , entry in the Internet Encyclopedia of Philosophy
• Time , entry in Wikipedia
• Time Travel , entry in the Internet Encyclopedia of Philosophy

causation: backward | consciousness: temporal | fatalism | future contingents: medieval theories of | Heidegger, Martin | Husserl, Edmund | indexicals | logic: temporal | McTaggart, John M. E. | Newton, Isaac: views on space, time, and motion | presentism | Prior, Arthur | propositions: singular | quantum theory: quantum gravity | space and time: absolute and relational space and motion, post-Newtonian theories | space and time: being and becoming in modern physics | space and time: conventionality of simultaneity | temporal parts | time: the experience and perception of | time: thermodynamic asymmetry in | time travel | time travel: and modern physics | Zeno of Elea: Zeno’s paradoxes

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• Published: 04 October 2016

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A brief history of time travel

• Dave Goldberg  1  

Nature Physics volume  12 ,  page 892 ( 2016 ) Cite this article

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Time Travel: A History

• James Gleick

In his brisk and entertaining new book, Time Travel: A History , James Gleick serves as an enthusiastic guide through the fourth dimension as seen in literature and popular culture. I spoke with Gleick to discuss his book, the interface between science and science fiction and what makes genuinely compelling time travel.

Gleick begins his story around the turn of the twentieth century, which saw both H. G. Wells's publication of The Time Machine and, not entirely coincidentally, Albert Einstein's annus mirabilis papers. Wells wasn't a scientist, and nobody — least of all Gleick — would make the argument that The Time Machine presaged special relativity, but he argues that language itself made spacetime almost an inevitable concept; we use the same word for the flow of time and the flow of a river, for example. “Physicists didn't invent that analogy; it was partly built into the language because we don't have words for everything. But the words connecting space and time became more powerful over time.”

The nineteenth century was a golden age of geology and Wells in particular was fascinated by it. There is a very literal sense in which the physical strata of Earth represent movement through time. That, along with the synchronization of clocks required for successful railroads, started many thinkers on the path of considering what time really is. Gleick notes, “Wells's hero, the time traveller, makes a speech explaining why time travel is possible in a way that to modern ears is almost comically pedantic. We can see that his vision of the Universe is almost standard — it's Einstein's version of the Universe”.

But time as a river, or a dimension or any of the familiar analogies is more than simple language at play. We know, and Einstein showed, that space and time really can be rotated into one another with a suitable choice of frame. Wells's innovation was to consider seriously the premise of freely traversing time as we would any other dimension.

Wells is a seminal figure in the development of fictional time travel, but he wasn't the first. Mark Twain's A Connecticut Yankee in King Arthur's Court , for example, predated The Time Machine by six years. But Twain's Connecticut Yankee is more of a tourist than what we now consider a time traveller.

Gleick makes the pitch that the precursors of modern time travel include many ideas that we wouldn't call time travel at all: sleeping into the future, for example, in the tradition of Rip Van Winkle or in prophecies going back to the ancients. In the early decades of the twentieth century, the time capsule fad represented a way of speaking directly to the future — albeit only in one dimension.

Wells's time traveller, however, wasn't simply a passive observer. He could affect the future (and Wells seemed to be uniquely interested in travelling into the future), prompting Gleick to propose a parlour game similar to the 'flight or invisibility?' litmus test for superpowers: “if you had one shot with a time machine, would you go into the future or the past, provided you could go and return safely?”

The future is full of promise, but in the past you could — perhaps — fix your mistakes. Gleick's tour of the development of what are by now are familiar paradoxes brings into focus the giants of early science fiction. One example is Ray Bradbury's A Sound of Thunder , which showed that changes in the past (in the story, the crushing of a butterfly in the Jurassic) can lead inexorably to dramatic changes in the present.

Temporal manipulations can create paradoxes, the most famous of which amounts to the fact that you can't kill your grandfather and prevent your own birth — or can you? From a storytelling perspective, time travel, or at least backwards time travel, seems to preclude the possibility of free will.

In the 1980s, the Russian physicist Igor Novikov put forth what is now known as the 'self-consistency conjecture', which posited, in short, that the probability of inconsistent histories in closed time-like curves was exactly zero. Quantum events should play themselves out the same way the second time around. Gleick notes that “this was why writing the book was so much fun. Watching first the science fiction writers and then the physicists grapple with these questions. What we love about science is the very way it undermines our common sense and it's a great joy whenever it happens.” He cites classic sci-fi movies like The Terminator, 12 Monkeys (itself a remake of La Jetée , a film he lovingly describes in detail in his book) and, in a particularly neat resolution, 'Blink' (an episode of the classic sci-fi series Doctor Who ).

The 'trick' in great time travel literature becomes the way in which the author closes the loop. As Gleick puts it, “the job of a good time travel writer is to explore the paradox by taking it seriously ... but when you go back and think about it, you can see the trick”.

What distinguishes Time Travel: A History from other books more focused on the philosophy or physics of time travel is that Gleick allows himself into the realm of the aesthetic. It is irrelevant whether the physical mechanism for time travel is driven by a steampunk bicycle or a wormhole — you have to believe in the dramatic stakes.

As to the practical possibility of time travel, Gleick is something of a sceptic. Common sense, he argues, suggests that the past really is immutable, no matter how clever the theoretical models that imply otherwise. And despite the apparent symmetry of the microscopic laws of physics, there really is, he argues, something different about the future and the past. “The future hasn't been written yet. When did that become controversial?”

DAVE GOLDBERG  Dave Goldberg is in the Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, USA.   e-mail: [email protected]

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Goldberg , D. A brief history of time travel. Nature Phys 12 , 892 (2016). https://doi.org/10.1038/nphys3910

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The Complex Role of Mental Time Travel in Depressive and Anxiety Disorders: An Ensemble Perspective

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The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

The ensemble hypothesis proposes that uniquely human cognitive abilities depend on more than just language. Besides overt language, inner speech, and causal interpretations, executive attention, mental time travel, and theory of mind abilities are essential parts that combine additively and even multiplicatively. In this review, we consider the implications of the ensemble hypothesis for the psychopathologies of anxiety and depression. Generalized anxiety disorder (GAD) and major depressive disorder (MDD) are two of the most common mental disorders worldwide. The mechanisms that differentiate them are difficult to identify, however. Mental time travel has been implicated in models of depressive and anxiety disorders, but here we argue that at least two other ensemble components, namely, interpreter biases and executive attention, must also be considered. Depressive and anxiety disorders have both been found to show impairments in all three of these components, but the precise relationships seem to distinguish the two kinds of disorders. In reviewing the literature, we develop models for depression and anxiety that take into account an ensemble of mental components that are unique for each disorder. We specify how the relations among mental time travel, interpreter biases, and executive attentional control differ in depression and anxiety. We conclude by considering the implications of these models for treating and conceptualizing anxiety and depression.

Introduction

Depressive and anxiety disorders are two major categories of psychopathology, yet they have proven difficult to differentiate in some respects. As will be documented below, both are characterized by dysfunctional executive attention and pessimistic attributional styles, with a high degree of comorbidity. Here we build on the premise of Roepke and Seligman (2016) that the core problem in depression is a difficulty in mental time travel, specifically, an inability to envision positive events in the future. We consider the role of mental time travel in differentiating the two disorders and conclude that this component of human cognition is by itself insufficient. Mental time travel, we suggest, is moderated by problems with executive attention and an interpretive component responsible for causal attributions and inner speech.

