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The Question

(Submitted July 11, 2001)

What would happen if two Black Holes merged? Would superpositioning of the gravitational fields make the event horizon radii shrink?

The Answer

The size of the event horizon is determined solely by the mass and spin (if it happens to be spinning) of the black hole. If two were to orbit each other (or any two massive bodies such as neutron stars) a lot of energy in the form of gravity waves would be emitted. This will leak energy out of the system, until the two objects merge. It would appear that the smaller one was swallowed by the larger, but it really is a merger around their common center of mass, and the event horizon would grow according to the new higher mass.

Also there is more information on this at:

Two experiments which hope to detect and confirm this can be seen at:

Hope this helps,
Michael Arida for Ask an Astrophysicist

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