If no singularity, what’s inside a big black hole?

[jimgraber]

Both string theory and loop quantum gravity claim possible elimination of the black hole singularities. If that is true, what do they predict the inside of a stellar size black hole contains? Is it some new ultra dense state of matter, or something else?

I will try to ask various authorities this question at the APS meeting in St. Louis next week. But what’s your opinion? Has anything been published?

The only concrete proposal I am aware of is the Mathur fuzzball (hep-th/0502050).

Jim Graber

 

[shalayka]

http://arxiv.org/abs/0712.0817

"We show that the singularity is replaced by a bounce at which quantum effects are important and that the extent of the region at the bounce where one departs from classical general relativity depends on the initial data."

Might be helpful to you.

 

[marcus]

Good suggestion Shalayka!
http://arxiv.org/abs/0712.0817 is a recent paper of Gambini Pullin and Campiglia.
Jim Graber, you ask "Has anything been published?" Plenty has been published in that general direction. here are some references. This is far from complete. I am excluding papers that deal only with the horizon or the exterior. This is a sample of loop papers having to do with the black hole interior.

A fair number of these papers were published in Physical Review Letters or in Physical Review D.
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+a+Bojowald+AND+DK+BLACK+HOLE+AND+DATE+%3E2004&FORMAT=www&SEQUENCE=citecount%28d%29

1) Quantum geometry and the Schwarzschild singularity.
Abhay Ashtekar (Penn State U. & Potsdam, Max Planck Inst.) , Martin Bojowald (Potsdam, Max Planck Inst. & Penn State U.) . IGPG-05-09-01, AEI-2005-132, Sep 2005. 31pp.
Published in Class.Quant.Grav.23:391-411,2006.
e-Print: gr-qc/0509075
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 48 times

2) Black hole evaporation: A Paradigm.
Abhay Ashtekar (Penn State U.) , Martin Bojowald (Potsdam, Max Planck Inst. & Penn State U.) . IGPG04-8-4, AEI-2004-072, Apr 2005. 18pp.
Published in Class.Quant.Grav.22:3349-3362,2005.
e-Print: gr-qc/0504029
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 39 times

3) A Black hole mass threshold from non-singular quantum gravitational collapse.
Martin Bojowald (Potsdam, Max Planck Inst.) , Rituparno Goswami (Tata Inst.) , Roy Maartens (Portsmouth U., ICG) , Parampreet Singh (Penn State U.) . AEI-2005-020, IGPG-05-3-3, Mar 2005. 4pp.
Published in Phys.Rev.Lett.95:091302,2005.
e-Print: gr-qc/0503041
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 34 times

4) Nonsingular black holes and degrees of freedom in quantum gravity.
Martin Bojowald (Potsdam, Max Planck Inst.) . AEI-2005-115, Jun 2005. 4pp.
Published in Phys.Rev.Lett.95:061301,2005.
e-Print: gr-qc/0506128
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 27 times

5) Spherically symmetric quantum geometry: Hamiltonian constraint.
Martin Bojowald, Rafal Swiderski (Potsdam, Max Planck Inst.) . AEI-2005-171, NI05065, Nov 2005. 33pp.
Published in Class.Quant.Grav.23:2129-2154,2006.
e-Print: gr-qc/0511108
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 19 times

Some more here:
FIND DK LOOP SPACE AND DK BLACK HOLE AND DATE >2005

6) Wave functions for the Schwarzschild black hole interior.
Daniel Cartin (Naval Acad. Prep. School, Newport) , Gaurav Khanna (Massachusetts U., North Dartmouth) . Feb 2006. 14pp.
Published in Phys.Rev.D73:104009,2006.
e-Print: gr-qc/0602025
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 5 times

9) Loop Quantum Dynamics of the Schwarzschild Interior.
Christian G. Boehmer (University Coll. London & Portsmouth U., ICG) , Kevin Vandersloot (Portsmouth U., ICG & Penn State U.) . Sep 2007. 15pp.
Published in Phys.Rev.D76:104030,2007.
e-Print: arXiv:0709.2129 [gr-qc]
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 5 times

