NASA/CXC, M.Weiss
Darn. When you wake up, just like in the nightmare films, you’re dead or a zombie or something…
Starting with an intriguing story about how a German soldier stationed on the Eastern Front in 1916 (World War I) produced exact solutions to Einstein’s equations of general relativity, we are told that it depends on whether black holes are space-like or timelike:
The physicists most wanted to find out whether their quasi-realistic black holes have a central singularity — a fact that had only been established for certain for simple Schwarzschild black holes. And if there is a central singularity, they wanted to determine whether it is “spacelike” or “timelike.” These terms derive from the fact that once a particle approaches a spacelike singularity, it is not possible to evolve the equations of general relativity forward in time; evolution is only allowed along the space direction. Conversely, a particle approaching a timelike singularity will not inexorably be drawn inside; it still has a possible future and can therefore move forward in time, although its position in space is fixed. Outside observers cannot see spacelike singularities because light waves always move into them and never come out. Light waves can come out of timelike singularities, however, making them visible to outsiders. …
The group found that for both types of black holes they examined, there is indeed a central singularity, and it is always spacelike. That was assumed to be the case by many, if not most, astrophysicists who held an opinion, Chesler noted, “but it was not known for certain.”
Steve Nadis, “Black Hole Singularities Are as Inescapable as Expected” at Quanta
Paper (open access) and subsequent paper. (open access).
The explanation below is: “Black holes capture the future.”
See also: A black hole that just shouldn’t exist
and
Sabine Hossenfelder: Black holes vs. quantum mechanics: Something has to give
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