In an important paper, Roepke and Seligman (2016) argued that prospection, or the mental representation of future events, plays a major role in depression. Human episodic memory enables mental time travel, that is to say, the ability both to recall past autobiographical events and to imagine possible future events ( Tulving, 2002 ). Roepke and Seligman suggested that the negative beliefs about the future and feelings of hopelessness that characterize depressive disorders ( Beck, 1974 ) can be directly linked to faulty prospection, an inability to envision possible futures and poor evaluation of possible futures. In their view, “.faulty prospection is the core causal process of much depression” (p. 24). A similar proposal was advanced by Miloyan et al. (2014) to account for depression; they also extended the analysis by suggesting that a different form of faulty prospection, centered on worry rather than pessimism and hopelessness, lay at the core of anxiety disorders.

We agree that problems with mental time travel are central to psychopathology, but we question whether this component can be isolated from other cognitive components to ascertain its relative contribution. Instead, we argue that other fundamental components of human cognition are concurrently at work in both depressive and anxiety disorders. In our view, it is important to consider how other components impair or even enhance the functioning of mental time travel. To develop this perspective, we draw on the ensemble hypothesis, which holds that human cognition depends on five core systems or components that interact in non-additive ways ( Kellogg, 2013 ; Kellogg and Evans, 2019 ). Mental time travel is necessary but not sufficient for explaining either the remarkable competencies of human cognition or its breakdowns in disorders such as anxiety and depression. An advanced executive form of working memory, a theory of mind augmenting social cognition, language, the ability to interpret information using inner speech, and causal inference are necessary, as well as an episodic memory capable of mental time travel. Kellogg (2013) introduced the ensemble hypothesis in the context of understanding the exceptional cognitive abilities in the evolution of our species, Homo sapiens . The book provides the reasons for considering the five components and their interactions in normally developing and functioning human beings. Kellogg and Evans (2019) offered further evidence in support of the hypothesis from behavioral studies, lesion studies, and studies involving neuro-atypical populations.

The key claim of the ensemble hypothesis is that two or more mental capacities can interact in a multiplicative fashion to yield competencies in a well-functioning human being that exceed their simple additive effects. For example, delay of gratification is a phenomenon that entails an ability both to prospectively consider the future and to exercise cognitive control using executive attention. In typically developing children, growth in the capacity of executive attention for self-regulation boosts the ability to delay rewards in anticipation of a larger future reward ( Mischel et al., 1989 ). Similarly, planning in problem solving requires future thinking and a normally functioning system of executive attention. Frontal lobe injuries that damage networks of executive attention often impair planning ( Kellogg and Evans, 2019 ). In normally functioning adults, retrospective memory for a list of words presented in a laboratory task requires an intact hippocampus and medial temporal lobe, but it is also boosted by maintenance and elaborative rehearsal strategies that depend on executive attention. Failing to deploy attentional resources to an encoding strategy impairs the recall of a list of words presented in a laboratory task in individuals with depression ( Hertel and Rude, 1991 ). As will be considered in detail later, the normal functioning of mental time travel can be altered by depression because of its effects, in part, on executive attention.

The purpose of the present paper is to consider the implications of the ensemble hypothesis for two broad categories of psychopathology: depression and anxiety. We suggest that much of the phenomenology and symptoms that underlie depressive and anxiety disorders can best be understood as an interaction of components of the hypothesized ensemble. We wish to extend the insights provided by Miloyan et al. (2014) and Roepke and Seligman (2016) by demonstrating how the interpreter and executive attention influence mental time travel. As will be seen, language is considered in the form of inner speech, but the broader concept of language as interpersonal communication falls outside the scope of the current paper. Similarly, as will be addressed in the limitation section of our paper, an extensive literature on theory of mind and social cognition in depression ultimately needs to be accounted for. Even so, our focus on the interpreter, executive attention, and mental time travel documents the importance of the interactions posited by the ensemble hypothesis.

To illustrate, consider the case of depression (see Figure 1 ), as exemplified by major depressive disorder (MDD). As will be discussed in detail later, the interpreter shown in Figure 1 refers to the inner voice and causal inference capacity of the left hemisphere of the human brain that enables attributions about the self and other people ( Gazzaniga, 2000 ; Kellogg, 2013 ). In depression, the interpreter is biased to assign blame to the self for negative experiences. This pessimistic and personally negative explanatory style ( Petersen and Seligman, 1984 ) causes the depressed individual to focus attention on negative past events and have difficulty envisioning anything positive about the future. Further, there is evidence that depression is associated with a concurrent deficit in executive attention ( Ólafsson et al., 2011 ), causing impaired cognitive control over mental time travel resulting in persistent negative rumination. Thus, the influence of both a bias in interpretation and a deficit in executive attention, we propose, could underlie faulty prospection in depressed individuals. The interactive model of Figure 1 differs from the position of Roepke and Seligman (2016) with respect to effective approaches to treatment for depression. They advocate for treatments targeting mental time travel, specifically, the core problem with prospection. Alternately, we contend that efforts to improve executive attention and to correct the pessimistic explanatory style of the interpreter ought not be neglected, because they can alter the functioning of mental time travel.

Model of major depressive disorder.

The plan of the paper is, first, to introduce several components of the ensemble hypothesis that are central to our analysis of depressive and anxiety disorders. Second, we consider evidence on the role of mental time travel in depressive and anxiety disorders. Third, we discuss literature regarding the pessimistic explanatory style in depression and suggest that anxious individuals are characterized by a related but distinct dysfunctional style of explaining events as threatening to the self. The emphasis on loss in depression and threats in anxiety can influence the functioning of mental time travel, we propose. Fourth, we document that both kinds of disorders are associated with impairments in executive attention that may compound problems with mental time travel. Fifth, we discuss how the symptoms of depression versus anxiety can best be understood by considering mental time travel, the interpreter, and executive attention as an integrated ensemble. We conclude by considering the implications of the ensemble perspective regarding effective therapies for depressive and anxiety disorders.

Mental Time Travel, the Interpreter, and Executive Attention

Mental time travel is the unusual form of human episodic memory that allows the mind to recollect the specific time and place of a past event in one’s personal history ( Tulving, 2002 ; Suddendorf and Corballis, 2007 ). It is conceived as mental time travel because the same neural systems are involved in imagining future events as well as recollecting past events. The brain systems involved in mental time travel include the hippocampus and medial temporal lobe structures as well as the default mode network activated in resting state conditions when no external task is presented ( Buckner et al., 2008 ). The ability to construct spatially coherent scenes in which an event takes place is essential in both recollecting the past and imagining the future. It has been suggested that scene construction is a core function of the hippocampus ( Clark and Maguire, 2016 ).

A uniquely human mental ability appears to be the interpretive capacity of the left hemisphere ( Gazzaniga, 2000 ). Over the course of human evolution, our oral language capacity became internalized as inner speech, mediated by language networks in the left hemisphere. Vygotsky (1962) emphasized that speech begins in early childhood as a means for communication, but as speech is internalized, it also becomes a means for planning and problem solving. Self-directed inner speech, then, has long been recognized as an important vehicle for thinking and appraising situations and events. The interpreter constructs a personal narrative that explains why we feel and behave as we do. Inner speech is combined with a specialization of the left hemisphere for a specific kind of thinking. The left hemisphere is not only specialized for the use of language, including self-directed language of inner speech, but it is also specialized for forming hypotheses ( Wolford et al., 2000 ) and making inferences about causal relationships ( Roser et al., 2005 ). Similarly, the ability to reason deductively is known to be impaired in patients with left frontal lesions but not right frontal lesions ( Reverberi et al., 2010 ).