10) Gravitational collapse in loop quantum gravity.
Leonardo Modesto (Bologna U. & INFN, Bologna) . Oct 2006. 16pp.
Published in Int.J.Theor.Phys.47:357-373,2008.
e-Print: gr-qc/0610074
References | LaTeX(US) | LaTeX(EU) | Harvmac | BibTeX | Keywords | Cited 4 times

 

[jimgraber]

shalayka and Marcus,
Thank you for the very good references. I have printed them out and am starting to study them. At first glance, the answer to my question which most of these papers seem to be promoting is a very dense Planck scale "spacetime foam" or "quantum geometry breakdown of spacetime" surrounded by a Kantowski-Sachs vacuum solution. Do you agree?
Thanks again.
Jim Graber

 

[marcus]

shalayka and Marcus,
Thank you for the very good references. I have printed them out and am starting to study them. At first glance, the answer to my question which most of these papers seem to be promoting is a very dense Planck scale "spacetime foam" or "quantum geometry breakdown of spacetime" surrounded by a Kantowski-Sachs vacuum solution. Do you agree?
Thanks again.
Jim Graber

Heavens! I don't know as I would recommend anyone to read these articles. Except what Shalayka said, maybe. The Gambini Pullin Campiglia one is recent and IIRC comparatively brief and clear. Jim I admire your energy and curiosity and I am very glad you plan to ask the appropriate people at the APS meeting.

Jim you asked was anything published about this? I listed that stuff just to show concretely a sample of what had been published. But I don't BELIEVE we humans have any very good notion of what happens down a black hole and all this work must necessarily be preliminary. The people doing it, if they are good, probably realize this better than anyone else.

What I believe is that geometry and matter are fundamentally the same and arise from the same basic microscopic D.O.F. stuff.
When it is very compressed, the distinction between matter and space disappears and one gets down to a stew of microscopic degrees of freedom, which we don't yet know how to model mathematically.

I believe it is the same kind of stuff that was there at bigbang time, at the beginning of expansion. That is, the stuff precedes classical cosmology. And the reason people call it FOAM is because by Heisenberg the geometry would have been very chaotic and unsmooth and hard to pin down.

The endpoint of something more and more complicated is something perfectly simple---call it foam, or Planck goo, or the Fire of Heracleitus. Or don't call it anything because we don't yet have a credible mathematical model so there is no convincing metaphor. It isn't "like" anything, yet.

that's just my attitude for the time being.

========================
Heracleitus born c. 540 BC, Ephesus, in Anatolia [now Selçuk, Tur.] died c. 480

Greek philosopher remembered for his cosmology, in which fire forms the basic material principle of an orderly universe. Little is known about his life, and the one book he apparently wrote is lost. His views survive in the short fragments quoted and attributed to him by later authors...

 

[BenTheMan]

Hi jim---

Marcus is a bit biased towards one way of thinking (as I am, admittedly). You may find more interesting the following paper about the ``fuzzball'' paradigm proposed by Samir Mathur.

http://arxiv.org/abs/0804.0552

The picture solves many of the problems with traditional black hole physics, some of which are generically present in other approaches to quantum gravity.

 

[shalayka]

One other point of view given recently is that of Christoph Schiller (the person responsible for the free physics textbook "Motion Mountain").

http://www.motionmountain.net/research/research.html

In the second paper of his series of four ("General relativity, gravitons and cosmology deduced from extended entities") he goes on to describe a method which logically removes black hole and universal (big bang) singularities.

Opinion may vary, and as far as I know, this work is not peer-reviewed yet. Even then, I found that it may be helpful to describe the thought patterns behind attempts at quantizing spacetime, and is also a good review of the more important tried-and-"true" equations related to General Relativity.

