# Black hole questions

Discussion in 'Physics & Math' started by ScRaMbLe, Dec 29, 2003.

1. ### ScRaMbLeChaos Inc.Registered Senior Member

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If a black hole creates an infinitely deep gravity sink in 4d space then any matter added to it would make the hole infinity + 1 deep. Does this mean the black hole is moving further away from 4d space as matter is added? What is supporting the mass since 4d space no longer can? Why doesn't it cease to exist the instant it is formed? Does this imply that gravity is merely an inverse measurement of the density of the 4d space? Do these questions even make sense?

Last edited: Dec 29, 2003

3. ### AndersHermanssonRegistered Senior Member

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It's not very practical to ask what goes on beyond the event horizon because nobody knows.

5. ### letheRegistered Senior Member

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beyond the event horizon of a classical black hole, the radial coordinate becomes timelike, and all worldlines end at the singularity.

7. ### NanoTecRegistered Senior Member

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By 4D space do you refer to space-time? Forces support the mass against gravitational collapse, and mass remains through the conservation of energy. It may be said gravity is a measure of energy density.

(Of course necessary details remainâ€¦)

8. ### ScRaMbLeChaos Inc.Registered Senior Member

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Yes, I was refering to the fabric of space-time when I said 4d space.

C'mon dude, why dont you just say "coz it just does". I'm looking for a more detailed answer. Which forces? Conservation of energy only tells us it MUST happen not HOW it happens.

When I say "inverse measurement of 4d density" I'm trying to say that the more mass that an object has, the lower the "pressure system" of the space-time surrounding it. Therefore light shifts when passing massive objects coz its just following the path of least resistance thru space-time. Hence gravity is a product of space-time energy pressure. Is this the right way of looking at it?

How can mass be stored by time alone without 3d? Does this imply that time in itself must be a form of energy and therefore must somehow be equatable with mass?

9. ### James RJust this guy, you know?Staff Member

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ScRaMbLe:

<i>If a black hole creates an infinitely deep gravity sink in 4d space then any matter added to it would make the hole infinity + 1 deep.</i>

A few errors here.

A black hole is not "infinitely deep"; it is infinitely curved, in the technical sense of the term "curved", though only at the singularity. Adding matter doesn't make it deeper. It just makes the event horizon larger.

<i>Does this mean the black hole is moving further away from 4d space as matter is added?</i>

I have no idea what you mean by moving away from 4d space.

<i>What is supporting the mass since 4d space no longer can?</i>

Huh?

<i>Why doesn't it cease to exist the instant it is formed?</i>

Why doesn't what cease to exist? The black hole? Because the hole itself is a <b>fossil</b> field. Spacetime has a kind of "memory" of the mass that used to be there, due to the nonlinearity of spacetime.

<i>Does this imply that gravity is merely an inverse measurement of the density of the 4d space?</i>

No.

<i>Do these questions even make sense?</i>

Not a whole lot of sense, no.

10. ### ScRaMbLeChaos Inc.Registered Senior Member

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he he he... cheers dude, didn't think so. I'll try and explain myself. Go easy on me, I'm not making any claims just trying to understand. Bear in mind the difference between "stupid" and "uneducated"

I get this bit.

This is what I was getting at by saying "moving" but I understand that they are completely different!

By this I mean, when you see the classic "funnel" diagram of a black hole, the bottom end is shown to be open. What does this signify? I understand that in mathematical terms, that because the space is infinitely curved the "sides" can never meet, but how does this translate into real terms?

Yes, the mass of the black hole. You say "used" to be there. Do you mean the matter no longer exists in real terms or is merely irretrievable?

If you haven't got the time to explain, perhaps some links?

Thanx

11. ### James RJust this guy, you know?Staff Member

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37,758
ScRaMbLe:

<i>By this I mean, when you see the classic "funnel" diagram of a black hole, the bottom end is shown to be open. What does this signify? I understand that in mathematical terms, that because the space is infinitely curved the "sides" can never meet, but how does this translate into real terms?</i>

The funnel diagram misses out one spatial dimension, so it can be a bit misleading. What is important is to ask what happens to matter which falls into the hole. From the point of view of an observer outside the hole, the matter never reaches the event horizon, let alone the singularity. It just fades from view as the light it emits is red-shifted to infinity. If, on the other hand, you rode on the piece of matter as it fell into the hole, you would very quickly reach the singularity, where the matter (and you) would be crushed out of existence.

However, conditions right <b>at</b> the singularity are not described by current physical theories. For that, we'd need a quantum theory of gravity, which doesn't exist yet.

<i>Yes, the mass of the black hole. You say "used" to be there. Do you mean the matter no longer exists in real terms or is merely irretrievable?</i>

Nobody knows. All we know is that if the mass is still anywhere within the black hole it must be at the singularity.

12. ### ScRaMbLeChaos Inc.Registered Senior Member

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Well isn't it about time we found out?

