There’s been some speculation lately that time travel may indeed be possible to some degree or another, and that within our own lifetimes, people, not just particles, would be able to travel as well.

Of course the problem with time travel is the grandfather paradox. Could you go back in time and kill your own grandfather before your parents were born? If so, then you would have never existed, and your grandfather wouldn’t have been killed, allowing you to be born so you could go back and kill your grandfather (poor grandpa!). Since the popularization of quantum theory, the solution to this problem has been parallel universes, or the so-called multiverse. Whenever more than one thing could happen, both do, each its it own universe. When two universes differ only very slightly, they might recombine, but in general, each second that goes generates a partial infinity of new universes. So if you went back in time and killed your own grandfather, you would simply generate a new universe, and you would have essentially moved from one universe (in which you did exist) to another (in which you would not have, except that your time machine brought you there). Viola – no paradox.

This concept saddens me somewhat, because even if time travel were to ever become a reality, we wouldn’t learn anything from it. Any time traveller who went into the past, whether he killed his grandfather or not, would inevitably create enough change that he would only be able to move forward in a different universe; from our perspective, he would be lost. He might return to a time when another version of us exists, but we would never see him again or learn anything he learned.

Travellers to the future might return to their own universe, but we would have no way to know reliably that the future they visited is the same universe we’ll end up in – the further into the future they go, the less likely it is that we’ll manage our way into that same universe (that’s not so sad, though – at least we can get our chrononaut back.)

We can be reasonably confident that we currently live in a universe that no time travellers from another universe might return to, because there were no time travellers in our past (unless they were very, very discreet). At least in this model, research into time travel does us no good.

I recently finished reading Time Enough for Love (review-in-brief: Good (Author is Heinlein: Double-good!)), and part of the story has the character time-travelling back to his past. Granted, when he wrote the story (1973), multiversing was really thought about too much (definitely not by me), and though Heinlein himself would go on to write novels exploring the idea, it’s not really dealt with in that novel. Instead, the way he deals with the possibility of the paradox is with a kind of “post-destination” – paradoxes won’t happen because they didn’t happen. I don’t travel back and kill my grandfather because he didn’t die that way. Or if he was killed by a mysterious stranger and I turn out to be the one, maybe he was wearing the horns at the time. It’s not that there is some kind of deus-ex-machina magic to prevent you from doing it, just the simple fact that what happened in the past happened in the past, and that didn’t happen.

It’s like pre-destination (which many scientists and philosophers supported as the only rational theory, both before and after Quantum theory), except we are also privy to the destination.

Perhaps this is an obvious concept to many, but I found it somewhat comforting when it finally worked it’s way into my head, especially as I find it, as a theory, equally plausible (and perhaps more so from an Occamite point of view) with the multiverse. Time travellers needn’t stress themselves over stepping on the wrong bug or saying the wrong thing to someone else – the universe has already taken care of itself. Of course, they should be careful not to get themselves killed, but they should worry too much about accidentally running over Grandpa. (Which makes me wonder – how would the statute of limitations work on a crime committed in the past?)

One could conceivably extend the same sense of peace to the present. Today’s present is tomorrow’s post-destination – whatever happens will have happened tomorrow, and from that perspective, is inevitable. Try not to get yourself killed or make the girls cry, but for goodness’ sake, relax a little!