One of the most bizarre features of general relativity, Einstein's theory of gravity, is that it permits the existence of tunnels through higher dimensional space. If such "wormholes" could be constructed, they would dramatically reduce cosmic commuting times. An intrepid space voyager might enter the mouth of a wormhole in the vicinity of, say, Earth and, after travelling a few kilometres down a starless tunnel, emerge from the other end in the vicinity of Alpha Centauri, Sirius or even the Andromeda Galaxy.
Not surprisingly, wormholes have long been the preserve of science fiction writers rather than real scientists. However, that could soon change.
A group of American physicists says that if advanced extraterrestrial civilisations have constructed a network of wormholes in our Milky Way, and are zipping back and forth across space through a sort of gigantic pan-galactic subway system, there is a way we could find out.
The technique being suggested by Michael Morris of Butler University in Indianapolis, Indiana, and his colleagues involves the monitoring of millions upon millions of distant stars over a period of many years.
In theory, if a mouth of a wormhole -- the interstellar equivalent of an Underground station -- happens by chance to pass between the Earth and any one of the stars, it will cause that star's light to fluctuate in a very distinctive manner.
In fact, for several years now, three international teams of astronomers have been routinely monitoring millions of stars in two places: in a nearby galaxy called the Large Magellanic Cloud and at the centre of our own Milky Way.
They have been looking not for the signature of wormholes, but of dark objects which make up the mysterious "dark matter" that surrounds the Milky Way. Whenever a dark object passes between the Earth and one of the stars, its gravity acts like a converging lens, bending and magnifying the star's light. As a result, the star brightens for a few days then fades again, an effect which has now been triumphantly observed at least a dozen times.
"A WORMHOLE mouth would also affect the light of a distant star, but in a quite different way," says Morris. The reason is that a wormhole mouth must contain a large amount of matter with a negative mass. According to theory, only matter possessing this extraordinary property has the repulsive gravity necessary to keep a wormhole mouth from snapping shut. No one knows whether such matter can really exist. However, it is not ruled out by the known laws of physics.
Morris and his colleagues have not worked out what would happen to a star's light if a negative mass wormhole passed in front of it. Contrary to their expectations, they found that it would not act like a diverging lens, causing the star's light to dim temporarily. Instead, it would cause the star to jump suddenly in brightness, return to normal, then go through the whole cycle again. "The 'twin peak' signature of a wormhole mouth is very distinctive," says Morris.
Wormhole physicists have suggested to teams looking for the signature of dark matter objects that they also look out for the wormhole signature, and they are currently doing so. Morris and his colleagues were very excited when they saw a double peak signature in data recorded by the Polish OGLE team in 1994. Unfortunately, it was a false alarm. A closer examination showed that it was the signature not of an eclipsing wormhole but of two black holes in orbit around each other.
Morris and his colleagues are well aware that wormholes will be difficult to find, "It's a very long shot, I admit," says Morris, "But who knows what's out there."
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