Here's a few comments on some of the issues discussed in the thread. All of this is theoretical.
Roy Kerr fiorst theorised that spinning black holes may well offer a way to travel back in time. In fact, it's now thought that all black holes spin - so all could be time-machines.
Black holes can exist (theoretically) with quantum-scale radii. In fact it is believed that the universe is peppered with microscopic black holes created in the instant of the big-bang.
The gravity gradient depends on the mass of the black hole. A very large mass black hole will have a large radius (the radius of its event horizon) and will have a relatively shallow gradient. If the black hole were big enough, its gradient would be so shallow as to be not only survivable but almost undetectable at the moment the observer crossed the event horizon. In fact, in some ways our entire universe acts like a gigantic black hole, with everthing trapped inside its event horizon.
I've not come across any recognised scientific theory that dark matter can be converted to ordinary (baryonic) matter (or vice versa). However, it is possible that dark matter is simply baryonic matter - that just happens to be non-radiating (such as dead stars, or extra-solar planets, rocks, dust, etc.).
The radiation (Hawking radiation) emitted by a black hole is not actually derived from the matter falling into it. That matter only imparts momentum to the black hole, but otherwise all "informational" content of it is utterly annihilated or lost (what happens to this "information" is one of the abiding mysteries of black holes).
Hawking radiation has a slightly more exotic origin. It derives from the "virtual particles" that are constantly popping into and out of existence in the vacuum of space (giving rise to the so-called zero-point energy of a vacuum). These particles are "allowed" into existance owing to the uncertainty principle of quantum mechanics. So long as their existance is brief enough, they do not violate any laws. Their energy is allowed to be conjured out of nothing because of its inherent uncertainty in accordance with the Heisenberg principle of quantum mechanics. And their charge and mass is zero since they are always produced in pairs (a particle and an anti-particle).
The vacuum is thought to be a seething mass of such virtual particles constantly popping into and out of existance. In fact, the Casimir effect seems to prove the reality of such particles (but that's another story).
Anyhooooo... since these virtual pairs are appearing constantly everywhere, they are also appearing in the region of a black holes. Imagine such a pair appearing at the event horizon. Whereas they would normally exist for a tiny fraction of a second before recombining and popping back out of existance, what now happens is that one of the pair can become trapped within the event horizon, the other remaining outside. The one that remains outside is a particle of Hawking radiation. It is this which is detected as an 'evaporation' of the black hole.
Interestingly, the rate of 'emission' increases over time. This leads to a runaway effect that in the final moments manifests as an apparent "explosion" of the black hole. I am not aware of any other theories of explosion.
On the Einstein thing - black-holes (i.e. the concept that something may so warp space/time that nothing - not even light - could escape) are predicted by his own theory of general relativity. I expect that what worried him most was the singularity issue - that something in the physical universe could have infinite density.
Personally I believe that this is indeed impossible, and the resolution to the issue lies in the fact that the black hole does not exist in our normal universe. It exists in a shrinking bubble of space/time. From that bubble's frame of reference, time is passing normally, with matter shrinking steadily and achieving steadily higher densities. This persists for ever. In that frame of reference, infinite density would only be achieved after an infinite amount of time.
But from our frame of reference outside the event-horizon, all of this - this eternity of shrinking - seems to take place in a finite space of time. So we appear to have the paradoxical situation of an infinitely dense body existing.
2old
