Expert: Jeffrey Eldred - 11/1/2010

Question
Dear expert,


Let's imagine I decide to go in the future, in year 2035 more precisely.

and I write down in a book everything people will do.
For instance, i'll see Obama cheating his wife.

Then I come back to the present with my book.
Would my book be in contradiction with Obama free will?


Sincerely

Answer
Introduction:
I'm not quite sure why this is an atheism question and not a philosophy question but I am happy to answer it. I believe we live in a deterministic universe, and therefore there is no “free will” to ever be contradicted. But I your question is more about whether time travel involves a contradiction with free willed universe, and that is what I will endeavor to answer. The short answer is that because time travel is consistent with either a deterministic or a free willed universe, time travel doesn't help the philosophical dilemma much.

I am aware of three different ways time travel is proposed to work, and the explanation  depends on which way it works. The paradox comes from the nature of the relationship between the universe that the time-machine comes from (let's call it A) and the universe that the time machine enters (let's call it B), and whether or not you can change the future. I will section this discussion under the headings “Alternate Timeline” (A and B are separate), “Self-fulfilled Prophecy” (A and B are the same), and “Event-invariant Time Machine” (There is a necessary overlap between A and B).

And let's get some self-consistent definitions of determinism and free will. A “deterministic universe” usually means “individuals in the same exact situation always make the same exact choice” and “free willed universe” to mean “individuals in the same exact situation may or may not make the same exact choice (they will probably pick similar but not exactly the same)”. I'll leave QM notions of determinism and indeterminism out of it for simplicity's sake[1].

I have to take an aside here though that it is not at all clear to me why people prefer a free-willed universe to a deterministic universe. I would rather expect that my decision is informed by (and therefore determined by) what I know of the universe - that my reasons for my decisions depend only on the situation. In fact, the decisions of people who make evidenced based decisions would look nearly deterministic even in a free-willed universe. But to assume that a free-willed universe is the one we live in is to talk about “what I would do if I did not do what I did”, which seems like a nonsensical contradiction. It is only in the context of time travel (or predicting the future) that this phrase could mean anything at all. I think people get confused between planning for predicted consequences of their actions and perceiving alternate futures that they are choosing between. When people use “free will” as part of a theodicy (an attempt to explain away the Problem of Evil[]), I always make sure they express it in terms of a constraint God's power or moral standards rather than figure out what it means for myself.


Alternate Timeline:
In this type of time-travel the future (universe A) puts no constraint on the past (universe B). When you (the time travel) travel to 2035 and read about yourself, you find that you disappeared in 2010 and haven't been seen since. When you travel back to 2010, you find yourself in a universe identical to the one you remember from but with one notable exception – you are in 2010 with knowledge from the future (and you don't disappear). This small change may or may not immediately affect what will happen in any dramatic way, but your mere existence is a fact of the universe that the rest of the universe becomes almost immediately aware of [3].  It will go on to be 2035 in universe B and you won't expect to meet yourself spying on Obama – because you visited 2035 of universe A and are not witnessing 2035 of universe B. Because we've changed the decision scenario slightly and there is no causal relationship between universe A and universe B, then Obama may not act as you expect and this would be true whether or not we live in a deterministic or free willed universe.

I think it could be said, however, that Obama is much more likely to act predictably in a deterministic universe. So the question of how useful your information is from universe A is in universe B depends on whether it is a deterministic universe or free willed universe but you could never be sure from this experiment which it was. One thing that is certain, there is not contradiction of free will here. There may be, however, a violation of conservation laws[4].


Self-fulfilled Prophecy:
In this type of time-travel, the universe that you leave is the universe that you return to. This means that while in 2035 you read about yourself returning to 2010 with knowledge of 2035 (including what you are reading right now) and changing 2010 to be the 2035 that you are currently visiting. It would be impossible for Obama to not cheat if you observed it in 2035, even if you tried to go back and stop him. Because any events you observe in 2035 take into account your plan to go back and try to stop him. It's not like either you or Obama would feel like a helpless observer watching actions you don't control, though. Your actions would feel just as free as they usually do, just the events that occur in 2010 coincidentally create the situation observed in 2035. If you don't think this could be possible (say you think all humans are too perverse by nature to do what they already knew they were going to do), then this type of time-travel is simply not possible. If you think it is only in rare cases that it is possible (say you trip while traveling into the time of 2035 knocking yourself unconscious until you automatically return to 2010), then it is only these rare cases that it could occur.

