When they are far apart, they interact just like any other bodies acting under weak gravitational fields. For black holes with about the mass of the sun, their sizes are about 2 kilometers in diameter. When they get within a few dozen miles, their shapes begin to deform, meaning their Event Horizons are distorted from a spherical shape ( if they are non-rotating black holes). As they get closer together, they are under enormous acceleration and since all accelerating bodies emit gravitational radiation, the combined black hole system begins to shed some of its energy in gravity waves. Since energy and mass are equivalent, this also means that the sum of the effective masses of the black holes during the collision is steadily decreasing compared to the sum of their masses when they were far apart. Within a few minutes, the black hole Event horizons begin to interpenetrate as the above supercomputer calculation shows.
If you were watching this all happen, you would see the black holes merge together to become a new black hole. The mass of this new hole would be a little less than the sum of the black hole masses before the encounter because of the loss of mass due to gravitational radiation. According to supercomputer calculations, the new mass is something like 10 percent less than the original sum of the masses. This means that, in terms of the surface area of the new black hole, it would not be exactly the sum of the surface areas of the two black holes, but would instead be slightly smaller.
Copyright 1997 Dr. Sten Odenwald
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