13. ### AndersHermanssonRegistered Senior Member

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This is predicted by GR? I admit that my oppinion doesn't count for much yet, but I was under the impression that GR cannot be trusted beyond the event horizon, because it isn't accurate under those conditions.

Last edited: Dec 30, 2003
14. ### letheRegistered Senior Member

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this is the prediction of GR. most people believe that GR breaks down at the singularity, but for very large black holes, the tidal forces just behind the event horizon can be quite weak, in which case we expect classical GR to work just fine behind the event horizon.

in short: we can say things about what happens behind the event horizon.

15. ### cephas1012Registered Senior Member

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I am wondering a few things:

First isn't there some equation for determining the minimum density required to creat a black hole, something about the "swartzchild radius". (dont know if that is spelled right).

Second, my understanding of a black hole is that it is matter packed so tightly that the gravitional force overwhelms all other repulsive forces. However, would it be possible to "break" a black hole? As in smashing some highly velocity particles into it intentionally giving the internal components enough energy to break apart?

16. ### letheRegistered Senior Member

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yes, the Schwartzschild radius (at least i think that is how its spelled) is 2GM/c^2. so if you have mass M in a region of that radius or less, you have a black hole. this is the radius where the event horizon is found.

simple answer is: it is not known. what exactly happens to the matter in a black hole is not clear. the classical theory says that all the matter collapses to the singularity, a single geometric point, overriding all other forces. as you approach the singularity, the spacetime curvature becomes infinite.

but the classical theory doesn t really apply to such strong regions of gravitation. one needs to take quantum effects into account.

is the singularity forever? can you somehow smash the black hole apart? what happens when the black hole evaporates? is there matter left over?

a theory of quantum gravity should be able to answer these questions. heck, maybe string theory has answers to these questions, i m not sure.

17. ### ScRaMbLeChaos Inc.Registered Senior Member

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Ok, but how can mass be reduced to a single dimension? I'm not looking for proofs but you guys must have your opinions on what happens.
If the mass disappears doesnt this break the law of conservation of energy? If it doesn't cease to exist and is merely converted into pure energy then does this mean that gravity cannot be attributed to mass alone? "Fossil force" sounds like a cop out to me, can you explain the reasoning behind it?

18. ### MarkRegistered Senior Member

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You ask for opinions. A black hole is a GR thing (predicted by General Relativity and to some extent confirmed by observations) and so to understand it you need to quantize GR which stringy theories dont try to do.

(they dont preserve the essential background independence of GR, the dynamic geometry)

The place to look for answers is probably in the LQG direction. There has been some success in quantizing General Relativity and removing the big bang singularity.
Also there are results on all ordinary sorts of black holes
(stringy approaches deal with very peculiar black holes with a high electric charge, "extremal" or near-extremal BH, not the common garden-variety Schwarzschild etc.)

Loop Gravity researchers have not removed the BH singularity yet. But you can see how it is expected to go: The Big Bang singularity was proved not to exist----there is a collapse down to a minimally-allowed volume and then a bounce. Quantizing volume and density leads to a maximum density (or minimum volume per amount of energy) so the prior phase of the universe could not actually collapse to a point but had to bounce.

There are dozens of journal articles about this, it has been a hot research topic for 2 or 3 years now. Just search arxiv for stuff by Bojowald. the main author's name will do. Or even google Bojowald. You get articles like "Absence of Singularity in Loop Quantum Cosmology"

The singularity in a black hole will probably yield to Loop Gravity analysis for the same reason-----a quantum bound on density.
Also the spin network description of the quantum states of space has all matter particles live on nodes of the graph and as the nodes collapse one can imagine reaching a state where every node is occupied by a particle----planck density---a new state of matter perhaps. This is just speculation (but you asked for our opinion!)----I suspect that spin networks will not only override the classical BH singularity but provide a picture of the collapse process and what actually goes on inside the BH event horizon.

For a good introductory discussion of the spin network model of space, read the January 2004 Scientific American article by Lee Smolin.

19. ### letheRegistered Senior Member

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the singularity is a point. a point is zero dimensional, so the mass is not reduced to a single dimension, but in fact to no dimensions at all.

how is this possible? well, most people believe that it is not. most people believe that classical GR is wrong, and must be quantized.

20. ### ScRaMbLeChaos Inc.Registered Senior Member

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Oops, I meant zero dimensional (brain fade) cheers for the correction.

The singularity in a black hole will probably yield to Loop Gravity analysis for the same reason-----a quantum bound on density.
Also the spin network description of the quantum states of space has all matter particles live on nodes of the graph and as the nodes collapse one can imagine reaching a state where every node is occupied by a particle----planck density---a new state of matter perhaps. This is just speculation (but you asked for our opinion!)----I suspect that spin networks will not only override the classical BH singularity but provide a picture of the collapse process and what actually goes on inside the BH event horizon.

Couldn't this only occur at zero deg kelvin tho, as there would be no room for kinetic energy movement? I dont understand all the terms used in the above paragraph, tho, so I've prob mis-interpreted...

edit...forgot "quotes" above