Desert, free will, determinism

[davidm]

The backward traveller can form a mental picture of himself killing gramps; but he knows he cannot actualize this mental picture, no matter how many times he tries. Can he really be said to have free will if his attempts are 100% doomed to failure?

Sure. After all, he may come to realize, even before entering the time machine, that he was going to try to do something that is logically impossible, and therefore he calls off the trip. There is a free choice right there, between two alternate possibilities: go back in time, or not. If he chooses not, then there was never a time that he was in the past and there was never an attempt on gramps.

Or he may not realize the logical impossibility of what he is trying to do and make the attempt, and be dumfounded at his failure. He then may abandon the project, another free choice, for he could have chosen to try again. Or he may choose in the past not to kill gramps after all, having the man clearly in his gunsights. That’s another free choice.

The only thing he is precluded from doing is bringing about a logical contradiction.

[Positor]

There is a free choice right there, between two alternate possibilities: go back in time, or not. If he chooses not, then there was never a time that he was in the past and there was never an attempt on gramps.

But whether he was in the past or not (as an older person) is a matter of historical record. Suppose that he was. How can he then choose not to go back in time?

Or he may not realize the logical impossibility of what he is trying to do and make the attempt, and be dumfounded at his failure.

Again, whether he even made the attempt is already a historical fact (regardless of whether anyone saw him do so). If in fact he did not make the attempt, he cannot now choose to do so.

This lack of choice extends even to his state of mind. If, as a historical fact, he thought X at time T, he cannot now choose to think anything other than X at time T, as that would change the past. His lack of choice seems absolute, extending to his very decision as to whether to travel to the past.

[davidm]

There is a free choice right there, between two alternate possibilities: go back in time, or not. If he chooses not, then there was never a time that he was in the past and there was never an attempt on gramps.

But whether he was in the past or not (as an older person) is a matter of historical record. Suppose that he was. How can he then choose not to go back in time?

Or he may not realize the logical impossibility of what he is trying to do and make the attempt, and be dumfounded at his failure.

Again, whether he even made the attempt is already a historical fact (regardless of whether anyone saw him do so). If in fact he did not make the attempt, he cannot now choose to do so.

This lack of choice extends even to his state of mind. If, as a historical fact, he thought X at time T, he cannot now choose to think anything other than X at time T, as that would change the past. His lack of choice seems absolute, extending to his very decision as to whether to travel to the past.

I think we can make your objection here more concrete and formal, putting aside the fact that there are no complete records of the past, so presumptively a time traveler likely wouldn’t know whether he hd been in the past or not. Earlier you wrote:

So let's consider this scenario from his point of view. First he experiences the period 2000-2018, then he experiences going back to 1999, then he experiences the period 1999-2000, then the period 2000-2018 a second time, but as a physically older person with a longer memory. This time he does not experience going back; instead (from his own point of view) he stays on in his second timeframe, and experiences the period 2018-2020 and onwards until his death.

Yes, this is a logically consistent story. And from this alone we can concoct any number of bizarre scenarios.

Let’s call the time traveler Bob. Even though they are the same person, we could designate younger Bob, who is born in 2000 and enters the time machine in 2018, B1. Later Bob, who goes back to 1999 and then lives out the rest of his life, is B2.

Now, after the time travel has taken place, B1 and B2’s temporal parts will coexist. This means B2 could try to contact and interact with B1.

Suppose, when B1 turns 18, the scientist who invented the time machine tells B1: “You can use this machine to go back in time, and interact with your younger self. However, because I desire to screw with reality and logic itself, here is what I want you to do: If you go back in time and interact with your younger self, you must surely now remember this remarkable event. If you do remember it, I want you not to enter the time machine, and thus not interact with your younger self. If you do not remember any such interaction, I want you to enter the time machine, and interact with your younger self.”

It seems this is a direct challenge to the thesis that time travel as such is logically coherent. For if B1 has the memory he won’t enter the machine; but if he doesn’t enter the machine he won’t have the memory, in which case he will enter the machine, but then he’ll have the memory and won’t enter the machine and … so on.

Either that, or it is a direct challenge to the compatibility of time travel and free will. One can now argue that B1 will be forced to choose in a way that keeps history logically consistent, which will mean that his choice is not free. Alternatively, he can act in such a way as to instantiate a logical contradiction. Give these two choices, it seems free will must yield, since it is not possible to instantiate a logical contradiction. But I think this is a bifurcation fallacy and there are a number of ways to elucidate this situation that sticks to the main thesis: (1.) No one can change the past (or present or future) and this fact does not rule out free will; and (2.) time travel is logically possible, but it is not logically possible to both travel to the past and change it. So more about this in a future post.

[TheVat]

Yes, when you mentioned Bradbury's famous story earlier, I recalled that logic problem that it seemed to have. The hapless time traveler who slips off the walkway should have been unable to crush that butterfly. Even if he had deliberately tried, something should have thwarted the attempt. Free will has never meant that we can implement any course of action which we can imagine. We can't teleport ourselves overseas, just by willing it. For SF writers, I think an interesting question would be: will a backwards travel time machine even work? If I stepped through a portal into the Cretaceous Period, wouldn't even a brief moment of standing there displace air molecules and set in motion some small changes?

