Mass Loss in Black Hole Merger? How Does That Work?

Question

Let's say we have two black holes, one 20 solar masses and the other 50 solar masses, and they get close enough and merge. In the process, they convert a solar mass or two into gravitational waves, which radiate off into the distance.Let me restate that, to emphasize what I think is really weird: A solar mass or two that is inside the event horizon got converted to energy and radiated outside the event horizon.E=mc^2. That part doesn't bother me. It's the "inside the event horizon" to "outside the event horizon" part that bothers me. Can someone ELI5 how that is possible? (Or even ELI20 with an undergrad in physics.)

sageserpent · Accepted Answer

Sorry, I am not informed about general relativity, but I can say this: the mass (and the angular momentum, and the electric charge IIRC) are all ‘visible’ through the event horizon - indeed, if they weren’t, the a black hole would in some sense wink out of existence as it formed - no gravitational pull, no frame-dragging.
I am oversimplifying, and treating the black hole as already collapsed inside the event horizon from the point of view of somebody well away from it; that’s not accurate. Perhaps thinking of that mass as being ‘just about to collapse into the event horizon’ is the best way to think about it, because that’s what that somebody would perceived from the outside who is not in free-fall into it.
Need a proper physicist to follow up on this, my training stopped at special relativity and Newtonian gravity…
(PS: studying tensor calculus and general relativity is on my bucket list - thanks for reminding me to get a move on with it!)