BLACK HOLE PART 2

                                               BLACK HOLE

Main approaches to the solution of the paradox

Information is irretrievably lost
Advantage: Seems to be a direct consequence of relatively non-controversial calculation based on semiclassical gravity.
Disadvantage: Violates unitarity, as well as energy conservation or causality.

Information gradually leaks out during the black-hole evaporation
Advantage: Intuitively appealing because it qualitatively resembles information recovery in a classical process of burning.
Disadvantage: Requires a large deviation from classical and semiclassical gravity (which do not allow information to leak out from the black hole) even for macroscopic black holes for which classical and semiclassical approximations are expected to be good approximations.

Information suddenly escapes out during the final stage of black-hole evaporation
Advantage: A significant deviation from classical and semiclassical gravity is needed only in the regime in which the effects of quantum gravity are expected to dominate.
Disadvantage: Just before the sudden escape of information, a very small black hole must be able to store an arbitrary amount of information, which violates the Bekenstein bound.

Information is stored in a Planck-sized remnant
Advantage: No mechanism for information escape is needed.
Disadvantage: To contain the information from any evaporated black hole, the remnants would need to have an infinite number of internal states. It has been argued that it would be possible to produce an infinite amount of pairs of these remnants since they are small and indistinguishable from the perspective of the low-energy effective theory.

Information is stored in a baby universe that separates from our own universe.
Advantage: This scenario is predicted by the Einstein–Cartan theory of gravity which extends general relativity to matter with intrinsic angular momentum (spin). No violation of known general principles of physics is needed.
Disadvantage: It is difficult to test the Einstein–Cartan theory because its predictions are significantly different from general-relativistic ones only at extremely high densities.

Information is encoded in the correlations between future and past
Advantage: Semiclassical gravity is sufficient, i.e., the solution does not depend on details of (still not well understood) quantum gravity.
Disadvantage: Contradicts the intuitive view of nature as an entity that evolves with time.