Tuesday, 15 September 2015

Do black holes violate T-symmetry?


One can fall into a black hole but not fall out of it. Does this mean black holes violate T-symmetry?



The closest thing I found to this is this section on Wikipedia, but it doesn't cite any sources and sounds dubious.



  • Our laws of physics might break down at the singularity, but not at the event horizon, and I remember reading that in a Schwarzschild black hole, all geodesics lead to the singularity - i.e. there's still a forward concept of time.

  • The section also mentions white holes, but that doesn't seem to solve the problem: even if white holes exist, they would just violate T-symmetry in another way - one can fall out of it but not into it.

  • It says that the modern view of black hole irreversibility is related to the second law of thermodynamics, but as far as I understand it, the second law of thermodynamics doesn't violate T-symmetry because on a microscopic level all the motion is T-reversible.


Other results I found focus on the black hole information paradox, which is not what I'm asking about.


If the answer is "yes", how is this possible given that only the weak force of the four known forces violates T-symmetry?



Answer



EDIT: the short answer to this question is that a time-reversed black hole is a white hole, full stop, so if you apply time-reversal to a particle falling into a black hole, you get a particle falling out of a white hole, but we don't physically expect to observe white holes.



Original text:


A blackhole space-time does not violate T-symmetry because, the extended Kruskal solution also contains a white hole:


https://en.wikipedia.org/wiki/Kruskal–Szekeres_coordinates


so, if you time-reverse the portion of a curve falling into the black hole, it will become a portion of a curve falling out of the white hole.


Now, we expect that the universe was created with initial conditions that don't allow white holes to exist, but this would mean that the T-symmetry in GR is spontaneously broken by some quantum theory that is not GR. It is absolutely present in the schwarzschild and Kerr spacetimes, though, thanks to the extend kruskal coordinates trick.


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