history paper-time travel for bethuel best

Time Travel, Coincidences and Counterfactuals

Author(s): Theodore Sider

Source: Philosophical Studies: An International Journal for Philosophy in the Analytic

Tradition, Vol. 110, No. 2 (Aug., 2002), pp. 115-138 Published by: Springer

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THEODORE SIDER

TIME TRAVEL, COINCIDENCES AND COUNTERFACTUALS

(Received 3 May 2002)

ABSTRACT. In no possible world does a time traveler succeed in killing her earlier self before she ever enters a time machine. So if many, many time travelers went back in time trying to kill their unprotected former selves, the time travelers would fail in many strange, "coincidental" ways, slipping on banana peels, killing the wrong victim, and so on. Such cases produce doubts about time travel. How could "coincidences" be guaranteed to happen? And wouldn't the certainty of coincidental failure imply that time travelers are notfree to kill their earlier selves? But if so, what would inhibit their freedom? Despite initial appearances, these and other doubts may be answered: the possibility of time travel survives yet another challenge.

1. FAILURES IN AUTOINFANTICIDE

Imagine you had a time machine. Nothing would stop you from going back in time and killing yourself as an infant, before you ever entered the time machine. But then a contradiction would ensue: you would never have entered any time machine since you were killed before doing so (let "killing" be understood throughout as implying permanent death), and yet you would have entered a time machine, in order to travel back in time to kill yourself. Some conclude that time travel is impossible, since it would lead to this contradiction.

There is nothing special about autoinfanticide: similar problems arise whenever a time traveler resolves to go back in time and do something that did not in fact occur. A time traveler who remem-

bers owning a 1974 Plymouth Gold Duster could, it would seem, go back into the past and prevent herself from ever owning such a fine automobile; a time traveler could, it would seem, go back and

prevent Lincoln from giving the Gettysburg address, and so on. But autoinfanticide is an especially vivid example.

As it stands, this argument is very weak. All it shows is that autoinfanticide is impossible, as are related scenarios, such as one

La PhilosophicalStudies 110: 115-138,2002.

T 02002 Kluwer Academic Publishers. Printed in the Netherlands.

TIME TRAVEL, COINCIDENCES AND COUNTERFACTUALS 117

philosophies of time according to which the future is "op no one other than a fatalist thinks this undermines my abi A. For Lewis, one has the ability to do A in w if one's doi consistent with the relevant facts about w (where what fa as relevant varies according to the context of the speaker); time traveler's victim is the time traveler herself is (typica contextually relevant fact.

We have arrived at the following familiar position: desp impossibility of autoinfanticide, time travel is possible. M though time travelers do not kill their earlier selves, the cally have the ability to do so. This position is, I believe, c But consider the following challenge. In an interesting th experiment due to Horwich (1987, Chapter 7; 1995), time t repeatedly go back in time with the goal of killing their selves. Imagine a futuristic Institute for Autoinfanticide se legions of assassins. (Perhaps these assassins have been emb by the failures of repeated attempts on their lives in their ch and fear nothing, not even the rumored cataclysmic dest of the world that would result from a violation of the la logic.) Since autoinfanticide is impossible, each assassin fa change their minds, others slip and fall on banana peels, y kill the wrong target, and so on. But there is something o the idea that such "coincidences" are guaranteed to happen and again!

There are, in fact, a few different arguments in the vicinity. While some have been made explicitly in the literature, I suspect others to lie below the surface of continuing inchoate resistance to time travel. Each is seductive, but none, I think, succeeds in undermining its possibility or likelihood. The purpose of this paper is to present and then rebut these arguments.

2. WHY BOTHER?

Before expending too much energy on the topic, it is worth thin a bit about its point. Beyond the (perfectly legitimate) desire the record straight, is there any reason to care about time travel

The most straightforward reason to care is that today's ph community cares. Whether the actual laws of nature permit

118 THEODORE SIDER

travel is a live debate in contemporary physics journals (see Earman, 1995). Suppose the arguments to be discussed in this paper against the possibility of time travel (without shackles) succeeded.

