What Happens When Two Black Holes Collide?

We know a few things about what happens when a black hole encounters another object in space. But what would happen if that object was, in fact, also a black hole?

Today, we were joined by theoretical physicist Chris Adami, who answered our questions about black holes — including this one about just what we could expect to happen if one black hole ran smack into the other.

The answer is, apparently, one incredibly gigantic black hole:

As you will probably find out from answers to other questions about black holes, we do not have a definitive answers to this question. However, some things are known. For example, we know that (at least if the black holes collide head-on) that a new black hole is formed with the mass of both black holes, and a surface area that is at least the surface area of the two black holes combined. This is because there is an "area" law of black hole thermodynamics that say that the total surface area of a set of black holes can only increase. It is like the second law of ordinary thermodynamics.

Image: Artist's conception of two black holes merging into one massive black hole / NASA-JPL