time travel summary essay

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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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causation: backward | free will: divine foreknowledge and | identity: over time | location and mereology | temporal parts | time | time machines | time travel: and modern physics

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Library of Congress Catalog Data: ISSN 1095-5054

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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Past, present, paradox: writing about time travel, crafting a believable time travel story requires careful consideration of the logic at play. let's crack the temporal code on traveling through time in fiction.

Table of Contents

Time travel in fiction can open your story to infinite possibilities. Ever wondered what it would be like if somebody taught the Romans how to make a nuclear bomb? Do you need to retcon an event in your story? Time travel!

It may seem simple for your time-traveling characters to hop in Tony’s Terrific Temporal Transport and whiz through time, but there are many hurdles to overcome when writing about time travel.

Chief among these is dealing with time travel paradoxes, so let’s look at those, discuss how you can write convincing time travel stories, and explore how some popular stories handle it.

The Problem With Time Travel

Consider an ordinary day in your life. It follows a sequence of events where one thing leads to another. This is called causality , the concept that everything that happens results from events that happened before it. The problem with time travel in fiction, especially travel to the past, is that it often breaks the rules of causality.

This can lead to time travel paradoxes and unforeseen results , including:

• Continuity paradoxes: The act of time travel renders itself impossible.
• Closed causal loop paradoxes: Traveling to the past creates a condition where an idea, object, or person has no identifiable origin and exists in a closed loop in time that repeats infinitely.
• The butterfly effect: Even the smallest action can have massive consequences.

With all that in mind, let’s embark on a journey through time and explore these further!

Grandfather Paradox

This thought experiment posits the idea of somebody traveling back in time and killing their grandfather before their parents were born. Because the grandfather never has children, the time traveler—his grandchild—cannot exist.

However, if the time traveler never existed, they couldn’t kill their grandfather, so he would go on to have children and grandchildren. One of those grandchildren is the time traveler, though, who might go back in time and kill their grandfather. If that seems confusing, it’s okay—it’s supposed to be.

The bottom line is that if somebody travels to the past and changes something that prevents them from ever traveling to the past, they have broken the timeline's continuity.

Polchinski’s Paradox

American theoretical physicist Joseph Polchinski removed human intervention from the time travel equation.

Imagine a billiard ball travels into a wormhole, tunnels through time in a closed loop, and emerges from the same wormhole just in time to knock its past self away.

Doing so prevents it from ever entering the wormhole and traveling through time, to begin with. However, if it does not travel back in time, it cannot emerge to knock itself out of the way, giving it a clear path to travel back in time.

Bootstrap Paradox

The Bootstrap Paradox is the first closed causal loop paradox we will explore. This presents a situation where an object, idea, or person traveling to the past creates the conditions for their existence, leading to it having no identifiable origin in the timeline.

Imagine sending the schematics for your time machine to your past self, from which you create a time machine. Where did the knowledge of how to create the time machine begin?

Predestination Paradox

The most nihilistic of paradoxes explores the idea that nothing we do matters, no matter what. Events are predetermined to still occur regardless of when and where you travel in time.

Suppose you time travel to the past to talk Alexander the Great out of invading Persia, but he hadn’t even considered this until you mentioned it. By traveling to the past to prevent Alexander’s conquest, you caused it.

Butterfly Effec t

Less of a paradox and more an exploration of unintended consequences, the butterfly effect explores the idea that any action can have sweeping repercussions, no matter how small.

In the 1960s, meteorologist Edward Lorenz discovered that adding tiny changes to computer-based meteorological models resulted in unpredictable changes far from the origin point. In traveling back in time, we don’t know what effect even minor changes might have on the timeline.

How to Write Convincing Time Travel Stories

Time travel can be pretty complex at the best of times, but that doesn't mean writing about it has to be a challenge. Here are a few practical tips to craft narratives that crack the temporal code.

Ask Yourself, "Why Time Travel?"

If your story has time travel, to begin with, it likely plays a pretty significant role in the narrative. Define the purpose that time travel has in your story by asking yourself questions like:

• How and why is time travel possible in your setting?
• What does it mean for your story and your characters?
• What are your characters meant to use time travel for?
• Is the actual practice of time travel different from its intent?

If you can't be clear about time travel's purpose in your story, how can you convincingly write about it? To get crafty with time, you first need to master its relevant mechanics.

Keep a Record of Everything

You're asking your reader to potentially make several mental leaps when time travel is involved in a story, so it's imperative to have all of your details sorted. Do the work of planning out dates and events ahead of time by creating a time map for yourself—like a mindmap, but for a timeline.

You'll be able to keep a birds-eye view of the narrative at all times, be more strategic about moving the order of events around, and ensure that you never miss a detail. You may even want to have multiple versions—a strictly linear timeline and a more loosely structured time map where you draw connections between events and in the order they appear in the narrative.

In Campfire, you can do both with the Timeline Module —create as many Timelines as you want by using the Page feature in the element. You can also connect your Timeline(s) to a custom calendar from the Calendar Module for extra fun with time wonkiness in your world.

If a new idea pops up while writing, don't stress! You'll have your handy time map already laid out so you can easily see if a new scene or chapter makes sense, as well as where it will best fit into the narrative.

Never Forget Causality

I mentioned this concept earlier in the article, but it should be reiterated: The most important rule of time travel is that every action results in a consequence. Remember cause and effect : an action is taken (your character time travels to the past), and causes an effect, the consequence (the timeline is forever changed).

"Consequence" doesn't have to be a negative thing, either, even though the word has that connotation. The resulting consequence of a given action could be a positive effect, too.

Regardless, seek to maintain causality so you don't confuse your readers (or yourself, for that matter). Establishing clear rules for how time travel works in your setting and sticking to them will help you keep your time logic consistent and avoid running into narrative dead ends or plot holes.

Tips & Tricks For the Time-Traveling Author

Now that we’ve examined several obstacles you can encounter when writing about time travel, let’s see how you can either avoid them or exploit them. That’s right! Even time travel paradoxes present opportunities for superb storytelling.

Focus on the Future

Fortunately, all the named paradoxes here involve the past, so the easy way to avoid them is to not go there! Thanks to Einstein’s theory of special relativity, you don’t even have to invent a clever way to travel instead to the future.

An aspect of Einstein's theory is time dilation , in which the faster an object moves through space, the slower it moves through time. With this, you need only zip around at near the speed of light for a few weeks or months, and when you come back to Earth, years or centuries will have gone by.

Create a Multiverse

A popular trope in science fiction today, and a theory gaining popularity among theoretical quantum physicists, is the multiverse concept. According to multiverse theory, whenever an event occurs, every possible outcome of the event happens simultaneously, splitting the universe into parallels that each contain differing outcomes.

Since all these realities exist, perhaps changing the past is simply a way for time travelers to travel between realities, shifting their perspective to a timeline where things occurred differently than in their original reality.

Get Creative With Consequences

Instead of avoiding paradoxes, maybe you want them to occur. Leading to some fascinating stories, this can be approached in a variety of ways. Perhaps you want to examine the unintended consequences of the butterfly effect, create a time-traveling police force that enforces the laws of time travel, or simply break time itself and revel in the chaos that ensues.

Just be sure to remember the action-consequence rule and keep your timeline handy for easy reference—especially if you're toying around with multiple timelines!

