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Imagine a black hole originally formed from, for example, Rubidium atoms. On the other hand, one made from, for example, Helium atoms. Will it be there any difference between the two? Or perhaps once formed it all turns into the same type of matter?

Can the "composition" of the matter-energy in the black hole be inferred somehow?

Could one say that "black hole substance" is a distinct form or state of matter different from other forms or states of matter?

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    $\begingroup$ Neutron stars consist of neutrons (in some way), not of different kinds of elements. $\endgroup$ – Pieter Apr 23 '18 at 20:28
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    $\begingroup$ See the no-hair theorem. $\endgroup$ – Ruslan Apr 23 '18 at 20:31
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    $\begingroup$ Related: physics.stackexchange.com/q/18981/2451 and links therein. $\endgroup$ – Qmechanic Apr 23 '18 at 21:38
  • $\begingroup$ @Pieter Neutron stars are not pure neutronium. See en.wikipedia.org/wiki/Neutron_star#Structure $\endgroup$ – PM 2Ring Jun 18 '18 at 12:51
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    $\begingroup$ @PM2Ring That is why I had added "in some way". And it is still true that the abundances of nuclei in a neutron star would not depend what elements there were in the parent star. $\endgroup$ – Pieter Jun 18 '18 at 14:37
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A not-entirely-silly answer: Black holes seem to act like a very, very heavy fundamental particle, described only by its mass and quantum numbers like spin and charge. Those numbers just happen to be very large compared to typical fundamental particles.

If you made them out of different kinds of atoms, they'd have the sum of the quantum properties of those atoms. If you're using neutral atoms, the black hole will be neutral, but if some of the atoms are ionized, you'll end up with a positively charged black hole. If you use entirely atoms that are prepared in a way that makes them mostly spin-up, the black hole will be spin-up. Most of the information about what you put in disappears, though.

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    $\begingroup$ "Black holes seem to act like very, very heavy fundamental particles, described only by their mass and quantum numbers like spin and charge."--Except that they are diametrically opposed to the fundamental particles with respect to entropic considerations. They have the highest possible entropy for the mass they have, unlike fundamental particles who have zero entropy. $\endgroup$ – Dvij Mankad Apr 16 at 15:01
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Assuming they have same the mass, net electric charge, and angular momentum any two black holes will be identical. There isn't any way to analyze a black hole and figure out what kind of atoms have crossed its event horizon.

We do not know, in terms of empirical data, what happens to the matter that crosses the event horizon, because no information about what is happening 'inside' the black hole can cross over the horizon into our part of the universe. Presumably, it arrives very quickly at the singularity, separated by tidal forces into a stream of fundamental particles.

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  • $\begingroup$ "Assuming they have same the mass, net electric charge, and angular momentum any two black holes will be identical."--Isn't it kind of important to amend this by saying that this is true only when we speak thermodynamically? In reality, of course, two black holes with the same macroscopic description can differ drastically, to wit, the ensemble of the black hole(s) described with a given set of $M, J, Q$ has very high entropy. $\endgroup$ – Dvij Mankad Apr 16 at 15:05

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