# Black hole anatomy

by bill alsept
Tags: anatomy, black, hole
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 P: 124 I understand pressure is required to supports a balloons surface but not so simple on a bowling ball. Why is pressure needed on a complely solid object?
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 Quote by bill alsept I understand pressure is required to supports a balloons surface but not so simple on a bowling ball. Why is pressure needed on a complely solid object?
Well, consider a chunk of the matter. There will be a force inward caused by the gravitational attraction. If that force is not balanced by something, that force inward will cause the matter to accelerate inward. So if the matter is not collapsing, there must be a force outward to counterbalance the inward force of gravity. That force is a pressure.
P: 1,261
 Quote by Chalnoth While it is indeed true that General Relativity provides a sensible description of space-time inside the event horizon but outside the singularity, this doesn't mean we can trust it. In order to avoid the singularity, after all, General Relativity has to give an incorrect description of the black hole some distance outside that singularity. How far outside? We don't yet know.
That's a really good point. (My personal guess it much closer to the singularity ;)
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 Quote by zhermes That's a really good point. (My personal guess it much closer to the singularity ;)
For a slightly more detailed answer as to why I think it may need modification even just inside the event horizon, consider this.

From the point of view of the outside observer, an infalling object actually never crosses the event horizon. In fact, Hawking radiation causes the black hole to become smaller before the infalling object ever reaches it. So, what happens to the object if it is never allowed to even cross the event horizon?

Note that if we accept classical General Relativity, from the point of view of the infalling object, it does crash into the singularity in finite time. And this is also what happens even if the black hole is an evaporating black hole that has always existed. Last time I looked this up, however, nobody had managed to figure out what happens in a black hole that forms and evaporates in finite time with General Relativity.

Finally, consider that the Hawking Radiation encodes the information of whatever fell into the black hole, so that the radiation which leaves is physically connected to matter that entered into the black hole.

So my supposition is that the black hole can actually be seen as sort of a collision of matter occurring with an extreme amount of time dilation that is so destructive that it almost perfectly thermalizes any and all matter which enters the collision. This is, however, just supposition.
P: 124
 Quote by Chalnoth Well, consider a chunk of the matter. There will be a force inward caused by the gravitational attraction. If that force is not balanced by something, that force inward will cause the matter to accelerate inward. So if the matter is not collapsing, there must be a force outward to counterbalance the inward force of gravity. That force is a pressure.
Maybe the smallest particles in the universe only have space between them because they are each liberated and going through some repeated cycles of their own. When they are finally corralled and pushed together to the point they can no longer be liberated or move they loose any effect or phenomena they caused before. Could it even be possible that a black hole goes completely cold at the center? And would need no support again leading to the idea that the singularity may have a diameter instead of being a point?
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 Quote by bill alsept Maybe the smallest particles in the universe only have space between them because they are each liberated and going through some repeated cycles of their own. When they are finally corralled and pushed together to the point they can no longer be liberated or move they loose any effect or phenomena they caused before. Could it even be possible that a black hole goes completely cold at the center? And would need no support again leading to the idea that the singularity may have a diameter instead of being a point?
This is a real phenomena, and it happens inside white dwarfs and neutron stars. It's known as degeneracy pressure, because the fermions that make up these particles cannot occupy the same space at the same time. But this degeneracy pressure is limited, and eventually it is simply insufficient to keep these objects from collapsing inward. In the case of the white dwarf, it is the degeneracy pressure of the electrons in the atoms that make up the star. When that pressure gets too great, the electrons combine with the neutrons to form a neutron star. When the neutron degeneracy pressure of the neutron star is insufficient, it collapses to form a black hole. And no amount of pressure can prevent the collapse of the matter inside the event horizon.
 P: 124 Sorry, I am not trying to beat a dead horse here but I am talking about after neutron degeneracy pressure of the neutron star and any other stages of collapse a body of mass may go through including the stage of converting to a black hole. Which I think is the same as all the other stages where the escape velocity just rises another notch. In this case it rises above the speed of light. But after that stage what stages are there and eventually you get to a particle that are so much smaller than anything else. There would be no other place to collapse to.
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 Quote by bill alsept Sorry, I am not trying to beat a dead horse here but I am talking about after neutron degeneracy pressure of the neutron star and any other stages of collapse a body of mass may go through including the stage of converting to a black hole. Which I think is the same as all the other stages where the escape velocity just rises another notch. In this case it rises above the speed of light. But after that stage what stages are there and eventually you get to a particle that are so much smaller than anything else. There would be no other place to collapse to.
Except that not all matter is fermionic. If the particles collapse to a bosonic state, then they can collapse as far as you like.
 P: 720 Chalnoth what other matter is there besides fermions?
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 Quote by Tanelorn Chalnoth what other matter is there besides fermions?
At the very least, the photon, gluon, W and Z bosons, and the Higgs boson.
P: 1,261
 Quote by Chalnoth From the point of view of the outside observer, an infalling object actually never crosses the event horizon. In fact, Hawking radiation causes the black hole to become smaller before the infalling object ever reaches it. So, what happens to the object if it is never allowed to even cross the event horizon?
That's not an issue in GR. First, for all practical purposes the event horizon is going to increase is size from the newly accreted matter, not decrease in size---only the smallest of black-holes (with no known formation mechanism) decrease in size at all.
Second, the same argument would apply to the classic undergrad question 'how does the black-hole increase in mass at all?' --- which is a non-issue. The object crosses the event horizon without a problem, its just never observed.

 Quote by Chalnoth Note that if we accept classical General Relativity, from the point of view of the infalling object, it does crash into the singularity in finite time. And this is also what happens even if the black hole is an evaporating black hole that has always existed. Last time I looked this up, however, nobody had managed to figure out what happens in a black hole that forms and evaporates in finite time with General Relativity.
Yes, the singularity is of course an issue---but I don't see how adding a finite-aged black-hole complicates the issue. And note that you don't have evaporation with just general relativity.