Best Time Travel Stories

What follows are what I think are some of the best time travel stories. As you will see, the first two fall victim to time travel paradoxes, while the other two do a great job of exploring various elements we’ve discussed.

Terminator 2: Judgment Day

The corporation Cyberdyne Systems has remnants of the Terminator from the first movie, which they use to create an artificial intelligence system called Skynet. Skynet then actually creates the terminators and sends one back in time. Thus, it gives humanity the technology to create itself in a classic example of a bootstrap paradox.

Back to the Future

In this film, Marty McFly travels to the past and inadvertently interrupts the event where his parents first meet. This causes a chain of events where Marty’s parents never get married and have children, threatening to erase Marty and his siblings from the timeline.

Some argue that the McFly offspring ceasing to exist is a great exploration of the consequences of time travel. However, they would never have been at risk had Marty not been in the past to impede their parents’ romance. And if he ceases to exist, he’ll never go back and get in the way, thus creating a grandfather paradox.

War of the Twins

In this second volume of the Dragonlance Legends trilogy by Margaret Weis and Tracy Hickman, the mage Raistlin Majere travels into the past, kills a wizard named Fistandantilus in a battle for power, and assumes his identity. Throughout the book, Raistlin unwittingly follows the historical fate of Fistandantilus, in a wonderful exploration of the predestination paradox.

It’s hard to talk about time travel in fiction these days without mentioning Loki. The show explores two suggestions from my list above: the multiverse and policing the timeline. In this series, varying outcomes of events lead to branching timelines, creating a multiverse of possibilities. However, an agency called the Time Variance Authority exists to prevent this from happening, and they set out to eliminate any branches separate from what they consider the Sacred Timeline.

Bon Voyage!

I hope this exploration of time travel leaves you prepared to tackle these obstacles and opportunities that naturally present themselves when playing around with time.

Just knowing about the complexities of time travel and the paradoxes it can bring about is the best way to avoid trouble and create innovative storytelling moments. So, dust off your DeLorean, polish your paradox-proof plot, and get ready to write your adventure through the ages!

Learn more about making a timeline with Campfire in the dedicated Timeline Module tutorial . And be sure to check out the other plotting and planning articles and videos here on Learn, for advice on how to plan your very own time travel adventures!

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Paradoxes of Time Travel

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

Reviewed by John W. Carroll, North Carolina State

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Sure I can; here it is.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Time Travel by David Wittenberg LAST REVIEWED: 29 July 2020 LAST MODIFIED: 29 July 2020 DOI: 10.1093/obo/9780199827251-0212

Time travel is of interest in several academic research fields that overlap and communicate with one another to varying degrees. Primarily, of course, time travel comprises a literary subgenre of science fiction literature and popular film. As a motif or plot type, it is also a frequent element in romance fiction, nongeneric speculative literature, postmodern literature, and experimental cinema. As such, time travel is potentially a focus for practitioners in many branches of criticism and theory, including genre studies, cultural studies, critical theory, film theory, psychoanalytic criticism, and, especially, narrative theory and narratology. Nevertheless, literary and cultural theory on the topic of time travel has been surprisingly sparse, and only a handful of dedicated studies are available at either book or article length. By contrast, in research areas outside literature and popular culture, time travel has received something closer to its due. In analytic philosophy, time travel is a familiar topic of inquiry or debate, usually serving as a thought experiment for logicians and philosophers of language or history, and as a test case for constructing meaningful or consistent claims about objects and events (See the Oxford Bibliographies in Philosophy article “ Time Travel ”). For physicists, time travel has played a similar role as a test for postulates of relativity and quantum mechanics, but it has also sporadically arisen as a real physical possibility within current theory, albeit a possibility that might require as-yet undiscovered theories or exotic materials. In particular, recent multiverse cosmologies and quantum computing models sometimes include time travel or multiple worlds as essential components or entailments. Finally, in historiographical theory, counterfactuals and possible-worlds models have been productive tools for theorizing historical events, creating a potentially rich area of overlap with literary and narrative theory.

Only a handful of full-length monographs are available on time travel in fiction and visual media, although the subject arises sporadically in many other critical texts. This section includes works fully devoted to time travel as a historical, theoretical, or cultural issue. Nahin 1999 and Gleick 2016 concentrate on conceptual and logical connections between science fiction time travel, physics, and analytic philosophy, while Foote 1991 focuses on the inheritance of past-directed time travel within science fiction. Wittenberg 2013 gives a broad theoretical analysis of time travel as a problem for narrative theory and the philosophy of time.

Foote, Bud. The Connecticut Yankee in the Twentieth Century: Travel to the Past in Science Fiction . Santa Barbara, CA: Greenwood Press, 1991.

For a long time, Foote’s monograph was possibly the only full-length critical study of time travel literature, unusual also for its focus on Mark Twain rather than H. G. Wells as the genre’s prime originator. Although it is to some degree superseded by more recent literary-critical work, Foote’s book is still a solid and useful read.

Gleick, James. Time Travel: A History . New York: Vintage, 2016.

A broad-ranging and readable survey of time travel examples in popular literature, film, physics, and some philosophy. Gleick has considerable experience as a popular science writer and usefully discusses how questions about spacetime and the physical possibility of time travel get adapted in popular literature and media.

Nahin, Paul J. Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction . New York: Springer Verlag, 1999.

DOI: 10.1007/978-1-4757-3088-3

A comprehensive study of science fiction time travel and its conceptual and historical connections to physics and philosophy. A thorough and informative book, although Nahin favors time travel stories that are strictly consistent with logic and physical theory, somewhat delimiting his body of research. The book contains an extensive bibliography. This is the revised version of a first edition that appeared in 1993.

Wittenberg, David. Time Travel: The Popular Philosophy of Narrative . New York: Fordham University Press, 2013.

An extended analysis of both fiction and visual media, proposing time travel as a testing ground for problems in narrative theory and the philosophy of time. So far, this is the only book-length study devoted to time travel as a narrative-theoretical problem.

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Time Travel: A History by James Gleick review – from mechanical to mental

This roving study of our enduring fascination with time travel covers well trodden ground but finds the concept constantly evolving

A re we trapped in the present, free to move in space yet unable to travel in the fourth dimension? Or is there a chance, a glimmer of a possibility, that the past and future could unfurl to our physical experience at will? Despite the punchline being apparent from the off – lest we forget, such journeys are impossible – James Gleick’s latest offering sets out to question the questions, probing how the idea of time travel emerged, gripped our imaginations and shaped our society.

From the start it is apparent who’s the hero of this journey. “One way or another, the inventions of HG Wells colour every time-travel story that followed,” writes Gleick, pointing out that while a smattering of earlier stories explored utopian futures, it was Wells who, with his 1895 work, got to the nuts and bolts of the matter in knocking up a time machine.

It’s easy to forget that time travel is a relatively recent notion. As Gleick points out, for most of human history, change was incremental – yesterday looked much like today, today much like tomorrow. “Before futurism could be born, people had to believe in progress,” he writes. The development of technology, culminating in the industrial revolution, made that possible. As change abounded, the future, and what it might look like, became a subject for speculation. And as archaeology burgeoned, writers like E Nesbit began flights of fancy to the past, too.

