Question: what happens when two black holes collide??

Sara Imari Walker answered on 20 Jun 2011:

Cool! Hi xam25! Drew’s link here looks pretty interesting. That basic description sounds fairly accurate though I am a little suspicious of this line “When two of these bodies collide, they emit more energy than the light of all the stars in the universe combined.” I can’t see any way this statement is true. If it were we would see bright lights all over our night sky from black hole mergers. In particular when galaxies collide the supermassive black holes at their centers merge – and we don’t see this kind of radiation emitted. Black holes are very similar to any other gravitational object outside of their event horizon (i.e. the point from which not even light can return). In fact if you replaced the sun with a black hole of identical mass nothing would happen to the Earth except we wouldn’t have sunlight. That would be bad for life but the gravity would be the same so the Earth’s orbit wouldn’t change! Since black holes interact like all over massive objects, I think the mergers would produce comparable energy. Certainly not anything to outshine the entire universe – although maybe it could outshine a galaxy. Supernovas (exploding stars) certainly do that.