In clinical psychology, the interpreter is important in understanding the role of inner speech and causal inference in how people respond to stressful life events. How an individual cognitively appraises stressors can either attenuate or exacerbate the strain that they cause. This role for causal attributions has long been recognized in understanding depressive and anxiety disorders. For example, Petersen and Seligman (1984) highlighted that depression is characterized by a personalized and pessimistic explanatory style. The individual attributes personal, pervasive, and permanent causes to negative personal experiences, committing what social psychologists call the fundamental attribution error. The role played by the interpreter in explaining why things happen and what significance events have for the self is central to both depression and anxiety, as will be detailed later in the paper.

The executive attention component of working memory enables the coordination and regulation of representations held in verbal, visual, and spatial stores of short-term memory. Working memory, planning, cognitive control, self-regulation, and response inhibition have all been referred to as executive functions that have traditionally been viewed as dependent on the frontal lobe ( Alvarez and Emory, 2006 ; Posner and Rothbart, 2007 ; Diamond, 2013 ; Ajilchi and Nejati, 2017 ). A more complex understanding has emerged in the literature with two distinct brain networks involved in executive attention; these include but are not limited to regions in the frontal lobe ( Posner and Peterson, 1990 ; Petersen and Posner, 2012 ).

By studying a battery of executive functioning tasks, Miyake et al. (2000) identified three correlated but distinctive processes underlying performance. Updating the contents of working memory, shifting goals as required in multitasking, and inhibiting irrelevant information are considered three essential and irreducible functions of executive attention. A widely used test of individual differences in working memory capacity, called the Operation Span (OSPAN) test, indicates that the ability to inhibit irrelevant information is especially important and shows a strong correlation with general fluid intelligence or the ability to solve novel problems ( Engle et al., 1999 ).

Mental time travel, the interpreter, and executive attention are three fundamental components of human cognition. Kellogg (2013) proposed that these components, together with theory of mind and language, comprise an ensemble that renders human cognition unique and qualitatively different from non-human cognition. Importantly, his hypothesis suggests that it is the interaction of these components that yields the unique properties of human cognition. If that is so, then it stands to reason that common forms of psychopathology should reveal such interactions, too. In persons experiencing anxiety or depression, a deficit in one component can cascade to degrade the functioning of another component, despite that the latter component is not necessarily dysfunctional.

Mental Time Travel Impairments

Roepke and Seligman (2016) reviewed a variety of evidence that faulty prospection lies at the heart of depression. First, persons experiencing depression can envision negative future scenarios more readily, compared to non-depressed persons ( MacLeod and Byrne, 1996 ). This characteristic is also shared with those experiencing anxiety, indicating it is not a unique dysfunction of mental time travel associated with depression. Miloyan et al. (2014) suggested that anxious as well as depressed individuals anticipate negative future events but that each disorder shows a unique profile of faulty prospection. Individuals with anxiety anticipate more negative experiences, but not fewer positive experiences, relative to control participants without a history of psychiatric diagnosis, according to some studies ( MacLeod and Byrne, 1996 ; MacLeod et al., 1997b ). Depression, on the other hand, is associated with a failure to anticipate positive future events ( Miranda and Mennin, 2006 ; Pomerantz and Rose, 2014 ). When depressed psychiatric outpatients were asked to describe a distressing personal problem and to imagine and rate the likelihood of both the worst and best possible outcomes, they rated the worst outcome as being more likely and the best outcome as being less likely, relative to generalized anxiety disorder (GAD) and control groups ( Beck et al., 2006 ).

Thus, it is possible that a faulty form of prospection found in depression results in a diminished ability to envision positive future events ( MacLeod and Salaminiou, 2001 ). However, both this finding and the finding that individuals with depression envision more negative future events than do controls can also be linked to a pessimistic explanatory style. MacLeod et al. (1997a) found that both depressed and anxious patients not only judged future negative events to be more likely, relative to controls; they also provided more supportive as opposed to contradictory reasons for their occurrence. As MacLeod et al. (p. 22) concluded, “…mood-disturbed subjects were pessimistic about what would happen to them in the future, and this was supported by their causal thinking about those events.” Thus, the pessimistic explanatory style of the interpreter rather than a malfunction in mental time travel per se could explain the findings. They could also be linked to the deficits in executive attention that are associated with depression ( Ólafsson et al., 2011 ). As will be argued in later sections of the paper, problems with mental time travel may arise because of the moderating influences of the interpreter and executive attention.

An important exception regarding memory impairment in depression is the tendency to focus and elaborate upon sad events ( Williams et al., 1997 ). A case can be made for mood congruent memory in depression ( Mineka and Nugent, 1995 ). For example, in a study by Derry and Kuiper (1981) , a list of depression-related adjectives (e.g., bleak, dismal, helpless) and non-depression-related adjectives (e.g., amiable, curious, loyal) were presented in an incidental learning task. The nature of the orienting task was manipulated, with one way being whether the adjective applied to the self. On a subsequent recall test, this self-reference orienting task resulted in a greater proportion of depressed-content words recalled (41%) than non-depressed content (16%) for depressed patients. Strikingly, this pattern was completely reversed for normal controls, who recalled more non-depressed content (43%) compared with depressed content (8%). Even a group of psychiatric controls showed a reversal with more non-depressed content (36%) relative to depressed content (18%). None of these effects were observed for structural (small letters?) and semantic (means the same?) orienting tasks, indicating that they are contingent on judging the word as relevant to the self.

Similarly, in another study, after being shown a list of words including pleasant, unpleasant, and neutral words, individuals with depression recalled more unpleasant words compared with pleasant words ( McDowall, 1984 ). A non-depressed control group as well as another control group made up of psychiatric patients with a diagnosis other than depression did not show this bias toward improved memory for unpleasant words. The depressed patients’ free recall of unpleasant words was at the same level as that for the two control groups, whereas they showed a memory impairment for pleasant words. This indicates that the mood congruent benefit of remembering unpleasant words can offset the usual memory impairment found in depression.

Clark and Teasdale (1982) found that autobiographical experiences also reveal mood congruency even within the same group of individuals with depression. The investigators compared the recall of personal memories at two different times of day to capitalize on diurnal variations in mood among psychiatric patients experiencing depression. The percentage of unhappy memories (52.3%) was reliably greater when the individual reported being more depressed compared with less depressed (36.7%). Happy memories (37.7 versus 51.1%) showed exactly the reverse pattern.

The above studies show that depression can bias retrospection in the direction of remembering sad events more readily than happy events. Would such findings also hold for prospection? MacLeod et al. (1997b) measured the recall of past experiences and the anticipation of future experiences in anxious, depressed, and control individuals. The study prompted the participants to remember or anticipate either positive experiences or negative experiences. This prompt variable allowed the comparison of the number of positive events versus negative events produced under conditions of both retrospection and prospection. Their findings showed no difference between the retrospection and prospection conditions for either disorder. Of importance, individuals with depression produced fewer events compared with controls—both positive and negative—both in recalling their past and in anticipating their future.

An analogous outcome has been found in laboratory studies of the retrospective recall of word lists versus prospective memory for future actions. Hertel and Rude (1991) found poorer free recall of a list of words presented earlier for currently depressed patients compared with recovered patients and control individuals with no history of depression in a retrospective task. Rude et al. (1999) similarly reported that depressed individuals perform poorly on a prospective memory task requiring the ability to self-initiate an action in the future. Their difficulties with “remembering to remember” to act in the future were parallel to impairments found in retrospective tasks, according to the authors. Of course, these tasks are different from the autobiographical reports examined by MacLeod et al., but the conclusions reached are consistent. MacLeod et al. (1997a) also found that anxious individuals did not differ from controls either in remembering or in anticipating positive events. However, they generated more negative events compared with controls regardless of whether they were engaged in retrospection or prospection. Their findings thus confirm that anxiety is primarily a disorder of worrying about negative outcomes ( Barlow, 1988 ). Whereas MacLeod et al.’s control participants both recalled and anticipated about 44% more positive life events than negative ones, the anxious participants only recalled 15% more positive events. Compared to participants with depression, the participants with anxiety recalled and anticipated about 67% more negative events.