Embraced by novelists, wrestled with by philosophers and informed by science – not least Hermann Minkowski ’s revelation, following Einstein’s breakthrough, of four-dimensional spacetime – the possibility that the arrow of time could be tinkered with became a meme. From pulp fiction writers such as Robert Heinlein , to F Scott Fitzgerald and his The Curious Case of Benjamin Button , from Terminator to Dr Who , science fiction blossomed.

As Gleick reveals, problems and paradoxes immediately spewed forth. Is our future governed by fate, or free will? Does time travel always mean ending up naked, your clothes left in the present? What would happen if a time traveller killed his or her grandfather when he was a child? “All the paradoxes are time loops,” writes Gleick as he canvasses the myriad responses to such conundrums. “They all force us to think about causality.”

Among those doing the thinking are philosophers and scientists. Attempting to tackle the idea that wormholes – tunnels in spacetime – could be turned into time machines allowing journeys into the past, even Stephen Hawking has entered the fray, concluding in his “chronology protection conjecture” that the laws of physics prevent it.

The consummate temporal tour guide, Gleick deftly navigates the twists and turns of our fascination with time travel, investigating its evolution in literature, exploring scientific principles that have hinted at or scotched the idea, and teasing apart the curious spell it cast across society with its suggestion of immortality.

But, as he notes, not every product of this obsession with time was profound. “The time capsule is a characteristically 20th-century invention: a tragicomic time machine. It lacks an engine, goes nowhere, sits and waits,” he writes, surveying various attempts to send snapshots of civilisation into the future. Indeed, it’s hard to know whether disappointment, amusement or simply bafflement will be the dominant emotion when the Crypt of Civilisation time capsule at Oglethorpe in Atlanta is opened in 8113 AD. Created in 1936, its contents, Gleick reveals, include a movie magazine, an electric toaster and a “lady’s breast form”.

Not every idea of time travel is rooted in the physical, and Gleick explores how in the act of storytelling we mess with chronology. “We don’t have enough tenses. Or rather, we don’t have enough names for all the tenses we create,” he writes of the complexity unleashed in literature by the concept. Readers, too, become time travellers, able to move at will backwards and forwards through a story. More than that, Gleick argues, books cannot be detached from time. “Even if you know a book well – even if you can recite it entire, like the Homeric poet – you cannot experience it as a timeless object,” he writes.

In probing the role of imagination and memory, Gleick also gives space to the concept of mental time travel, the phenomenon which allows us to immerse ourselves in our past and muse upon what the future might hold. But there, too, Gleick, drawing on Proust, uncovers complexity. “Our intelligence writes and rewrites again the story it is trying to recall,” he warns.

Time Travel is intoxicating, but that is only in part down to Gleick’s execution. Much of this is well trodden ground, our enduring fascination with the notion sown long ago by many adroit hands. At times, Gleick seems to get lost in his own, sometimes opaque, musings. Parts of the book are frustratingly repetitive, while his practice of paraphrasing obscure time travel stories before analysing their finer points too often feels like the dinner party anecdote that rather feebly concludes “Well, you had to be there really”.

Exasperations aside, Time Travel reminds us that our relationship with the slippery concept of time is far from static: technology continues to shape our view, even now. “With persistent connectedness time gets tangled,” Gleick notes. With the internet, it seems, time ahead and time past are both brought into the present.

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Summary of “The Paradoxes of Time Travel” by Lewis

Time travel is a fascinating fantasy idea that has a logical justification in addition to its obviously entertaining function. In particular, such travel is inextricably associated with the endless paradoxes generated whenever the traveler decides to move into the past or the future. This raises legitimate questions about whether the traveler can change the course of events, about the objective measure of time, and what justifies the discrepancy between the two temporal functions.

These are all questions Lewis sought to answer in his reflections on “The Paradoxes of Time Travel.” More specifically, the author conducted an eclectic analysis of the phenomenon of time travel from the perspective of precise terminology, logical validity, and explanations of apparent paradoxes. The article is full of Lewis’s personal statements and illustrative cases to facilitate understanding such confusing material.

It is paramount to recognize that Lewis’s article is a summarizing and multifaceted work that adequately covers the basic philosophical and logical questions outlined as early as the first paragraphs of the material. Broadly speaking, this includes defining the permissibility of time travel, finding a terminological description of the basic elements, and explaining the geometry of time from the perspective of an outside observer and time traveler. Thus, the reader reading “The Paradoxes of Time Travel” may feel the initial confusion and logical incoherence of the text’s components, but through the scenarios and descriptions given by the author that accompany almost every page, the overall point is reached.

Consequently, Lewis began his work by attempting to explain the spatial geometry of time. Acknowledging the fiction of time travel — but, more importantly, not pointing out the impossibility of it — the researcher makes a cursory comparison of linear, planar, and four-dimensional time. Lewis originally adhered to the concept of a Cartesian model of time, in which the traveler is free to move between axes, whereas the average person lives by a linear function symmetrical about two axes.

However, this idea of two-dimensional time is not supported by logic from the point of view that the same event cannot be dissected into two axes. In other words, the nature of time is inseparable and represents one line rather without division into directions: straight or cyclic. Straight time aligns perfectly with what Lewis proposed, namely the four-dimensional model. Aligning the three spatial coordinates with the time axis allows the traveler to cascade along with one of the axes without splitting events into different versions at different times.

The difference between the stages of travel in time constitutes change. In attempting to formulate a definition of qualitative change, Lewis was particularly careful to refer to illustrations. In particular, a person’s growing up or hair growth determines the passage of time, and in this sense, the traveler can go back to a time when he was still a child without hair, for example, this forms the Cambridge change model (Lewis 146). However, some elements are constant at all times, such as numbers or the laws of physics. In this sense, even when traveling through time, the traveler will find that unchanging elements or events have not been subject to transformation.

A key idea, set at the beginning of the paper and found up to the last paragraph, was the division of the nature of time into personal and external. To better convey the meaning of this division, Lewis gives an example concerning the time it took to travel. For instance, if a traveler moves one thousand years into the past, the journey itself may take him about an hour, but the final destination will have a chronological difference from the point of departure of minus a thousand years.

Thus, to respond to the seeming inconsistency of time — Lewis, as mentioned above, adheres to the hypothesis of the inseparability of time along the axes — the author introduces the terms “personal” and “external” time. The hour elapsed for the traveler determines his personal account, indicated on his wristwatch, while external time characterizes the difference that objectively exists between the beginning and the end of the journey.

Near the middle of his text, Lewis raises the most intriguing questions concerning time teleportations: the phenomenon of the traveler’s personal identity and the grandfather paradox. There is no doubt that the individual who has returned to the not-too-distant past is in the same reality system as the younger version of him. For example, a twenty-year-old man who has gone back ten years may be in contact with a ten-year-old himself. Lewis urges the reader not to make the mistake of differentiating between the two individuals and especially emphasizes that both the young and adult versions of the individual are the same person, bound together by a mental connection. For outsiders and even the young version of the traveler, all given events occur according to the course of external time, while the return to the past is the man’s personal experience.

On the other hand, such an effect raises the question of the existence of a causal link between the past and the present. In particular, in the case when an adult traveler tells a young one about the device of a time machine, this knowledge as a timeless phenomenon is transferred from the present to the past so that it then turns into the present. Simply put, a young explorer could not have created the time machine without encountering a more adult copy of himself. Lewis emphasizes that similar connections produce casual cycles and loops, which, however, do not explain the origin of the time machine’s knowledge.

