The causal structure of black holes

[sharp_blue]

A commenter writing about my earlier post, "Relativity, FTL and Causality" said:

As far as FTL being equivalent to time travel, the above explanation is correct. As far as FTL being impossible at present, it is not quite. Richard's explanation lacks a mention of black holes. An object (with non-zero mass), falling into a black hole from rest, will

Comments

[ashley_y]

So black holes are not symmetrical with respect to time inversion. How does a time-inverted black hole behave, and have any been observed?

[sharp_blue]

Black holes that result from the collapse of stars aren't time symmetrical, which is why I chose to discuss them rather than the full Schwarzschild solution, which is the spacetime of a static, eternally existing, uncharged spherically symmetric black hole. The full Schwarzschild solution has a more complex causal structure with both past and future singularities and two asymptotically flat exterior vacuum regions, and is symmetric under time reversal. The spacetime outside any uncharged, non-rotating spherical body is just like a patch of the Schwarzschild solution and this is typically joined to a different metric within the body. (As the body collapses the non-Schwarzschild region shrinks away to nothing

[ashley_y]

As I understand it, a black hole cannot be destroyed, except by evaporation through Hawking radiation. Would that mean that a white hole cannot be created except by somehow time-reversing Hawking radiation?

Secondly, I assume a black hole can exist without gaining mass if there's nothing around to absorb. So presumably a white hole can exist and not spew anything out, and thus not decrease entropy? This seems to be the most likely behaviour of a white hole were one to exist in this universe of increasing entropy.