Finally, MacLeod et al. expected that individuals with depression would show a mood congruent effect by remembering or anticipating more negative events compared with positive events. In contrast to prior studies reviewed earlier, this outcome did not occur. Rather, negative events were remembered by patients with depression at about the same rate as found in the controls. This rate was equivalent to the number of positive events remembered by those with depression, who were 75% less likely to remember positive events than were patients with anxiety and controls. This is reminiscent of the findings with the free recall of word lists reported by McDowall (1984) . Unpleasant words were remembered as well by patients with depression as by controls, but recall for pleasant words showed a marked impairment.

The above findings on memory could depend on the severity of the depressive disorder. It is important to note in that regard that MacLeod et al. (1997b) examined patients who met the diagnostic criteria for panic disorder and MDD. Similarly, the studies by Derry and Kuiper (1981) , Clark and Teasdale (1982) , McDowall (1984) , Hertel and Rude (1991) , and Rude et al. (1999) examined psychiatric inpatients or patients with depression in the community with screening done to insure they met the diagnostic criterion for depression. By contrast, in a non-clinical student population, neither trait anxiety nor trait depression was associated with difficulties in a measure of prospective memory ( Arnold et al., 2014 ). Thus, the severity of the disorder probably plays a role in the effects of depression and anxiety on mental time travel.

In contrast to the picture for clinical depression, the findings on retrospective memory for anxiety disorders are mixed. MacLeod and McLaughlin (1995) found that individuals currently receiving treatment for GAD performed worse than those in a control group on an explicit recognition test for words presented in a laboratory setting. By contrast, on explicit memory tests of cued recall ( Mathews et al., 1989 ) and free recall ( Becker et al., 1999 ), anxious individuals performed at the same level as control participants. For threatening words included among the lists presented in the laboratory, GAD patients showed no advantage in recall or recognition, but they did show superior performance on various implicit memory tests compared with controls ( Mathews et al., 1989 ; MacLeod and McLaughlin, 1995 ). A similar heightened explicit memory for threatening words was found by Becker et al. (1999) for individuals diagnosed with panic disorder but not with social phobia or GAD. In a review of the literature on memory and anxiety disorders, Mineka and Nugent (1995) concluded that the evidence for an explicit memory bias for threatening events is weak, difficult to replicate, and unconvincing, at least with respect to persons experiencing GAD.

We conclude from this sample of findings in the literature that while depression impairs mental time travel ability, it does not seem to be a selective difficulty with prospection. Judging from the findings of MacLeod et al. (1997a) , at least for positive events, anxious individuals do not appear to show any impairment in mental time travel, either in its prospective or in its retrospective form. In fact, they appear to recollect past negative events and envision future negative events more often than is found in both non-anxious controls and depressed patients. However, other studies indicate that such memory bias for threatening events is tenuous at best in anxiety. Patients with depression, on the other hand, forget positive events more readily than is found in non-depressed controls. A central question is what accounts for these differences in the functioning of the mental time travel component. We propose that considering the role played by the interpreter and executive attention helps to understand the pattern of results found for mental time travel.

Interpreter Biases

As noted earlier, the interpreter in individuals with depression employs a pessimistic explanatory style ( Petersen and Seligman, 1984 ). An inability to envision a positive future and a facility with envisioning a negative future could be understood as a dysfunction of the explanatory style found in individuals with depression rather than a fault with mental time travel per se ( MacLeod et al., 1997a ). A negative style of explaining why things happen as they do is a prime reason for feelings of hopelessness in depressed people ( Alloy et al., 1988 ). Individuals with depression tend to attribute the reasons for events in life to internalized causes about the self that are stable over time and that are global or pervasive in multiple situations. Another compounding factor is a negative attributional style that attributes negative events to uncontrollable causes ( Sanjuán and Magallares, 2009 ). As a consequence, persons with depression might be able to recollect or imagine an event that most people would regard as positive (e.g., getting a job promotion) but then interpret it as negative. Individuals with depression might appraise the promotion as full of pitfalls—more responsibility, longer working hours, and greater stress. Remembering or anticipating a job promotion may not be the problem but, rather, its pessimistic interpretation.

The interpreter, therefore, has a prominent, if not central, role in depression. Indeed, Beck (1974) designed cognitive behavioral therapy (CBT) to confront and modify a depressed person’s inner speech of hopelessness and self-deprecation. By altering the person’s cognitive appraisals of situations and causal explanations of events, mood improves as a result. Beck’s concept of the cognitive triad included a negative view of the self, negative interpretations of ongoing experiences, and a negative view of the future. Although the latter could be caused by faulty prospection, the first two stem from the distorted explanations of the interpreter.

Comparisons of the interpretative style of depressed versus anxious individuals have yielded conflicting results, however. For example, Heimberg et al. (1989) , by contrast, found that the attributional style found in the learned helplessness of individuals with depression was also characteristic of multiple anxiety disorders, such as social phobia, agoraphobia, and panic disorder. In their study, the two disorders differed only in that depression produced global and unstable attributions regarding the causes of positive events, whereas anxiety was associated with the same kind of attributions for negative events. Still other findings indicate that attributing internal, stable, and global causes to negative events is in fact found in currently depressed individuals, but especially in those with comorbid anxiety ( Fresco et al., 2006 ). Similarly, Luten et al. (1997) concluded that a pessimistic attributional style is not specific to depression but, rather, is correlated with high levels of negative affect as is also found in in persons with anxiety disorders. Ahrens and Haaga (1993) even reported that a negative event attributional style was only found with anxiety disorders rather than with depression.

Thus, it seems that pessimistic forms of causal inference about life’s events are a non-specific risk factor for anxiety and depression. This commonality with respect to the dysfunction of the interpreter is a likely reason why depressive and anxiety disorders share a high degree of comorbidity ( Gotlib, 1984 ; Kessler et al., 2007 ; Grisanzio et al., 2018 ).

Despite their similarities and high rates of comorbidity, there may be some unique aspects to the interpreter’s dysfunction in anxiety disorders, however. Riskind and Williams (2005) identified a looming cognitive style in which individuals overestimate the progression of a potential threat in terms of both spatial and temporal dimensions. Individuals with a high score on their looming cognitive scale misinterpret potential threats as catastrophic threats. A study by Reardon and Williams (2007) showed that this looming cognitive style is uniquely associated with anxiety disorders. A pessimistic cognitive style contributed to both anxiety disorders and depressive disorders, but individuals predisposed to anxiety disorders also were prone to a looming cognitive style that magnifies potential threats. Anxiety disorders also feature highly persistent negative self-talk. The excessive worry that characterizes anxiety is largely verbal in nature ( Borkovec et al., 1998 ). Instead of imagining a threat in a visual–spatial context, anxious individuals talk to themselves about it. Finally, it has long been recognized that the causal inferences made in depression are associated with personal failures and self-deprecation ( Beck, 1974 ). This contrasts with worries about uncertainties and potential dangers in the case of anxiety disorders ( Beck et al., 1987 ; Clark et al., 1990 ).

As shown in Figure 2 , the anxious interpreter views events as threatening to the self rather than as a negative reflection of the self as in depression ( Figure 1 ). Kendall and Ingram (1989) differentiated the two disorders precisely in terms of their characteristic attributions. The interpretations of the depressive person often are “self-referent, definitive, past-oriented cognitions of sadness, failure, degradation, and loss,” in contrast to the “future oriented ‘questioning’ cognitions” found in anxiety disorders ( Kendall and Ingram, 1989 ; p. 36).

