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Time Travel and Mortality - A Problem with Subjective Time.
By: Brian Crabb

Most so-called ‘paradoxes’ of time travel are taken as evidence that time travel is impossible. If I were able to go back in time and kill my grandmother before she had my mother, I would never have been born. So I would not be here now, and would not be able to take the trip. Or, for that matter, anyone might have gone back and killed anyone else’s grandmother, with the consequence that a grandchild who does exist now would not. Indeed, taking this line of argument, it might be suggested that the very idea of going back in time presents an immediate factual contradiction; I did not live in the 18th Century, but by travelling back to that time I also ensure that I did. I cannot both live and not live at the same time. In any case, the general point is that time travel would require such contradictions to occur, and so just cannot be possible.

The motive for writing this paper is that I do not find any argument of the above form convincing. What such arguments show, if anything, is not that time travel as such is impossible, but rather more modestly, that vicious regressions – regressions that lead to contradictions – can never happen. But this claim seems to me no more challenging to common sense than the everyday observation that as we each go about our daily lives we never do anything that results in such a contradiction. Thus, once it is a fact that I stay indoors because of the rain, it is then also a fact that I do not go out. It is not impossible for me to go out, and nor is it impossible for me to stay indoors, although it is impossible for me to both go out and stay indoors. Similarly, then, I see that if I stay indoors on the Wednesday, I do not also travel back in time from the following Friday and go out on the Wednesday. I could have gone out on the Wednesday, but only if I had not stayed in. So while declaring that to do what we do not do constitutes a logical contradiction, it commits us to nothing about the practical possibility of taking either course of action. Thus, we can work out that it would be impossible to go back and kill our own grandmother, because we could not both be born and not be born. So the problem with the idea of travelling back in time, as I see it, is not that it would generate contradictions; logic determines that no such trips could take place. The real problem is the practical one – of why they could not take place.

We imagine the traveller revisiting the previous Wednesday as a fully conscious, autonomous individual, capable of choosing as he pleases. If he chooses to murder his grandmother, we can see no practical reason why he should be unable in principle to do so. And yet we know from logical considerations that he never does, and it is hard to see why. It seems surprising, although logically inevitable, that everyone who will ever go back in time with the intention of killing his grandmother will fail to do so. What practical obstacles would be bound to stand in his way? My preference in dealing with such puzzles is to view the matter thus: In order to be in a position to travel back to his grandmother’s younger days, the traveller must already not have killed her. With that fact in mind, it is as easy to understand why he does not kill her as it is to understand why I do not go out in the rain. I do not go out in the rain because I stay indoors. The traveller does not kill his grandmother because he allows her to live. No restraint except from the bare rules of logic is required to explain why. The inferable generalisation that no-one ever will actually decide to go back and then succeed in killing their own grandmother (because no-one would be here to do so) is surprising, I admit, and it might carry serious implications for our commonsense notion of free-will, but it does not demonstrate that time travel per se entails contradiction. For just as we can happily subscribe to the observation that no-one both stays in and goes out at the same time, we can just as easily accept that no-one who has a grandmother also does not. As a matter if fact, therefore, no-one ever will go back and kill his own grandmother. To ask why not is akin to asking why no-one who stays in doors will also go out in the rain.

My own ‘paradox of time travel’, which I find far more compelling, differs from the above in that it appeals to conflicting intuitions about subjective, or ‘personal’, time. Allow me to lay the groundwork by drawing the relevant distinction. If Smith and Jones are in the library chatting over a cup of tea there are some uncontroversial facts of the matter. It is, in objective terms, noon on the Friday, so there is no doubt that the conversation is occurring at that objective time. Smith, Jones, and their conversation, all occupy the library simultaneously at noon on the Friday. So far, so good. Running alongside this objective sequence of events, however, Smith and Jones each have their respective experience of subjective time. As Smith is speaking, he has the subjective experience of his act of speech being in the present. As Jones listens to him, he has the subjective experience of hearing Smith’s speech in the present. And we tend, uncritically, to associate these subjective ‘presents’ with the objective time of the associated occurrence. Roughly, during the objective event of Smith and Jones conversing, we just assume that they each have the experience of that conversation at the time of the conversation, and therefore simultaneously. More generally, whenever we observe that someone is alive at present, we assume that his experience of being alive is going on at that time.

The First Intuition.