There are two ways to look at this though. One way to look at this is as an infinite loop in which Obama and you are continually put in the exact same situations and you make the exact same choices every time. Clearly this is possible in a deterministic universe but not a free-willed one. The other way to look at this is that the events from 2010 to 2035 are only determined once and are (somewhat) simultaneous. In that case it could be deterministic or free willed – so long as the actions that are freely chosen are self-fulfilling. You could imagine the multiverse trying to resolve the contradiction by forcing the decision to be repeated over and over until an (accidental) agreement could be made between all parties. But some people also posit a stronger version of free will, which means individuals not only have the capability to do things in response to the same scenario, but the capability to do anything in response to any scenario. In this case the self-fulfilling condition would be a constraint which could not be permitted (by definition).


Event-invariant Time Machine:
In this type of time-travel starts out like the Alternate Timeline one: you visit 2035 (in universe A), you return to 2010 (in universe B) and you watch the events unfold in a slightly different manner the in universe A. The only difference with the Event-invariant Time Machine in the that once you get back to 2035 you meet yourself visiting (from universe A) and it is required that you return to 2010 exactly as you did before time changed. So Obama is free to cheat or not cheat on his wife like in Alternate Timeline, but the time-travel event has to happen exactly the same in all alternate timelines like with Self-fulfilled Prophecy. Ultimately though, because it is so hard to do the exact same thing the exact same way twice, this scenario doesn't make have much room to differentiate itself. The time-travel event could be exactly the same only if everything is exactly the same (in which case it degenerates to Self-fulfilled Prophecy), or if the requirements for everything to be the same aren't so strict after all (which degenerates to Alternate Timeline).


Conclusion:
So hopefully my explanations made as much sense to you as they did to me. It is possible to come up with time-travel scenarios which do and which do not contradict free-will. The larger point though is that free-will and determinism are in the domain of philosophy simply because we cannot separate them experimentally. If it were possible to do a time-travel experiment, the definitions of these words would be defined in context of that experiment rather than that experiment actually testing any useful predictions.


Links:
[1] Because this discussion about human agency predates quantum mechanics, I am leaving the QM of  wave-function collapses out of the discussion on the assumption you are looking for the philosophical implications of the scenario and not an assessment of the scenario itself. We can actually envision four scenarios, though, one in which humans and QM are deterministic, one in which neither are, one in which humans are but QM isn't, and one in which humans aren't but QM is. The whole thing is really weird to analyze from a physics perspective, because physics tells us time travel should not be possible, (if you want to hear more about that, here is a previous AllExperts of mine[]).
[2] http://en.allexperts.com/q/Atheism-2724/2009/12/Best-best-1.htm
[3] The wavefunction of every particle in the universe it touching the wavefunction of every other particle in the universe. So changing one particle in the universe anywhere has ever so slight changes to the behavior of particles everywhere. Keep in mind though that the effect from one particle on another just a meter away is so minuscule that I would need the logarithm of a logarithm to express it, but it is there. Keep in mind also that what happens to one particle at one point in time can only affect another particle a rate equal to the speed of light or slower. So presumably your transportation into a particular point of time is accompanied by a wave traveling at the speed of light which allows the wavefunctions of all particles touched by the wave to detect your presence.
[4] Universe A loses you but universe B gains you so there is an influx of matter, energy, and information from one universe into the other. Assuming that it makes sense to talk about each universe existing in it's own right , then before and after 2035 the total of matter, energy, and information of the both universes is conserved, but during 2035 you are in both universes (even if you die in universe B before 2035, your carbon atoms still exist). This is probably problematic, because it means that the energy of the universe multiplied by the length of time that energy exists is no longer the same, which is analogous to having more total energy.
[] http://en.allexperts.com/q/Atheism-2724/2009/7/intuition-predictions.htm