In science-fiction movies, wormholes--bridges between two regions of spacetime--are handy devices for traveling halfway across the universe in the blink of an eye. Many scientists think that wormholes are physically plausible, but most doubt that spaceships would be able to cross the bridge. Now, a team of physicists concludes that it might be surprisingly easy to make a wormhole traversable.
As outlandish as it might seem to move ships and people faster than the speed of light, Einstein's rules permit it. Even so, theorists suspected that spacetime bridges would have to exact a heavy toll. That's because relativity also shows that any wormhole can be turned into a time machine. By passing through one, a person might go back in time and kill his own ancestors before he was born, causing a paradox. So physicists were not surprised to discover that wormholes, like black holes, have "event horizons"--regions beyond which not even light can escape. The event horizon, sitting squarely in the throat of the wormhole, would prevent a traveler from leaving the wormhole.
But in 1988, physicist Kip Thorne of the California Institute of Technology in Pasadena and his students discovered a way to get rid of the event horizon. All that was needed was "exotic matter" with negative energy--that is, less energy than an equivalent volume of empty space. By placing a chunk of exotic matter in the throat of the wormhole, a spacetime traveler could banish the event horizon and make the wormhole passable. Exotic matter really exists, in a sense; indeed, the rules of quantum mechanics require that it is forever being created and annihilated on tiny scales. Unfortunately for sci-fi buffs, those quantum doses seemed inadequate to get rid of that pesky event horizon.
New calculations might get hopes up again. In the 23 May issue of Physical Review Letters, Visser and his colleagues investigated a specific type of wormhole and chugged through extensive calculations to figure out how much exotic matter was needed to wipe out the event horizon. Contrary to conventional wormhole wisdom, they claim that an arbitrarily small smidgen of the stuff--as tiny as the amount that is constantly created and destroyed on quantum scales--would make a black hole traversable.
Other wormhole researchers are cautious about interpreting Visser's calculations. "I'd be a little careful in saying you need only a small quantity of exotic matter" to destroy the event horizon, says Ulvi Yurtsever, a physicist at the Jet Propulsion Laboratory in Pasadena. Instead, he says, the equations might indicate that a wormhole still requires a large amount of exotic matter, but that large chunk could be offset by a slightly smaller chunk of ordinary matter.