essay on time travel to future

1000-Word Philosophy: An Introductory Anthology

Philosophy, One Thousand Words at a Time

Time Travel

Author: Taylor W. Cyr Category: Metaphysics Word Count: 1000

Time travel is familiar from science fiction and is interesting to philosophers because of the metaphysical issues it raises: the nature of time, causation, personal identity, and freedom, among others. [1]

It’s widely accepted that time travel to the future is possible, but the possibility of backward time travel remains hotly debated. [2] This article will sketch some models of backward time travel (hereafter simply “time travel”) before addressing the main objections to its possibility. [3]

1. Models of Time Travel

According to the standard model of time travel, time is linear so a time traveler’s journey may be depicted along a single timeline, with some events that occur earlier in the timeline’s being experienced as later by the traveler: [4]

On another model, time travel results in the creation of a new universe that branches out from the same trunk (shared past) as the original:

A third model of time travel maintains that there is a second temporal dimension, and so, in addition to times, there are “hyper-times.” [5] On this model, time is more like a plane than like a line, and a time traveler may, in returning to an earlier time, reach that time at a later hyper-time, with the result that the aforementioned time bears different properties at the different hyper-times: [6]

2. Changing the Past

It is natural to suppose that time travel would change the past, which many believe is impossible. Changing the past would require that the past have a certain property at one “time” and then lack that property at another “time.” This is incoherent on the standard model of time travel, which maintains that time is linear (there is no “second time around”), so the standard model precludes changing the past.

But time travel doesn’t require changing the past. We may distinguish changing the past from affecting the past, where the latter requires only that the time traveler’s travels have effects in the past. [7] For example, suppose a time traveler finds her younger self and attempts to convince herself not to time travel. [8] Assuming the standard model of time travel, she will fail to prevent herself from time traveling, but the attempt will affect how the past was “all along,” so to speak. From the outside, the scene will look like an ordinary conversation between two people, but, assuming the time traveler remembers the scene, she will remember an older version of herself trying to convince her not to time travel. [9]

Moreover, according to the other two models of time travel, one and the same time may exist in two different universes or hyper-times, and so it isn’t obviously incoherent to state that some past time may have a property at one “time” (either in one universe, or at one hyper-time) that it lacks at another “time” (in another universe, or at another hyper-time). [10]

3. Causal Loops

Consider some events from the television show Lost . [11] At one point, Richard gives a compass to Locke, telling him to return it the next time they meet. Locke then travels back in time, sees a younger Richard, and returns the compass, which Richard keeps until he gives it to Locke in the aforementioned meeting.

The Lost compass is strange. It was not created in the usual way—in fact, it has no creator! It appeared (with Locke) at time t1 (when it was given to Richard), remained with Richard at a later time t2, and then was given to Locke at t3, when Locke set out for t1, resulting in a “causal loop.” At each time t1-t3, there is a causal explanation for the compass’s presence by reference to the prior stage in the loop. But no explanation can be given for the loop itself. (Where did the compass come from to begin with? There is no answer.)

Now, if such cases are impossible, this might cast doubt on the possibility of time travel. As David Lewis says in response, however, such cases “are not too different from inexplicabilities we are already inured to” such as “God, or the Big Bang, or the decay of a tritium atom,” all of which are “uncaused and inexplicable” (1976: 149).

Note that this objection assumes the standard model of time travel, since these strange loops do not necessarily result from time travel on the other models. Moreover, it may be possible for there to be cases of time travel that don’t generate causal loops even assuming the standard model. [12]

4. Time Travelers’ Abilities

Suppose Tim time travels and attempts to kill his Grandfather before his parents are conceived. Assuming Tim has a gun, is a good shot, etc., it would seem that Tim can kill Grandfather. But Tim can’t kill Grandfather, for doing so would preclude his own existence. Tim both can and can’t kill Grandfather: that’s a contradiction, so we should give up the assumption that led to it, namely that time travel is possible.