Approaching the second half of the article, Lewis discusses the grandfather paradox, which best covers time travel’s causal mechanism. In particular, if a grandson from his own time returns to 1921 to kill his hated grandfather, this obviously raises a number of questions about the permissibility of such a situation (Lewis 148). On the one hand, the grandson’s personal reality does not exclude changes in the past, so there is no contradiction in killing his grandfather.

However, this version is untenable from the point of view that the events in time are interconnected. The murder of the grandfather, according to Lewis, would lead to the impossibility of the birth of the father and then of the grandchild proper in the future. In other words, the grandchild would not have to exist and could not go back in time to kill the grandfather. Consequently, the grandfather would not die, and the grandson would survive. Obviously, such thinking leads to untenable conclusions and logical fallacies, which is why this effect is called the time paradox.

One of the last thoughts mentioned in the paper is the hypothesis of the branching of time when events occur during travel. Although this idea has been mentioned cursorily, it is an important thought that allows considering the phenomenon of time in an alternative way (Lewis 152). Thus, time is not linear and does not represent an axis on a four-dimensional coordinate plane; instead, it can branch. The grandfather paradox perfectly describes this model: in the case of murder, the world continues to develop along one path, but the unkilled grandfather forms a parallel, alternate reality.

Lewis, David. “The Paradoxes of Time Travel.” American Philosophical Quarterly , vol. 13, no. 2, 1976, pp. 145-152.

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Editor’s Note: We know that many of you are looking for help writing travel experience essays for school or simply writing about a trip for your friends or family. To inspire you and help you write your next trip essay—whether it’s an essay about a trip with family or simply a way to remember your best trip ever (so far)—we enlisted the help of Professor Kathleen Boardman, whose decades of teaching have helped many college students learn the fine art of autobiography and life writing. Here’s advice on how to turn a simple “my best trip” essay into a story that will inspire others to explore the world.

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Trip Essays: What to Avoid

Please don’t tell me everything about your trip. I don’t want to know your travel schedule or the names of all the castles or restaurants you visited. I don’t care about the plane trip that got you there (unless, of course, that trip is the story).

I have a friend who, when I return from a trip, never asks me, “How was your trip?” She knows that I would give her a long, rambling answer: “… and then … and then … and then.” So instead, she says, “Tell me about one thing that really stood out for you.” That’s what I’d like you to do in this travel essay you’re writing.

The Power of Compelling Scenes

One or two “snapshots” are enough—but make them great. Many good writers jump right into the middle of their account with a vivid written “snapshot” of an important scene. Then, having aroused their readers’ interest or curiosity, they fill in the story or background. I think this technique works great for travel writing; at least, I would rather enjoy a vivid snapshot than read through a day-to-day summary of somebody’s travel journal.

Write About a Trip Using Vivid Descriptions

Take your time. Tell a story. So what if you saw things that were “incredible,” did things that were “amazing,” observed actions that you thought “weird”? These words don’t mean anything to me unless you show me, in a story or a vivid description, the experience that made you want to use those adjectives.

I’d like to see the place, the people, or the journey through your eyes, not someone else’s. Please don’t rewrite someone else’s account of visiting the place. Please don’t try to imitate a travel guide or travelogue or someone’s blog or Facebook entry. You are not writing a real travel essay unless you are describing, as clearly and honestly as possible, yourself in the place you visited. What did you see, hear, taste, say? Don’t worry if your “take” on your experience doesn’t match what everyone else says about it. (I’ve already read what THEY have to say.)

The Importance of Self-Editing Your Trip Essay

Don’t give me your first draft to read. Instead, set it aside and then reread it. Reread it again. Where might I need more explanation? What parts of your account are likely to confuse me? (After all, I wasn’t there.) Where might you be wasting my time by repeating or rambling on about something you’ve already told me?

Make me feel, make me laugh, help me learn something. But don’t overdo it: Please don’t preach to me about broadening my horizons or understanding other cultures. Instead, let me in on your feelings, your change of heart and mind, even your fear and uncertainty, as you confronted something you’d never experienced before. If you can, surprise me with something I didn’t know or couldn’t have suspected.

You Can Do It: Turning Your Trip into a Great Travel Experience Essay

I hope you will take yourself seriously as a traveler and as a writer. Through what—and how—you write about just a small portion of your travel experience, show me that you are an interesting, thoughtful, observant person. I will come back to you, begging for more of your travel essays.

Take Notes in a Cute Journal

Keep track of all the crucial details- and even the ones you might forget, in a durable and refillable journal.

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The Paradoxes of Time Travel by David Lewis Term Paper

Introduction, lewis on time travel, the criticism, lewis’ revision strategy of the story, works cited.

David Lewis, in his work, The Paradoxes of Time Travel, posits that time travel is possible and adds that; paradoxes surrounding time travel are not impossibilities but oddments. In this paper, the writer imagines that an author writes a science fiction story about time travel.

In the story, a poor scientist in 2010 uses a time machine to travel back to 2008, where he/she tells his younger self the winning lottery numbers for 2009. The time traveler uses the time machine to return to 2010, where he is now rich. The writer then uses Lewis’ arguments to criticize the story and suggest how Lewis would revise it. To understand this story, it is important to understand some of Lewis’ arguments.

Lewis observes that, “time must not be a line but a plane” (146). This implies if two events are separated more than once in time dimensions, then they can have two one-sided separations. In normal life, people live on straightforward aslope lines cutting across the plane of time. However, a time traveler lives on bent-line slopes on the same time plane. Moreover, according to Gott, time traveler has a personal time that does not comply with the rules of the normal time, also called external time (5).

In this case, the time traveler’s personal time can go back into ancient time in the present external time. Nevertheless, this phenomenon is relative and Lewis notes that the probability of a time traveler going back in time to change the past depends on some set facts. Lewis therefore would find a foothold to criticize the aforementioned fiction.

As aforementioned, the author writes a story where a poor scientist in 2010, goes back to 2008, reveals to his younger self the winning numbers of a 2009 lottery; wins the lottery, and comes back to 2010 where he or she is rich. Lewis would say that this scenario could not happen because of inconsistency.

Lewis would consider some few facts here. The poor scientist cannot be rich in 2010. This poor scientist is poor right now, therefore going back in 2008 and reveal to his/her younger self the winning numbers of a 2009 lottery, would be tantamount to changing the past, which cannot change.

Lewis would argue that, events surrounding “past moments are not sub divisible into temporal parts; therefore, cannot change” (151). Events of 2008 can either timelessly include the poor scientist revealing to his/her younger self the lottery winning numbers or timelessly do not include the events; however, the two events cannot occur simultaneously.

If the fiction story were to be considered true, the possibility of describing two events referring to same thing would be inevitable. In this case, there would be ‘original’ 2008 and ‘new’ 2008.

The ‘original’ 2008 would represent the actual time when the poor scientist lived and did not know anything about the winning lottery numbers; on the other hand, the ‘new’ 2008 would represent a counterfactual time when the poor scientist is revealing to his/her younger self the winning numbers of the lottery. In the time traveling world of this poor scientist, both the ‘original’ and the ‘new’ 2008 exist in his/her extended timeline; however, in the external time people would be referring to the same thing.

Unfortunately, one event cannot be defined or described by two different events. If the poor scientist did not reveal to his or her younger self the winning numbers in the ‘original’ 2008, but he reveals the numbers in the ‘new’ 2008, then he/she must both reveal and not reveal the winning numbers in 2008, because there can only be one 2008 which is both the ‘new’ and the ‘original’ 2008.

