View Full Version : Black Hole Question

iAWESOME

2011-Feb-21, 06:51 AM

This may sound ridiculous I know but I've never (ever) received a real answers to this. If a black hole is simply the distortion of the space time fabric (and the gravity), how "deep" is the distortion? For example, if the Moon was a black hole (scaled to size) would the entire moon be the black hole, or just the moons surface? And how "deep" does the space fabric distortion reach? If a black hole has a ten mile diameter, is it 10 miles in every direction, or is it only 10 miles on the surface? The answer is probably simple, and I'm just missing something, so please, correct me if necessary..

astromark

2011-Feb-21, 09:23 AM

I have a cunning and devious plan. I will hide the answer in the following text... and it's not ridiculous to ask a question. the answer could be the problem...

A Black Hole is not 'just a distortion of space time'..

It is the collapsed remnants of a giant star. Where it has shed most of its outer layers and the gravity of the remaining mass has collapsed into such a state as to become a plasma ball of super hot massive density mass... where the gravity of its mass is so strong as to require a escape velocity greater than c. Where a star that would normally have been a 100 times larger than our sun now is as small as the Moon. It's a black hole., and then 'How deep' ?

As the mass has become a BH approaching it you would find its gravity acting upon you... pulling and eccelorating you down the gravity well. You will not notice the point where you entered the BH., but you can no longer escape. right on down to where your component molecules are converted to raw elements and vapours of super hot plasmas... and that little question about the ten mile ? No. Its a star. Its hot its dense all the way through... and we do not know of the state of matter at its centre.

Bob Angstrom

2011-Feb-21, 10:03 AM

The "surface" of a black hole is called the "event horizon" and it is a mathematical boundary- not to be thought of as a solid or observable surface. It is the collection of points in space where the gravitational escape velocity of the black hole is equal to the speed of light and it is described by the Schwartzschild equation where the radius of a body is Rs = GM/c2. Rs is the Schwarzschild radius of the body, G is Newton's gravitational, M is the mass of the object, and c is the speed of light.

Strange

2011-Feb-21, 01:22 PM

I am not quite sure what "model" of a black hole you have in mind. But it doesn't sound like the right one :) It sounds a bit as if you are thinking of it as a "hole" in space, which probably isn't very useful. Actually, I don't think the idea of "curvature of space" (actually spacetime) is very useful either as it isn't curvature as we think of it.

A black hole is basically a sphere that light cannot escape from. The "surface" of that sphere is, as Bob says, the event horizon. Nothing can pass from inside the event horizon to the outside so we can never know what is happening, or what the state of matter is inside.

If you think of the curvature of space in a black hole, what it really means is that any path away from the center gets curved around to the center again. So if you fire a beam of light away from the center of a black hole, it follows that curved path until it ends up at the center.

Alternatively, you can think of the black hole as sucking "space" into it at the speed of light (at the event horizon); so any light that tries to escape will get "dragged back" in. But that is a poor description as well because there isn't really a "thing" called "space" that gets sucked in...

As to how "deep" a black hole is, it goes from the event horizon to the center. So if the diameter of the black hole is 10 miles then it is 5 miles deep (i.e. the same as any other sphere).

Hope that helps you get to grips with it a bit better...

cosmocrazy

2011-Feb-21, 09:29 PM

As to how "deep" a black hole is, it goes from the event horizon to the center. So if the diameter of the black hole is 10 miles then it is 5 miles deep (i.e. the same as any other sphere).

Classically yes we can calculate this to be correct, but in the case of a BH can we really be confident 100% for this to be true? We cannot observe behond the EH there fore we cannot truly measure behond this also. We can only assume from the prediction of classical math what it should measure.

caveman1917

2011-Feb-21, 09:41 PM

Classically yes we can calculate this to be correct, but in the case of a BH can we really be confident 100% for this to be true? We cannot observe behond the EH there fore we cannot truly measure behond this also. We can only assume from the prediction of classical math what it should measure.

Good point. Especially since to an external schwarzschild observer the radial dimension inside the EH turns into a timelike dimension, so that quite messes up our questions about "how deep" it is. I suppose we can say that a BH takes away a sphere of radius x from the surrounding space, but talking about how "deep" it is on the inside gives a plethora of issues, not in the least the one you mention that the entire thing is beyond testability in the first place.

Strange

2011-Feb-21, 10:01 PM

Good point. Especially since to an external schwarzschild observer the radial dimension inside the EH turns into a timelike dimension, so that quite messes up our questions about "how deep" it is. I suppose we can say that a BH takes away a sphere of radius x from the surrounding space, but talking about how "deep" it is on the inside gives a plethora of issues, not in the least the one you mention that the entire thing is beyond testability in the first place.

I was pretty sure it would be more complex once you start taking general relativity into account... Do you fancy trying to give an intuitive description of "timelike"? Would it make sense to say the depth would be measured in seconds rather than miles?

pzkpfw

2011-Feb-21, 10:40 PM

General mod comment: this is one of those cases where the answer ought to be pitched at the questioner. Looking at the OP, I'd say the "10 mile diameter, 5 mile deep" answer is just fine. A deeper discussion than that could occur - but maybe over in the science and tech sub-forum.

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