This is the Grandfather Paradox, and it is the main objection to the possibility of time travel. Here are two responses, both of which assume the standard model of time travel. [13]

First, one might understand “can” claims like “Tim can kill Grandfather” as claims about what is possible in view of certain facts—and which facts are held fixed is determined by the context of utterance. [14] For example, in view of Tim’s possession of a gun, his reliable aim, etc., it is true that Tim can kill Grandfather. But if we also hold fixed the fact that Grandfather lives , then Tim’s killing Grandfather isn’t possible, and thus he can’t kill Grandfather. So, there is no contradiction; it is true that Tim can kill Grandfather holding certain facts fixed, and it is false holding more fixed, but the claim is not both true and false in the same context. [15]

A second approach denies that Tim can kill Grandfather. [16] This denial follows from certain independently motivated views of agents’ abilities, and it avoids the Paradox by restricting the freedom of time travelers.

5. Conclusion

Perhaps time travel is (metaphysically) possible, but it doesn’t follow that it’s technologically feasible, or that it will ever actually occur. Only time will tell.

[1]   While not the first philosophical discussion of time travel, David Lewis’s classic 1976 essay “The Paradoxes of Time Travel” popularized the subject in metaphysics. For a recent philosophical discussion of time travel—an excellent summary of several facets of the debate, as well as some new developments—see Wasserman (2018).

[2]   By “possibility” I mean metaphysical possibility—consistency with the laws of metaphysics, such as the laws of causation, identity, etc. For more on the discussion of the various senses of possibility we might be asking about in connection with time travel, see Wasserman (2018, chapter 1), and see the rest of the same book for a summary of the debate about the metaphysical possibility of backward time travel.

[3]   There are other objections, but there isn’t space to consider all of them here. One objection concerns its likelihood rather than its possibility . As we will see below, there are certain things that it would seem time travelers cannot do, and so if time travelers attempted the impossible, something would prevent them from succeeding (perhaps the time traveler would have a change of heart, or perhaps she would slip on a banana peel, or…). Horwich (1987) argues that since backward time travel would result in such improbable events, this casts doubt on the likelihood of time travel. See Smith (1997) for discussion and a response to Horwich.

[4]   See the first figure. Reprinted from Wasserman (2018, chapter 3) with permission of Ryan Wasserman and Oxford University Press.

[5]   For developments of the hyper-time model, see Meiland (1974), Goddu (2003), and van Inwagen (2010).

[6]   If we graphed the two dimensions of time on a plane, with the temporal dimension along the x- axis and the hyper-temporal dimension along the y -axis, as in the third figure, time travel would amount to moving leftward (back in time) and upward (forward in hyper-time).

[7]   As Brier explains, “One cannot change the past or undo what has been done. Rather, what is at issue is whether one can affect the past; that is, by a present action cause something to have happened which would not have happened otherwise” (1973: 361).

[8]   For a simple example of this from science-fiction, see the film Interstellar . After leaving Earth, Cooper is able to send messages back in time, and he uses his first message to try to get his daughter to make him stay on Earth, as seen here .

[9]   For another example of affecting (but not changing) the past, see J. K. Rowling’s Harry Potter and the Prisoner of Azkaban . An especially excellent case of time travel occurs toward the end of the book when Hermione takes Harry back in time, allowing him to save himself from Dementors. In the film version, we see Harry attacked by (but saved from) Dementors here , and then we see Hermione take Harry back in time here , and finally, we see Harry save himself here .

[10]   It is contentious whether these models of time travel really allow for changing the past. See Smith (1997, 2015) and Baron (2017) for arguments against, and see Law (Forthcoming) for a response.

[11]   The first of these occurs in the third episode of season five, “Jughead,” from 39:44-41:19,  and the second scene occurs in the first episode of season five, “Because You Left,” from 29:30-34:34.

