The only way anything "escapes" from black holes is Hawking Radiation - virtual pairs of particles that appear and disappear constantly - can be split when they happen on event horizon - and that slowly drains the energy from black hole.

https://en.wikipedia.org/wiki/Hawking_radiation

If 2 black holes collide - they join together.

> If 2 black holes collide - they join together.

Presumably though some of the matter "escapes" as energy released during the merger, in the form of gravitational waves.

Yes. If memory serves this is the first event that LIGO detected. Two black holes of 20-30 solar masses each, joining and releasing 1-2 solar masses worth of gravity.

Which resulted in the earth (and space) being stretched by less than a fraction of a proton.

And we measured it...

Interferometers are just magical in their accuracy.

Merging black holes release more energy than anything else in the universe: https://bigthink.com/starts-with-a-bang/merging-supermassive...

I thought the energy came from the gravitational potential between the two black holes.

I know this comment is old, but I was wondering if you could randomly explain something I've wondered about for some time.

Doesn't Hawking radiation, since it exits a black hole, violate the idea that nothing escapes a black hole?

Essentially, yes, but not exactly.

My understanding is that particles and anti-particles pop into existence randomly, but quickly cancel each other out. Normally this doesn't affect anything, but if they happen to pop into existance right on the border of the event horizon, one will fall in and one will fall away. Thus matter is very very very slowly "leaving" the black hole. But not because matter inside is making it out, but rather because of some interesting quantum features of the fabric of spacetime.

Thanks for replying and your explanation!