Model of generalized anxiety disorder.

In our view, the interpreter biases play a role in the problems observed in mental time travel with depressed and anxious individuals ( Eysenck et al., 2006 ). For depression ( Figure 1 ), the pessimism of the interpreter causes individuals with depression to remember and ruminate about negative life experiences that reinforce feelings of loss and self-blame. Perhaps the difficulty with imagining a positive future is a direct consequence of depressed individuals focusing on negative past events. Roepke and Seligman (2016 , p. 27), in fact, suggest the possibility that persons experiencing depression “struggle to recall a good past,” with few positive memories ( Williams and Scott, 1988 ).

For GAD ( Figure 2 ), the interpreter is biased to detect threats to the self. This might cause one to recall, see, and foresee dangers rather than losses. Instead of mental time travel being impaired relative to control individuals who are neither depressed nor anxious, there is, if anything, an excessive prospection and retrospection of negative events. This outcome can be seen in the study by MacLeod et al. (1997b) , who reported that anxious individuals both retrospectively and prospectively generated more negative events than did controls and even individuals with depression. Similarly, when asked to recall recent life events, 77% of anxious participants remembered danger events compared to 44% of depressed participants ( Finlay-Jones and Brown, 1981 ). Loss events, on the other hand, were more frequently remembered by participants with depression (65%) compared with anxiety (15%). Comparable findings for autobiographical recall have been reported by Witheridge et al. (2010) .

Thus, in our view, the interpreter plays a key role in the functioning of the mental time travel component. The content of the events that are remembered in depression is more likely to deal with loss rather than danger. This, we suggest, occurs because of the bias of the interpreter on mental time travel. In anxiety disorders, retrospection is not impaired; if anything, there is excessive rather than impaired prospection and retrospection about threatening events. In addition to mental time travel being moderated by the interpreter, we further consider in the next section the possible influence of executive attention.

Executive Attention Deficits in Depression and Anxiety

A number of studies have shown that anxiety and depression are associated with impaired performance on a variety of neuropsychological tests that measure for executive control functions ( Reinholdt-Dunne et al., 2013 ; Devito et al., 2018 ). This supports the notion that both disorders are associated with impairments in executive attentional control. In this article, we refer to executive attention as executive attentional control and attentional control, interchangeably. In accordance with Stefanopoulou et al. (2014 , p. 330), attentional control can be defined as “the ability to sustain focus on tasks in the face of competing activities or to shift attention from one task to another.” However, depression and anxiety do not show the same pattern of executive attention deficits.

The Attentional Control Scale (ACS) is a self-reported attention control measure that is comprised of two components: focusing and shifting ( Reinholdt-Dunne et al., 2013 ). Ólafsson et al. (2011 , p.77) define attentional focusing as “the capacity to intentionally hold the attentional focus on desired channels and thereby resist unintentional shifting to irrelevant or distracting channels” and define attentional shifting as “the capacity to intentionally shift the attentional focus to desired channels, thereby avoiding unintentional focusing on particular channels.” Because it has been noted that those with anxiety show attentional impairment in relation to shifting and focusing ( Devito et al., 2018 ), the ACS has been used to compare the relationship between attentional focusing, attentional shifting, and levels of anxiety and depression in adults. Ólafsson et al. (2011) found that when controlled for depression, the focusing ACS subscale significantly predicted anxiety ratings, whereas when anxiety ratings were controlled for, the shifting subscale significantly predicted depression ratings. Reinholdt-Dunne et al. (2013) supported these findings when they found ACS focusing to be associated with lower anxiety and ACS shifting to be associated with fewer depression symptoms. These findings support the claim that anxiety is more associated with attentional focusing and depression is more associated with attentional shifting.

Shi et al. (2019) performed a meta-analysis to investigate the size and nature of attentional control deficits in participants with anxiety versus non-anxious participants. They found that anxiety-producing deficits were supported in processing efficiency, rather than effectiveness, on a variety of behavioral tasks. However, they also found that when looking at task switching studies alone, both efficiency and effectiveness produced anxiety-related deficits in attentional control. Their results also showed that studies requiring participants to operate under high cognitive load conditions showed greater anxiety-related attentional control deficits compared to studies where participants were under normal cognitive load conditions ( Shi et al., 2019 ).

Although attentional control deficits have been related to anxiety disorders, these deficits are prominently seen in individuals diagnosed with GAD, characterized by uncontrollable worry. This uncontrollable worry has been connected to deficits of the central executive function of working memory, which includes attentional control as a key component of working memory ( Stefanopoulou et al., 2014 ). Uncontrollable worrying can be attention-demanding and, consequently, consumes voluntary attentional resources required ( Eysenck et al., 2007 ). This links uncontrollable worry to impairments in attentional control.

Stefanopoulou et al. (2014) used the key-pressing task to assess the extent to which attentional resources were depleted by worry in individuals with GAD. Stefanopoulou et al. (2014) found that GAD individuals were less random on the key-pressing task while worrying compared to when thinking of a positive topic, indicating that fewer residual attentional control resources were available during the worrying process. However, the performance of the healthy participants did not differ between conditions. GAD participants also reported having more negative thoughts and anxiety during this task compared to healthy participants. This same study also used the N-back task, which “varies in difficulty and is sensitive to subtle difference in ability to handle increasing demands on attentional control” ( Stefanopoulou et al., 2014 , p. 330). During this task, GAD participants exhibited longer reaction times compared to healthy participants for the higher load conditions. These results together indicate a greater difficulty in sustaining focus in conditions requiring a higher degree of attentional control, suggesting that poor attentional control may partially explain the excessive worry seen in individuals with GAD.

Further, there appears to be a bidirectional relationship between attentional control and anxiety ( Devito et al., 2018 ). Impairments in attentional control may increase one’s risk for developing anxiety, and anxiety symptoms may prevent executive components of attention from being recruited. We indicate this bidirectional relationship between the interpreter and executive attention in Figure 2 . The pessimistic explanatory style and negative self-talk of the interpreter consume limited attentional resources. The resulting deficit in executive attention weakens the ability to inhibit the dysfunctional thinking of the interpreter in anxiety disorders.

Whether a similar bidirectional relationship occurs in depression is unclear. An argument against this takes into account the speech and inner speech of depressed versus anxious individuals based on the symptoms outlined in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition ( DSM-V ). The hallmark of GAD is excessive worry in the form of inner speech. By contrast, in MDD, fatigue and tiredness occur on nearly a daily basis, and this can be accompanied by slowed speech, long pauses before responding, and a decrease in the amount and variety of speech content ( American Psychiatric Association, 2013 , p. 132). These suggest that inner speech in MDD is more likely to be inhibited or overly regulated rather than exaggerated, as is apparent in GAD. Moreover, in a review of the literature on inner speech, Alderson-Day and Fernyhough (2015) noted that the evidence for inner speech playing a central role in anxiety disorders is stronger and more specific than it is with depression. The verbalized worry of anxiety is, in their words (p. 948), “…repetitive thinking that is.negative, uncontrollable, and aimed at some ill-defined problem solving, such as a problem with a clear solution.” We propose that the bidirectional links between executive attention and the interpreter produce worry in GAD that is indeed out of control (see Figure 2 ). A positive feedback loop ensues in which worry depletes attention, which in turn worsens worry. In depression, the negative impact of depleting attention does not appear to feed back on the interpreter. Instead, we suggest, it feeds forward to impact mental time travel. Specifically, the deficit in executive attention found in depression results in a loss of control in mental time travel (see Figure 1 ). The arrows shown in Figures 1 , ​ ,2 2 are intended to reflect the major pathways of influence from one component to another. From the perspective of the ensemble hypothesis, all possible links among components are potentially relevant, including bidirectional relationships. In a normally developed and well-functioning adult human being, each of these components influences the others. Our aim in these figures is to take a minimalist approach by highlighting only strong interactions that differ from normal under a diagnosis of psychopathology. The purpose is to differentiate as clearly as possible how MDD and GAD differ from each other. For example, we intentionally omit an influence of executive attention on mental time travel in GAD. Although it is known that the availability of executive attention affects the functioning of mental time travel even in healthy individuals, we only indicate interactions that are unique to GAD or MDD.