[12]   For example, suppose I travel back in time by twenty seconds but set my machine to a destination on the other side of the planet. Presumably, my appearance in the past will not have any causal consequences across the globe, despite its occurring twenty seconds earlier than my departure, and thus no causal loop will be generated. For a similar example, see Hanley (2004: 130).

[13]   On the other models, there is no reason to think that Tim can’t kill Grandfather, for doing so would preclude Tim’s future birth in the new timeline (the new branch or hyper-time), but Grandfather would not have been killed in the original, and thus Tim is still born in that timeline.

[14] See Kratzer (1977).

[15]   While Lewis’s (1776: 149-152) influential response to the Paradox also relies on the Kratzer semantics for “can,” his proposed resolution is slightly different, for he sees the fact that Grandfather lives as one that it would be illegitimate to hold fixed. Holding it fixed, he thinks, amounts to “fatalist trickery,” as such a fact “is an irrelevant fact about the future masquerading as a relevant fact about the past” (1976: 151).

[16]   See Vihvelin (1996).

Baron, Sam (2017). “Back to the Unchanging Past,” Pacific Philosophical Quarterly 98: 129–147.

Brier, Bob (1973). “Magicians, Alarm Clocks, and Backward Causation,” Southern Journal of Philosophy 11: 359-364.

Goddu, G. C. (2003). “Time Travel and Changing the Past (or How to Kill Yourself and Live to Tell the Tale),” Ratio 16: 16-32.

Hanley, Richard (2004). “No End in Sight: Causal Loops in Philosophy, Physics, and Fiction,” Synthese 141: 123-152.

Horwich, Paul (1997). Asymmetries In Time: Problems In the Philosophy of Science . Cambridge, MA: MIT Press.

Kratzer, Angelika (1977). “What ‘Must’ and ‘Can’ Must and Can Mean,” Linguistics and Philosophy 1: 337-355.

Law, Andrew (Forthcoming). “The Puzzle of Hyper-Change,” Ratio .

Lewis, David (1976). “The Paradoxes of Time Travel,” American Philosophical Quarterly 13: 145-152.

Meiland, Jack (1974). “A Two-Dimensional Passage Model of Time for Time Travel,” Philosophical Studies 26: 152-173.

Smith, Nicholas J. J. (1997). “Bananas Enough for Time Travel?” The British Journal for the Philosophy of Science 48: 363-389.

Smith, Nicholas J. J. (2015). “Why Time Travellers (Still) Cannot Change the Past,” Revista Portuguesa de Filosofia 71: 677–694.

van Inwagen, Peter (2010). “Changing the Past,” in D. Zimmerman, ed., Oxford Studies in Metaphysics , vol. 5. Oxford: Oxford University Press.

Vihvelin, Kadri (1996). “What Time Travelers Cannot Do,” Philosophical Studies 81: 315-330.

Wasserman, Ryan (2018). Paradoxes of Time Travel . New York: Oxford University Press.

Related Essays

Philosophy of Space and Time: Are  the  Past and Future Real ?  by Dan Peterson

Personal Identity by Chad Vance

Free Will and Free Choice  by Jonah Nagashima

Translation

This essay has been translated into Italian for the Italian cultural magazine  L’Indiscreto .

About the Author

Taylor W. Cyr is an Assistant Professor of Philosophy at Samford University. His main research interests lie at the intersection of ethics and metaphysics, including such topics as free will, moral responsibility, death, and time. His work has appeared in such journals as Ethics , Philosophical Studies , Philosophical Quarterly , and Erkenntnis . TaylorWCyr.com

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May 14, 2020

Time travel into the future is totally possible

by Paul M. Sutter, Universe Today

Believe it or not, time travel is possible.

In fact, you're doing it right now. Every single second of every single day, you are advancing into your own future. You are literally moving through time, the same way you would move through space. It may seem pedantic, but it's a very important point. Movement through time is still movement, and you are reaching your own future (whether you like it or not).

And what's even cooler is that you can skip forward in time if you feel like it.

Well, let me be clear, you need to do a little bit of engineering first.