Suppose now that Jones decides to travel back in time with the intention of conversing with Socrates, in 420 BC. What can we say about his experiences? If we refer to his wristwatch as an indicator of Jones’ subjective time, he must fully expect it to continue working as normal. As he sets off in his time machine, then, his watch continues to count the minutes off one by one. When he feels that he has been in the machine for about ten minutes, he will glance at his watch and find that, indeed, ten minutes or so have passed. He has experienced ten minutes of subjective time since he got in. Objectively, however, he finds that the time outside the machine is regressing to 420BC. Let us say that Socrates first appears outside his window after ten minutes have elapsed as measured by his wristwatch, whereupon he steps out onto the ancient soil and the two thinkers enter into an impromptu philosophical exchange. As they do so, Jones’ subjective time, which has moved on by ten subjective minutes, will be located in 420BC; he will experience himself as conversing in the subjective present, ten minutes after he set off, and the objective time when he does so will be 420BC. So once again the two participants in a conversation may be thought of quite naturally as sharing a common subjective time. For just as it made sense to assume that Smith and Jones shared a common subjective present while they conversed in the library, it will now make exactly the same sense to assume that Jones shares a common subjective present with Socrates. It makes no sense at all, in contrast, to assume that Socrates and Smith share a common subjective present. As he fondly remembers his friend’s last conversation in the library, Smith will recognise that while he, Smith, is continuing to experience his subjective present in 2008, Socrates and Jones are experiencing no subjective present at all; they are dead and gone. For it was already a matter of common sense that Socrates is dead and gone, and since Jones is having a conversation with him, Jones must be no better of than Socrates.

It seems impossible to doubt that there is a meaningful distinction here. Smith knows perfectly well that Jones was alive and conscious and experiencing his subjective present while Smith was conversing with him, and he also knows perfectly well that at the very same time Socrates was experiencing nothing at all. That is the tragedy of death; the subjective presents of the departed are all in the past; they are no longer available to be experienced. If this were not so, we would feel comfortable with the thought that ‘Socrates is dead now, but other than that he is having a fine time. He is having an interesting conversation with Jones’. So the meaningful distinction we should embrace is just this: In 2008, the people who are alive in that year are sharing a common subjective time. The people who lived in 420 BC, on the other hand, are experiencing nothing at all. And here is the crucial point: They are not dead merely in relation to 2008; in 2008 they are just dead. This is my first intuition.

A common misunderstanding on this point runs as follows: It is true in 2008 that Socrates is dead and gone – that he experiences nothing at all – but it is equally true that in 420BC he did experience something. He was alive and well, and experiencing his subject time then just as we do now. So while the Jones of 420BC is dead now, he was not dead then. That is all we need for time travel to be successful. So long as Jones’ subjective present relocates successfully to the objective time 420BC, he loses nothing. His subjective time proceeds just as ours does, minute-by-minute as registered on his watch, day-to-day by his calendar, and on into our subjective futures. The only difference is that his subjective future begins to occur within the objective time 420BC. So it seems that there is no particular (time-related) reason to prefer a life in 2008 to a life in 420BC, and therefore no reason to suppose that Jones has made a poor decision. So long as Jones’ subjective experience of time contimues just as Smith’s does, it makes no difference whether it does so within the objective context of 2008 or 420BC. Indeed, if Jones were to draw down the blinds in his time machine he might not even know the difference. This is an extremely compelling objection, at least prima facie, but I want to explain what is wrong with it.

Essentially, the above objection amounts to this: For any true statement I can make about time in 2008, there is an exactly equivalent statement I could have made in 420BC. Thus, “I am alive and well” will be true in 2008 of anyone who is alive and well in 2008. Correspondingly, the very same statement would be true in 420BC of anyone who was alive and well in 420BC. In just the same way, “2008 is a good time to be alive, because it is now” will have its exact equivalent, with just the year altered, at any other time. So the idea is that Jones’ decision to relocate to 420BC is a perfectly good one. Whatever we can say about living in 2008, we can also say about living in 420BC. The idea that Jones is heading for oblivion is just nonsense, based on a failure to see the essentially relational nature of our statements. In particular, the statement:

“It is 2008 now. If Jones goes back to 420BC he will be dead”

is true, but only as a statement uttered in 2008. Being dead in 2008 is fully compatible with being alive and well in 420BC. So Jones’ response to the plea that he should not go back, because it will amount to an act of suicide, will be that while to anyone located in 2008 he will be dead, what people in 2008 think, or what is true for them, is of no interest to him living in 420BC. He will be fully alive in 420BC, even though dead and gone at later times. And since an exactly parallel observation can be made about 2008, as considered from much earlier or later dates, it follows that there is nothing intrinsically preferable about a life in 2008. It just happens to be where we are now.

