[evanb]

I was always surprised that when we talk about BHs mass, charge, and spin that we really mean U(1) (electromagnetic) gauge charge and not charges from global symmetries. (If BHs had global charge, you could at least say that this or that black hole was made out of N baryons, or whatever.)

But it's really so---according to GR, black holes don't have global charges. So even if you see a star made out of baryons collapse into a black hole, once the BH settles down into a steady state you can't say it's "really" got baryons inside: the baryon number gets destroyed.

(Of course, a different model of gravity that preserves unitarity might upset this understanding.)

[daxfohl]

And that a BH made from matter and one made from antimatter are mathematically identical, and merging them would not cause any explosion.

[actionfromafar]

Thanks! That made me (superficially of course) understand it. Super weird stuff.

[daxfohl]

Basically, the event horizon is the event horizon is the event horizon. If two non-steady-state matter/anti black holes merge (like before everything has hit the singularities), it will cause explosions inside the BH, but energy is mass is energy is mass, so for an external observer it's indiscernable. It will look no different from two all-matter BHs merging.