We know through the physics of Einstein's special theory of relativity that you can trade motion in space for motion in time. If you're standing perfectly still, you're moving through the dimension of time at a particular speed (the speed of light , for those of you who are curious). As soon as you start moving through space, however, you slow down your rate of moving through time.

In other words, the faster you move in space , the slower you move in time.

This means that moving objects, like a clock on a rocket, run a little bit slow. One second for someone in a moving spaceship lasts a little bit longer than a second for someone staying still.

The trick is that in order for this to have any sort of noticeable impact, you have to get close to the speed of light, which is really hard to do—to give you some perspective, astronauts that orbit the Earth at tens of thousands of miles per hour are off by only a microsecond or so from our clocks on the ground.

Our fastest human spacecraft don't even crack a tenth of a percent of the speed of light. But if you could somehow spend a good amount of time hugging close to that ultimate speed limit in the universe, the slower your clock will run. You will travel through time into the future. Nothing will feel different for you, but after a couple of years' journey you would return to the Earth to find our clocks advanced by thousands or even tens of thousands of years, depending on how fast you go.

So the future is yours, and you get to choose how quickly you reach it.

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Time Travel

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

1.1 Time Discrepancy

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2. The Grandfather Paradox

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

We can set out Horwich’s argument this way:

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

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

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

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

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

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

3. Causation

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

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

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

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

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

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

4. Time and Change

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

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

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

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

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

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

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

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

Earlier we posed two questions:

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

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

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

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

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

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

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

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

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

• Time Travel , entry by Joel Hunter (Truckee Meadows Community College) in the Internet Encyclopedia of Philosophy .

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

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

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

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

Turns out, King might have been on to something.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• grandfather paradox
• time travel

Essay on Time Travel

Students are often asked to write an essay on Time Travel in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Time Travel

What is time travel.

Time travel means moving back or forward in time. It’s like skipping chapters in a book. Instead of reading in order, you jump to the past or future. Imagine going back to see dinosaurs or forward to meet robots. It’s a popular idea in stories and movies.

Time Machines

In stories, people use time machines to travel in time. These machines can be anything from cars to watches. The idea is to control time as easily as turning a page in a book. But in real life, we can’t do this yet.

Why Time Travel Fascinates Us

Time travel excites us because it offers endless possibilities. We could fix mistakes or see what the future holds. It’s about exploring unknowns and asking “what if?” This curiosity makes time travel a captivating concept for all ages.

Also check:

• Paragraph on Time Travel

250 Words Essay on Time Travel

What is time travel.

Time travel is the idea of being able to move forward or backward in time. It’s a popular concept in science fiction stories, but is it actually possible?

Is time travel possible?

We don’t know for sure if time travel is possible. Some scientists believe that it might be possible, but others think that it’s impossible. There are a lot of different theories about how time travel might work, but none of them have been proven.

How would time travel work?

There are a lot of different ideas about how time travel might work. One idea is that it might be possible to travel through a wormhole. A wormhole is a kind of shortcut through space-time. If you could find a wormhole, you could use it to travel to another time.

Would time travel be dangerous?

Time travel could be very dangerous. If you traveled to the past, you could accidentally change something that would have a big impact on the future. You could also end up in a time where you don’t know anyone or anything.

Time travel is a fascinating idea, but it’s important to remember that it’s just a theory. We don’t know for sure if it’s possible, and we don’t know how it would work. Even if it is possible, it would be very dangerous.

500 Words Essay on Time Travel

Time travel is a concept that has captured the imagination of humans for centuries. It is the idea of being able to move through time, either forward or backward, to a different point in history. While time travel remains a theoretical concept, it has been a popular theme in science fiction books, movies, and TV shows.

Theories of Time Travel

There are several different theories about how time travel might be possible. Some theories suggest that it could be possible to travel through wormholes, which are hypothetical tunnels through spacetime. Other theories suggest that it might be possible to travel through time by bending spacetime, or by creating a closed timelike curve.

