Time Travel

I'll just be up front about this -- I like the Harry Potter movies. And I'm not ashamed.

That said, I can continue with the more substantive stuff. (The following is a bit of a spoiler for a part of the movie, so don't read it if you'd rather be in the dark.) In the most recent installment, Harry Potter and the Prisoner of Azkaban, Harry and his friend Hermione travel back through time. It's a rather impressive part of the story, because as far as I can tell, it is consistent. (Unlike, say, Back to the Future.) Anyway, watching this got me thinking about the relationship between time travel and free will.

In the movie, Harry gets saved by time-travelling Harry (though the first Harry doesn't know it's his future self) and then after he gets saved goes back in time for a different reason. While he's in the past, he is watching his past self get attacked and realizes that the person who saved him in the past was actually himself (the time traveller). So, time-travelling Harry steps up to the plate and saves his past self.

Harry was quite lucky that his future self was there to come to his rescue! But the question is -- after past Harry was rescued by his future self (though he didn't know who rescued him), was he free NOT to go back in time? Intuitively (to me at least) it seems like he HAD to go back in time. If he didn't go back in time, he wouldn't have been there to save himself -- paradox! So he had no choice.

So, was he free or not?

Comments

[christian]

I don't know if time travel stories are consistent. There is a time such that Harry is both sitting and standing at that time. Contradiction? Can temporal parts help? Who is the fusion of the co-temporal stages at some time where they are both present?

[Rob]

The necessity suggested by Neal -- that Harry "HAD to go back in time" -- has something of the fatally perplexing circularity which constitutes the core a priori case against not only the possibility but the very intelligibility of "free will": namely, that ultimate responsibility requires self-creation (-- this latter point made by Nietzsche throughout his corpus and to which Galen Stawson's essays on the subject can be read as lucid footnotes.)

[Charles]

Christian: why isn't it more plausible that what you say is an argument that the two Harrys aren't the same (identical), rather than that they are the same and there's a contradiction?

[christian]

Charles, that's a good question. I think I change my mind. I now want to say that such a case would provide us with evidence that the two Harry's are not the same thing. But, I still think time travel is a problem for those that think properties are essentially relativized to times. Many 3dist. I also think typical 4dist will have to explain the two Harry's difference in a plausible way, it seems there must be some kind of counterpart relation that must hold between the two Harrys existing at the same time, rather than say a stage-like relation (Harry one essentially exists before Harry2). But this counterpart relation is a problem because counterpart theory is false. Maybe. Maybe the relation between the two Harry's is straightforward. They are fully located at different places at the same time. At least this isn't contradictory.

[Jeremy Pierce]

The 4D view is straightforward, and Lewis has a famous paper on this from 1976 dealing with that. You simply have two stages of the same worm appearing at the same time in what he calls external time, but in the worm's personal time (i.e. orderedness) they're not simultaneous but one is later than the other.

He also defends freedom in the face of time travel by saying freedom is always with respect to something that the context determines. In most contexts, we ignore the future. In a time travel context, you can't get away with that so easily. In the end, he says you are free with respect to the intrinsic state of the world at that time, but you're not free with respect to your own personal past, which happens to be the future of the world. We normally ignore it because of the future. The reason we wonder whether we're free is because we confuse future and past.

The 3D view can be made consistent very easily, but you end up saying strange things. But then time travel is strange! You have to say that you're wholly present at two places at the same time. It's a contradiction for two things to be wholly present in different places at the same time (just try to put it into a form of 'p and not-p'). It's not a contradiction for something to be both standing and sitting at the same time (try it with this also -- you won't be able to do it without supplying an additional premise). What you can't say is that you're both standing and not standing, but that's not the case here anyway. You're standing. You're sitting. It's false that you're not standing, because you are standing. It's false that you're not sitting, because you are sitting.

The strangest part, to me anyway, is when you ask how many people are there. It can't be two if the 3D view is right, but it seems as if you've got two people there. The 4Der can deal with this by saying you've got two things there -- two stages of the same worm, whereas there's also one thing there with two parts -- the worm itself. There are some complications regarding who you are when you separate what Lewis says from what Ted Sider says, but that's the overal picture. (Ted, by the way, has a whole worked-out temporal counterpart theory in his book, and arguing against it merely by stating it's false isn't going to get you very far.) The 3Der, on the other hand, can only capture one of our two conflicting intuitions about time travel cases and counting.

[christian]

Well,

I don't think Lewis's proposal is straightforward. I don't understand the distinction between personal and external time. Secondly, I don't think it's a contradiction for two distinct things to be wholly present in two distinct places at the same time. A 4D partist accepts that this is possible. I don't understand how a 3Dist can make these statements consistent:

1. Joe is sitting at t and Joe is standing at t.

Sure, we need this too.

If Joe is standing at t, then Joe is not sitting at t. But this is necessary. So, 1 and it are inconsistent and I don't know how a 3Der should respond.

Maybe, they can say it is not Joe, but someone else as similar to Joe as anything in Joes world that is sitting at t while Joes is standing at t. I don't see how this helps though, we want to know about Joe, not someone really similar to Joe.

I think the counting problem is tough too. Perhaps we just count in two ways, sometimes stages and at others worms. In the time travel scenario we see two stages and count two, but really there is one worm of which they are parts. If we are counting people and only worms are people, then we will probably miscount the time traveler and his earlier self as two persons. We need to assume some restriction on personhood though, something like no person can have another person as a proper part. This seems plausible.

The big problem for the 4Der is that they have to deny, I think, too maintain that time travel stories are consistent, the following plausible thesis:

If Stage T and Stage S are distinct temporal stages of the same worm, then T and S do not overlap (exist at the same time).

I think time travel fictions, within one's own lifespan, are cases of overlap which appears to me to be inconsistent with the above plausible analytic principle.