From what I can gather, Schiller's method may classify as a fuzzball.

P.S. I am naturally biased towards fuzzball methods mostly because I really love cats and they are generally cute little fuzzballs as well. Except for those hairless ones -- they're more like a naked singularity, I guess. :)

 

[jimgraber]

Quantum gravity singularity elimination talks.

I did hear Abhay’s and Gary’s talks yesterday. Basically, nothing was said that has not previously been published. Both agreed that singularities are probably eliminated by quantum gravity. Abhay talked mostly about cosmology, but said during the question period that similar things applied to black holes but the work was not as advanced and that the picture on the post collapse side still needed details worked out. But he said he was sure there was no singularity. Gary also said recent work favored no singularities, but he said he only had two strong arguments, not a proof. One argument, primarily due to Eva Siverstein, was perturbative and based on tachyon condensation smoothly pinching off the space before the singularity was reached. The other nonperturbative argument was based on Maldacena duality and concluded that a black hole could not exist because an infinite redshift event horizon could not exist without contradicting the Maldacena dual description. During the comment period Abhay said he doubted the second argument because it was too dependent on analyticity. He went on to say however that he also thought there was no event horizon, only an isolated horizon or a dynamic horizon. I had read or heard this before, but I thought this was only a mathematical technicality. However, Abhay seemed to think it was a necessary part of singularity elimination. I’ll probably post more later about the size of the “Planck goo”.
The key phrase is “Planck density”, not “Planck length.”
Bye for now.
Jim Graber

 

[marcus]

Thanks! It's nice to get an up-to-date report.

 

[Bernie G]

I think the question basically is: Can intense radiation pressure be the support mechanism inside a black hole? I think it is logical that when a star above several solar masses collapses, the neutrons in the core disintegrate into radiation and some quark matter. As the collapse continues and temperature rises still further virtually all matter converts to radiation. If the radiation is contained in the system, the pressure of the radiation should be P = pc*2 , where p is the equivalent mass density of the radiation. The contained radiation, which has mass, basically acts like a compressed gas that can generate pressures exceeding neutron collapse pressure.

As I understand the TOV equation, dP/dr is proportional to p + P, which means if P is high enough there is runaway collapse. I don't think Einstein accepted this equation because he didn't believe in a point singularity.

 

[tom.stoer]

According to LQG the interior of a black hole horizon is nothing else but a huge intertwiner, which means ordinary space.

 

[marcus]

According to LQG the interior of a black hole horizon is nothing else but a huge intertwiner, which means ordinary space.

That reminds me of a paper by Krasnov and Rovelli where they concluded that "for an observer at infinity" the black hole was described somewhat in that way. The Hilbert space of BH states was the space of intertwiners of some size determined by the area of the horizon. So a BH state (for an observer at infinity) would be an intertwiner.

But maybe there isn't yet one clear answer in the LQG context. I did a spires search on keywords "black hole and quantum gravity, loop space" and came up, for instance, with this:

http://arXiv.org/abs/0805.1187
Black holes in loop quantum gravity: the complete space-time
Rodolfo Gambini, Jorge Pullin
4 pages, 2 figures
(Submitted on 8 May 2008)
"We consider the quantization of the complete extension of the Schwarzschild space-time using spherically symmetric loop quantum gravity. We find an exact solution corresponding to the semi-classical theory. The singularity is eliminated but the space-time still contains a horizon. Although the solution is known partially numerically and therefore a proper global analysis is not possible, a global structure akin to a singularity-free Reissner--Nordström space-time including a Cauchy horizon is suggested."

