Is time travel possible?
Here's a question that's bugged me for all eternity — is time travel possible, even in theory?
Cecil's gut feeling on this question was no, based on the following logic: if it were possible, the people of the future would have done it; and if they did, why haven't they shown up yet? But then I thought: come on, is this a century you'd want to come back to?
The possibilities of deductive reasoning thus having been pretty much exhausted, I consulted the Straight Dope Science Advisory Board. While the question is far from settled, a few optimists do think time travel may be possible. In fact, a widely noted paper was published a few years ago proposing a hypothetical time machine. The machine requires some outrageous theoretical leaps, though, and the technology is totally beyond our present capabilities. So don't start making plans to visit the Pleistocene epoch just yet.
The centerpiece of the hypothetical time machine, which was proposed by physicists Kip Thorne, Michael Morris, and Ulvi Yurtsever, is something called a "wormhole." A wormhole is a place where the space-time continuum is so warped it basically doubles back on itself, creating a cosmic shortcut. (Think of it as a tunnel between two points on an apple, the surface of the apple being the ordinary space-time continuum.) Up till now the only known way a wormhole could be created was inside a black hole. But these wormholes were of a cheesy quality and tended to collapse too quickly to be usable. Also, since they're in black holes, once you get in you can't get out.
That's where we get to Great Leap #1. Thorne et al propose (rather cavalierly, as we shall see) that a black hole-less wormhole could simply be plucked from the "quantum foam." The quantum foam is the micro-level of the universe beneath which things are so small we can't observe them. Since we have no idea what's going on down there, there's no reason to think the usual laws of physics apply. So Thorne and friends say, hey, let's assume that at quantum level, freestanding wormholes exist. Fine, it's a free country. But then they say, now let's assume the holes can be enlarged to usable size. How? God knows. But theoretical physicists pride themselves in not getting hung up on details.
Then we get to Great Leap #2. To keep your enlarged wormhole from collapsing, you need to brace it with special weird materials that would give it negative mass and energy. Only problem is, physicists aren't so sure you can have negative mass and energy, except under exotic circumstances. No problem. With a few deft equations, Thorne and company calmly suggest circumstances don't get much more exotic than this. Negative M&E it is.
All right, suppose we have our stable wormhole, with the two mouths located near one another. While keeping Mouth A stationary, we accelerate Mouth B to near the speed of light, then bring it back to its starting point. (Don't ask how we do this; it's Great Leap #3.) According to the Theory of Special Relativity, this causes Mouth B to age less than Mouth A. Mouth B may thus be said to have been yanked into the future. (Take my word for it.) By traveling through the wormhole from Mouth A to Mouth B we travel forward in time, and by going from B to A we travel backward.
Thornean time travel has its limitations. You can use a wormhole to travel into the future and back, but you can't travel into the past. Furthermore, you won't necessarily get those time travel paradoxes so beloved of sci-fi writers. (You know: if you went back in time and killed your grandmother, you would never have been born, but if you weren't born, you couldn't have killed your grandmother, and so you would have been born ... you get the idea.)
"Causality violation," as it's called, would totally kibosh our current understanding of how the universe works, and a lot of physicists hope desperately that it can be shown to be impossible. In fact, if no way can be gotten around it, a lot of them have half a mind to rule out the possibility of time travel altogether. Physicists can be pretty wild and crazy sometimes, as the above wormhole exercise shows. But other times they're no fun at all.