Given that many physicists tell us otherwise, that would be a problem! Whatever else metaphysicians must do, they should at least try to make metaphysical sense out of what physicists take seriously.

Secondly, time travel is tied up with larger issues in metaphysics and philosophy of science concerning the direction of time, causa tion, and so on. The possibility of time travel limits the space of acceptable theories in these areas.

Finally, time travel is connected with important issues in the philosophy of persistence. I have argued elsewhere (2001, Chapter 4, Section 7) that the possibility of time travel undermines "three dimensionalism", the view that objects persist over time by being "wholly present" or "enduring", rather than by "perduring", i.e., persisting by means of temporal parts.

3. THE IMPROBABILITY OF COINCIDENCES

Having resolved to care about time travel, let us consider what argu ments might be based on Horwich's thought experiment. There is first the argument that Horwich himself advances (1987, Chapter 7). Repeated attempts at autoinfanticide would lead to repeated "coinci dental" failures - slips on banana peels, failures of nerve, etc. But we have empirical reason to think that such repeated coincidences do not occur. We do notice the odd slip on a banana peel en route to a murder, but such slips are rare indeed. Since we have strong inductive evidence against the existence of a rash of coincidences of this sort, we have reason to think that time travel into the recent past does not occur.

The argument establishes at most that we have defeasible reason to believe that time travel into the recent past does not actu ally occur. The argument concerns only the actual world because the evidence against coincidences is contingent; clearly strings of coincidences might have occurred. The argument provides only defeasible evidence because the evidence is inductive: the future existence of strings of coincidences is logically compatible with

120 THEODORE SIDER

travelers with attendant strings of "coincidences", since the present absence of coincidences does not seem to be projectible. To date, we have observed no rash of coincidences. But this observation does not warrant postulating any law of nature, only a certain matter of

particular fact: that no legion of assassins has descended upon the present time (either because time travel will never be discovered, or because no future Institute for Autoinfanticide has attended to our time). There is no reason to expect such a particular matter of fact to continue to obtain (absent independent evidence against the existence of time machines); it is only observed lawlike patterns that are projectible into the future.

For all we have learned from the probabilistic argument, time travel might yet occur in our world. But let us leave the actual world to the physicists and return to the question of whether there are conceptual or metaphysical challenges to the very possibility of time travel. As noted, the probabilistic argument provides no such challenge. But further interesting challenges to time travel may be based on Horwich's thought experiment.

4. COUNTERFACTUALS OF COINCIDENCE

Imagine that time travel is indeed possible, and that The Corporate Board contemplates forming an Institute for Autoinfanticide. In fact they decide against its formation. But what would have happened had the Institute been formed? There would have been an incre dible series of coincidences. Had the institute been formed, there would have been a long string of slips on banana peels, serendipitous changes of heart, and the like.

We are all familiar with might-conditionals of this sort: if I had gotten up from the couch today, I might have tripped on a banana peel. But few of us think that in normal cases, would-conditionals of this sort are ever true. Unless one's couch is surrounded by banana peels, garden rakes and hanging cymbals, counterfactuals of this sort seem false:

If I had gotten up from the couch, I would have encountered some "coincidental" disaster

At most what is true is:

122 THEODORE SIDER

requires strange shackles on the time traveler. Vihvelin herself does not draw this conclusion, but it is hard to see why. Once the inability of the time traveler to kill her former self is admitted, one wonders what prevents her from doing so. Vihvelin is willing to grant that a time traveler is free to do many things other than kill her earlier self, but what prevents the time traveler from doing this thing?

6. COINCIDENCES AND FREEDOM

A final argument would be that the repeated slips on banana peels are too predictable and regular to be coincidences. There must be some force or mechanism causing the slips, perhaps due in some way to the fact that the would-be assassins are time travelers. But if that's so, then again we have a challenge to the freedom of the time-travelers, for such a force would presumably be inconsistent with their freedom. Of course, this is no threat to the possibility of time travel itself, for one can always spin a time-travel yarn with a convenient guardian of logic ready to cause slips on banana peels when inconsistency threatens. But the argument nevertheless threatens those of us who think that time travel is possible without such shackles on time travelers. It moreover threatens the possibility of time travel in worlds like our own, in which time travelers would presumably be unshackled.