The Grandfather Paradox

One of the biggest challenges to the idea of time travel is the grandfather paradox. This paradox states that if someone were to travel back in time and kill their own grandfather, it would create a logical contradiction. If the grandfather is killed, then the person who traveled back in time would never have been born. However, if the person was never born, then they would not have been able to travel back in time to kill their grandfather in the first place.

The Possibility of Time Travel

Despite the challenges, some scientists believe that time travel might be possible. They argue that the laws of physics do not explicitly rule out the possibility of time travel. However, they also admit that it is very unlikely that time travel will ever be possible for humans.

The Implications of Time Travel

If time travel were ever to become possible, it would have profound implications for our understanding of history, science, and the universe. It would allow us to learn about the past in a way that is not currently possible. It would also allow us to test scientific theories in a way that is not currently possible. And it would allow us to explore the universe in a way that is not currently possible.

That’s it! I hope the essay helped you.

If you’re looking for more, here are essays on other interesting topics:

• Essay on Time Is Gold
• Essay on Time In Life
• Essay on Tibet

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Time Travel: Dream or Possible Reality?

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Time travel is a favorite plot device in science fiction stories and movies. Perhaps the most famous recent series is Dr. Who , with its traveling Time Lords who whisk throughout time as if traveling by jet. In other stories, the time travel is due to unexplainable circumstances such as a too-close approach to a very massive object like a black hole. In Star Trek: The Voyage Home , the plot device was a trip around the Sun that hurled Kirk and Spock back to 20th century Earth. In the popular movie series Back to the Future , the characters traveled both backward and forward in time. However it is described in stories, traveling through time seems to pique people's interest and ignite their imaginations. But, is such a thing possible? 

The Nature of Time

It's important to remember that we are always traveling into the future. That's the nature of space-time. This is why we remember the past (instead of "remembering" the future). The future is largely unpredictable because it hasn't happened yet, but everyone is headed into it all the time.

To speed up the process, to peer further into the future, to experience events more quickly than those around us, what would or could anyone do to make it happen? It's a good question without a definitive answer. Right now, no one has built a working time machine to travel temporally.

Traveling into the Future

While it's not possible (yet) to travel to the future fast than the rate at which we're doing it now, it is possible to speed up the passage of time. But, it only happens in small increments of time. And, it has only happened (so far) to very few people who have traveled off Earth's surface. For them, time moves at an infinitesimally different rate. Could it happen over longer time spans? 

It might, theoretically. According to Einstein's theory of special relativity , the passage of time is relative to an object's speed. The more quickly an object moves through space, the more slowly time passes for it compared to an observer traveling at a slower pace. 

The classic example of traveling into the future is the twin paradox . It works like this: take a pair of twins, each 20 years old. They live on Earth. One takes off on a spaceship on a five-year journey traveling at nearly the speed of light . The traveling twin ages five years while on the journey and returns to Earth at the age of 25. However, the twin who stayed behind is 95 years old! The twin on the ship experienced only five years of time passing, but returns to an Earth that is much farther into the future.

Using Gravity as a Means of Time Travel

In much the same way that traveling at speeds close to the speed of light can slow down perceived time, intense gravitational fields can have the same effect.

Gravity only affects the movement of space, but also the flow of time. Time passes more slowly for an observer inside a massive object's gravitational well. The stronger the gravity, the more it affects the flow of time. 

Astronauts on the  International Space Station experience a combination of these effects, though on a much smaller scale. Since they are traveling quite quickly and orbiting around Earth (a massive body with significant gravity), time slows down for them compared to people on Earth. The difference is much less than a second over the course of their time in space. But, it is measurable.

Could We Ever Travel into the Future?

Until we can figure out a way to approach the speed of light (and warp drive doesn't count , not that we know how to do that at this point, either), or travel near black holes (or travel to black holes for that matter) without falling in, we won't be able to do time travel any significant way into the future. 

