Tuesday, February 26, 2013

Can A-theorists believe in time travel?

I used to think that A-theorists cannot consistently believe in time travel. I think I was mistaken. As best as I can reconstruct my line of thought it was this. Time travel requires a distinction between external and internal time. If I go into a time machine, then maybe in five minutes I'll be a thousand years ago. That's a contradiction given non-circular time unless one distinguishes as follows: internally in five minutes I'll be a thousand years ago externally. But now I think that what I must have been thinking was that in five internal minutes my internal present will no longer line up with the world's objective present, since in five internal minutes my internal present will be about thousand years behind the world's external present. I don't know for sure if that's the thought I had, but if it was, it would have been a howler. For on the view, internally in five minutes, I will be at the time at which the world's external present was about a thousand years ago. Or, to put it from the external point of view, a thousand years ago I was five minutes older than I am now (age is measured internally). Even presentists can say that.

To see that this is coherent, consider a theory that takes external time to governed by the A-theory but internal time to be entirely governed by the B-theory. Thus, superimposed on the external A-series of past, present and future, there is an indexical B-series of earlier-for-me and later-for-me, where these relations are perhaps defined by internal causal relations (earlier states causing later ones). There is no more need for these two series to line up than there would be a need for the two series to line up if the external series were a B-series.

However, while this is coherent, maybe it undercuts one of the main motivations for the A-theory. For if there is a distinction between internal and external time, as there must be for time travel to be possible, all the changes we actually experience are changes with respect to internal time. In other words, they are B-type changes. But the typical A-theorist thinks B-type changes--it (internally) earlier being one way, and (internally) later another--are not what we experience when we experience "real change". Indeed, if time travel is possible, it is possible to live all of one's life at one external time, but moving through external space. Basically, just imagine that at each moment you travel to some external time t0, but to a different spatial location in it. Maybe you have a backpack time-machine which is permanently stuck on t0, but with the spatial locations changing. You'd experience change, because your state at internally earlier times will be different from your state at internally later times. But it would be mere B-type change, since it would all be happening at one and the same objective time.

I suppose one could say that in time-travel scenarios, especially the preceding one of living all of one's life at one external time, our experiences of change become non-veridical, for a condition on the veridicality of our experiences of change is that our internal clock lines up correctly with external time, and time-travel causes a misalignment. Maybe.

But in any case, now that we have the possibility of living all of one's life--a life that presumably could have rich causal interconnections--at one objective external time, just moving "sideways" to new spatial locations, I do think that the motivations for the A-theory decrease. For we see that what matters for the diachronic richness of our lives is that our lives be stretched over internal time, not over external time. It also matters that other people's internal times be sufficiently lined up with ours. But that doesn't call for the A-theory, either.

So, all in all, while A-theorists can believe in time travel, thinking time travel through would undercut much of the motivation for the A-theory.


Jeremy Pierce said...

A-theory time travel cases do require denying one thing that A-theorists like to say. If I go back in time and meet my earlier self, I can't say anymore that I'm wholly present in one place. I'm actually present in two places in that time. (Am I wholly present twice over? That's what I'd be inclined to say, but it's weird to say that all of me is in both places fully, when each instance of me is not all of me at that time.)

And I can't speak in terms of earlier and later stages of me, as Lewis does, unless we do some very weird things to combine the A-theory with four-dimensionalism. You can work it out logically, but you have to say some very strange things to make it work. I think there's a section of Ted Sider's book that explores that. I know there was in the drafts he gave us when he was writing it, but I don't remember if he cut it out for the final version. So some of his way of dealing with personal and external time isn't going to work unless you go 4D while affirming the A-theory, which is especially weird if that means parts of you don't exist, as it would if you're either a presentist or a growing (or shrinking) block theorist.

Alexander R Pruss said...

