Positor » September 19th, 2018, 7:07 pm wrote:davidm » September 19th, 2018, 2:42 pm wrote: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?
davidm wrote: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.