Memory Impairment From an Ensemble Perspective

As shown in Figure 1 , we suggest that both retrospection and prospection will be impaired as a result of a deficit in executive attention ( Hertel and Rude, 1991 ; Rude et al., 1999 ). Evidence for a causal role played by attention comes from an intervention designed by Hertel and Rude (1991) to remediate the attentional deficits. Hertel and Rude studied three groups of individuals who were currently depressed, recovered from depression, or without a history of depression in an incidental learning and memory task. The participants’ ability to recall a list of target words that they had viewed in the first phase of the experiment was markedly impaired in the individuals with depression compared with recovered and healthy controls. But this outcome only occurred when their attention to the words during learning was unconstrained by the demands of the task. For half of the participants, the investigators required the participants to repeat the target words aloud on each trial, as a means of focusing their attention. Strikingly, this manipulation eliminated the memory impairment of the depressed patients entirely. This result suggests that retrospection per se is not necessarily deficient in depression, but a memory deficit can be observed as a result of the influence of executive attention not being appropriately allocated to the task at hand.

A comparable finding was reported by McDowall (1984) . On a free recall test, inpatients with depression performed markedly worse than did a control group consisting of non-depressed psychiatric inpatients in remembering pleasant words. However, when given an orienting task of rating each word for pleasantness as was shown during the study phase, patients with depression showed no difference in recall between pleasant and unpleasant words and performed no worse than did the psychiatric control group doing the same task. Their mean recall of 5.6 words out of 12 was only slightly less than was found for a non-psychiatric control group (6.8 words), again with no difference between pleasant versus unpleasant words. As with the word repetition technique used by Hertel and Rude (1991) , the orienting task directed attention to the words in a way that eliminated most, if not all, of the memory impairment for individuals with depression.

Ruminating on negative life experiences is part and parcel of the sense of loss, hopelessness, and self-deprecation frequently seen in persons experiencing depression. In our view, these phenomena are the direct result of the interpreter bias found in depression. It is the influence of the interpreter with mental time travel that contributes to the inability of individuals with depression to think about positive life experiences, whether they lie in the past, the present, or the future. Further, the persistence and intrusiveness of negative memories in depression could reflect an inability to inhibit them because of executive attention deficits (see Figure 1 ). Poor cognitive control may combine with the loss bias of the interpreter to produce the profile of memory problems found in depression.

As shown in Figure 2 , for GAD, the mental time travel system is biased to focus on the uncertainties and threats of life experiences. Instead of loss and self-blame, the content of memories predominately concerns threats to the self in anxiety disorders to the extent that they are biased at all. This can account for why negative events are, at times, better remembered or anticipated by anxious individuals (e.g., MacLeod et al., 1997b ). However, in contrast to the memory bias effects for losses observed in depression, similar effects for threatening events in anxiety disorders are harder to detect reliably ( Mineka and Nugent, 1995 ). They might be found in panic disorder but not GAD ( Becker et al., 1999 ). Or they can be observed with implicit memory tests but not explicit tests of recall or recognition ( Mathews et al., 1989 ; MacLeod and McLaughlin, 1995 ). They might also be observed when people are asked to recall autobiographical events of personal relevance ( Finlay-Jones and Brown, 1981 ; Witheridge et al., 2010 ) but not when they are asked to remember word lists that contain some threatening versus neutral words ( Levy and Mineka, 1998 ).

We suggest that the mixed picture for memory bias in anxiety disorders occurs because executive attention deficits do not generally disrupt mental time travel in persons experiencing GAD, which is not the case for MDD (see Figure 2 ). The deficit in executive attention causes a loss of control with the interpreter but not with mental time travel. Without both a loss of cognitive control and a threat bias from the interpreter, the mental time travel system functions relatively normally in GAD. That implicit tests of memory reveal bias effects for negative information implies that a threat bias from the interpreter is at work. But for the declarative memory system of episodic memory to show such effects, it requires both the threat bias and a loss of cognitive control over mental time travel. Perhaps only in severe cases of anxiety disorders, such as panic disorder, does the loss of cognitive control from deficits in executive attention spill over to affect mental time travel, much as it does in depression. This could account for the results of Becker et al. (1999) for panic disorder in contrast with other forms of anxiety disorder. It is worth noting that MacLeod et al. (1997b) studied anxious participants who all met the criteria for panic disorder. Thus, the characteristics of their sample might have explained why they observed a bias for negative events when so many other studies have been unable to do so, as they noted in their discussion section.

In summary, accounting for the consistent memory bias for losses or a lack of positive memories in MDD seems to depend on distorting inputs from both executive attention and the interpreter (see Figure 1 ). For persons experiencing GAD without panic disorder, the input from executive attention is weak or non-existent. Without this concomitant symptomatology, the bias of the interpreter for threatening events does not distort either retrospective or prospective memory, although it shows up on implicit, non-declarative forms of memory.

Limitations, Implications, and Future Directions

As noted previously, our explication of the complex role of mental time travel in explaining the phenomenology and research findings related to MDD and GAD has focused on interrelationships between three of the five components of the ensemble hypothesis. In focusing on these three constructs, we acknowledge the limited attention we have given to the importance of the two remaining ensemble components—overt use of language and social cognition—in accounting for differences and similarities in MDD and GAD. Reviewing the broader concept of language as interpersonal communication falls outside the scope of the current paper. Similarly, the extensive literature on theory of mind and social cognition in disorders such as MDD and GAD merits careful consideration that is not undertaken by our current analysis. Research indicates that theory of mind, a specialized aspect of social cognition ( Frith and Frith, 2007 ), plays a complex role in presentations of depression and anxiety where aspects of social cognition are prominent ( Bora and Berk, 2016 ; Washburn et al., 2016 ). Examples would include depression in the context of discordant relationships or bereavement, and social anxiety disorder. Exploring the interrelationships between social cognition and other components of the ensemble hypothesis is a fruitful direction for further theorizing and research.

Also, our paper is limited in scope, in that we focused on accounting for differences between disorders such as MDD and GAD, rather than examining similarities in their phenomenology and accounting for the high comorbidity of these conditions. We believe that further analysis of the interrelationships among the ensemble of mental components in MDD and GAD may help account for the comorbidity of these two disorders. For example, the high incidence of comorbidity might be accounted for by the reciprocal relations between the cognitive ensemble components and symptoms that constitute pathways that connect the disorders ( Borsboom and Cramer, 2013 ). It is worth noting the strong similarities of MDD and GAD as portrayed in Figures 1 , ​ ,2. 2 . Both disorders involve several components of the ensemble hypothesis, including executive attention and the interpreter, in addition to mental time travel. The specific characteristics of memory functioning seem to depend on these interrelated cognitive components of the ensemble perspective. Thus, future theorizing and research should explore the interrelated components of the ensemble hypothesis as they relate to comorbid presentations of MDD and GAD.