The Krasnov Rovelli paper gives a different picture (specialized as it is from a particular observer's point of view):
http://arXiv.org/abs/0905.4916
Black holes in full quantum gravity
Kirill Krasnov, Carlo Rovelli
5 pages
(Submitted on 29 May 2009)
"Quantum black holes have been studied extensively in quantum gravity and string theory, using various semiclassical or background dependent approaches. We explore the possibility of studying black holes in the full non-perturbative quantum theory, without recurring to semiclassical considerations, and in the context of loop quantum gravity. We propose a definition of a quantum black hole as the collection of the quantum degrees of freedom that do not influence observables at infinity. From this definition, it follows that for an observer at infinity a black hole is described by an SU(2) intertwining operator. The dimension of the Hilbert space of such intertwiners grows exponentially with the horizon area. These considerations shed some light on the physical nature of the microstates contributing to the black hole entropy. In particular, it can be seen that the microstates being counted for the entropy have the interpretation of describing different horizon shapes. The space of black hole microstates described here is related to the one arrived at recently by Engle, Noui and Perez, and sometime ago by Smolin, but obtained here directly within the full quantum theory."

The spires search, if anyone wants to see all the LQG black hole papers with date > 2004:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+gravity%2C+LOOP+SPACE+AND+DK+BLACK+HOLE+AND+DATE+%3E2004&FORMAT=www&SEQUENCE=citecount%28d%29
Spires finds 76 papers

 

[marcus]

Tom, I decided there was too much old stuff in that search, given how much the field has changed in the past 3 years. So instead of setting the date at 2004, I changed to 2007:

http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+AND+DK+BLACK+HOLE+AND+DATE+%3E2007&FORMAT=www&SEQUENCE=citecount%28d%29

Now it gives 45 papers and all have the date > 2007.

It may surprise readers to see which papers are the most-cited. The more highly cited ones are listed first.

 

[Ben Niehoff]

Both string theory and loop quantum gravity claim possible elimination of the black hole singularities. If that is true, what do they predict the inside of a stellar size black hole contains? Is it some new ultra dense state of matter, or something else?

I will try to ask various authorities this question at the APS meeting in St. Louis next week. But what’s your opinion? Has anything been published?

The only concrete proposal I am aware of is the Mathur fuzzball (hep-th/0502050).

Jim Graber

Hi jim---

Marcus is a bit biased towards one way of thinking (as I am, admittedly). You may find more interesting the following paper about the ``fuzzball'' paradigm proposed by Samir Mathur.

http://arxiv.org/abs/0804.0552

The picture solves many of the problems with traditional black hole physics, some of which are generically present in other approaches to quantum gravity.

I am actively working on this fuzzball thing. We are able to find what we call "bubbling solutions" for very special kinds of 5-dimensional black holes. These are solutions that from infinity look like a black hole, but the geometry is regular everywhere and there are no horizons. Instead of a singularity, the solutions have a deep throat with pieces of nontrivial topology at the bottom, which we call "bubbles". There are no pointy bits (singularities), but only smooth bits of 2-homology.

The idea is that these solutions represent microstates, and the horizon of a physical black hole arises as an emergent phenomenon when you consider the statistical fluctuations of the geometry at the bottom of the throat. Here is the canonical review paper, if you're interested:

http://arxiv.org/abs/hep-th/0701216

Right now we're working on finding bubbling solutions for slightly less special kinds of black holes...however, this is quite a bit more difficult, as the equations get rather nasty.

 

[unusualname]

Motl had an interesting blog article about this, a while back, in relation to whether AdS/CFT can say anything interesting, he concluded "At the center of black holes, something bad is happening to time which is much more drastic an event than when space shrinks..."

 

[lmodesto]

There are at least a couple of papers where
(in a particular model inspired by LQG) to find the answer :
1) arXiv:0905.3170
Self-dual Black Holes in LQG: Theory and Phenomenology
Leonardo Modesto, Isabeau Prémont-Schwarz
Journal-ref: Phys.Rev.D80:064041,2009;
2) arXiv:0811.2196
Space-Time Structure of Loop Quantum Black Hole
Leonardo Modesto Int.J.Theor.Phys.49:1649-1683,2010.

 

[Bernie G]

I'm not thrilled with the theories about what's inside a black hole. A point singularity seems to be one of the worst explanations; where there is a lack of understanding a poor simplification is given as logical.