Travel into the Past

Moving into the past is also impossible given our current technology. If it were possible, some peculiar effects could occur. These include the famous "go back in time and kill your grandfather" paradox. If you did do it, you couldn't do it, because you already killed him, so therefore you don't exist and can't go back in time to do the dastardly deed. Confusing, isn't it? 

Key Takeaways

• Time travel is a science fiction trope that may possibly be technically possible. But, no one has achieved it.
• We do travel into the future all our lives, at a second per second. To do it faster requires technology we don't have.
• Travel to the past is also impossible at the present time.
• Is Time Travel Possible?| Explore , www.physics.org/article-questions.asp?id=131.
• NASA , NASA, spaceplace.nasa.gov/review/dr-marc-space/time-travel.html.
• “Time Travel.”  TV Tropes , tvtropes.org/pmwiki/pmwiki.php/Main/TimeTravel.

Edited by Carolyn Collins Petersen .

• Is Time Travel Possible?
• Is Warp Drive From 'Star Trek' Possible?
• Learn About the True Speed of Light and How It's Used
• Can We Travel Through Time to the Past?
• What Is the Twin Paradox? Real Time Travel
• An Introduction to Black Holes
• Amazing Astronomy Facts
• Cosmic Rays
• Einstein's Theory of Relativity
• Learn about the Doppler Effect
• Should We Build a Moon Base?
• The Future of Human Space Exploration
• The History of Gravity
• Can a Planet Make a Sound in Space?
• Understanding Cosmology and Its Impact
• What are Rotation and Revolution?

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How would time travel affect life as we know it?

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Key Takeaways

• If unrestricted time travel were possible, it would lead to a complete breakdown of the rational order of things.
• The ability to travel to both the past and future would upend our understanding of time.
• Stephen Hawking's "chronology protection hypothesis" suggests there might be natural laws preventing unrestricted time travel.

Science fiction has thoroughly covered the topic of time travel, starting with H.G. Wells' "The Time Machine" in 1895 and continuing right up to modern movies like " Déjà Vu " starring Denzel Washington. But physicists have also explored the nature of time and the plausibility of time travel for more than century, beginning with Albert Einstein's theories of relativity. Thanks to Einstein, scientists know that time slows as moving objects approach the speed of light. Gravity also slows time. This means that, in one sense, all of us can already consider ourselves time travelers in a limited way because we experience a tiny time warp (a difference of only nanoseconds) when we, for example, take a flight on an airplane. But physicists who study time travel today search for plausible ways to create a time warp large enough to allow noticeable travel into the past or future.

In his book "How to Build a Time Machine," physicist Paul Davies writes, "The theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel -- to any epoch, past or future -- might just be possible, too." This astonishing statement begs an important question: If time travel did indeed become a reality, how would it affect our world as we currently experience it?

First, it's important to realize that building a time machine would likely involve enormous expense, and the sheer complexity of such an apparatus would mean only a limited group of time travelers would have access to it. But even a small group of "astronauts" traveling through time and space could conceivably have a tremendous impact on life as we know it today. The possibilities, in fact, seem almost infinite.

Let's begin by assuming that it's possible to create a complete loop in time travel -- that time travelers could travel back into the past and then return to the future (or vice versa). Although scientists view traveling to the future as a much less problematic proposition than traveling to the past, our daily lives wouldn't change much if we could only send time travelers backward or forward in time, unable to recall them to the present. If we could, in fact, complete this loop of time travel, we can conjure up an incredible array of possible effects.

Possibilities and Paradoxes of Time Travel

Time travel turned total mayhem.

Imagine sending a time traveling astronaut 100 years into the future. The time traveler could witness technological advancements that we can only dream of today, much as people at the turn of the 20th century likely couldn't imagine the items we take for granted in 2010, such as iPods or laptop computers. The time traveler could also gain insight into medical advancements, such as new medicines, treatments and surgical techniques. If the time traveler could bring this knowledge backward in time to the present, the time from which he or she came, society could effectively leap forward in terms of its technical and scientific knowledge.