Therefore, logically speaking, the poor scientist cannot reveal to his/her younger self the winning numbers of the lottery; consequently, he or she cannot be rich in 2010.

Instead of giving a one sided story, Lewis would opt to give it two sides considering what the poor scientist could do and what he/she could not. The first scenario is that of the poor scientist not revealing to his/her younger self the winning numbers of the lottery as explicated in Lewis’ criticism. In revising this story, Lewis would argue that the poor scientist would reveal to his/her younger self the winning of the lottery.

Here are some facts that would facilitate this occasion. The poor scientist would change his/her poverty status in the past by revealing to his younger self the winning lottery numbers; however, he/she would fail to do that, not because of any impossibility but because of some inefficiencies. Given the fact that the poor scientist did not reveal the winning numbers in the original 2008, consistency requires that he/she does not reveal them in the ‘new’ 2008; why?

There has to be a reason why the poor scientist could not reveal the winning numbers to his/her younger self. Maybe he/she lost the paper containing the numbers or simply doubted the authenticity of the numbers. In this case, the poor scientist has the potential to reveal to his/her younger self the winning lottery numbers; however, something crops up which changes the fate of this poor scientist. This is normal in life; people try hard to do things that they would wish to; however, fate has it that they fail.

Not because it is impossible to do such things, it is only that luck does not allow it. In this case, some eminent contradictions would sabotage the consistency of the story. One, the poor scientist does not reveal the numbers even though he can for he/she has them. Two, the poor scientist does not reveal the numbers, and he/she cannot for the past is unchangeable. According to Sider, Lewis would argue that, ‘can’ is equivocal; hence, the two scenarios are compatible (1).

To say the poor scientist ‘could’ reveal the winning lottery numbers is compossible with contextual facts; that is, he had the numbers. The poor scientist could reveal to his/her younger self the winning lottery numbers just the way a teacher can read out answers to students. However, the poor scientist could not reveal the numbers to his young self because this scenario is not compossible with some other facts, he/she is poor in 2010, and this is the fact.

Nevertheless, interpreting these two scenarios calls to choose either a wide delineation and conclude that the poor scientist cannot reveal the numbers or a narrow delineation of relevant facts and conclude that he/she can reveal the numbers.

Relativity takes precedence here and either of the arguments can pass as true; however, one cannot afford to conclude that the poor scientist could and could not reveal the numbers simultaneously. The call to make choice here is to root out contradiction, which would otherwise refute the possibility of time travel.

Lewis points out that time travel is possible; however, one has to make a choice and argue his/her case out based on relativity of facts surrounding the subject under study. In the case of a poor scientist in 2010 traveling through a time machine to 2008, revealing to his younger self the winning numbers of a 2009 lottery, winning it and becoming rich in 2010, Lewis would criticize it on basis that, the past is unchangeable.

However, Lewis would revise the story and throw in a possibility of such an event happening depending on the relativity of facts surrounding it. The poor scientist did not reveal the numbers to his younger self but he/she could do so because he/she had the numbers; however, he/she failed for he/she either misplaced the paper containing the numbers or simply doubted the authenticity of the same.

On the other hand, the poor scientist did not reveal the numbers to his younger self and cannot because the past is unchangeable. The fact is, in 2010, the scientist is poor, and it depends on the stand that one takes in interpreting the possibility of this poor scientist going back to 2008 to reveal the winning numbers to his young self. Nevertheless, one cannot say that the poor scientist can and cannot travel back to 2008 simultaneously.

Gott, Richard. “Time Travel in Einstein’s Universe: The Physical Possibilities of Travel.” New York; Houghton Mifflin Company, 2001.

Lewis, David. “The Paradoxes of Time Travel.” American Philosophical Quarterly. 1976: 13(2); 146-152.

Sider, Ted. “Lewis on Time Travel.” Nd. Web.

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IvyPanda. (2024, January 31). The Paradoxes of Time Travel by David Lewis. https://ivypanda.com/essays/time-travel/

"The Paradoxes of Time Travel by David Lewis." IvyPanda , 31 Jan. 2024, ivypanda.com/essays/time-travel/.

IvyPanda . (2024) 'The Paradoxes of Time Travel by David Lewis'. 31 January.

IvyPanda . 2024. "The Paradoxes of Time Travel by David Lewis." January 31, 2024. https://ivypanda.com/essays/time-travel/.

1. IvyPanda . "The Paradoxes of Time Travel by David Lewis." January 31, 2024. https://ivypanda.com/essays/time-travel/.

Bibliography

IvyPanda . "The Paradoxes of Time Travel by David Lewis." January 31, 2024. https://ivypanda.com/essays/time-travel/.

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Star Trek's Time Travel Rules Aren't Nearly as Confusing as They Seem

Over the six-decade history of Star Trek, there's been a lot of time travel and, confusing it as it may seem, there're clear rules about how it works.

• Star Trek time travel operates with genuine logic, impacting the past and future consistently within the narrative framework.
• Major series and films heavily rely on time travel, introducing rules like the "slingshot" effect and parallel realities.
• Changes in the past affect the future of the Prime Timeline, except for the creation of the Kelvin Timeline as a parallel reality.

As a mainstay of science fiction for six decades, Star Trek is no stranger to time travel stories. On the surface, it may seem as if the rules of time travel are loose in Gene Roddenberry's universe. The first chronological use of time travel comes at the end of Star Trek: The Original Series Season 1, Episode 6, "The Naked Time," when the USS Enterprise is sent three days back in time. Since then, every major Star Trek series has featured time travel, with some even relying heavily on the convention.

Time travel also shows up in the films, from Star Trek IV: The Voyage Home to Star Trek: First Contact . The 2009 reboot Star Trek movie by J.J. Abrams also relies on time travel to create its brand-new universe. Star Trek also uses parallel realities, such as its famously evil Mirror Universe. While there are rules in its universe, the real-life way Star Trek time travel works is, essentially, narrative convenience. What the storytellers want to happen is what happens, but through these choices made by disparate storytellers, a framework emerges.

Time Travel in Star Trek: The Original Series Set the Tone

Star trek has been quietly influencing one of the most popular franchises of all time.

"The Naked Time" and the Enterprise being flung into the past was meant to be the first part of a two-part story. The second part was "Tomorrow Is Yesterday," eventually the 21st episode of the first season. In this story, the USS Enterprise is accidentally flung back to 1969. This episode developed the "slingshot" effect, in which the ship warps around the sun and uses that to break the "time barrier." In this story, the Enterprise is able to successfully erase its presence in the past by undoing the changes they made. In Season 2, Episode 26, "Assignment: Earth," The Enterprise is again in the past on what's supposed to be a "routine" observation mission. Instead, they help Gary Seven -- a mysterious human called a Watcher" -- prevent a nuclear detonation.

In that episode, Spock discovers the Enterprise was always supposed to be a part of the events that day. This is a closed time-loop in which the presence of time travelers was always part of what happened . However, in the famous episode "City on the Edge of Forever," a crazed Doctor McCoy accidentally goes back in time through the "Guardian of Forever" gate and changes history. The USS Enterprise disappeared, but the away team on the planet did not. This was the first time that changes in the past were immediately reflected in the timeline, which is the de facto way time travel manifests in Star Trek . Similarly, in Star Trek: The Animated Series , the Guardian of Forever is used to correct a mistake that erased Spock from history.

