Wormholes and time machines.

Wormholes and time machines

Captain Kirk, beware! A sufficiently advanced civilization might be able to construct and maintain wormholes -- short-cuts through space-time -- for rapid interstellar travel between widely separated parts of the universe. And, if the laws of physics permit such a construction, then a wormhole could also be converted into a time machine.

These speculations surface not in a script for a "Star Trek" episode but in a recent issue of a respectable physics journal. In the Sept. 26 PHYSICAL REVIEW LETTERS, Michael S. Morris, kip S. Thorne and Ulvi Yurtsever of the California Institute of Technology in Pasadena present mathematical arguments supporting the notion that the presence of a wormhole could locally scramble the relationship between cause and effect.

According to present theories, when a star several times more massive than the sun collapses to form a black hole, under certain circumstances, the black hole can turn inside out and poke itself into another part of space-time. The result is a tunnel through space-time -- a wormhole -- linking two black holes. For most theorists, wormholes hardly seem worth thinking about because they squeeze shut before there is any time for a spacecraft or even information to pass through.

Morris and his colleagues, however, suggest that a wormhole could be kept open if two identical, perfectly conducting, equally charged spheres were placed on either side of its throat. The presence of the charged spheres invokes a quantum process known as the Casimir effect, which changes the nature of the vacuum between the spheres, allowing the wormhole to stay open. With such a technology, an advanced civilization could use a wormhole to transmit messages and travel across the universe.

If a wormhole's two mouths initially happen to lie close together, then the wormhole can be turned into a kind of time machine. By using electrical or gravitational force to pull one mouth away from its stationary neighbor at a speed verging on the speed of light, then reverse its direction and return the mouth to its original position, extraterrestrial technicians would cause it to "age" less than its partner. Consequently, by traversing the wormhole from the younger to the older mouth, "one can travel backward in time . . . and thereby, perhaps, violate causality," the researchers write. "This wormhole space-time may serve as a useful test bed for ideas about causality, free will and the quantum theory of measurement."

The entire argument hinges on whether the laws of physics, as presently formulated, permit the creation of traversable wormholes. That question, in turn, raises deep, ill-understood issues about cosmic censorship, quantum gravity and quantum field theory. One plausible scenario for wormhole creation entails "quantum foam" -- fluctuations in the fabric of space-time many times smaller than the size of a subatomic particle. "There might be some way to amplify these quantum effects, although we don't know how," the physicists say. "One can imagine an advanced civilization pulling a wormhole out of the quantum foam and enlarging it to classical size."

"There are many uncertainties," says Morris, now at the University of Wisconsin-Milwaukee. "The calculations are only preliminary." For instance, although it may be possible to construct a traversable wormhole, it could be unstable. As in the case of a precariously balanced pencil standing on its point, any slight nudge would topple the system.

"It's all very speculative," Morris says. "Any new idea could crush the whole thing in the sense of proving that it would be impossible. But any one of these directions of speculation, including results that crush the idea, would be very interesting." Such speculations give physicists ways to probe their understanding of physical laws, illuminating ill-defined concepts and gaps in scientific knowledge.