Studying GR, sometimes one asks himself/herself where GR fails to describe physics. For simplicity, take a Schwarzschild solution (nonrotating chargeless black holes), what of the following conditions are expected to be true and/or equivalent in order to solve the downfall of GR that GR itself predicts? Consider BH is an effective theory very close to "the true theory" but not the whole story. Then, are the following cases equivalent?
a) Does if fail at the event horizon? Case 1: BH horizons are only apparent or quantum atmospheres make EH fuzzy.
b) Does if fail whenever you cross the event horizon, since metric spacetime signature flips at that boundary? Case 2: after crossing BH EH the metric flips space-like and time-like coordinates, thus signature is relative and we should seek a theory of relativity for metric signatures.
c) Does it fail only at the center, when we find out a spacetime singularity? After all, a change of coordinates shows us that only r=0 is a problem for curvature and density. Can we really think a BH has an infinitely dense object when it is yet extended over the event horizon scale? Case 3: the only real problem is at the center, in the black hole singularity, and quantum gravity will erase this problem.
d) Maybe GR does not fail but only gets wrong answers since we are not yet armored with quantum gravity and the proper TOE to understand the fate of BH singularities, beyond the scope of a GR approximation. Case 4: GR is only an effective field theory, thus, this question is irrelevant.
e) Does GR fail in the firewall before the EH reaches? Case 5: firewalls are inevitable.
f) Does GR fail whenever we reach the fuzzball scale from the string theory proposal? Case 6: fuzzballs will solve the divergences and the information problem
g) Does GR fail whenever we reach the spin network scale, close to the planck length(radius)? Case 7: Planck scale and/or Loop Quantum Gravity will enter into the game to clarify the GR downfall.
Should we disregard BH singularities as they would imply and infinite resource or infinite information recording machine?
BONUS: Can the point where GR fails be related to the information paradox or is that an independent problem?