Thursday, September 3, 2009

Does the Possibility of Time Travel imply the Impossibility of Free Will? (I)

There seems to be wide agreement among theorists that while time travel is logically possible, altering the past is not. In other words, it's coherent to believe that you might travel into the past one day, but incoherent to believe that while on this journey you might kill your father. If that's right, it means problems for libertarian conceptions of free will.

The initial argument is simple: Let's say that

i) In 1966 George and Lorraine have a child, Marty.

ii) In 1985, Marty time travels to 1955 and runs his father, George McFly, over with a DeLorean, killing him.

iii) In 1966, George is dead and Lorraine is an alcohol-raddled spinster.

Thus, iv) In 1966 Marty is both born and not born.

(iv) is obviously self-contradictory, and therefore logically impossible. Furthermore, since Marty killing his own father is logically impossible, a fortiori it is physically impossible.

If this doesn't seem right, it helps to remember that Marty is a time traveler, not a world traveler. This is the point David Lewis stresses at the end of his important essay ('The Pardoxes of Time Travel'). There is only one 1955, just as there is only one 1966 and one 1985. Marty can travel from one time to another with about as much ease as you or I can drive from home to the supermarket, but just as when you or I visit the local supermarket it is always the same supermarket, so when Marty visits 1955 it is the same 1955 it has always been. Thus, in the 1955 in question, it is timelessly true that Marty was there, and that he was the instrument of his parents' romance, just as it is timelessly true that he is born in 1966 and travels back to 1955 from 1985. There is not one 1955 without Marty, and another 1955 with Marty. Marty is always there, in 1955, even though he is not born until 1966. This only seems to be paradoxical (How can he exist at a time before he is born?). Lewis recommends distinguishing between personal time and external time: For Marty, his arrival in 1955 follows his birth, whereas to an external observer, living through the normal course of time, Marty just appears one day in 1955, while his infant self is born eleven years later in 1966. (This of course poses problems for identity theory, but not for time). If the initial argument is correct, then Marty is, in the strogest sense--logically, physically--unable to kill George in 1955. If one nevertheless wants to insist the contrary, this would have to be in a different 1955, a 1955 which is followed by a 1966 in which no one is born to George and Lorraine, which in turn is followed by a 1985 in which no one identical to Marty travels back in time. In other words, we have here two worlds, not two times--a world in which George and Lorraine meet and have Marty who then time travels in 1985, and another world in which George is killed and no one identical to Marty is ever born. If Marty ends up in that 1966 where George has been killed and no one identical to himself is born, it has not been through traveling back and altering time, but by traveling between one world and another.

This suggests the following argument against libertarianism:

a) A free agent is able to cause one rather than another possible outcome.

b) Marty is not able to cause one rather than another possible outcome.

c) Marty is not a free agent.

d) Since Marty is not special in anyway, no one is a free agent.

Marty, we know, cannot kill George. He cannot do so by accident, but more to the point, he cannot do so intentionally. He may--perhaps overwhelmed by newly-blossomed incestuous longings--desperately want to kill George in 1955, but he cannot and thus will not do so, for we know that he is born in 1966 and that George is his father. But if Marty cannot alter the past and affect the present, so too neither he nor anyone else could alter the present and affect the future. For Marty's inability to kill his own father is not unique to Marty. If Marty is unable, despite the firmest of desires, goals and plans, to thwart George and Lorraine's consummation, then so too are George and Lorraine, whatever their desires, goals and plans, unable to alter that same consummation. George and Lorraine will give birth to Marty in 1966, and there is nothing George, Lorraine nor Marty can do about it. Suppose that Marty informs George that Lorraine becomes an insufferable alcoholic, that their kids are complete losers, etc.--no matter, George will not be able to avert that future, for if he does avert that future, then in 1966 Marty is not born, and so cannot time travel in 1985 and so will not be there to inform George in 1955 get the picture. Finally, if there is nothing special about Marty in 1955, and if there is nothing special about George and Lorraine in 1966, then there is nothing special about you or I today. Que sera, sera.

A libertarian might object at this point: even if we grant that it is impossible to perform actions today that are in contradiction to the future, surely we are free to do other things today not in contradiction with the future. For example, nothing is stopping Marty from moving the teapot in Doc's house from one burner to the other, because nothing about his later travel back to the past is inconsistent with that changed fact. Hence, Marty is free to move the teapot, even while he is not free to kill George. This objection misses the point. Nothing Marty does can change the future, not because the future is just somehow perfectly rigid, but because the future after 1955 results from whatever occurs in 1955, and Marty has always been in 1955, i.e., if Marty moves the teapot, then the teapot's being moved is just a timeless fact about 1955. What is compossible with moving the teapot is Marty's wanting to move the teapot. What is not compossible with the moving of the teapot is the not moving of the teapot. We can say that Marty is free to move to the teapot insofar as he is not stopped from doing what he wants to do, or insofar as his second-order intention towards his decision to move the teapot is an approving one, but we cannot say that Marty is free to move the teapot if by that we mean that Marty could have done otherwise.

Lewis' attempts to get out of this predicament seem only half-hearted. Lewis argues that, if we restrict the range of facts we are considering, then Marty is able to kill George. For example, Marty's killing George is compossible with Marty's desire to be his mother's lover, George's oafishness, the fact that George is weak and easily fooled, his ability to purchase a gun, etc., and insofar as we restrict ourselves only to facts like these, there is a sense in which it is appropriate to say that Marty could kill George . But as we expand the pool of facts that need to be included, the range of possible actions decreases, and once we consider all the facts, then it is impossible that Marty kills George. Lewis, in other words, does not really show that free will makes sense, he only manages to stipulate a use of 'could' or 'possibility' that is not baldly self-defeating; nothing Lewis says can be taken as proof that we really are free.

Anyway, I will address some further complications in a later post. I suspect there is something wrong or missing in the above argument, but I can't quite identify what it is. One point I will address is the following: the above account of time-travel certainly seems to suggest a four-dimensionist picture of time, and there have been attempts to make libertarianism compatible with four-dimensionalism. I don't find these persuasive. First off, I think the argument fails. Secondly, from what I have read, libertarian four-dimensionalists at best succeed in supplying a way in which we can think of choices as random events or points. I'm not persuaded: it seems to me that, even among libertarians, it's important not only that the agent be the cause of an action, but that s/he be responsible for that action, yet if an action is truly random, it may be the cause of some event, but I don't see why the agent would be responsible.


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