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A black hole is so dense that a sphere around it called the event horizon has a greater escape velocity than the speed of light, making it black. So why do astronomers think that there is anything weird (or lack of anything Inc space) inside the event horizon. Why isn't simple the limit to where light can escape and in the middle of event horizon (which physically isnt a surface) is just a hyper dense ball of the matter that's been sucked in and can't escape just like light. Why is it thought that the laws of physics don't exist in the event horizon?

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I'm not aware of any prevailing belief that the laws of physics don't exist inside a black hole's event horizon. Sure, things get weird, but as far as I know, general relativity is still expected to work as long as you're not at (or perhaps very close to) the singularity itself. Did you have a specific reference in mind where someone says otherwise? – David Z Nov 10 '10 at 0:08
My astronomy teacher. But he could be wrong. – Jonathan. Nov 10 '10 at 0:47
Well, do you know/remember exactly what he said? Although I'd argue that normal physics does exist inside an event horizon (as far as we can tell), perhaps he was actually talking about something slightly different. – David Z Nov 10 '10 at 2:23
Are you sure he was not speaking of the singularity instead, where all the mass is located? – Mark C Nov 10 '10 at 6:06
up vote 3 down vote accepted

For large enough black holes, space is still weakly curved at the event horizon, so of course we should expect that normal physics still exists there. An infalling observer wouldn't experience anything out of the ordinary when crossing an event horizon.

What is true is that for an outside observer, it's impossible to probe what's happening inside the event horizon of a black hole. (The best you can do is wait a long time and collect the outgoing Hawking radiation.) So from the point of view of such an observer, you can't really tell the difference between living in a world where spacetime keeps going across the horizon, or living in a world where space just ends there and some radiation emerges. This might be the sort of idea your teacher was getting at. You might want to look up "black hole complementarity" to learn more.

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"An infalling observer wouldn't experience anything out of the ordinary when crossing an event horizon" you mean apart from being torn to pieces, or do you consider that ordinary? :p (+1) – Tobias Kienzler Nov 16 '10 at 11:49
You wouldn't be torn to pieces when crossing an event horizon for a large enough black hole, because space is weakly curved there. (For a small one, you would. It's just a different condition than the one that determines where the horizon is, so in general you can't conclude anything like that.) – Matt Reece Nov 17 '10 at 1:05

If you are inside the event horizon of a spinning black hole, there are closed timelike curves--paths through spacetime which, when you progress them into the future, they end up in their past. These curves clearly violate the principle of causality, and thus, the region inside the black hole can be considered unphysical in a sense.

Also, of course, the spacetime singularity also lives inside of the black hole's horizon. Classically, it is a point or ring of infinite density at which the curvature of spacetime is infinite. Spacetime histories intersecting the singularity must end if you believe in classical mechanics. So, there is clearly some new physics there--most relativity researchers believe in something called the Cosmic Censorship conjecture, which says that nearly all* physically reasonable solutions of Einstein's equations hide their black hole singularities behind a horizon.

*There are some known exact solutions where physically reasonable matter distributions collapse to horizonless singularities, but they are believed to form a set of measure zero in the space of matter distributions.

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For example, for starters, outside a Schwarzschild black hole horizon, particles can move in any direction, but time only goes one way. i.e. forward. Inside the horizon, particles can only move inward toward the central singularity, i.e. one way, but time can go either forward or backwards. This is a result of the "light cones tipping over". That's different.

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