Source: New Scientist January 6, 1999

Claire Bowles claire.bowles@rbi.co.uk

Add A Few Time-Travelling Tachyons And Black Holes Make Sense

DO BLACK holes spit out particles that move faster than light and travel backwards in time? Two physicists in New Jersey say that particles with these bizarre properties could come to the rescue of a promising theory about what happens in the heart of a black hole.

The world of very massive objects is ruled by general relativity theory, while quantum mechanics governs the realm of very small scales. Black holes -- enormous stars crushed into no space at all -- fall into both categories, as they are both massive and tiny. Much to the chagrin of physicists, general relativity and quantum mechanics appear incompatible, so nobody is sure what equations hold true at the centre of a black hole.

But in the past few decades, a new set of theories has raised the hopes of physicists struggling to understand the interior of these bodies. String theories, for instance, portray black holes and particles as wiggling strings. The disappearance of a particle into a black hole would simply be the result of two different strings being spliced together.

What's even more exciting is that physicists are combining these theories into one large "M-theory", which explains a lot about the interior of black hole. But there is still a flaw: although black holes seem to devour everything that comes their way, this cannot happen under the rules of M-theory. Just as oil and water refuse to mix, so energetic particles cannot merge with black holes, according to the theory.

"If you send a particle in towards the black hole, and it gets sufficiently close, it needs some mechanism to be absorbed," says Daniel Kabat, a physicist at the Institute for Advanced Study in Princeton, New Jersey. "Once it gets too close to the black hole, it becomes unstable." Without some way of getting rid of that instability, the black hole would spit out the particle-something that doesn't happen in nature.

But now Kabat and Princeton University's Gilad Lifschytz believe they have figured out how to keep M-theory intact, while at the same time explaining how a black hole can keep down its lunch. The answer lies in tachyons. These are particles with imaginary mass that can be thought of as travelling backwards in time. They move faster than light, and slowing down to the speed of light would be as impossible for them as it is for us to accelerate to light speed. Physicists also use the term "tachyon" for a whole family of instabilities that quickly decay.

Though nobody has ever seen one, the researchers have shown that tachyons might get rid of a particle's excess energy, making it palatable to the black hole. In an article due to appear in The Journal of High Energy Physics, they suggest that particles spit out tachyons as they merge with a black hole. "These tachyons would be important for the dynamics inside the black hole, but I don't think an observer outside the black hole would be able to see them," says Kabat.

If tachyons really do solve M-theory's problem, physicists may at last have a way to build up a coherent picture of what happens inside a black hole. "You're able to do precise calculations, and such calculations are hard to come by," says Kabat. "It's a pretty compelling picture of a black hole."

"This paper is very interesting, and it's potentially important," comments Michael Douglas, a physicist at Rutgers University in New Brunswick, New Jersey. "It poses a lot of ideas."