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Crash Testing Black Holes: A Colossal Collision

July/August 1996

What do you get when two black holes crash into each other?

A bigger black hole!

At least that's what University of Texas physicist Richard Matzner and a team of collaborators got when they built a computer simulation of a black hole collision.

Theoretically, a black hole is a collapsed star with a gravitational force so immense that even light can't escape. While the idea is widely accepted, scientists have no direct evidence of the phenomenon. This and other simulations, created by the NSF-supported Grand Challenge Binary Black Hole Alliance, will help scientists recognize a black hole and a black hole collision when they find them.

The simulation starts with two symmetrical spheres describing these black holes. When the program ends, only one black hole survives. In between, the gravitational giants battle for supremacy.

By definition, a black hole has an irresistible surface gravity. Nothing, not even light, escapes from its symmetrical boundary. But a second black hole of equal size has an equally strong gravitational field.

Each hole pulls on the other. Like the moon's tidal effect on the earth, each hole's gravity yanks its counterpart out of the symmetrical shape. Bulges appear. The two become tear-shaped, and then oblong -- stretching each other out like a taffy pull.

Finally the holes' tips touch and they collapse together, making one, big, round, black hole.

It is not a peaceful process. Matzner estimates that in such an encounter, the colliding black holes each send off as much energy as our Sun burns in 100 million years, or one percent of its life span.

It is that energy that astronomers hope to see. Unfortunately, the power is not released in flashy light waves, but in quieter gravitational waves.

Much as throwing a stone into a pond produces waves that radiate to the edges, gravitational waves, in theory, start from the black hole collision point and radiate out across the universe. Like pond waves, gravitational waves become weaker as they get farther away from the source, becoming mere ripples by the time they reach the detectors on Earth.

Scientists expect to be able to identify the stronger gravitational waves with the new NSF-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) which goes on-line in 1999. The system of lasers and mirrors will measure the minute changes that occur when gravitational waves roll over Earth.

Researchers will use the models, such as the one created by the Alliance, to identify the cataclysmic events that cause the waves they see through LIGO.

While waiting for LIGO, the Alliance is working on further simulations where the black holes don't collide head-on, but spiral into each other. This scenario is more difficult to model, says Matzner, but it is also more likely to reflect what is really going on. He expects his group to complete the second model in the next two years.

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