In "City on the Edge of Forever," proximity to the Guardian prevented the crew from being affected by the changes. Yet, in "Yesteryear," the crew by the Guardian were not immune, not recognizing Spock when he emerged. Lastly, in Star Trek IV the crew changes the past , but its effects are left vague. Scotty gives a random manufacturer the formula for transparent aluminum, while Chekov leaves behind his communicator and phaser when captured by the US military. How these changes affected the future are not known, but it's presumed that they would have been both instantaneous and unnoticeable upon the Enterprise's return to their present.

How The Next Generation Era Solidified Star Trek Time Travel Rules

One of star trek's best new characters reclaimed a problematic trope.

There were a number of episodes in the second wave of Star Trek that dealt with time travel, including its consequences on the future. In a Star Trek: Voyager two-parter, Ed Begley, Jr. played an important character who was, essentially, responsible for the 1980s and 1990s computer age the Star Trek: The Originl Series storytellers didn't see coming. In Star Trek: Deep Space Nine 's "Past Tense," Captain Sisko has to step in for historical figure Gabriel Bell in the "Bell Riots," an important historical event. Also, Deep Space Nine revealed that Quark, Nog and Rom were the "Rosewell aliens" thanks to a time travel snafu. However, one episode further solidified how changes to the timeline worked.

In Star Trek: The Next Generation 's third season episode, "Yesterday's Enterprise," the USS Enterprise-C was displaced from its time into the 24th Century. The moment it arrived there, the timeline changed and none of the Enterprise-D crew was the wiser. Only Guinan was subtly made aware of these changes due to her El Aurian ancestry. When the Enterprise C went back into the temporal rift, that reality snapped back not just to normal but the moment it first emerged. This was despite the fact that days passed in the alternate present the ship's arrival created. This matches how the temporal changes in the sequel series worked as well. Sometimes other characters were exempt from the changes, and while there were sci-fi reasons given, it was, again, narrative convenience.

In Star Trek: First Contact , the Borg successfully altered the past when it sent a sphere back to 2063. The USS Enterprise-E was immune from the timeline changes because of its proximity to the temporal singularity that eventually allowed them to follow . Since the version of Zefram Cochrane that appeared in The Original Series was ignorant of the Federation, it's safe to assume the presence of The Next Generation 's characters changed that past from what it once was. When they returned to their present, presumably, any permanent changes to the present were, again, unnoticeable.

Star Trek: Enterprise and the Kelvin Timeline Changed Time Travel Rules Further

Star trek: discovery fixed one big mistake the kelvin timelines movies made.

The idea of a temporal war was introduced in Star Trek: Enterprise , again suggesting the version of events viewers saw was not the history the characters in the past series knew. From the war with the Xindi in Season 3 to the various temporal incursions in the earlier seasons, the NX-01 Enterprise experienced a different history than it should've. This suggests that every time travel adventure in Star Trek changed the past, save perhaps for the events in "Assignment: Earth." However, that episode is an outlier because it was conceived as a backdoor pilot for a spinoff starring Gary Seven. At some point, according to both Star Trek: Discovery and Star Trek: Strange New Worlds , the Temporal War went from "cold" to "hot," resulting in even more significant changes.

Still, in almost all cases, changing something in the past affected the future of the Prime Timeline. There is one notable exception, which also exists for narrative convenience. When Ambassador Spock and the Narada traveled into the past after the destruction of Romulus, it created an alternate timeline that became a parallel reality running concurrent with the Prime Timeline . Unlike other time travel events that changed Star Trek canon , the Kelvin Timeline continued forward without undoing the events in the Prime Timeline. Star Trek: Picard Season 1 proved this by confirming the destruction of Romulus. And, when those characters time traveled in Season 2, the changes in the past were again reflected in the Prime Timeline's future rather than creating a branch reality.

For those looking for a sci-fi reason for this, one could say that the mysterious "Red Matter" Spock used had something to do with it. However, the Kelvin Timeline isn't necessarily the only parallel reality created by time-travel. In Star Trek: Discovery , the Guardian of Forever implied to Philippa Georgiou that there was a period in the Mirror Universe's past where it was the same timeline as the one fans know. Though, Enterprise showed that divergence happened, at least, as far back as Vulcan First Contact. Still, save for it and the Kelvin Timeline, Star Trek 's rules of time travel suggest changes in the past affect the future of the Prime Timeline.

There Are Still Unanswered Mysteries About How Time Travel Works in Star Trek

The new star trek series could erase the kelvin timeline, but shouldn't.

When it comes to the real-world story reasons for creating an alternate timeline versus a parallel reality, it's all down to narrative needs. The Kelvin Timeline needed a future unencumbered by existing canon, thus it existed without changing the Prime Timeline. However, most of the time travel incursions in the Star Trek universe don't work that way. Yet, events change for other, inexplicable reasons such as the birth and rise of Khan Noonien Singh . In "Space Seed," the The Original Series Season 1 episode that introduced the character, his reign of terror happened in the 1990s . Yet, Strange New Worlds fixed that in Season 2.

Khan's descendent, La'an Noonien Singh, was recruited by a Temporal Agent to fix an attack in the past. When he passed his Time Gizmo to her, the timeline changed around her, including erasing the Temporal Agent's body. While in the past, she encountered a Romulan temporal soldier sent back to the 1990s to kill Khan. Without him, Starfleet and the Federation would never form. Yet, when she got there, she found no evidence of Khan's existence. Eventually, she discovered him as a child in the mid-21st Century. She implied to La'an that certain "canon events" will happen regardless. Perhaps because of the changes made to the time in Voyager , Khan's creation didn't happen until later.

While these aren't the clearest rules, time travel can explain many things, including Strange New Worlds ' advanced technology . These changes alter the look and capabilities of Star Trek 's future, but the "big" things still happen. Christopher Pike captains the Enterprise and is debilitated in an accident. James T. Kirk replaces him, preventing a Romulan war. This can also mean that time travel events that don't happen on screen can also change the timeline in ways the characters (and the audience) would never realize . In fact, all these various stories told by different generations of storytellers over decades fit together even this neatly is a kind of miracle in itself.

Star Trek's series and Kelvin Timeline films are available to stream on Paramount+, while the first 10 films stream on Max. All are available to own on DVD, Blu-ray and digital.

The Star Trek universe encompasses multiple series, each offering a unique lens through which to experience the wonders and perils of space travel. Join Captain Kirk and his crew on the Original Series' voyages of discovery, encounter the utopian vision of the Federation in The Next Generation, or delve into the darker corners of galactic politics in Deep Space Nine. No matter your preference, there's a Star Trek adventure waiting to ignite your imagination.

Here are seven charts to make sense of the Victorian state budget

The Victorian government handed down its budget for 2024-25 yesterday.

Some of the big announcements include $400 credits for children at government schools, the delay of the Airport Rail Link, and billions of dollars invested into upgrading major hospitals  — but plenty more can be gleaned from the data.

Here are seven charts to help make sense of the numbers.

Debt is predicted to grow

The Victorian government is in debt, and that's only predicted to increase.

After a sharp rise in the last few years, attributed to the COVID pandemic and big infrastructure project spending, the debt is expected to grow to $156.2 billion.