Yes, you do need multipresence then--I have forgotten that A-theorists often don't like that. (But on the other hand, a lot of A-theorists are theists, and theists are committed to God being present--and surely not merely partly--everywhere.) So I suppose you lose the presentist solution to the problem of temporary intrinsics, and that's another motivation for A-theory gone. It wasn't a very good motivation, anyway. :-)

You don't need stages, though. One can make internal time be determined by causal connections between events. Thus, suppose that the event of your existing (i.e., having existed, existing or will be existing) at a spacetime location x2 partly internally causes (i.e., causes through a chain of causes internal to you) the event of your existing at spacetime location x1 (we might need something more robust than this, but this is just a toy version). Then you're at x2 internally earlier than at x1. Likewise, you're a toddler internally earlier than you're an adult because states internally simultaneous with being a toddler partly internally cause states internally simultaneous with being an adult.

This is going to be tricky with presentism, given that at most one of these events is real on presentism, but I assume the presentist has some favored solution to the problem of diachronic causation and can simply press that to work here.

Alexander R Pruss said...

And you can define being wholly present in a location L at external time T: there exists an internal time t which is at T and is such that all the parts/aspects that you have (i.e., had, have or will have, where these tenses are external) at t are in L. When you do this, you can be wholly present in more than one disjoint location at the same external time, though you cannot be wholly present in more than one disjoint location at the same internal time.

rgrydns said...

Interesting analysis. I'm curious where the distinction is drawn between internal and external. Along an ontological division between mind and body, or a empirical division between body and world. If the former, do you return to the past as a disembodied mind and procure a new body? If the later, how do you account for the great amount of material continuity between body and world.

In other words, where do I begin and the world end, such that five minutes can pass internally, while I lost 2000 years externally?

Alexander R Pruss said...

Well, the distinction is between all the things that are a part of me and all the things that are not a part of me. :-)

What if you say that's vague? Maybe it is.

If so, then the internal/external distinction is vague. In that case, it might be a vague matter whether one has time-traveled.

I don't think the distinction is vague. There is a non-vague distinction between all the matter that is informed by my form and all the matter that is uninformed by my form.

Dagmara Lizlovs said...

"And you can define being wholly present in a location L at external time T: there exists an internal time t which is at T and is such that all the parts/aspects that you have (i.e., had, have or will have, where these tenses are external) at t are in L. When you do this, you can be wholly present in more than one disjoint location at the same external time, though you cannot be wholly present in more than one disjoint location at the same internal time." - Is this how Padre Pio was able to bi-locate. One of my friends is Orthodox and has married an Egyptian Copt. He has told me that many miracles such as bi-location have occured in Egypt.

Now I have this question about being in two different locations at external time T. How would this stand up in court, if one of those locations was at the crime scene and the other location was not, and you have ample witness who can truthfully testify to your being at both locations?

As for time travel, isn't there something of a paradox. Let's say you went back in time and accidentally/incidentally killed your great grandfather before he could father any children. (I say accidentally/incidentally because I assume no one would want to do this deliberately.) What would happen to you? Would you then morph into someone else because your great grandmother will then marry someone else? What about your brothers, sisters and cousins at the present location in time? Do they just vanish into thin air, morph into other people with different last names or what? Now for the ultimate paradox, what if you went back in time, ran into your younger self and let's say killed him accidentally. What would that do to you? Would you cease to be at that point? On the possitive side, let's say you went back in time and saw some one drowning and rescued him. Had you not gone back in time you would not have been there and he would have drowned. Let's say he has not fathered any children when you rescued him. Let's say he now goes out and fathers a whole bunch of kids. Now what? I do not know if the nature of things would allow for such paradoxes. I think Stephen King in his novel "The Langoliers" (It is in a book entitled "Four Past Midnight") has the best solution to these paradoxes. If you haven't read "The Langoliers" or seen the TV movie version, and you don't have time to read the whole thing, and you don't mind plot spoilers here is the synopsis:


Husain Alshehhi said...

Could you please write on how a B-theorist would answer objections that rise from time travel senarios (such as a son killing a mother before giving birth)?

Jeremy Pierce said...

A B-theorist says that's impossible, so it's not an issue. If the past is real, then going to it just means you fulfill the past. There's no changing it. For B-theorists, there's no possibility of the scenario you describe.