time travel topics essay

Essay on Time Travel

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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.

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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

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

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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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30 Time Travel Writing Prompts

Ever feel stuck in the present moment?

Wishing you could grab a pizza with Cleopatra, mastermind a plan with Machiavelli, or explore a future city filled with gadgets we can’t even imagine?

Hold onto your hats because we’re about to blow the doors off time travel stories!

Forget boring timelines, paradoxes are your new BFFs. These prompts will have you rewriting reality, jumping through history, and making the rules of time as bendy as a pretzel.

Let’s check them out.

Time Travel Writing Prompts and Story Ideas

Winds of the Lost Era: A gust of wind has the unique ability to transport anyone it touches to a different time for just one hour. When a young woman gets caught in this wind, she finds herself in the midst of a pivotal historical event. She has exactly sixty minutes to observe, interact, or possibly change history. What event does she witness? And what choices does she make?

The Last Sunset Before Eternity: The world’s leading scientists discover that there’s only one sunset left before the earth stops rotating, plunging half the world into perpetual day and the other half into eternal night. They’ve also created a one-time-use machine that can send one person back in time to prevent this. Who is chosen? And what historical point do they go back to to make things right?

Messages Across Time: A young woman discovers she can send messages to her younger self through dreams. Each message can only be a short sentence, but it’s enough to give her past self clues or warnings about the future. However, every change she makes creates ripples in her present, and sometimes, the outcomes are not what she expects. What message does she send first? And what unforeseen consequences emerge?

The Day the Clocks Stopped: Time everywhere freezes, except for one city. Its inhabitants live through days, months, and years while the rest of the world remains static. As they advance technologically and culturally at a rapid rate, they prepare for the day time might resume for everyone. How do they plan to reintroduce their city to the world, which is now centuries behind?

Chrono-Tourists: A company starts offering ‘Time Travel Tourism’. You can’t change major events, but you can witness them. A young couple decides to go on their honeymoon to witness a peaceful, beautiful moment from the past. But when they arrive, they realize it’s the eve of a significant, unrecorded disaster. How do they reconcile their experience with the joy they were expecting?

The Museum of Moments: In the future, there are no traditional museums. Instead, there’s a museum where people can experience any moment from the past in full sensory detail. A teenager decides to relive the ‘most peaceful day on Earth’, only to realize that peace is often a matter of perspective. What does he truly witness on this so-called peaceful day?

Time’s Locket: A locket passed down through generations in a family doesn’t just contain pictures; it allows the wearer to briefly live in the moments the pictures were taken. A young girl decides to experience a day in her great-grandmother’s life, hoping to understand her better. However, she discovers a family secret that has been hidden for decades, changing her understanding of her lineage.

Letters in the Sands of Time: In a small coastal town, messages mysteriously appear at sunrise on the beach, always addressed to someone present there. These are letters from their future selves. A man, skeptical at first, starts reading a letter addressed to him and discovers details of a choice he’ll soon have to make. What decision looms in his future, and how does this knowledge affect him?

The Two Lifetimes of Ms. Daniels: On her 30th birthday, Ms. Daniels wakes up to find herself back in her 10-year-old body but with all her memories intact. She lives her life again, making different choices based on her memories, until she reaches 30 again. And then, she’s back in her original timeline. How do her dual experiences shape her perspective? Which life felt more real to her?

Echoes of the Time Vortex: A cavern hidden in the mountains is said to echo not sounds from the present, but conversations from the past and whispers of the future. When a grieving mother enters the cave, she hears the voice of her departed child from a future that never happened. What message does she receive, and how does it change her healing process?

The Train at Midnight: There’s a legend of a train that passes through a town at midnight. Those who board it are taken to any point in time they desire, but they can only observe and feel emotions—they can’t interact. A woman boards to revisit a day she considers her life’s biggest mistake. What day does she return to , and what closure does she seek?