By 2027-28, it's forecast to rise further, to $187.8 billion.

However, the debt isn't growing as fast as it has in previous years, which is perhaps better seen by comparing the size of the debt to Victoria's economy as a whole.

The size of Victoria's economy is measured using the Gross State Product (GSP), which is calculated by adding up all the goods and services a state produces.

When analysing these figures together, we can see Victoria's debt will reach 24.4 per cent of its economy in the next financial year.

In budget forecasting, it's expected to peak at 25.2 per cent in 2026-27 — then fall by 0.1 per cent the year after.

In his budget speech, Treasurer Tim Pallas said this predicted decrease would be the first since the pandemic, and would indicate the "strength" in the growth of Victoria's economy.

But it's only a forecast figure at this stage.

Goal to reach budget surplus by June 2026

The Victorian government not only predicts a reduction in the debt ratio — it's forecasting a budget surplus.

This means the government will be taking in more money than it's spending.

After years of being in deficit through the COVID response years, the government predicts there will be one more year of deficit before a surplus of $1.5 billion in 2025-26.

The budget surplus is predicted to grow from there, increasing to $1.64 billion in 2026-27, and $1.94 billion the year after that. 

Infrastructure spending predicted to drop

One way the government has tried to keep future budgets in surplus is by reducing its spending on capital projects.

These projects encompass all the building and repairing of infrastructure that happens across the state, from major road projects, to new schools, and hospital upgrades.

There was a massive increase in infrastructure spending in the past few years, including level crossing removal projects across Melbourne, the West Gate Tunnel, the Metro Tunnel, and early works on the Suburban Rail Loop.

But now the government is looking to curb this.

The budget predicts annual infrastructure spending will reach a peak of $23.3 billion in the coming financial year, and gradually decrease to $15.6 billion in 2027-28.

Pallas says this is intentional, as the aim is to decrease the amount spent on infrastructure to pre-COVID levels, "to better align with the ability of our economy to deliver".

Health and education dominates spending

To get an idea as to what the state government spends its money on, a breakdown of expenditure by government function is a useful measure.

These functions are defined by the Australian government , and are useful when comparing the expenditure of different states and territories.

Health spending dominates, comprising 31 per cent of the 2024-25 budget's operating expenses.

Next comes education, which equates to about a quarter of spending.

That's followed by public order and safety, which includes policing, general public services, encompassing public servant wages, and transport.

These proportions stay similar from year to year.

Big reductions across government functions

Grouping government spending into these functions can also show where the government might look to save money in coming years.

This can be done by taking the amount allocated for the coming financial year, and comparing it with the forecast allocation for 2027-28. 

For example, 'general public services' will see a $3 billion annual increase in spending between this budget and 2027-28.

This allocation includes all the costs of running a government, such as paying MPs and public servants.

The biggest decrease is in 'economic affairs', which includes agriculture, forestry, mining, manufacturing, construction and communication. There will be more than a 50 per cent reduction in spending in this sector between this budget and 2027-28.

The next biggest reductions are in 'recreation, culture and religion', which includes sports funding, which the state government predicts will halve over the next four years.

But the biggest increase is in a classification called 'not allocated by function'. As its name suggests, this money has not yet been set aside for a particular purpose.

Payroll taxes are the state's biggest

And finally, a look at how the government makes its money from taxes.

The biggest tax is payroll tax, which is calculated based on wages paid to employees by an employer above a threshold.

Revenue from this tax is expected to grow, and is bolstered by the COVID Debt Levy, and the Mental Health and Wellbeing Levy, with these levies paid by businesses with a national payroll of more than $10 million.

Revenue from stamp duty and land taxes is also predicted to increase, while revenue from gambling taxes is expected to fall.

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The Morning

When travel plans go awry.

There are ways of keeping ourselves anchored, even when we enter a parallel universe disconnected from time.

By Melissa Kirsch

The weekend trip is, in theory, the perfect break. Two nights someplace else, just a small duffel bag and limited logistics standing between you and a reset. Leave on Friday, come back Sunday, fill the hours in between with enough that’s novel and return refreshed, or at least with a slightly altered perspective. You might take a weekend trip for vacation or work or to see family, but the effect is the same. You’re a little changed on return. You see your regular life a little bit differently.

I took what was meant to be a quick trip last weekend to attend a college graduation, and it was, strictly speaking, quick: I was scarcely away for 48 hours, but extreme weather marooned me for most of those hours in the liminal spaces of transit — airports, grounded planes, traffic jams — where time loses legibility. An old friend used to call these neither-here-nor-there realms the “zero world” for the way they feel unfastened from reality, parallel to daily life but separate. The flight cabin after an announcement of a fourth lightning delay is a world detached from the one you know, a temporary society populated by temporary citizens with perhaps not much in common save one deeply held belief: We need to get out of here.

I was as cranky and impatient as the rest of my fellow travelers at each complication in our journeys, but also fascinated by the communities and customs and Cibo Express markets of the zero world. Each of us was, at any given time, one captain’s announcement away from a temper tantrum, but we were also competitively careful to be polite to one another and to the airline staff, as if determined to demonstrate that those wild videos of short-tempered passengers being duct-taped to their seats did not represent us, the makeshift civilization of this departure lounge.

Graduation, when I finally arrived, was a joyous affair despite the glitches. The speaker, an astronaut, showed a photo of the farm where she grew up, the place she thought of as home for much of her life. Then she showed a photo of the limb of the Earth, the glowing edge of the atmosphere, and described how, when she went to space, home was no longer a town on a map but this planet, a shift in perspective so massive I felt a little queasy contemplating it.

On Hour 3 in the airport bar on Sunday morning, beside two German travelers practicing Spanish, I ordered an omelet and imagined my own home, which felt very far away and lit by its own otherworldly halo. What would I be doing if I were there? Reading, texting, catching up on emails — the same things I was doing here. What was so bad about this? Was it the lack of choice? The lack of fresh air?

It was all those things, and also the feeling of being trapped in a warp between origin and destination. My emotions felt out of proportion to the situation: I hadn’t traveled very far for very long, was in no peril and would still arrive in New York with enough day left to do whatever needed to be done, but I felt on the verge of tears, loosed from my moorings, floating between fixed points, dislocated. I put on my headphones, put on a favorite band whose songs are so familiar they provide a home base no matter where I am. I listened to the same album on repeat for the duration of the flight, in the car on the way home, even at home once I finally made it there.

There’s a story in The Times today about how A.S.M.R., the pleasant, brain-tingling feeling we get when hearing certain sounds or watching certain comforting scenes, has become a feature of all viral internet content, not just specialized videos devoted to inducing the sensation. You can still put on a very specific video of someone whispering into a microphone or crinkling paper, but you’re just as likely to find the stimuli in videos of people cooking or cleaning their pools. This seems like a logical extension. We’re restless beasts in need of soothing. Sometimes we’re dramatically homesick, sometimes it’s just a bad day. Why not imbue the mundane with the choreography of comfort? Why not add pleasure whenever and wherever we can?

For weekend travel inspiration: The Times’s 36 Hours series.

How to deal with the increasing unpredictability of travel .

Stunning views of Earth from space .

How A.S.M.R. became a sensation.

THE WEEK IN CULTURE

The final round of the Eurovision Song Contest takes place in Sweden today. This year’s favorites include a Croatian techno act called Baby Lasagna. Read , or listen to , a guide to the competition.

