Escaping Black Holes

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Danger said:
For some reason, I'm very familiar with the name 'Kip Thorne', but I can't for the life of me remember why.
Possibly from MTW? (Charles Misner, Kip Thorne, and John Wheeler's Gravitation). Thorne was a student of Wheeler's and has done a lot of work on black holes and worm holes. If you've read any Stephen Hawking books, Thorne's name probably showed up as well.
 

Danger

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?llipse said:
Possibly from MTW? (Charles Misner, Kip Thorne, and John Wheeler's Gravitation). Thorne was a student of Wheeler's and has done a lot of work on black holes and worm holes. If you've read any Stephen Hawking books, Thorne's name probably showed up as well.
Thanks, Ellipse (I don't know how to make that character in your name). I just checked 'A Brief History of Time' and saw that Hawking had bet Thorne that there's no black hole Cygnus X-1. That's not why I knew the name, but your response did trigger the memory. He's been cited repeatedly by other authors of gravity-related articles in Sci-Am, and I believe that he's written a few of them.
 

EnumaElish

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pervect said:
Because r is time-like, there is only one direction that an object inside the event horizon can go. This is in the direction of its future, the direction of increasing r.
What in physics theory predicts that time flows one-way only? My impression is that there is no logical proof, only our experience. In other words, time may well flow backwards in a theoretical sense. Aren't you risking to have projected your humanly experience of this side of a black hole into theoretical physics when you make that statement?
 

pervect

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There's only one way to be really sure - go jump in a black hole for yourself :-).

Meanwhile, I think I've given a reasonable quick over-view of the relevant theory given our current state of knowledge. This state of knowledge does not include much empirical testing, for obvious reasons.

Several assumptions are made, including that GR is an accurate theory of gravity. The idea that time does flow only one way is only one assumption of many - an assumption that's necessary before the question of why things can't come out of a BH can even be asked. (If one does not assume that time has a preferred direction, this question makes no sense).

One thing that does bother me a bit is that in one post I use the Schwarzschild metric to make a point, in another post I point out that the Schwarzschild metric doesn't really work in a real (non-rotating) black hole, that the actual solution is likely to be a so-called BKL singularity.

http://scienceworld.wolfram.com/physics/BKLSingularity.html

This suggests that there should be a deeper argument for why things can't come out of a black hole, one that does not rely on the exact form of the metric inside a black hole. This probably relates to one of Hawking's singularity theorems and/or global methods. Meanwhile, I think that even the simpler explanation I gave is probably already complicated enough for a general audience.
 
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melknin said:
Well I think it's enough to answer my question that you can't ignore relativity. (I have seen that Simpsons episode, I love that show ^.^) But if anyone can explain the relativistic effects, I'd love to learn.
The best way of thinking about it that I’ve seen is here:

From Schwarzschild Geometry:
Free-fall coordinates reveal that the Schwarzschild geometry looks like ordinary flat space, with the distinctive feature that space itself is flowing radially inwards at the Newtonian escape velocity v = sqrt(2GM / r). The infall velocity v passes the speed of light c at the horizon.

Picture space as flowing like a river into the black hole. Imagine light rays, photons, as canoes paddling fiercely in the current. Outside the horizon, photon-canoes paddling upstream can make way against the flow. But inside the horizon, the space river is flowing inward so fast that it beats all canoes, carrying them inevitably towards their ultimate fate, the central singularity.

Does the notion that space inside the horizon of a black hole falls faster than the speed of light violate Einstein's law that nothing can move faster than light? No. Einstein's law applies to the velocity of objects moving in spacetime as measured with respect to locally inertial frames. Here it is space itself that is moving.
 

pervect

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Zanket said:
The best way of thinking about it that I’ve seen is here:

Picture space as flowing like a river into the black hole.
While this picture does have many good points as far as visualizing what happens around a black hole, it does have one weak point. That is the fact that there is no way to measure the "flow of space". If it helps you understand how objects behave around Black Holes, great - but don't take it too literally, because there is no way to actually build a "space flowmeter". In fact, the principle of relativity prohibits one from building such a device, as it would be able to measure absolute velocities.
 

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