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Comment by t8sr

10 hours ago

(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.

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t8sr

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evanb  10 hours ago

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  10 hours ago

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

bbor  5 hours ago

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! ;)