7. A CLOSER LOOK AT COUNTERFACTUALS OF COINCIDENCE

In fact, each of these arguments may be rebutted. I begin with the argument of Section 4, that would-counterfactuals of coincidence are never true. Let us leave time travel for the moment and consider a more mundane case. Suppose I tried to throw a heavy stone at a fragile window. Since I have good aim and a strong arm, the window would break. I might, I suppose, slip on a banana peel, or hit a bird passing by with the rock, or have my throw deflected by a great gust of wind, or have a sudden failure of aim despite my many years of training in stone-throwing. But at the very least, it surely is not the case that one of these strange coincidences would happen. The would-counterfactual of coincidence:

TIME TRAVEL, COINCIDENCES AND COUNTERFACTUALS 123

If I were to try to throw the stone at the window, I would slip on a hit a passing bird or...

is false.

But now let us consider a different counterfactual:

(C) If I were to try to throw the stone at the window but the window did not subsequently break, then I would slip on a banana peel or hit a passing bird or ...

Here I have built my failure into the antecedent; the counterfactual concerns what would have happened had I tried and failed. Here, I think, our sense is that the counterfactual is now true. Given the background facts, the only way for me to fail to hit the window would be for some strange coincidence to occur. Though most ordinary would-counterfactuals of coincidence are false, some are true, namely those whose antecedents describe circumstances that could only come about by an "unlikely coincidence". We can think of the antecedents of these conditionals as describing states of affairs that embed a certain "tension", states of affairs that are "difficult" to make true. To include such a state of affairs, a possible world must include some strange coincidence. (Or something even stranger, perhaps a lurking guardian of the window ready to spring out and intercept the rock. Since no such guardian is present in the actual world, surely no guardian would have existed had I thrown the rock.)

We should all agree that there are true would-counterfactuals of coincidence like (C) whose antecedents "embed tension" in this way. But in fact, the counterfactuals in the time travel case are similar. We ask what would have happened if a time traveler had tried to kill her earlier self. If the time traveler is in fact a capable assassin and has the appropriate resolve and information, then making this antecedent true is very "difficult"; it is hard to find a non-coincidental reason why the time traveler would fail. And yet

there is no possible world in which the time traveler successfully kills her earlier self. Thus, the would-counterfactual of coincidence:

If a certain time traveler had tried to kill her earlier self, she would have slipped on a banana peel or had a sudden change of heart or ... looks a lot like (C). But we admitted that (C) is true. We should say the same thing about this counterfactual. Coincidences would have happened, in the right circumstances.

TIME TRAVEL, COINCIDENCES AND COUNTERFACTUALS 125

the case that P then it would have been the case that Q1 is true at w iff Q is true in the possible world most similar to w in which P is true.5 The relevant similarity relation is one determined in part by the conversational context of the speaker, and may weight some contextually salient factors more heavily than others. In the context in which (T) seems true, we are holding constant the fact that Tina is a time traveler, and the fact that the little girl standing in front of her at t is Tina herself. So in this context, the counterfactual is a lot like (C), in containing an antecedent that is very "difficult" to make true in worlds similar to the actual world, given that in all those worlds Tina is a time traveler attempting to kill her earlier self. (T)'s truth is therefore no more surprising than (C)'s.

8. SELECTIVE ATTENTION

The truth of certain would-counterfactuals of coincidence in cases of time travel has been defended. But what of their connection with freedom? How can the members of the Institute for Autoinfanticide be free of unusual constraints during time travel, when if they were to attempt to kill their former selves they would repeatedly fail?

Let us again examine an analogous case having nothing to do with time travel.6 Suppose we define a permanent bachelor as a man

who never gets married. When we survey the class of permanent

bachelors across the space of all possible worlds, we find that they fail to get married for a variety of reasons. Some never have the inclination, others wish to be married but never find a suitable partner, others slip on banana peels and fatally injure themselves while walking down the aisle, and so on. No anti-nuptial force need

be postulated to account for this: by our definition of "permanent bachelor" we selectively attend to a certain class of possible indi viduals when we ask for the class of permanent bachelors. Many of these permanent bachelors could have gotten married. No force stands in their way; had they gotten married, they would no longer

have counted as permanent bachelors.

