: : Consider a collapsing star. If it is more than 1.3 solar masses, it becomes a neutron star, shrinking stopped by Pauli principle (neutrons are Fermions). If it is somewhat larger it becomes a black hole (blackhole radius is proportional to mass, geometric radius for constant density is proportional to the cube root of mass). : : According to gen. rel., the black hole should collapse into a singularity. What happened to the Pauli principle? What happens to the neutrons (or constituent quarks) on a microscopic scale? Is it some exotic form of matter, yet unknown?

: AS you said, you can think of the Pauli Principle as the source of a pressure. With the addition of more mass, gravitation : overwhelms the pressure and, in effect, the electrons are squeezed into the protons to create neutrons and a neutron star. Add : yet more mass and the structure collapses into a black hole. All the neutrons go to the singularity at the center. What happens : there? GR says they are crushed to a point. QM says "No way!" Since we do not have a theory of quantum gravity, we cannot : say what happens, and anyway it is screened by the horizon.

Actually there is a quantum gravity theory that does offer an explanation for what happens. It is string theory. I'm not saying its right. But there is an answer out there.

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