Chronicles of the Hourglass City: In a city shaped like an hourglass, the top half lives in the past and the bottom half in the future. A bridge connects the two, and citizens are allowed a single journey across it. A young woman from the future decides to cross into the past. What or who is she seeking, and what challenges await her in this duality?

Diary from Tomorrow: A man finds a diary on his doorstep, and to his astonishment, it’s filled with detailed entries from the next year of his life. Each day he reads about tomorrow, and each time he’s faced with the moral dilemma of acting on or ignoring the knowledge. What major revelation does the diary hold, and does he dare to change its course?

The Timestream Weaver: In an old attic, a loom is found that doesn’t weave fabric but moments in time. When operated, it can merge moments from different times into one. A curious teenager weaves together a day from her childhood and one from her elder years. What harmonies or conflicts emerge from this singular day?

Guardians of the Temporal Oasis: Deep in the desert lies an oasis where every drop of water lets you relive a moment from your past. But, you can’t choose the moment—it chooses you. A traveler, seeking refuge, drinks from the oasis and is thrust into a forgotten memory. What long-buried moment resurfaces, and how does it change his path forward?

The Timeless Town Square: A secluded town has a central square where time doesn’t flow linearly. Every day at dawn, the square chooses a random day from the past or future to reflect. Townspeople can enter to relive memories or see snippets of what’s to come. One day, the entire town gathers as the square displays a date significant to all. What date is shown , and how does it bind the community?

Candles of Yesteryears: A boutique sells candles, each corresponding to a year in history. When lit, the room transforms, enveloping the person in the ambiance of that year. An elderly woman buys a candle corresponding to a year she wants to forget. As it burns, what secret memory unfolds, and what catharsis does it bring?

Threads of Time: In a mystical land, there are multiple threaded structures that showcase the entire timeline of the universe. When touched, a person can feel the emotions of any moment stitched into it. A young prince touches a seemingly insignificant thread and is overwhelmed by its intensity. What hidden moment did he discover, and how does it reshape his understanding of history?

The Mirror of Moments Past: A mirror in an antique store doesn’t show the present but a past version of whoever stands before it. A woman sees herself as a child, interacting with someone she doesn’t remember from her childhood. Who is this mysterious figure , and why have her memories of them been erased?

Temporal Tunes of the Old Gramophone: An old gramophone has the power to play not just songs but also ambient sounds from specific moments in time. A listener can immerse themselves in the atmosphere of that moment. A man hears the background noise of a place and time he never visited but finds strangely familiar. Where does this sound take him , and what lost connection does he rediscover?

The Garden of Future Blooms: In a secret garden, flowers bloom showing visions of potential futures. One rare flower is said to bloom only once every century, showing a vision crucial for humanity. As it blossoms, many gather to witness its vision. What future does the flower reveal , and how do those who see it react?

Clockwork of Cosmic Consequences: A clockmaker designs a timepiece that can turn back time, but for every minute turned back, it fast-forwards another person’s timeline by a year. The creator, desperate to rectify a personal mistake, uses it, but at what cost? Whose life gets fast-forwarded , and how does this unintended consequence play out?

Whispers of the Time-Touched Tree: A tree in a forest is said to be touched by time. Those who sleep under it dream of a moment from their past, but from the perspective of someone else who was there. A soldier, burdened by guilt, sleeps under the tree, hoping to understand a decision he made in battle. Whose eyes does he see through , and how does this new perspective aid his quest for forgiveness?

Shadows of the Sundial: In an abandoned village, there’s a sundial said to cast shadows not of the current time but of times gone by. When a researcher places her hand where the shadow falls, she’s momentarily transported to the moment the shadow represents. She inadvertently touches a shadow that takes her to a day the village wishes to forget. What dark secret is unveiled, and how does she reconcile with the truth?

Temporal Café: A café opens downtown where each table is set in a different era. Patrons can’t interact with the past or future directly but can witness and hear conversations. A detective sits at a table set in the future, trying to solve a case that’s stumped him. What revelation about the case does he overhear, and how does it change the course of his investigation?

