When Black Holes Collide

Black holes are the densest objects in the universe, with gravitational fields powerful enough to trap light and anything else that strays too close by; so it takes a lot of nerve to grab one of these behemoths and cut out its heart. But that's what one group of physicists is doing, numerically speaking. The goal of this bit of mathematical mutilation is to understand the dynamics of black hole collisions and the gravitational waves that such events create.

The mathematics that describe black hole interactions is so complex that no one is completely sure what the resulting gravitational waves will look like. Although computer simulations can help, many algorithms fail when they address regions near black hole singularities where the gravitational fields theoretically approach infinity.

A group of researchers affiliated with the Universities of Texas, Pittsburgh, British Columbia, and Penn State University are avoiding the difficulties of singularities by snipping the troublesome data out of their simulations. Only the portions inside the black holes' event horizons are ignored. Because an event horizon lies at the distance from a black hole where gravity is so intense that even light cannot escape, it is impossible for information to pass outward through the horizon. The information impasse means that cutting out the inner portion of a black hole doesn't affect computer solutions for the regions outside the holes.

In a recent simulation the researchers (Luis Lehner, University of British Columbia, 604-822-1383, luisl@sgi1.physics.ubc.ca) considered a grazing collision of two black holes. The holes merged into one, radiated energy in the form of gravity waves, and oscillated like a glob of gelatin (for figure go to Physics News Graphics). Calculations such as these will eventually help scientists interpret signals from a new generation of gravity wave detectors, including the recently completed Laser Interferometer Gravitational Observatory (Update 442), which will scan the heavens looking for interactions involving black holes, large stars, and other very massive objects. (S. Brandt et al, Physical Review Letters, 25 December 2000; text at Physics News Select.)