Black hole anatomy

Chalnoth

The only thing that sets how far it can collapse is the supporting pressure.

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?

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.

zhermes

That's a really good point. (My personal guess it much closer to the singularity ;)

Chalnoth

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.

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?

Chalnoth

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.

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.

Chalnoth

Except that not all matter is fermionic. If the particles collapse to a bosonic state, then they can collapse as far as you like.

Tanelorn

Chalnoth what other matter is there besides fermions?

Chalnoth

At the very least, the photon, gluon, W and Z bosons, and the Higgs boson.

zhermes

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.

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.

Again, I don't see an issue here.

I don't follow what you're saying.

zhermes

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.

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.

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.

zhermes

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).

Chalnoth

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.

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 [itex]10^{66}[/itex] to [itex]10^{100}[/itex] 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.

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.