Tuesday, February 13, 2007

(Real) Time Travellers Should Believe in Presentism

Okay so this is really inchoate, but I figured that's sorta what this is for. As far as I know, most philosophers are comfortable with the metaphysical possibility of Lewisian time travel. Lewisian time travel is just time travel as Lewis characterized it; roughly, a discrepancy between personal time and (external) time. I think this notion is perfectly coherent, but it does not capture another idea that seems probably worthy of the name 'time travel'. It's this. Often, as a kid, I wished I could go back and redo something I did in a different way. This usually happened when I got in trouble for doing something stupid and I deeply desired that I could undo my mistake. (I have wished similar things as an adult, but it occurred much more frequently when I was a kid.) It seems like something that I wanted to do might be naturally described as going back in time and doing things differently. Now forget whether I could do things differently without causing contradictions to be true for now. Note that the sort of time travel I wanted to do was not Lewisian time travel. I did not want my older self to go back and aid my younger self. I wanted to relive the experience (and change its outcome). In semi-Lewisian terms, I wanted to make some earlier personal time present again. It would have been cool, but not at all what I wanted, for a later "stage" of me in personal time to be simultaneous with a younger "stage" of me. (4d talk makes this easier to describe, but I use scare quotes to distance myself from the view--I don't think that one needs to accept stages, etc in order to make sense of all this.)

So: Whatever kind of time travel this is, it's not Lewisian time travel. Call it 'Real time travel', for lack of a better name.

My problem is this. I believe Real Time Travel is a coherent notion. But I cannot for the life of me make sense of it without invoking A-properties. It seems that A-properties are needed to even make sense of Real Time Travel.

Furthermore, if I can engage in Real Time Travel and change what happened, then it seems to me some version of presentism must be true. Here I am assuming the following:

a. Travel in 2d time is not sufficient for time travel.
b. Branching universe is not sufficient for time travel.
c. It is not the case that in every possible case in which Real Time Travel occurs, the time traveller is just mistaken about what happened "the first time around" (so as to avoid contradiction between H's happening at t "the first time around" and H's not happening at t "the second time around".

So I guess I think that someone who is on board with (a-c) and thinks Real Time Travel is possible should be a presentist. But I am not a presentist and I am inclined toward (a-c) and the possibility of Real Time Travel. Help!


Blogger Joshua said...

Here is a view. I am not sure if it is considered a kind of branching time or not. Suppose that I paint a house pink and later come to regret my decision. I go back in time and make it such that the house is painted purple instead of pink. Once I have succeeded in my time travel endeaver, I and the house and everything else are located in a different portion of the same time that we once traveled through. In fact, in some sense, we are multiply located. We are in the portion of time where the house is pink, but we are also in the portion of time where the house is purple.

I claim that this is coherent. It requires that there are various portions of time. But I don't see why this should be a problem. These portions might be space times the contents of which are causally isolated from one another (in some sense) but that have a shared causal origin.

I would diagram this in the blackboard in the same way we do branching time. But, I would claim that (1) the entities on one branch are identical to those on another branch and (2) if a line is drawn vertically that crosses both branched, then the points at which it crosses are the same time (they are just different portions of it).

I am not sure anyone has defended such a model of time travel. I talked to Andrew about it last semester and I don't remember what he said at that time. But, I'm sure he has something to say about it since he's the resident time travel expert.

8:48 PM  
Blogger Chris Tillman said...

I honestly find time portions rather mysterious. I take a single time to be something like a region of spacetime such that x and y are parts of the same time iff x is simultaneous with y. (Something like global events with respect to a frame of reference.) There is a fair amount of metaphysical baggage here, but the underlying idea is clear enough it seems: any two events (whatever) that are not time-like separated with respect to a frame are parts of the same time with respect to that frame. Are your time portions consistent with this idea of times? I think if you say "yes" then the best way to make sense of the idea is to conceive of times as something like worlds and take the view to be a modal-realism-with-overlap model of time. Is something like that the idea? And if this makes any sense, then do we want to say, on this model, that you can real time travel to before you painted the house pink and paint it purple but then not real time travel to when it was pink? These are just ill-formed questions, not objections.

Another question: Though this view, I think, would avoid presentism, does it avoid A-properties? It seems something like A-properties are still needed here to make sense of what occurred with your experience: first your pre-paint experience was future, then it was present, then it was past, then it was present again, then it was past again. On a pure B-theory account I have a hard time seeing how to make sense of real time travel even if travel in the model you propose counts as genuine time travel.

3:23 PM  

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