 Quote by Chalnoth Finally, consider that the Hawking Radiation encodes the information of whatever fell into the black hole, so that the radiation which leaves is physically connected to matter that entered into the black hole.
Again, I don't see an issue here.

 Quote by Chalnoth So my supposition is that the black hole can actually be seen as sort of a collision of matter occurring with an extreme amount of time dilation that is so destructive that it almost perfectly thermalizes any and all matter which enters the collision. This is, however, just supposition.
I don't follow what you're saying.
P: 1,261
 Quote by bill alsept Sorry, I am not trying to beat a dead horse here but I am talking about after neutron degeneracy pressure of the neutron star and any other stages of collapse a body of mass may go through including the stage of converting to a black hole. .... But after that stage what stages are there and eventually you get to a particle that are so much smaller than anything else. There would be no other place to collapse to.
This is completely irrelevant as has already been stated many times. If you still don't understand, you should read a GR textbook. The nature of the material within the event horizon doesn't matter in general relativity. There is no such thing as 'a particle so much smaller than anything else'---you're argument is based on the premise that there is a fundamentally small thing, which cannot get smaller; you are then using that as an argument for the same point.
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 Quote by zhermes Yes, the singularity is of course an issue---but I don't see how adding a finite-aged black-hole complicates the issue. And note that you don't have evaporation with just general relativity.
The potential complication is that it is apparently unknown whether or not a singularity would form for a finite-aged black hole (in this case singularity simply meaning dense clump of matter at the center, instead of a mathematical singularity, since we'd be talking about a real black hole). That is, the time dilation may be so severe that the matter inside the event horizon just doesn't have enough time to collapse any significant amount before the black hole evaporates.

And yeah, obviously I was talking about a semi-classical black hole with regard to evaporation, which uses General Relativity to define the space-time but adds an evaporation mechanism.
P: 1,261
 Quote by Chalnoth The potential complication is that it is apparently unknown whether or not a singularity would form for a finite-aged black hole (in this case singularity simply meaning dense clump of matter at the center, instead of a mathematical singularity, since we'd be talking about a real black hole). That is, the time dilation may be so severe that the matter inside the event horizon just doesn't have enough time to collapse any significant amount before the black hole evaporates.
This is not an issue. Formation of black-holes and singularities has been thoroughly studied, by Papapetrou, Thorne, Penrose... Teukolsky I think. Again, in all astrophysically significant cases, evaporation can't occur----absorption of the CMB is far more rapid than the production of hawking radiation (and I can't recall for sure, but I think even very low ambient density environments, the bondi accretion is also more rapid than Hawking Radiation).
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 Quote by zhermes This is not an issue. Formation of black-holes and singularities has been thoroughly studied, by Papapetrou, Thorne, Penrose... Teukolsky I think.
Well, admittedly I haven't looked into this in great detail myself. But in this case I'm going off of my GR professor, Steve Carlip, who I think is generally quite good on this stuff. As far as I know, we're still a long way from really examining what GR has to say here for anything beyond the absolute simplest case. My understanding was that the only sort of black hole formation we could produce was due to symmetric infalling spherical shells, and that is a highly non-physical situation.

 Quote by zhermes Again, in all astrophysically significant cases, evaporation can't occur----absorption of the CMB is far more rapid than the production of hawking radiation (and I can't recall for sure, but I think even very low ambient density environments, the bondi accretion is also more rapid than Hawking Radiation).
Well, sure, but those accretion processes will all end in far, far less time than the expected black hole evaporation rate anyway. We're talking roughly $10^{66}$ to $10^{100}$ year timescales here. So I don't quite understand what this has to do with my point that we may not know much of anything about what's going on inside the black hole's event horizon.
P: 720
 Quote by Chalnoth At the very least, the photon, gluon, W and Z bosons, and the Higgs boson.
Chalnoth, are Bosons or force carriers conidered what is loosely called "ponderable matter". I am quite prepared to be wrong about this I just thought they werent.
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 Quote by Tanelorn Chalnoth, are Bosons or force carriers conidered what is loosely called "ponderable matter". I am quite prepared to be wrong about this I just thought they werent.
Well, they are either moving at the speed of light or are unstable. But when they are inside the event horizon of a black hole, moving at the speed of light doesn't prevent collapse, and the extreme space-time curvature may potentially make the W and Z bosons stable, in an analogous way to how the incredibly high pressure inside a neutron star makes neutrons stable.

Edit: Of course, this almost certainly isn't enough to produce a degenerate bosonic state at the center of a black hole, because you also have to get rid of the baryons, and baryon number is a conserved quantity in known physics. So we would probably need some beyond-standard-model interactions to get into a degenerate bosonic state, and that may open the door for still more bosons.
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It was specifically in reference to your comment that:
 Quote by Chalnoth That is, the time dilation may be so severe that the matter inside the event horizon just doesn't have enough time to collapse any significant amount before the black hole evaporates.
Those studies have shown singularity formation in finite coordinate (and proper) time---but you're right, I'm not sure exactly how long (but I think its on the order of a dynamical time), and in how diverse situations; good keeping me on my toes.

But going back to the initial point; the event horizon can definitely form stably. We agree that the singularity itself is completely questionable---so that part doesn't really matter. My overall argument is that we are really really confident event horizons can form; and quantum mechanics (string theory etc etc) only suggest GR breaks something like 50 orders of magnitude closer to the center of the BH than the event horizon... so there doesn't seem to be a reason to expect GR to break near the EH...

(Unless you're looking at a near-extremal kerr black-hole---which we believe exist---in which case the singularity itself can be very near the event horizon.

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