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Physics, Astronomy & Cosmology / Re: What happens inside a black hole, and how do black holes evaporate?
« on: 26/09/2020 13:21:38 »Thanks for the great answers. But I am unable to get the clear picture just yet even after watching the video that dealt with future light cones etc. Please explain in simple terms what would be happening to observer A falling into event horizon(here, uncharged non-rotating black hole only) and we watching from afar.That would be a second observer B (us). The two of us would obviously observe different things.
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1. Would observer A only appear to be stuck on event horizon due to his light getting red shifted to undetectable frequencies? While observer A himself would have crossed event horizon and towards singularity in proper time?Yes, to all. A crosses the event horizon in his own proper time. Proper time is observer dependent, and it pretty much means the time on the guy's watch, so if he crosses at his noon, observer B will see A's clock approach but never reach noon.
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2. Would observer A's all of particles and information be encoded into 3D black hole's 2D surface?Black holes don't have a surface, so no. The information preservation thing is not entirely solved for black holes, but in the coordinate space of observer B, A never crosses in, so his information up to noon does exist at any given moment in time, and I think that's what you mean by a 'surface'.
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Would the black hole grow very slightly due to observer A's falling into black hole?Of course.
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Would there be dual existence of observer A? From our perspective, he was stuck forever at the surface while he himself became a part of black hole in proper time?All observers are a worldline, not a point of existence. A's worldline continues on into the black hole. This is all assuming that A stays together, which eventually isn't the case. We're assuming he's falling into a big one and survives the approach. A stellar mass black hole will kill any human well before he gets to the event horizon, and will be reduced to a series of diverging worldlines.
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3. At the event horizon, would the observer A see all of time in the universe pass behind him?No twice. As I said above, he can't tell when he crosses the event horizon. Assuming he can observe all the way to the singularity, he can observe anything in his past light cone, which includes some events and not others. If B is sending news down to him from Earth, there will be a point at which that news will not reach A before time ends at the singularity. That means that A cannot see the history of the universe, but only a brief bit of time.
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Eg. at the event horizon, he looked behind and sees time of the universe infinitely accelerated.An observer hovering at the event horizon will see this, but not a falling observer. This is nothing special. If I get in a ship with infinite acceleration, I too will see the universe 'infinitely accelerated', at least in front of me.
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4. Is there infinite curving of space and dilating of time at the singularity, and hence infinite gravity?Time dilation is relative, so the proper time of A would be unchanged, by definition. Gravity is spacetime curvature, so it seems correct to equate infinite curvature with infinite gravity.
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Suppose there is 6 km black hole, is there infinite density at its singularity?Size matters not. The solution to the field equations yield infinite density of the singularity of a black hole of any mass. A unified field theory might have a better description than that, but we've no such theory at this time. Current mathematics yields a divide by zero at the singularity, which is why it is a singularity.
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Or is it false since black hole has finite mass?Density is mass/volume, and any small mass can have arbitrarily high density if the volume is reduced enough.
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