The example may be brought a step closer to relevance by considering certain counterfactuals.

TIME TRAVEL, COINCIDENCES AND COUNTERFACTUALS 127

In thinking about the freedom of permanent bachelors, we ought to distinguish between two sorts of claim. First, consider a permanent bachelor who never attempts to get married. Let this person be a perfectly ordinary, actual, permanent bachelor with no extraordinary social or psychological impediments to marriage. We clearly want to say that this person could have married, despite the truth of counterfactuals like: "if this person had been a permanent bachelor, and had tried to get married, he would have slipped on a banana peel or . . .". The actual freedom to get married is claimed, and his counterfactual failure (under the description "permanent

bachelor") is irrelevant to this claim. Contrast this with a claim of counterfactual freedom. One might claim also that, had this person been a permanent bachelor and tried to get married, he would have failed in one of a number of "coincidental ways", but would nevertheless have been free. Here one is claiming the permanent bachelor has counterfactual freedom, despite counterfactual failure. This claim of freedom would also be true, I think. As argued above, cases in which permanent bachelors try and fail to get married include cases of "coincidental failure" similar to cases with which we are familiar. Some people really do slip on banana peels on their way to the altar. Such slips involve bad luck, but no failure of freedom. The appearance to the contrary is due to neglecting the role of selective attention in the truth of would-counterfactuals of coincidence.

9. SELECTIVE ATTENTION, COINCIDENCES AND FREEDOM

Let us now return to time travel, beginning with the argument o Section 6. The argument was that the numerous mishaps faced b time travelers attempting autoinfanticide just couldn't be coinci dences. There must be some force or mechanism preventing the killings, perhaps a force causally linked to the would-be assassins status as time travelers. But then, those assassins are shackled in

implausible way, and would not be free.

In light of our remarks on selective attention, the argument loses its force. The many mishaps facing the class of permanent bache lors require no explanation beyond the fact that we delineated the class with our notion of a permanent bachelor. The class of possib

128 THEODORE SIDER

worlds containing time trave

ticide is similar. We have placed two constraints on this class of worlds that are very difficult to satisfy jointly. The first requires the worlds to contain large numbers of persons who want to kill certain persons, and have the means and the desire to do so. The second constraint is that the would-be killers are time travelers and the would-be victims are their former selves. This in effect requires that these persons fail in their missions. We thereby selectively attend to a class of worlds that contains large numbers of "coincidental failures". This requires no force compelling failure. We have delin eated the class of worlds so that it contains the failures; the failures can still be genuine coincidences. The freedom of the time travelers is not compromised.7

It may help to remember that logical space contains many worlds with segments that are qualitatively like cases in which time trave lers confront their former selves. In some, the would-be murderers fail, but in others they succeed. These latter cases are cases in which the victims are not the former selves of the murderers. Whether a given segment is embedded in a world in which a would be murderer counts as the later self of a victim determines the inclusion of the segment in the class of worlds to which we have selectively confined our attention, in discussion of time travel and autoinfanticide.

10. SELECTIVE ATTENTION AND VIHVELIN'S ARGUMENT

The other argument against the possibility of time travel (S

was that the freedom of time travelers is undermined by th

of would-counterfactuals of coincidence. The argument appealed to Vihvelin's principle that a person is genuinely free to do a certain thing only if it is true that if she had tried to do the thing, she would have or at least might have succeeded. This argument, too, is undermined by the phenomenon of selective attention, although the matter is delicate. The truth of counterfactuals like these:

(PB 1) For all x, if it had been the case that (x is a permanent bachelor who attempts to get married), it would have been the case that: (x slips on a banana peel and dies, or gets a bad case of cold feet, or .. .)