“I won’t let anything break me”: Eden Golan, Israel’s 20-year-old entrant, spoke to The Times about the campaign to exclude her country from the event because of the war in Gaza.

The stage crew has 50 seconds to disassemble and reassemble sets. Watch a video from The Wall Street Journal .

Film and TV

“It’s easy to get caught up in the bigness of it all”: Owen Teague, the star of the latest “Planet of the Apes” film, and Andy Serkis, the lead in the earlier movies, sat down for a conversation .

“Kingdom of the Planet of the Apes” is not as transporting as the previous trilogy of films, the Times critic Alissa Wilkinson writes , but “there’s still a tremendous amount to mull over.”

The latest season of “Doctor Who,” starring Ncuti Gatwa as the 15th actor to play the doctor, opened with a double episode. Read a recap.

Disney and Warner Bros. Discovery announced a plan to bundle their Disney+, Hulu and Max streaming services this summer

The recording engineer Steve Albini, who died this week at 61, was “arguably the most influential figure ever to emerge from indie rock,” Pitchfork wrote . Listen to 10 of his essential tracks , which shaped the sound of alternative rock music.

Kendrick Lamar and Drake’s rap beef crashed the website Genius , where users can annotate lyrics to songs. Times critics discussed where the rappers’ sonic conflict goes next .

Other Big Stories

A stage version of the beloved animated film “Spirited Away” is running in London, after premiering in Japan. The adaptation is opulent and impressive, but it could use more heart , our critic writes.

A federal judge granted a preliminary injunction that would bar the Des Moines Art Center from dismantling “Greenwood Pond: Double Site,” an environmental work by Mary Miss that includes wooden walkways and sitting areas in need of repair.

The owners of the Los Angeles house where Marilyn Monroe last lived, and died, sued the city, accusing officials of “backroom machinations” to save it from a planned demolition .

David Shapiro, a lyrical poet who appeared in a famous photograph from the 1968 uprising at Columbia University, died at 77 .

THE LATEST NEWS

Israel-Hamas War

A Biden administration report said that Israel may have broken international law in Gaza, but that Israel’s “credible and reliable” assurances mean the U.S. can keep sending weapons.

The Biden administration is still waiting for Israel to show how it plans to evacuate and protect civilians in Rafah ahead of a possible invasion.

The U.N. General Assembly voted to support Palestinian statehood , a symbolic move. The U.S. voted no, and Israel accused delegates of “shredding the U.N. charter.”

Michael Cohen, who paid Stormy Daniels hush money and whom prosecutors say Donald Trump reimbursed, is expected to testify Monday in Trump’s Manhattan criminal trial. The judge asked prosecutors to stop Cohen from criticizing Trump .

Russia tried to break through Ukrainian lines in the country’s north using shelling and armored columns. Ukraine said it had repelled the attacks.

Russia is upgrading a munitions depot in Belarus, possibly to house nuclear weapons , a Times analysis of satellite imagery found.

The Biden administration plans to raise tariffs on Chinese electric vehicles to protect U.S. auto manufacturers.

Apple is revamping Siri to offer more advanced A.I. responses , akin to ChatGPT.

An appeals court upheld Steve Bannon’s conviction for defying a subpoena from the House Jan. 6 committee. He could soon have to serve prison time.

A Virginia school board voted to restore the names of Confederate leaders — including Robert E. Lee and Stonewall Jackson — to two schools, reversing its 2020 decision to rename them .

CULTURE CALENDAR

By Desiree Ibekwe

🎥 Back to Black (Friday): You may well have seen the online discussion about this movie, an Amy Winehouse biopic directed by Sam Taylor-Johnson. The movie — which focuses on Winehouse’s relationship with Blake Fielder-Civil — was No. 1 at the British box office but divided viewers and critics, some of whom found fault with the appearance of its star, Marisa Abela. “I don’t need to convince people that they’re actually watching Amy,” Abela told The Times . “I need to remind people of her soul.”

RECIPE OF THE WEEK

By Melissa Clark

Strawberry Shortcake

It’s Mother’s Day tomorrow, and if your mom has a sweet tooth (and if so, I can relate), Jane Grigson’s strawberry shortcake as adapted by Nancy Harmon Jenkins might be just the thing for a celebratory brunch. Make the biscuit dough and cut out the rounds the day before (just keep them in the fridge until baking time). Then, while they’re in the oven, you can macerate the berries (any kind you like) with sugar and prep the whipped cream. Be sure to save any leftover biscuits. They’re excellent toasted for breakfast the next day.

REAL ESTATE

The hunt: An American took a chance on the Lake Geneva area of Eastern France, with a $300,000 budget. Which home did she buy? Play our game .

What you get for $900,000: A Frank Lloyd Wright house in Wilmette, Ill.; an 1879 three-bedroom house in Wilmington, N.C.; or a renovated ranch house in Scottsdale, Ariz.

Free help: A filmmaker, feeling unhelpful in her daily life, decided to offer small favors to passers-by in Union Square.

Made for walking: Brides are increasingly pairing cowboy boots with relaxed silhouetted dresses.

Scarlett Johansson: The actress shared her beauty regimen with T Magazine.

How to: Restoring a chair is easier than one might think . Here’s how a couple known as the Brownstone Boys did it.

ADVICE FROM WIRECUTTER

Food processors, blenders and choppers.

Countertop appliances can help you get a meal on the table faster, often with less work and a quicker cleanup. But deciding which gizmo is best for you can be a challenge. It depends on what kinds of foods you most frequently prepare, Wirecutter’s kitchen experts say. For example, if your main goal is to reduce the time you spend prepping ingredients, a food processor is likely your best bet. If you demand the smoothest, silkiest textures from your soups, sauces and smoothies (and have ample storage space), consider a full-size blender. Oh, and those TikTok-famous manual vegetable choppers ? No one needs those. — Rose Lorre

GAMES OF THE WEEK

W.N.B.A. season openers: A once-in-a-generation group enters the W.N.B.A. next week. You may already know their names: Caitlin Clark, Angel Reese, Cameron Brink, Kamilla Cardoso. Their college matchups shattered viewership records, and their pro draft last month did the same. The W.N.B.A. is trying to seize the moment: Nearly all of Clark’s games with the Indiana Fever will be national broadcasts , and some of her games are moving to bigger arenas to meet fan demand.

The season begins Tuesday, as Clark and the Fever face the Connecticut Sun and M.V.P. contender Alyssa Thomas. After that, the two-time defending champion Las Vegas Aces play Brittney Griner and the Phoenix Mercury. 7:30 p.m. and 10 p.m. Eastern on ESPN2

More coverage

Clark and Cardoso are featured in a documentary series , “Full Court Press,” airing on ABC this weekend, which follows them through their final season of college.

The W.N.B.A. is expanding : The league plans to add a 13th team, in the San Francisco area, next season, and a 14th, in Toronto, the year after.

NOW TIME TO PLAY

Here is today’s Spelling Bee . Yesterday’s pangram was uncloak .

Take the news quiz to see how well you followed this week’s headlines.

And here are today’s Mini Crossword , Wordle , Sudoku , Connections and Strands .

Thanks for spending part of your weekend with The Times. — Melissa

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Melissa Kirsch is the deputy editor of Culture and Lifestyle at The Times and writes The Morning newsletter on Saturdays. More about Melissa Kirsch