The futuristic time traveler could also bring back knowledge of what lay ahead for the world. He or she could warn of natural disasters, geopolitical conflicts, epidemics and other events of worldwide importance. This knowledge could potentially change the very way we operate. For example, what if a time traveler journeyed into the future and literally saw the effects that automobiles would eventually have on our planet? What if the time traveler witnessed an environment so polluted and damaged that it's unrecognizable? How might that change our willingness to use alternative forms of transportation?

Imagine that time travel became less restricted and more available to a larger population. Perhaps travel into the future would be exploited for personal gain. A futuristic time traveler could draw on knowledge of the stock market to guide his or her investment decisions, effectively using the granddaddy of all insider information to amass a fortune. Militaries might rely on time travel to gain valuable knowledge about the enemy's positioning and resources in future battles. Terrorists could use time travel to scout out the scenes of future attacks, allowing them to carefully plan with precise knowledge of future conditions.

The potential effects seem equally limitless in terms of the less likely possibility of time travel into the past. History books would no longer be based solely on exhaustive research and interpretation of ancient materials. Time travelers could resolve historical debates and verify how things did or didn't happen in the past. Imagine how different our understanding of the world might be if we could say definitively, for example, whether Moses actually parted the Red Sea or whether Lee Harvey Oswald acted alone in killing John F. Kennedy. A journey into the past could prove or disprove religious beliefs or result in face-to-face encounters with people such as Jesus, Buddha, Napoleon or Cleopatra -- or even the time traveler's former self. Perhaps time travelers could even bring back from the past things that had been lost, such as extinct species or dead and long-forgotten languages.

But here it's very important to raise the issue of self-consistent narratives and paradoxes. The concept of self-consistent narratives tells us that anything a time traveler would alter or affect in the past would have to remain consistent with the future from which he or she journeyed. Changing the past would effectively change the future, creating a causal loop. But such causal loops would only pose inherent problems if changes to the past resulted in a future different from the one the time traveler came from.

But perhaps the question of how time travel would affect life as we know it goes deeper than even a discussion of potential paradoxes and causal loops. Perhaps a discussion of specific effects of consequences on life as we know it makes little sense when faced with something that could change everything about the way in which we perceive our world.

Physicist Paul Davies gives a good example of a consistent causal loop in his book "How to Build a Time Machine." A mathematics professor uses a time machine to travel forward in time, where he discovers a new theorem. He returns back to the time he came from and gives one of his particularly gifted students the idea for that theorem. The student goes on to publish the theorem, and it turns out that it was this very student's work that the professor perused during his journey to the future. The narrative here is consistent.

On the other hand, with the grandfather paradox, a time traveler goes back in time and kills his grandfather. But if the time traveler's grandfather dies before the time traveler is born, how can he or she exist at all? And if the time traveler doesn't exist, how could he or she travel back in time to kill granddad?

As physicist Paul Davies describes it, unrestricted time travel -- meaning time travel that could form a complete loop to both the past and future -- would ultimately lead to total mayhem. In his words, "Time travel opens a view of the world that is a sort of madhouse where the rational order of things would no longer work. Under those circumstances, it's very hard to see how ordinary human life could continue."

In a world where the relationship between past, present and future is turned on its head, we would transcend the things that define our lives today. We would lose our notion of how time works, which could be so fundamentally damaging to our worldview that we would no longer care as much about the things that matter to us today: work, finances, making plans with friends and family, shopping -- you name it. These things just wouldn't be relevant in this crazy new world because we'd have a newfound preoccupation with simply making sense of a world without a set chronology -- we wouldn't know the order in which things occur.

It may be beside the point, then, to talk about resolving historical debates, saving endangered species or gaining technological, financial or military insight because those things might very likely fall by the wayside in the strange world that would follow the advent of unrestricted time travel.

