Incredible Stuff: Evading quantum barrier to time travel

Ruling out the possibility of traveling back in time has turned out to be trickier than many physicists had supposed. Two researchers of Princeton University have shown that quantum effects do not necessarily prevent the occurrence of loops in time.

Einstein's special theory of relativity unifies space and time as aspects of a single, four-dimensional entity known as space-time. His general theory of relativity describes how the presence of matter warps the fabric of space-time. Physicists have found that general relativity equations yield many solutions representing different space-times. Sometimes, sufficient warping of a particular space-time makes possible the existence of paths known as closed timelike curves. A traveler moving along such a path would find that his or her watch always runs forward, even though the traveler eventually ends up where - and when - he or she started.

In 1982, William Hiscock and Deborah Konkowski calculated the quantum state of the space, or vacuum, pervading a simple type of space-time called Misner space, which includes closed timelike curves. Their result indicated that such a combination of gravitational space-time and vacuum quantum state could not exist. Noting that result, Stephen Hawking (University of Cambridge) proposed the chronology protection conjecture, stating that physical laws do not permit the appearance of closed timelike curves. For example, quantum theory could conspire to prevent time travel by ruling out the existence of space-times with such paths.

Misner space can exhibit more than one type of vacuum state, however. Li-Xin Li and J. Richard Gott III of Princeton University demonstrate now that one of these states permits the occurrence of time loops. That state is self-consistent, meaning that going back in time doesn't alter what happens later in the system.

"We have found a counterexample to Hawking's conjecture," Gott says. "Quantum effects do not automatically enforce chronology protection in every case."

That doesn't mean a person could build a machine to travel back in time. The amount of space-time warping required for such a feat would lead to all sorts of practical problems. The calculations also involve crucial approximations and may not apply to the "real" cosmos. However, the possible existence of closed timelike curves under certain extreme conditions may offer a solution to the problem of what came before the Big Bang, which most cosmologists believe started our universe.

Li and Gott explore the question of whether anything in the laws of physics would prevent the universe from creating itself. "The universe wasn't made out of nothing," Gott suggests. "It arose out of something, and that something was itself. To do that, the trick you need is time travel." The researchers speculate that a universe undergoing the rapid early expansion known as inflation could give rise to baby universes, one of which (by means of a closed timelike curve) would turn out to be the original universe.

As they did for Misner space, the two physicists found a self-consistent vacuum state, demonstrating that closed time loops can occur under inflationary conditions in certain space-times. Hence, the researchers say,