132 THEODORE SIDER

(nor would she have her ordinary powers undermined). The point of doing so is to avoid the conclusion that the possibility of time travel could only be secured by postulating a kind of "force" or "guardian of logic" that shackles time travelers by ensuring that they perform certain tasks. Such guardians should be resisted, not because they are impossible (since surely they are not impossible), but rather because it is hard to believe that the sort of time travel the physicists consider an open possibility would require such exotic metaphy sical add-ons, and moreover because time travel seems possible in worlds that lack such guardians. If temporal part Killer is free to kill temporal part Victim, that is sufficient for time travel being possible without metaphysical add-ons; it shows that the causal order in a world including time travel need not contain any such guardians. This is the bar for success I set for myself: showing that time trave lers need not be shackled by metaphysical add-ons. It would not matter much if it turned out that what we ordinarily mean by "the time traveler can kill her earlier self" is false. (In fact I argue below that this does not turn out false.)

It is worth showing that Vihvelin's challenge to the freedom of time travelers can be answered without assuming the metaphysics of temporal parts. Pretend for a moment that objects "endure", i.e., persist "wholly present" through time. As we have learned, utili zing Vihvelin's principle requires consulting a counterfactual that does not contain the description "time traveler" in its scope. So let us name the time traveler "Katy", and consider the truth of the following counterfactual relative to a world, w, in which Katy travels back in time with the intention of killing her former self, but decides against doing so:

(K) If Katy had tried to kill Katy, she would have or at least might have succeeded

There may be similarity metrics under which (K) turns out false, but the question is whether (K) is true under a metric that does not hold constant facts about what happens after the attempted killing, for that is the only metric under which Vihvelin's principle is true. To answer this question we must compare the similarity of various worlds to the original world w. Those worlds include:

TIME TRAVEL, COINCIDENCES AND COUNTERFACTUALS 135

attempts to kill her former self are worlds containing suc dental failures, this does not imply the presence in w of mechanisms or the absence in w of enabling mechanisms. fore, the falsity of counterfactuals like (K) and (KV) in w not show that time travelers in w lack the ability to kill their selves. Vihvelin's principle fails because in these cases the f counterfactuals like (K) and (KV) lacks its usual import.

Remarks like those of the past few paragraphs also answer a challenge to the freedom of time travelers based on a principle weaker than Vihvelin's: that freedom to do X requires the metaphy sical possibility of doing X, whereas it is metaphysically impossible for Katy to kill Katy in circumstances intrinsically like the case of time travel.13 First, this principle allows that the temporal part Killer is free to kill the temporal part Victim, and that is freedom enough. Second, temporal parts aside, the worlds in Class 4 show that it is metaphysically possible that Katy kill Katy in circum stances intrinsically like those of the case of time travel. This is

not to say that there is a possible world in which Katy kills Katy in this way but is also a time traveler in that world. But the case of the permanent bachelors shows that freedom at most requires that the agent possibly do the action simpliciter, not that the agent possibly do the action under some description. Finally, if the exis tence of these possibilities is rejected, this would only show that their existence is not required for freedom, for the nonexistence of the possibilities would not have its usual import; it would not indicate the presence of disabling causal mechanisms or the absence of enabling mechanisms in worlds in which time travel occurs.

1 1. CONCLUSION

Time travel escapes again, unscathed. If many time travelers attempted autoinfanticide, an apparently miraculous series of coincidences would ensue. But this fact is unremarkable, and in no way undermines time travelers' freedom. As comparison with uncontroversial cases has shown, it is the result of the description

"time traveler who attempts autoinfanticide" focusing our attention on a certain class of possible worlds, a class of worlds that is guaran-

138 THEODORE SIDER

Smith, Nicholas J.J. (1997): 'Bananas enough for Time Travel', British Journal

for the Philosophy of Science 48, 363-389.

Stalnaker, Robert (1968): 'A Theory of Conditionals', in N. Rescher (ed.), Studies

in Logical Theory, Oxford: Blackwell.

Vihvelin, Kadri (1996): 'What Time Travelers Cannot Do', Philosophical Studies

81, 315-330.

Rutgers University, NJ

USA

E-mail: trsider@syredu

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This content downloaded from 132.174.250.24 on Mon, 03 Dec 2018 01:24:34 UTC

All use subject to https://about.jstor.org/terms

This content downloaded from 132.174.250.24 on Mon, 03 Dec 2018 01:24:34 UTC

All use subject to https://about.jstor.org/terms