Binary black hole systems orbiting around one another are particularly interesting because they lose energy due to emission of gravitational radiation, spiral inwards, and eventually merge into a single remnant black hole almost twice the size. The violence of the collision whips space itself into wild vibrations. These gravitational waves race outwards from the collision with the speed of light, carrying huge amounts of energy. Starting in 2002, a new generation of gravitational wave detectors (e.g. LIGO, GEO, VIRGO etc) hopes to catch these waves. Many astronomers believe that the first waves caught will be from merging black holes.
On the theoretical side the study of binary black hole mergers is now one of the most exciting and challenging topics in the astrophysical relativity community. Several theoretical approaches have been developed for treating these systems. The post-Newtonian approximation has provided a good understanding of the early slow adiabatic inspiral phase of these systems, applicable when the black holes are not too close, in what we may call the ``far limit''. Likewise, the ``close limit'' approximation, black hole perturbation theory, can successfully describe the system at late times when it is dynamically similar to a single black hole, which ``rings-down'' as it radiates away its distortions. Between these stages, when the system is near the ``innermost stable circular orbit'', orbital inspiral dynamics are expected to give way to a rapid plunge and coalescence. Here the two-body dynamics are most significant, obstructing any approximation method, and the system can be treated only by a fully non-linear numerical simulation of Einstein's gravitational field equations.
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