A more amusing consideration of the problem may be found in William Tenn's 1948 short story, "The Brooklyn Project."

[davidm]

Ha, the Tenn story is very amusing. It’s online here.

Tenn is actually Philip Klass, and I know I read that story as a kid, but couldn’t recall it. Like A Sound of Thunder, and as Tenn himself notes in the afterword, the story is really about politics, not science or philosophy. And that really is the strength of The Brooklyn Project, calling out the pretensions and proto-Fascism of the U.S. government at the dawn of the cold war. In that sense, but only in that sense, the story is visionary.

Still, it raises an interesting philosophical idea. Suppose you really could change the past, without violating the laws of logic? The way to do that, possibly, is you change the past, but all the changes are undetectable. In that way it resembles the old philosophical problem of whether, if the universe and everything in it suddenly doubled in size, could you be able detect this change? It seems not.

As amusing and interesting as this is, it really doesn’t work. Why would the chronar not change the past in such a way that no intelligent life evolved, thus ensuring that the oscillating balls would not be developed? That is what we ought to expect, if changing the past were a viable idea. But in that case we are back to the old logical conundrum: The chronar goes back. It changes the past in such a way that no one later invents it. If no one later invents it, it doesn’t go back to the past. But if it doesn’t go back to the past, someone later invents it, and … Grandfather’s paradox all over again.

[Positor]

Also, we talk of the actual process of time travel in a rather imprecise, hand-waving way: "Let's assume that it's somehow possible..." But can we assume that it is possible, even in a thought experiment? It may be that any attempt to rigorously describe the mechanics of a journey into the past or future reveals hidden logical contradictions.

Consider the actual moment of the traveller's departure. What is actually going on objectively in the affected volume of space, and at the boundary between that and the outside world? Does the traveller carry some space itself with him/her, or only matter (including air), particles, energy etc? What happens to the volume vacated by the traveller and capsule? Is a momentary vacuum created? Does a full description contain any contradiction with regard to 'before' and 'after'? Is it coherent to suppose that subjective and objective timelines can diverge in this way? (Even if we can draw such lines on a diagram, do they have any physical meaning?) And so on. Is the time journey itself (not just what one does 'after' it) a logical as well as a physical problem?

[davidm]

Several schemes for backward time travel have been mooted that are consistent with known physics, including traversable wormholes and closed time-like curves. None, in and of themselves, show any logical inconsistency, though they may prove physically impossible to create or sustain.

[davidm]

I would maintain that if we possess free will in the forward time direction — and this free will need not be libertarian, but compatibilist or neo-Humean — then we must possess it with respect to backward time travel, because the logic of both forward and backward time travel is the same.

In the problem of future contingents, we are invited to consider whether the proposition “tomorrow there will be a sea battle” is true at the moment of utterance. If it is true, it is feared that the sea battle tomorrow must happen — it is unavoidable. This is more than determinism, this is fatalism, and fatal to free will.

However, the solution to this alleged problem, sometimes called logical determinism or logical fatalism, is identical to the solution described earlier, in which it is shown that God’s foreknowledge cannot, as a matter of logic, impugn future freedom to choose.

If it’s true today that tomorrow there will be a sea battle, then of course tomorrow the sea battle will take place — but it does not have to take place, which is the fatalistic worry. Suppose it doesn’t take place? Then today, a different proposition will be true — that tomorrow, there won’t be a sea battle. Propositions always take their truths from the events that they describe, even future events, and never the other way around — propositions never force their truths upon the events that they describe. David Lewis discusses this fatalistic worry in his time travel paper to which I linked earlier.

In the case of Bob the time traveler, the problem of future contingents becomes the problem of past contingents — but the logic remains the same.

In my scenario mooted above, B1 is told that if he has a memory of interacting with B2, his future temporal parts, then he is not to enter the time machine, thus changing the past. If he has no memory of interacting with B2, he is to step into the time machine and interact with his past self — again, changing the past.

Bob cannot change the past.

Either it’s true today that yesterday B2 interacted with B1, or it isn’t true. The propositions are mutually exclusive and jointly exhaustive, regardless of whether Bob enters the time machine or not.

If B1 has no memory of interacting with B1 yesterday and therefore steps into the time machine to ensure that he will interact with his past self, his expedition is doomed to failure. Perhaps he will arrive in the past and fail to interact with his past self, likely for some mundane reason. Maybe he just can’t locate his past self.

If B1 recalls interacting with B2 in the past, and then declines to step into the time machine in the hope of erasing those memories and changing the past, it may be that his past memories were in fact false memories — quite common — fantasies, or dreams, or simply faulty recollections. Or it may be that the memories are authentic, and at some later date Bob will step into the time machine, interact with his past self and make those memories true. Any number of other solutions are on offer, all of them logically consistent.