Regarding one final limitation of our paper, we acknowledge that the ensemble component of “mental time travel” as it pertains to episodic foresight involves multiple constructs, each with substantive theoretical and empirical literatures that lie beyond the scope of our paper. Examples would include the role of mental time travel in future decision making involving delayed rewards ( Boyer, 2008 ) and the literature on “affective forecasting” ( Wilson and Gilbert, 2005 ) as it relates to the ensemble components in persons experiencing depression or anxiety. Once again, future theorizing and research should explore the interrelationships of such constructs with the ensemble components as they pertain to the etiology and phenomenology of MDD and GAD.

In review, we believe that similarities and differences between MDD and GAD are best conceptualized by considering an ensemble of mental components. Although mental time travel plays a role in both disorders, this component is influenced by the interpreter that assigns causal attributions to events and a dysfunction in executive attention.

If depression is primarily a problem with faulty prospection, then it is reasonable to target future thinking as perhaps the most effective form of treatment. Roepke and Seligman (2016) reviewed four variations of CBT that emphasize positive expectancies, hopeful thinking, a focus on future-oriented solutions to problems, and goal setting and planning. Initial results with each of these approaches have been positive and are worthy of additional study in randomized trials. Further, Roepke and Seligman (2016) suggest several new future-oriented interventions that might be considered (e.g., using visual imagery to imagine a route to future success).

While new approaches certainly merit exploration, we note that the premise underlying these—namely, that faulty prospection is the core causal process in depression—is open to debate. We believe that the effects of the interpreter and executive attention, in conjunction with mental time travel, should be considered to better understand both MDD ( Figure 1 ) and GAD ( Figure 2 ). From this ensemble perspective, therapies should target all three components rather than focusing only on mental time travel.

For example, mindfulness-based therapies including short-term meditation explicitly address deficits in executive attention. A short-term program (5 days of training for 20 min per day) has been shown to improve attention and self-regulation in a sample of healthy young adults ( Tang et al., 2007 ). Such mindfulness-based interventions have been shown to minimize relapse and offer promise in the treatment of acute symptoms of depression and anxiety, although more research is needed to clearly establish their clinical efficacy ( Edenfield and Saeed, 2012 ). In a different approach, training attention using computer-based tasks has been found beneficial in treating generalized social anxiety disorder ( Schmidt et al., 2009 ). If anxiety disorders as well as depression primarily are influenced by the mental time travel component ( Miloyan et al., 2014 ), then it is difficult to explain why treatments targeting the executive attention deficit would be effective. Yet, it is known that executive functioning matters. Although neurocognitive abilities can improve with CBT treatment for anxiety and depression, individuals with poor attentional control show decreased benefit from such treatment compared to those with adequate executive skills ( Devito et al., 2018 ).

Many techniques in traditional CBT build on the premise of altering the pessimistic explanatory styles employed by depressive and anxious individuals. These techniques are based on the premise that the symptoms and dysfunctional behaviors of these disorders are mediated by cognitive factors. The therapeutic goal, then, is to restructure the dysfunctional thinking and beliefs underlying the disorder. Cognitive distortions must be identified and refuted in restructuring the functions of the interpreter. The evidence supporting CBT as an effective treatment of both anxiety and depression is solid ( Butler et al., 2006 ). As Roepke and Seligman (2016) pointed out, CBT interventions already include a number of techniques that improve future thinking. Even so, the aim of CBT is to alter thinking patterns in general, including past and present thinking as well as future thinking. It is not clear that new approaches that emphasize future-oriented thinking only would be, or even should be, superior to standard CBT.

In terms of future directions, transdiagnostic psychotherapies for depression and anxiety ( Clark, 2009 ) could potentially be understood within and informed by the aspects of the ensemble hypothesis. The ensemble models shown in Figures 1 , ​ ,2 2 suggest that a unified approach to CBT plus mindfulness/attention training might well be plausible for treating both depressive and anxiety disorders. Finally, in recent years, network approaches to psychopathology have emphasized the interplay of symptoms across a variety of traditionally defined, yet comorbid, disorders ( Borsboom and Cramer, 2013 ). The psychopathology network approach contends that such emotional disorders arise from interactions among symptoms, as well as their reciprocally reinforcing relationships ( Borsboom, 2017 ). It may be possible to conceptualize these networks of psychopathology within the context of the ensemble hypothesis of human cognition considered here.

Data Availability Statement

Author contributions.

RK developed the concept of the paper. RK and CC wrote the first draft. JG contributed with advice and revisions to subsequent drafts. All authors reviewed the final manuscript.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding. The funds required for open access fees required to publish this article were provided through Faculty Development accounts available to the first and third authors from the Department of Psychology at Saint Louis University.

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Encyclopedia of Global Archaeology pp 1–11 Cite as

Archaeology and Time Travel

• Cornelius Holtorf 2 &
• Bodil Petersson 2  
• Living reference work entry
• First Online: 28 July 2018

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Time travel is a characteristically contemporary way of approaching the past. If reality is defined as the sum of human experiences and social practices, all reality is partly virtual, and all experienced and practiced time travel is real. In that sense, time travel experiences are not necessarily purely imaginary. Time travel experiences and associated social practices have become ubiquitous and popular, increasingly replacing more knowledge-oriented and critical approaches to the past. Our discussion covers some of the implications and problems associated with the ubiquity and popularity of time travelling including the benefits of methodical anachronism. The deliberate use of anachronism is an important method in understanding ourselves and the nature of knowledge gained about the past. We also discuss whether time travel is inherently conservative because of its escapist tendencies, or whether it might instead be considered as a fulfillment of the contemporary Experience or Dream Society. Whatever position one may take, time travel is a legitimate and timely object of study and critique because it represents a particularly significant way of bringing the past back to life in the present.

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Holtorf, C., Petersson, B. (2018). Archaeology and Time Travel. In: Encyclopedia of Global Archaeology. Springer, Cham. https://doi.org/10.1007/978-3-319-51726-1_2793-1

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The Physics of Time Travel

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Is it real, or is it fable?

In H.G. Wells’ novel, The Time Machine, our protagonist jumped into a special chair with blinking lights, spun a few dials, and found himself catapulted several hundred thousand years into the future, where England has long disappeared and is now inhabited by strange creatures called the Morlocks and Eloi. That may have made great fiction, but physicists have always scoffed at the idea of time travel, considering it to be the realm of cranks, mystics, and charlatans, and with good reason.

However, rather remarkable advances in quantum gravity are reviving the theory; it has now become fair game for theoretical physicists writing in the pages of Physical Review magazine. One stubborn problem with time travel is that it is riddled with several types of paradoxes. For example, there is the paradox of the man with no parents, i.e. what happens when you go back in time and kill your parents before you are born? Question: if your parents died before you were born, then how could you have been born to kill them in the first place?

There is also the paradox of the man with no past. For example, let’s say that a young inventor is trying futilely to build a time machine in his garage. Suddenly, an elderly man appears from nowhere and gives the youth the secret of building a time machine. The young man then becomes enormously rich playing the stock market, race tracks, and sporting events because he knows the future. Then, as an old man, he decides to make his final trip back to the past and give the secret of time travel to his youthful self. Question: where did the idea of the time machine come from?

There is also the paradox of the man who is own mother (my apologies to Heinlein.) “Jane” is left at an orphanage as a foundling. When “Jane” is a teenager, she falls in love with a drifter, who abandons her but leaves her pregnant. Then disaster strikes. She almost dies giving birth to a baby girl, who is then mysteriously kidnapped. The doctors find that Jane is bleeding badly, but, oddly enough, has both sex organs. So, to save her life, the doctors convert “Jane” to “Jim.”

