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Suppose two black holes where rotating around each other, perhaps spinning into each other, at a distance such that their event horizons merge into one event horizon. It seems to me that the event horizon of the combined objects would, for at least a short while, enclose only one barbell region of space. This event horizon would not be spherical. Is this typical of black holes? Are event horizons in reality not oblate spheres? And does that tell us something about the distribution of mass on the inside of the black hole?

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  • $\begingroup$ Black hole mergers are described in this pdf and on this Wikipedia page. Black holes that rotate have a more complex form than non-rotating black holes. $\endgroup$ – StephenG Apr 16 '17 at 3:19
  • $\begingroup$ The idea of black hole uniqueness theorems in 3+1 dimensions is precisely that! Every stationary solution is parameterized by mass, charge, and angular momentum, and nothing else (in classical em + classical gravity, at least). If its a dynamical situation then yeah, you can have all sorts of crazy things, but once the dust settles, you'll be left with an oblate sphereoid horizon. $\endgroup$ – user12029 Apr 16 '17 at 4:54

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