Of course, this is just wrong; temporal relationism taken beyond the facts. Suppose we, here and now, were to accept that there is no reason for Jones in 2008 to prefer a life in 2008 over a life in 420BC, even though the latter option involves living at the same time, both subjectively and objectively, as Socrates. Didn’t we agree that in 2008 it is true that Socrates is just dead and gone? How could we possibly argue that being dead is not a state to avoid? We would have to say that for Socrates to be dead now is not negative in any absolute sense; it is just for 2008 to fall outside of his lifespan on the objective timeline. When we say that Jones and Socrates in 420BC are now dead, then, we are merely commenting on the location of 2008 in relation to their lifespan; that the one does not fall within the other. The implication is that the following two statements are equivalent:

Statement 1. In 2008, Socrates is absolutely dead and gone.

Statement 2. In 2008, the present moment falls outside the lifespan of Socrates.

But clearly statement 1 is saying much more than statement 2. Jones the time-traveller really is, objectively, dead, and he is objectively dead because the world-time really is 2008 – not 420BC. If world-time is 420BC, it is better to live in 420BC. If world-time is 2008, it is better to live in 2008. But world-time actually is 2008, not 420BC, and everyone who died before now is really dead and gone. So it is better to live now; Jones should not take the trip.

The Second Intuition.

But if this is correct, and surely it must be if our departed loved-ones are truly dead and gone, then what can we say of Jones’ subjective experience of time? My second intuition was that as Jones travelled back in time he continued to experience his subjective present as usual. Subjectively, his present drifted through the conversation with Smith in the library, and then on to his experiences as a time-traveller, finally to lead into his experience of conversing with Socrates. We might imagine that Jones kept a calendar in his time capsule, and tore off a page every day. If asked how Jones is, Smith will say that he has gone back to have a talk with Socrates, and that he is probably having a fine time. There is no thought that Jones is just dead. How could he be? When a person dies, they come to the end of their stream of subjective presents, but Jones is doing nothing of the kind. His watch is ticking away as usual, the calendar is displaying progressively later dates, so his subjective time is progressing as usual. It is just that Jones is with Socrates while this is happening. So the conflict of intuitions is a vivid one. I want to say that the dead have come to the end of their subjective presents. That is why I feel sad about the loss of my parents. It is a fact now that they are experiencing nothing. On the other hand, I also want to say that Jones is continuing to live - in 420 BC – and therefore that there is nothing to be sad about. When he decided to travel back in time, he did not expect to die. He expected his subjective time to continue, but couched in the objective time of Socrates. So the conflict is this: Socrates is surely dead and gone, while Jones is alive and well. But they are having a conversation.

The commonsense response to this might be that of course Jones is dead now, but he did live on and experience the remainder of his subjective time alongside his idol, in 420 BC. They did, indeed, share a common subjective present while they conversed. But that will not do the trick. When Jones made his decision to go back in time, he was not deciding to have lived in the past. That would have been to commit suicide. He was deciding to continue to live, albeit in the past; to direct his ongoing subjective experience of life into a past objective setting. Somehow, if time travel is possible, we need to be able to say that although Jones is now dead and gone, his bid to continue his subjective time in the past was a success. I simply have no idea how I could do that. Everyone who lived in 420 BC is now dead and gone. Their subjective experience of time is at and end. But that has to include Jones. To the extent that I am sad because Socrates is dead, I must also be sad because Jones is dead. They lived at the same time, and died at the same time, and neither of them experiences anything now.

The Dilemma.

If, as the modern physics of black holes and wormholes suggests, time travel into the past is a real, practical possibility, it seems to follow that being in the past does not amount to being dead and gone. For as I have argued, at the very least, travelling into the past involves continuing to have a subjective timeline, whilst finding oneself doing so in an objective past. Nothing less than that would qualify as time travel. If this is true, then, the only remaining possibility as that my first intuition was wrong; that to have lived only in the past is not to be dead and gone. I do not know whether time travel into the past really is possible, and therefore I have to leave this dilemma unresolved. It is, however, a real dilemma, and it implies that if time travel is possible, then death lacks the absolute extinction we fear.

Article Source: http://journal.ilovephilosophy.com

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