“Jim” subsequently becomes a roaring drunk, until he meets a friendly bartender (actually a time traveler in disguise) who wisks “Jim” back way into the past. “Jim” meets a beautiful teenage girl, accidentally gets her pregnant with a baby girl. Out of guilt, he kidnaps the baby girl and drops her off at the orphanage. Later, “Jim” joins the time travelers corps, leads a distinguished life, and has one last dream: to disguise himself as a bartender to meet a certain drunk named “Jim” in the past. Question: who is “Jane’s” mother, father, brother, sister, grand- father, grandmother, and grandchild?

Not surprisingly, time travel has always been considered impossible. After all, Newton believed that time was like an arrow; once fired, it soared in a straight, undeviating line. One second on the earth was one second on Mars. Clocks scattered throughout the universe beat at the same rate. Einstein gave us a much more radical picture. According to Einstein, time was more like a river, which meandered around stars and galaxies, speeding up and slowing down as it passed around massive bodies. One second on the earth was Not one second on Mars. Clocks scattered throughout the universe beat to their own distant drummer.

However, before Einstein died, he was faced with an embarrassing problem. Einstein’s neighbor at Princeton, Kurt Goedel, perhaps the greatest mathematical logician of the past 500 years, found a new solution to Einstein’s own equations which allowed for time travel! The “river of time” now had whirlpools in which time could wrap itself into a circle. Goedel’s solution was quite ingenious: it postulated a universe filled with a rotating fluid. Anyone walking along the direction of rotation would find themselves back at the starting point, but backwards in time!

In his memoirs, Einstein wrote that he was disturbed that his equations contained solutions that allowed for time travel. But he finally concluded: the universe does not rotate, it ex- pands (i.e. as in the Big Bang theory) and hence Goedel’s solution could be thrown out for “physical reasons.” (Apparently, if the Big Bang was rotating, then time travel would be possible throughout the universe!)

Then in 1963, Roy Kerr, a New Zealand mathematician, found a solution of Einstein’s equations for a rotating black hole, which had bizarre properties. The black hole would not collapse to a point (as previously thought) but into a spinning ring (of neutrons). The ring would be circulating so rapidly that centrifugal force would keep the ring from collapsing under gravity. The ring, in turn, acts like the Looking Glass of Alice. Anyone walking through the ring would not die, but could pass through the ring into an alternate universe. Since then, hundreds of other “wormhole” solutions have been found to Einstein’s equations. These wormholes connect not only two regions of space (hence the name) but also two regions of time as well. In principle, they can be used as time machines.

Recently, attempts to add the quantum theory to gravity (and hence create a “theory of everything”) have given us some insight into the paradox problem. In the quantum theory, we can have multiple states of any object. For example, an electron can exist simultaneously in different orbits (a fact which is responsible for giving us the laws of chemistry). Similarly, Schrodinger’s famous cat can exist simultaneously in two possible states: dead and alive. So by going back in time and altering the past, we merely create a parallel universe. So we are changing someone ELSE’s past by saving, say, Abraham Lincoln from being assassinated at the Ford Theater, but our Lincoln is still dead. In this way, the river of time forks into two separate rivers. But does this mean that we will be able to jump into H.G. Wells’ machine, spin a dial, and soar several hundred thousand years into England’s future? No. There are a number of difficult hurdles to overcome.

First, the main problem is one of energy. In the same way that a car needs gasoline, a time machine needs to have fabulous amounts of energy. One either has to harness the power of a star, or to find something called “exotic” matter (which falls up, rather than down) or find a source of negative energy. (Physicists once thought that negative energy was impossible. But tiny amounts of negative energy have been experimentally verified for something called the Casimir effect, i.e. the energy created by two parallel plates). All of these are exceedingly difficult to obtain in large quantities, at least for several more centuries!

Then there is the problem of stability. The Kerr black hole, for example, may be unstable if one falls through it. Similarly, quantum effects may build up and destroy the wormhole before you enter it. Unfortunately, our mathematics is not powerful enough to answer the question of stability because you need a “theory of everything” which combines both quantum forces and gravity. At present, superstring theory is the leading candidate for such a theory (in fact, it is the ONLY candidate; it really has no rivals at all). But superstring theory, which happens to be my specialty, is still too difficult to solve completely. The theory is well-defined, but no one on earth is smart enough to solve it.

Interestingly enough, Stephen Hawking once opposed the idea of time travel. He even claimed he had “empirical” evidence against it. If time travel existed, he said, then we would have been visited by tourists from the future. Since we see no tourists from the future, ergo: time travel is not possible. Because of the enormous amount of work done by theoretical physicists within the last 5 years or so, Hawking has since changed his mind, and now believes that time travel is possible (although not necessarily practical). (Furthermore, perhaps we are simply not very interesting to these tourists from the future. Anyone who can harness the power of a star would consider us to be very primitive. Imagine your friends coming across an ant hill. Would they bend down to the ants and give them trinkets, books, medicine, and power? Or would some of your friends have the strange urge to step on a few of them?)

In conclusion, don’t turn someone away who knocks at your door one day and claims to be your future great-great-great grandchild. They may be right.

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The Scholarly Kitchen

What’s Hot and Cooking In Scholarly Publishing

A Time Travel Guide to Ancient Rome

Looking back at  The Scholarly Kitchen over the years, it becomes clear that rather than a blog about scholarly communications, it’s true purpose is to prepare readers for the eventualities of time travel. Previous posts include a review of Ryan North’s guide to reinventing civilization for the stranded time traveler, videos giving an overview of the English language through 13 centuries and showing how far back you could go and still understand people , and one offering a system for diagramming your temporal adventures .

All important subjects, but what if you want to travel back to Ancient Rome? Don’t worry, author and historian Garrett Ryan has you covered, with just the necessary video series. In Part 1, below, you’ll learn all the essentials for fitting in like a native, because as he puts it, “It’s gauche to explore Rome in sweatpants.” Instructions include when to go visit (not during plagues or the reign of Caligula), how to dress (tunics, not togas), what to bring (spices like cinnamon, worth its weight in gold at the time), and important Latin phrases (“Ubi est latrina?”). The series continues, but hopefully this will get you started, and I look forward to seeing you around the Colosseum.

David Crotty

David Crotty is a Senior Consultant at Clarke & Esposito, a boutique management consulting firm focused on strategic issues related to professional and academic publishing and information services. Previously, David was the Editorial Director, Journals Policy for Oxford University Press. He oversaw journal policy across OUP’s journals program, drove technological innovation, and served as an information officer. David acquired and managed a suite of research society-owned journals with OUP, and before that was the Executive Editor for Cold Spring Harbor Laboratory Press, where he created and edited new science books and journals, along with serving as a journal Editor-in-Chief. He has served on the Board of Directors for the STM Association, the Society for Scholarly Publishing and CHOR, Inc., as well as The AAP-PSP Executive Council. David received his PhD in Genetics from Columbia University and did developmental neuroscience research at Caltech before moving from the bench to publishing.

3 Thoughts on "A Time Travel Guide to Ancient Rome"

I think the email announcing this post wins the ‘marketing copy of the month’ award, for opening with: “The Scholarly Kitchen has, over the years, offered significant resources for the potential time traveler…”

• By Mark Carden
• Jul 29, 2022, 8:45 AM

People who like time travel stories with a comic twist should consider the Chronicles of St. Mary’s series by Jodi Taylor, perhaps starting with the short stories (including one set in ancient Rome): https://bookshop.org/books/the-long-and-short-of-it-stories-from-the-chronicles-of-st-mary-s/9781597809153 For a darker take on time travel, I recommend Time and Time Again by Ben Elton. https://www.goodreads.com/book/show/23164931-time-and-time-again

• Jul 29, 2022, 8:54 AM

And of course the Ian Mortimer Time Traveler Guides to medieval, renaissance and Restoration England….

• By Alex Barker
• Aug 1, 2022, 1:34 PM

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