 

[mickyboy]

just an idea, but what if a black hole is created by the universe moving outward like currents create whirlpools and that the singularity is not there at all.but instead it is spat back out the otherside to create a new energised matter. I also read a theory about the big bang starting from a singularity if that were the case wouldn't the universe be going the complete other direction, being drawn into, not being pushed away.just thinking.

 

[Bernie G]

That sounds too weird. I think simply a radiation ball exists inside the Schwarzschild Radius, with a density profile similar to that of a conventional star (denser closer to the core). I think this because neutrons can not exist at these high pressures and energies. Radiation pressure would be the supporting mechanism. This might be confirmed by observation within 20 years by observing the merger of 2 typical 8 solar mass black holes in other galaxies. (Roughly perhaps 100 mergers of neutron stars and black holes occur annually, so about one black hole - black hole merger should occur annually.) If the radiation ball size is ≥ 70% of the Schwarzschild radius, a large observable burst of radiation will occur, but if the black holes are a point singularity nothing will be ejected of course. I hope to have a good estimate soon for the size of this theoretical radiation ball, but have to get a formula for the radiation pressure P first. P should be equal to (1/3)pc^2 or (2/3)pc^2 or pc^2, where p is the radiation density.

 

[tom.stoer]

There are several proposals from ceratin theories of quantum gravity, i.e. the fuzzball proposal in string theory and a 'huge intertwiner' in loop quantum gravity. In both cases the singularity is resolved by quantum effects.

 

[tanzanos]

If the mass that makes it past the event horizon end up in an infinite spiral then how can we account for the massive gravitational force of a black hole? On the other hand if there is no singularity and no infinite spiral then again where does the gravity emanate from?

Does this mean that there is no limit to the mass that a black hole can "ingest"?
Surely mass cannot be infinitely compressed?

What if information remains on the event horizon but the mass disappears either in an endless spiral or ends up elsewhere then would it be wrong to claim that the G forces are emanating not from the core of the black hole but the event horizon where information remains?

Pardon my ignorance but although I am not a physicist I am still fascinated by these glorious beasts.

 

[phinds]

Does this mean that there is no limit to the mass that a black hole can "ingest"?
Surely mass cannot be infinitely compressed?
.

I can't see that infinite compression follows from more and more mass. Why do you think it should? "Infinite" is a word you need to use with great care.

 

[tanzanos]

I can't see that infinite compression follows from more and more mass. Why do you think it should? "Infinite" is a word you need to use with great care.

Ok I see what you mean so let me put it differently: is there a limit where mass cannot be compressed more? If yes then black holes cannot contain a singularity and continue to feed indefinitely. Perhaps you may shed some light on this?

Thanks!

 

[med17k]

I thought that superstring theory considers a black hole to be a p brane vibrating at a very high frequency .

 

[phinds]

Ok I see what you mean so let me put it differently: is there a limit where mass cannot be compressed more? If yes then black holes cannot contain a singularity and continue to feed indefinitely. Perhaps you may shed some light on this?

Thanks!

No, as far as I am aware, there is absolutely no reason why black holes cannot contintue to accrete matter. I don't believer there is an upper limit on mass or any reason why there should be. Mass just won't compress beyond a certain point, but that doesn't mean you can't add more.

Also, there are theories that the mass-singularity at the heart of a black hole is NOT actually what's there and that it becomes a dense soup of radiation. That may or may not be, but in any case do NOT let it lead you off into crackpot land where the singularity that started out universe was a black hole. Black holes don't have THAT much mass.

 

[tanzanos]

That may or may not be, but in any case do NOT let it lead you off into crackpot land where the singularity that started out universe was a black hole. Black holes don't have THAT much mass.