The Time-Torn Map: An explorer discovers a map that doesn’t just lead to places but to times. Marking a location and date transports the holder to the specified moment. The explorer chooses a date and place where a famous artifact went missing. What happens when he arrives , and how does this journey reshape historical narratives?

Waves of the Temporal Beach: There’s a beach where each wave that crashes ashore comes from a different era. Collecting items brought by the waves can provide glimpses into various moments in time. A historian finds an item linked to her family’s past. What story does the item tell , and how does it redefine her family’s legacy?

Melodies from the Time-Touched Violin: A violin is found that, when played, doesn’t produce sound but images from the past or future. A musician plays a tune, and a series of events unfold before her, hinting at a future personal dilemma. What decision does she foresee , and how does she prepare for it?

Stairs of the Epoch Tower: An ancient tower’s stairs are said to ascend through time. With each floor representing a different era, climbers can observe, but not interfere. A writer ascends, seeking inspiration, but finds himself on a floor mirroring a future event of his own life. What event does he witness , and how does it inspire his next work?

The Time Capsule’s Promise: A school’s time capsule is unearthed not by students from the present but by visitors from the future. They leave behind a message for the current generation about an impending global challenge. What warning do they give , and how does the world rally in response?

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A Day in The Life of a Time Traveler

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Words: 854 |

Published: Feb 7, 2024

Words: 854 | Pages: 2 | 5 min read

A leap into history, a brush with renaissance brilliance, a glimpse of future possibilities, a journey to the heart, a race against time, the return to present, a timeless journey.

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Essays on Time Travel

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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]

time travel art - train coming out of a fireplace, with a clock on mantel.

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]

Time travel. Hyper time graphic. Reprinted from Wasserman (2018, chapter 3) with kind permission of Ryan Wasserman and Oxford University Press.

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:

Time travel. Reprinted from Wasserman (2018, chapter 3) with kind permission of Ryan Wasserman and Oxford University Press.

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.

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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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April 26, 2023

Is Time Travel Possible?

The laws of physics allow time travel. So why haven’t people become chronological hoppers?

By Sarah Scoles

yuanyuan yan/Getty Images

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation .

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

Time traveling to the near future is easy: you’re doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein’s special theory of relativity, time’s flow depends on how fast you’re moving. The quicker you travel, the slower seconds pass. And according to Einstein’s general theory of relativity , gravity also affects clocks: the more forceful the gravity nearby, the slower time goes.

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“Near massive bodies—near the surface of neutron stars or even at the surface of the Earth, although it’s a tiny effect—time runs slower than it does far away,” says Dave Goldberg, a cosmologist at Drexel University.

If a person were to hang out near the edge of a black hole , where gravity is prodigious, Goldberg says, only a few hours might pass for them while 1,000 years went by for someone on Earth. If the person who was near the black hole returned to this planet, they would have effectively traveled to the future. “That is a real effect,” he says. “That is completely uncontroversial.”

Going backward in time gets thorny, though (thornier than getting ripped to shreds inside a black hole). Scientists have come up with a few ways it might be possible, and they have been aware of time travel paradoxes in general relativity for decades. Fabio Costa, a physicist at the Nordic Institute for Theoretical Physics, notes that an early solution with time travel began with a scenario written in the 1920s. That idea involved massive long cylinder that spun fast in the manner of straw rolled between your palms and that twisted spacetime along with it. The understanding that this object could act as a time machine allowing one to travel to the past only happened in the 1970s, a few decades after scientists had discovered a phenomenon called “closed timelike curves.”

“A closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, at some point finds themselves at the same place and time where they started, creating a loop,” Costa says. “This is possible in a region of spacetime that, warped by gravity, loops into itself.”

“Einstein read [about closed timelike curves] and was very disturbed by this idea,” he adds. The phenomenon nevertheless spurred later research.