As Davies makes clear, none of this fallout would occur from one-way travel. Hitching a one-way ride to the future or even the past (assuming we stick with self-consistent narratives) wouldn't cause this kind of profound reordering of the world as we currently experience it. But closing that loop of travel could be, in a word, disastrous.

Davies points out that science fiction normally focuses on the novelty aspect of time travel. But according to him, "It's not a novelty or a curiosity, it's something that strikes at the very rational basis of how we live and function. It's really hard to imagine that anything could be the same again." In his view, unrestricted time travel could change life as we know it so dramatically that we wouldn't even recognize it. Because chronology would have no meaning, we couldn't easily tell if something happened before or after, was a cause or an effect, and we would lose the ability to predict rationally the outcomes of our actions. In essence, it would be as though we had all gone insane.

These sobering potential effects of time travel have caused some scientists to wonder whether a principle exists in nature that would actually prevent unrestricted time travel, such as Stephen Hawking's "chronology protection hypothesis." This type of "theory of everything" might provide a scientific explanation as to why we could never unhinge the universe as we know it by making unrestricted time travel a reality. Scientists have yet to discover such a theory, but hearing Davies' take on the frightening effects of time travel makes one hope that they find it soon -- even if it means that we won't ever know for sure who killed JFK.

Frequently Asked Questions

How could time travel impact our understanding of history, what are the ethical implications of time travel, lots more information, related articles.

• What is the fourth dimension?
• What does the fourth dimension have to do with time travel?
• Does gravity distort space-time?
• Are there other universes like ours out there?
• Does time change speed?
• How Special Relativity Works
• How Time Travel Will Work
• Theory of Relativity

More Great Links

• NOVA Online: Time Travel
• Paul Davies' Web site
• ABC Science Online. "The Big Questions: The Riddle of Time." Jan. 17, 2002. (Oct. 7, 2010) http://www.abc.net.au/science/bigquestions/s460740.htm
• Davies, Paul. "How to Build a Time Machine." Penguin Books. 2002.
• Davies, Paul. Personal interview. Oct. 13, 2010.
• PBS Nova. "Sagan on Time Travel." October 1999. (Oct. 7, 2010) http://www.pbs.org/wgbh/nova/time/sagan.html
• Pickover, Clifford. "Time: a traveler's guide." Oxford University Press. 1999.

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Home / Essay Samples / Science / Physics / Time Travel

Time Travel Essay Examples

Comparative analysis of films about time travel: time bandits and back to the future.

Time Bandits directed by Terry Gilliam and Back to the Future directed by Robert Zemeckis are both 1980’s films about time travel, but that’s where the similarities stop. Time Bandits, released in 1981 tells the story of a boy named Kevin and his adventures through...

Mary Pope Osborne’s Magic Tree House and the Theory of Time Travel

Is time travel real? Mary Pope Osborne is the author of the renowned 'Magic Tree House' series. In which Mary writes about a brother (Jack) and sister (Annie) duo, who go on adventures through time to inform the youth who read light novels about that...

Time Travel Narrative and Its Features

Time travel can be viewed as a child's play. Being categorized as science fiction, the concept of this art has been universally known as the act of physically traveling to a different period and tends to involve some sort of transportation machine. Time travel was...

Discussion of Whether Time Travel is Ethical

Time travel is ethical because we can learn about the past, solve unsolved crimes, and visit lost loved ones. Time travel has shown that can be very helpful in most cases. Lacking knowledge about how the future will evolve given changes to the past is...

The Paradoxical Effects of Time Travel

The issue of travelling in time is not new because it has interested people for centuries and continues to be interesting to us today. However, in spite of people's high concern and eagerness to uncover time's mysteries, it still keeps its secrets as it did...

The Hypothesized Nature of Time Travel

Time travel is one of the best/the most interesting topic in science fiction. Time travel is usually depicted in films such as Back to the Future, and Star Trek, you will see people hopping in strange contraptions or using a device that will take them...

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