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Black holes are the simplest macroscopic objects, and provide unique tests ofGeneral Relativity. They have been compared to the Hydrogen atom in quantummechanics. Here, we establish a few facts about the simplest systems bound bygravity: black hole binaries. We provide strong evidence for the existence of`global' photosurfaces surrounding the binary, and of binary quasinormal modesleading to exponential decay of massless fields when the binary spacetime isslightly perturbed. These two properties go hand in hand, as they did forisolated black holes. The binary quasinormal modes have high quality factor andmay be prone to resonant excitations. Finally, we show that energy extractionfrom binaries is generic and we find evidence of a new mechanism -- akin to theFermi acceleration process -- whereby the binary transfers energy to itssurroundings in a cascading process. The mechanism is conjectured to work whenthe individual components spin, or are made of compact stars.

The physics of black hole binaries: geodesics, relaxation modes and energy extraction