Science began to take time travel seriously in the 1980s. In 1990, for instance, Russian physicist Igor Novikov and American physicist Kip Thorne collaborated on a research paper about closed time-like curves. “They started to study not only how one could try to build a time machine but also how it would work,” Costa says.

Just as importantly, though, they investigated the problems with time travel. What if, for instance, you tossed a billiard ball into a time machine, and it traveled to the past and then collided with its past self in a way that meant its present self could never enter the time machine? “That looks like a paradox,” Costa says.

Since the 1990s, he says, there’s been on-and-off interest in the topic yet no big breakthrough. The field isn’t very active today, in part because every proposed model of a time machine has problems. “It has some attractive features, possibly some potential, but then when one starts to sort of unravel the details, there ends up being some kind of a roadblock,” says Gaurav Khanna of the University of Rhode Island.

For instance, most time travel models require negative mass —and hence negative energy because, as Albert Einstein revealed when he discovered E = mc 2 , mass and energy are one and the same. In theory, at least, just as an electric charge can be positive or negative, so can mass—though no one’s ever found an example of negative mass. Why does time travel depend on such exotic matter? In many cases, it is needed to hold open a wormhole—a tunnel in spacetime predicted by general relativity that connects one point in the cosmos to another.

Without negative mass, gravity would cause this tunnel to collapse. “You can think of it as counteracting the positive mass or energy that wants to traverse the wormhole,” Goldberg says.

Khanna and Goldberg concur that it’s unlikely matter with negative mass even exists, although Khanna notes that some quantum phenomena show promise, for instance, for negative energy on very small scales. But that would be “nowhere close to the scale that would be needed” for a realistic time machine, he says.

These challenges explain why Khanna initially discouraged Caroline Mallary, then his graduate student at the University of Massachusetts Dartmouth, from doing a time travel project. Mallary and Khanna went forward anyway and came up with a theoretical time machine that didn’t require negative mass. In its simplistic form, Mallary’s idea involves two parallel cars, each made of regular matter. If you leave one parked and zoom the other with extreme acceleration, a closed timelike curve will form between them.

Easy, right? But while Mallary’s model gets rid of the need for negative matter, it adds another hurdle: it requires infinite density inside the cars for them to affect spacetime in a way that would be useful for time travel. Infinite density can be found inside a black hole, where gravity is so intense that it squishes matter into a mind-bogglingly small space called a singularity. In the model, each of the cars needs to contain such a singularity. “One of the reasons that there's not a lot of active research on this sort of thing is because of these constraints,” Mallary says.

Other researchers have created models of time travel that involve a wormhole, or a tunnel in spacetime from one point in the cosmos to another. “It's sort of a shortcut through the universe,” Goldberg says. Imagine accelerating one end of the wormhole to near the speed of light and then sending it back to where it came from. “Those two sides are no longer synced,” he says. “One is in the past; one is in the future.” Walk between them, and you’re time traveling.

You could accomplish something similar by moving one end of the wormhole near a big gravitational field—such as a black hole—while keeping the other end near a smaller gravitational force. In that way, time would slow down on the big gravity side, essentially allowing a particle or some other chunk of mass to reside in the past relative to the other side of the wormhole.

Making a wormhole requires pesky negative mass and energy, however. A wormhole created from normal mass would collapse because of gravity. “Most designs tend to have some similar sorts of issues,” Goldberg says. They’re theoretically possible, but there’s currently no feasible way to make them, kind of like a good-tasting pizza with no calories.

And maybe the problem is not just that we don’t know how to make time travel machines but also that it’s not possible to do so except on microscopic scales—a belief held by the late physicist Stephen Hawking. He proposed the chronology protection conjecture: The universe doesn’t allow time travel because it doesn’t allow alterations to the past. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” Hawking wrote in a 1992 paper in Physical Review D .

