Score a paper victory for time travel. Physicists have long pondered how to transport a person--let's say you, the reader--into the past, perhaps using as a slingshot the warped space-time around black holes. Current theory says you would have to venture so deep into a black hole--beyond the so-called "event horizon"--that you could never escape to reach an earlier Earth and revisit your youth. But that's not the case, report two physicists in the 6 April Physical Review Letters, who have devised what they think is a way to at least send your ionized, black hole-shredded remains backward in time.
Einstein's general theory of relativity allows physicists to imagine space-time curved by a massive object--a black hole, for instance--into a loop where you can travel into your past. (The wrap-around is "like when the explorers went west, west, west, and found themselves back in Europe," says Princeton's Richard Gott, except here you go back in time instead of space.) But these calculations have always been incompatible with the laws of quantum mechanics. Physicist Stephen Hawking of the University of Cambridge, for one, has conjectured that the laws of physics would always somehow thwart time travel.
But Gott and student Li-Xin Li found that if they started with a fancy solution to Einstein's relativity equation--one that allowed for more bend in space-time than what Hawking and colleagues had used--it didn't go haywire when they threw in quantum mechanics. Get enough mass together, Gott says, and you can puncture a hole in space-time. Travel along the hole's edges and you can turn back the clock, but not to a time before the hole was created and began to warp things. Still, says Gott, "this is not something you're going to be doing in your basement with bicycle parts." You would need a black hole the mass of a sun to venture back even a millionth of a second, he says, and even then you'd probably be torn apart by the black hole's gravitational forces.
There may be theoretical potholes as well. "I'm not too keen on this approach," says Joseph Lykken, a physicist at Fermilab in Batavia, Illinois. Lykken thinks it would take a full-blown theory that combined quantum mechanics and gravity to settle the matter. Still, he suspects time travel may indeed be possible. "It comes down to luck," he says, "if you find a little black hole nearby, you'd be in business."