In 1996, string theorists were able to give an account of how -theory (which is an extension of superstring theory) generates a number of the string-states for a certain class of black holes, and this number matched that given by the Bekenstein entropy. A counting of black hole states using loop quantum gravity has also recovered the Bekenstein entropy. It is philosophically noteworthy that this is treated as a significant success for these theories (i.e., it is presented as a reason for thinking that these theories are on the right track) even though Hawking radiation has never been experimentally observed (in part, because for macroscopic black holes the effect is minute).
This, then, gives us two characterizations of a spacetime singularity: (1) a singularity that cannot be removed by any choice of coordinates, and (2) limits in which curvature invariants blow up. While these criteria work for black holes, however, they are not sufficient to capture all spacetime singularities. The standard characterization of a spacetime singularity is more general. This criterion relies on the notion of the geodesics of a spacetime. Geodesics are the "straightest-possible" lines of a space-time. They are the paths that an object in free-fall (i.e., not subjected to any non-gravitational forces, like the thrust of a rocket engine, or a pull of a rope) will follow. For any geodesic, we can ask whether it is possible to extend it without limit. If this is not possible, then the geodesic path comes to an end in some finite distance. This gives us a characterization of a spacetime singularity in terms of "geodesic incompleteness": (3) A spacetime is singular if it contains geodesics that cannot be extended to infinity. In such cases, it seems that there is an "edge" or and "end" to spacetime, which lies some at some finite distance. Here again, for black holes it can be shown that geodesic paths can be extended through = 2M (so there is no true spacetime singularity there), but they cannot be extended through = 0 (so this is a spacetime singularity).
Black hole essay Geneva August 15, 2016
The "event horizon" of a black hole is the very last point at which a light signal can still escape to the external universe. For a standard (uncharged, non-rotating) black hole, the event horizon lies at the Schwarzschild radius. A flash of light that originates inside the black hole will not be able to escape, but will instead end up in the central singularity of the black hole. A light flash outside of the event horizon will escape, but it will be red-shifted to the extent that it is near the horizon. An outgoing beam of light that is on the horizon itself will, by definition, be there until the end of the universe.