Part of his reasoning involved the paradoxes time travel would create such as the aforementioned situation with a billiard ball and its more famous counterpart, the grandfather paradox : If you go back in time and kill your grandfather before he has children, you can’t be born, and therefore you can’t time travel, and therefore you couldn’t have killed your grandfather. And yet there you are.

Those complications are what interests Massachusetts Institute of Technology philosopher Agustin Rayo, however, because the paradoxes don’t just call causality and chronology into question. They also make free will seem suspect. If physics says you can go back in time, then why can’t you kill your grandfather? “What stops you?” he says. Are you not free?

Rayo suspects that time travel is consistent with free will, though. “What’s past is past,” he says. “So if, in fact, my grandfather survived long enough to have children, traveling back in time isn’t going to change that. Why will I fail if I try? I don’t know because I don’t have enough information about the past. What I do know is that I’ll fail somehow.”

If you went to kill your grandfather, in other words, you’d perhaps slip on a banana en route or miss the bus. “It's not like you would find some special force compelling you not to do it,” Costa says. “You would fail to do it for perfectly mundane reasons.”

In 2020 Costa worked with Germain Tobar, then his undergraduate student at the University of Queensland in Australia, on the math that would underlie a similar idea: that time travel is possible without paradoxes and with freedom of choice.

Goldberg agrees with them in a way. “I definitely fall into the category of [thinking that] if there is time travel, it will be constructed in such a way that it produces one self-consistent view of history,” he says. “Because that seems to be the way that all the rest of our physical laws are constructed.”

No one knows what the future of time travel to the past will hold. And so far, no time travelers have come to tell us about it.

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Time Travel - Free Essay Examples and Topic Ideas

Time travel refers to the concept of moving between different points in time, either through time dilation or by using some form of technology. It is a popular topic in science fiction literature and movies, and is often associated with the ability to change historical events or alter the course of the future. The idea of time travel has been explored by scientists, philosophers and writers for centuries, and remains a fascinating concept that continues to captivate people’s imaginations.

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Dreaming of Time Travel
Time Travel and Slavery: Octavia Butler’s Kindred
The value of travel time
Theme of Travel in Time in Literature
The Existence of Time Travel in Kindred by Octavia Butler
If you were to time travel all the way back to 1899
If You Could Go Back in Time
Tirelessly working day after day makes the anticipation for time
If I could travel back in time before the revolution I would
Valu of Time
About Medival Time
The Time Machine
How to Spend Your Free Time?
“The Time Machine” by H.G. Wells
Cuss Time Summary
Time Management
Tretheway’s “Theories of Space and Time”
Of Mice and Men- Candy character analysis
My Most Exciting Vacation Time
Time Management Skills
Concept of time
Ottoman Architecture: A Travel Through Time
Christianity and Time Period
Storytelling in Movie A Time To Kill
Increase in discretionary time
Time and Gatsby
The Time Machine: Retelling and Analysis
The Time Overrun
Woman on the Edge of Time
Children in the Philippines: No Time for Play

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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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Essay on Time Travel

100 Words Essay on Time Travel

Time Machines

Why Time Travel Fascinates Us

250 Words Essay on Time Travel

Is time travel possible?

How would time travel work?

Would time travel be dangerous?

500 Words Essay on Time Travel

Theories of Time Travel

The Possibility of Time Travel

The Implications of Time Travel

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Bibliography

Time Travel

1.1 Time Discrepancy

2. The Grandfather Paradox

3. Causation

4. Time and Change

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30 Time Travel Writing Prompts

Time Travel Writing Prompts and Story Ideas

A Day in The Life of a Time Traveler

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Is Time Travel Possible?

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Time Travel - Free Essay Examples and Topic Ideas

Essay examples

FAQ about Time Travel

Time Travel Essay Examples

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

Time Travel Narrative and Its Features

Discussion of Whether Time Travel is Ethical

The Paradoxical Effects of Time Travel

The Hypothesized Nature of Time Travel

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