[t8sr]

(I’m an astrophysics undergrad.) Black holes aren’t composed of anything, they’re just defined by their charge, spin and mass equivalent.

Dust clouds have those mass ranges. It’s not a galaxy-scale mass by any measure.

This thread has a lot of CS people being confident about physics.

[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.

[bbor]

I mean, I included a disclaimer... But regardless, you appear to be wrong on both counts (or at least contradicting Wikipedia):

1. "The presence of a black hole can be inferred through its interaction with OTHER MATTER and with electromagnetic radiation such as visible light." https://en.wikipedia.org/wiki/Black_hole

2. "A dwarf galaxy is a small galaxy composed of ABOUT 1000 up to several billion stars" https://en.wikipedia.org/wiki/Dwarf_galaxy

Darn astrophysics majors being confident about astronomy! ;)

[t8sr]

1. Your argument is about the grammar of a sentence about black holes on Wikipedia? This isn’t some kind of gotcha.

2. I missed the dwarf part, but think about what you’re arguing: the mass range of a loosely defined category (the lower bound of a few thousands is not one I’ve ever heard, btw) that has nothing to do with the paper in question. Collections of stars of any kind produce light. This doesn’t. What are you saying?

What do you think physicists do all day?

[9991]

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