The latest special issue of the Scientific American deals with Time and Brad DeLong has some interesting thoughts on the same subject. Both Brad and the philosophers writing in SciAm find it very puzzling and problematic that, according to relativity theory, there’s no objective way of dividing time into past, present and future.
I must say I don’t find this problematic at all, at least at a fundamental level. Of course, it’s surprising and counterintuitive that if you send a person into space at the speed of light, they’ll come back younger than a twin who remained on earth. But that’s only surprising in the ‘isn’t science amazing’ way that a microwave oven is surprising the first time you see it. I wouldn’t have thought you could boil water while not heating the cup containing the water, but now I don’t give it a second thought.
As regards past, present and future, the observation that has had the most effect on my thinking was not made by Einstein or Feynmann, but by David Hume, a couple of centuries ago.When asked how he, as an atheist and nonbeliever in eternal life, could be unafraid of death and non-existence, he replied that no-one seemed to be worried about the uncounted centuries of non-existence that had passed before they were born.
When I think about this now, the division that’s real for me is not between past and future, but between those things I can remember and those I can’t. My past is the things I can remember. Of course, I assume that the same is true for everyone else, and that, in the manner of an archeologist with tree rings, we can patch together a consistent chronology from our joint memories, and an even better one with the aid of scientific instruments. But there’s nothing in this to make me think that this chronology should match up exactly with one generated at the other end of the galaxy or by someone moving at 100 000km/second.
There’s obviously a sense in which the past is accessible and the future is not. But I don’t know that even this is fundamental. I feel a lot less fundamentally uncertain about tonight’s episode of The Bill than I do about what life was really like for David Hume and his contemporaries or what’s happening right now (modulo relativistic effects) in the neighborhood of Alpha Centauri.