In an article published March 15, 1995 in the scientific journal Physical Review D, physicists John G. Cramer, Gregory Benford, Geoffrey A. Landis, Robert Forward, Matt Visser and Michael Morris argue that the search for dark matter in the universe could be expanded to include exotic objects such as wormholes, topological connections between separated regions of space-time that might be described as "subways to the stars."

The article, "Natural Wormholes as Gravitational Lenses", explains one theory underlying the possible existence of such objects and describes the gravitational lensing "signature" that might enable us to detect them. The proposal stems from a workshop on relativity and quantum mechanics sponsored by NASA.

"A 'wormhole' is a theoretical object permitted by Einstein's theory of general relativity, where distant regions of space are connected by a shortcut," says Landis, a scientist at NASA's Lewis Research Center in Cleveland, OH. "Such wormholes could have been created in the distant past, in the time just following the 'big bang' that created the universe. What we discovered at the workshop was that if such wormholes did exist, they could be detected by the bending of light due to gravity, an effect known as the 'gravitational lens.' "

"Several research groups currently are using telescopes to monitor gravitational lensing events to search for massive compact halo objects (MACHOs) that have positive mass," Benford explains. Gregory Benford is a professor of physics at the University of California Irvine. "They want to count these objects because they could explain the vast amount of dark matter in our galaxy, or perhaps the whole universe.

"So far, these experiments are not surprising anyone. They're turning up a reasonable amount of possible dark matter, but nothing that's going to solve the dark matter problem. But we believe that by paying just a little more attention, by analyzing the MACHO search data for evidence of what we like to call GNACHOs (Gravitationally Negative Anomalous Compact Halo Objects), there's a chance to make a really profound discovery."

"Wormholes, although consistent with the theory of relativity, are theoretically unstable," says Dr. Landis. "Previous scientists have detailed the theory, but never suggested a plausible means by which one might be created. In this work, we realized that during the formation of the universe in the big-bang, wormholes could have been stabilized by loops of negative-mass cosmic string." Cosmic strings are discontinuities in the structure of space that have been theorized by cosmologists. "If so, they could still be here, and it is worthwhile to look for them. If we find one, the implications are enormous."

Wormholes provide one possible way to create astronomical objects of negative mass, according to theoretical calculations by Matt Visser, professor of physics at Washington University in St. Louis, a co-author of the article. A key element of the theory is the finding that, as matter passes through the wormhole, the "entrance" and "exit" mouths of the hole gain and lose mass.

"According to theory, either end of a wormhole can swallow mass, ejecting it out the other end," said Benford. "But a wormhole mouth in a dense region of matter swallows mass faster than its other end, if that end is in a sparse region. Mass emerging from an end curves space-time oppositely. It's as though the end loses mass, finally reducing to zero mass and then to negative. Gravitationally, that negative end looks like a negative mass, maybe even a large, stable one."

Expanding the current Optical Gravitation Lensing Experiment (OGLE) a collaborative project between the Warsaw University Observatory, Carnegie Observatory and Princeton University Observatory to search for these and other possible negative mass objects would be relatively simple, said Benford. This is because GNACHOs, if they exist, provide a distinctive light enhancement profile when passing between us and distant stars. This resembles the twinkling of starlight in the Earth's atmosphere.

Ordinary mass causes the sudden brightening of a star image, exerting a gravitational focussing of the light, like a focusing lens. "Ordinary mass causes a single peak in brightness because it focuses the light," said Benford, "but negative matter deflects the light rays, creating a shadowed umbra region where light from the source is extinguished. At the edges of the umbra the light rays accumulate to form what's called a caustic, giving a very large increase in light intensity in two visible peaks of brightness." This light signature is unique.

What would it mean if we were to discover a wormhole?

A wormhole is like a shortcut through space; a method of instantaneously travel between places that are spatially very distant, without contradicting the theory of relativity. You go in one end and you pop out somewhere else.

"Finding one would completely change our view of how galaxies form, because it would mean that wormhole transport of mass from one part of the universe to another has been a significant evolutionary factor. It would mean that space-time is intimately connected in a way we didn't know; that the whole universe is knitted together and that's a profound result."

But whether or not we find a wormhole, it's the search that's important, said Benford.

"The chances of negative mass wormholes being common are slim and nobody's going to get funded to go out and look for wormholes but the chance to find one as a bonus with data from existing dark matter experiments is enticing," he said.

"Our goal is to say `Let's see if we can make an observation.' Because there are still lots of mysteries in the universe, and we shouldn't take for granted that we know what's in it."