Inside a black hole? Consider a neutron star with mass 1 gram less than needed for it to become a black hole. Since the neutron star is not (yet) a black hole, we can 'see' it. Send in the 1 gram and watch while the neutron star converts to a black hole
where, as usually proposed, the mass that was the neutron star suddenly shrinks to the "singularity" at the center of the black hole.
Now, it appears that there is a huge change -- neutron star to a black hole -- from a small input, the 1 gram, that is, in math terms, there is a jump discontinuity.
There was something about the physics of the neutrons that kept the neutron star from shrinking to a singularity. Well, maybe that something also keeps that mass plus the 1 gram from shrinking to a singularity. That is, if there is no jump discontinuity, the inside of that black hole is essentially just like that neutron star.
> watch while the neutron star converts to a black hole where, as usually proposed, the mass that was the neutron star suddenly shrinks to the "singularity" at the center of the black hole.
This "conversion" doesn't imply matter transitioning from one state to another. The main thing happening during transition to a black hole is that the light can't escape anymore - you see the star in one moment, and can't see it in another moment. Not necessarily because of some matter transition, but because it stops radiating light.
Singularity is a mathematical artifact, we don't know what's happening in the blackhole with matter and don't really care since it has no effect on the outside world.
We don’t know that an object with 1 gram less than needed to become a black hole will be a neutron star. There may be other denser states or matter in between, like quark stars or strange stars that are still not dense enough to become black holes.
Under GR it doesn’t matter since as the mass increases beyond a critical point an event horizon will form and all that matter will be compressed into a singularity regardless.
I am trying to imagine what happens to a single contiguous mass that has both a part that is travelling at relativistic speeds, and a part that is not?
Maybe nothing much? From each parts point of view it is still simply in contact with it's neighbor and neither is moving relative to the other?
Three properties completely describe a black hole: mass, spin, and charge.
It does not appear that charge will be useful in the question of an extremal black hole.
In considering spin, as velocity of matter approaches c (the speed of light), more energy is required to achieve less gain as c is approached.
Can a black hole spin at the speed of light? Can it spin faster?