Topics Time

Question In class, our professor discussed the impossibility of time travel. He stated that if in the future, machines are made to travel back into time, then we would be seeing people from the future right now. His argument ended there but would this be true? Is this a valid argument to disprove the possibility of time traveling in the future?

Accepted:
August 27, 2008

Comments

I hope your professor was just trying to provoke you, because it's a terrible argument. For one thing, it's not clear why he's so sure that we aren't already seeing people from the future, who've traveled back to this time zone, as it were, and are doing a good job of blending in. And in any case, suppose that in 3008, someone figures out how to travel backward in time. Why is it so obvious that they would come to this time?" Why not a later time? Or a time when there were no humans at all? If we add the plausible conjecture that the process would be expensive, dangerous and not altogether reliable, what basis would we have at all for speculating about the likelihood that someone would have shown up somewhere that we'd know about?

More importantly, if something is actual, it's certainly possible, but the converse doesn't follow. Even if time travel is possible, it doesn't follow that it will ever actually happen. The world is and always will be pregnant with unrealized possibilities. Perhaps this is one of them.

And then there's the question "possible in what sense?" The equations of general relativity allow solutions that include so-called "closed timelike curves." Anything that followed such a curve could fairly be said to undergo time travel. But from the fact that this is physically possible, it doesn't follow at all that it's a practical possibility. The laws of physics allow many things that we'll never be in a position to do.

Though your professor doesn't seem to have this in mind, some people have argued that time travel is inherently paradoxical and therefore that the whole notion is incoherent. (For example: the idea of time travel is supposed to sanction the possibility that I could kill my own grandfather before he ever met my grandmother, which would mean that I could cause myself never to have existed.) I think this is confused, and I can do no better than recommend that you get a copy of David Lewis's paper "The Paradoxes of Time Travel" (American Philosophical Quarterly 13: 145-52. It's also reprinted in his Philosophical Papers Volume II. Oxford: Oxford University Press.)