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Ringing a black hole.

Invisible, elusive, bizarre. These adjectives go naturally with the notion of a black hole.

Kip S. Thorne of Caltech has described this strange object in the following words: "Of all the conceptions of the human mind from unicorns to gargoyles to the hydrogen bomb, perhaps the most fantastic is the black hole: a hole in space with a definite edge over which anything can fall and nothing can escape; a hole with a gravitational field so strong that even light is caught and held in its grip; a hole that curves space and warps time."

The general theory of relativity has forced physicists to take black holes seriously. No one who accepts general relativity has found any way to overturn the prediction that black holes can form from the gravitational collapse of sufficiently massive objects and that they ought to exist in the universe.

But solving the relativity equations to predict precisely how black holes would behave and what types of gravitational waves they might generate has proved extremely difficult. Using supercomputers, researchers have so far managed to work out only a handful of special cases.

At the National Center for Supercomputing Applications (NCSA) at the University of Illinois at Urbana-Champaign, a team of researchers has now solved

the Einstein equations to illustrate what happens in the brief interval just before a pair of two-dimensional black holes, rapidly spiraling in toward each other, finally merge.

At this stage, the two black holes generate such strongly varying gravitational fields that the situation is analogous to a huge gravitational wave sitting on top of a single black hole, says astrophysicist Larry L. Smarr, a member of the NCSA team.

This meant the researchers could study the complicated pattern of gravitational waves produced during the final stage in the coalescence of two black holes by looking at the interaction between a gravitational wave and a single black hole.

The results show that the wave's presence forces the black hole out of its stable, equilibrium state. Then, as the perturbing wave moves away, the black hole snaps back to its original state but overshoots it. The black hole ends up oscillating at a particular frequency, In other words, it rings like a bell. As it rings, the black hole itself absorbs and radiates gravitational waves.

Similarly, a pair of colliding black holes will quickly merge into a single black hole and begin oscillating with a characteristic frequency

The NCSA researchers have now rewritten their computer program to handle three-dimensional black holes. "By 1996 or so, we should have enough [computer] power to run the full, three-dimensional version," Smarr says.

NCSA's Edward Seidel described the two-dimensional simulations at Physics Computing '93, a conference held earlier this month in Albuquerque.
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Title Annotation:Cover Story; computer simulations of merging black holes
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Jun 26, 1993
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