True but if the universe were to experience a "Big Crunch" then suffice it to say a super impossibly dense black hole would arise. For as you say there is no limit on how much mass can be compressed then all the mass of the universe can be compressed into a singularity? Of course we are not sure in what way the end of the universe will be but were it to contract back then it is possible and upon evaporation a point will be reached where the singularity will release the mass in a "Big bang" thus repeating a cycle?

Just asking!

 

[phinds]

True but if the universe were to experience a "Big Crunch" then suffice it to say a super impossibly dense black hole would arise ...

Well the original singularity was a dense soup of radiation with no mass at all, so why would a big crunch necessarily be any different?

EDIT: actually that's not a correct statement. What I should say is that no more than one Plank time AFTER the singularity, the universe was a ... (we don't know WHAT the singularlity was or was like --- maybe if we ever get a viable theory of quantum gravity ... )

 

[Naveen123]

Does science offer any explanation or conjecture as to why the initial 'singularity' exploded.
Thanks.

 

[tom.stoer]

As of today there is no explanation, but there are some research directions indicating what could happen.

For example in LQC which is a "symmetry reduced" "approximation" to LQG one finds something like a "cyclic universe" where the big bang singularity is replaced by a big bounce. That means that a collapsing universe is bouncing back due to repulsive quantum gravity near Planck scale. It is problemativ to described this classically as the bounce itself is a quantum effect where notions like space and time do no longer apply.

(We should be rather careful as this is work in progress and by no means a well-established fact)

 

[Bernie G]

To continue on the size of the theoretical radiation ball inside a black hole: The radiation will generate a pressure proportional to density and therefore should have the same density profile as a conventional gas star, varying about as 1/(r^2). The gravitation potentional energy of this density profile is about (2GM^2)/R.

If the results of the viral theorem can be used here (and maybe the viral therom can’t be used here because the supporting energy is always constant here regardless of size), the supporting energy would equal half the gravitational potential energy, or (2GM^2)/2R = (GM^2)/R. If the supporting energy is (1/3)Mc^2, R would equal (3GM)/(c^2), which doesn’t work because R then would be larger than the Schwarzschild radius. I think the most likely value for supporting energy would be (2/3)Mc^2 since the measured value of reflected radiation pressure is (2/3)Mc^2 from laboratory measurements. If the supporting energy is (2/3)Mc^2 and half the gravitational energy is (GM^2)/R, then the radius R of the radiation ball would equal (3GM)/(2c^2).

Note that (3GM)/(2c^2) is 75% of the Schwarzschild radius, which means a large observable burst of radiation should occur when 2 typical 8 solar mass black holes merge. I hope this is confirmed by observation in the future.

 

[Bernie G]

I'm not terribly happy about the possible calculation above for the size of a theoretical radiation ball inside the Schwarzschild radius. Does anyone have a better simple non-relativistic formula for the radius of a gravitationally formed sphere of Mass M, with a density profile of 1/r^2, and a supporting pressure of (1/3)pc^2 or (2/3)pc^2 or pc^2, where p is the radiation density?

 

[Bernie G]

Maybe the maximum fraction of energy that can exerted by a neutron to generate pressure is (1/3)Mc^2 or (2/3)Mc^2, and above that a neutron disintegrates.

 

[mjacobsca]

There are theories for quark stars and preon stars, so why not further degenerate states such as photon-degenerate stars, string-degenerate, etc...? I suppose a photon-degenerate star would technically be a radiation soup as suggested above. But my gut says that the actual center of a black hole is going to be no more interesting than the center of any other degenerate star. No infinity. No wormholes. No baby universes. Just plain old photons/other tiny particle stuck together by gravity with a big old beware-of-danger sign on their front porch. I'm no expert, but it seems a degenerate state of matter beyond from which light cannot escape is simple, logical, and plausible without blowing up physics as we know it.

 

[tom.stoer]

There are theories for quark stars and preon stars, so why not further degenerate states such as photon-degenerate stars, string-degenerate, etc...?

There are such theories, i.e. fuzzballs in string theory.

Just plain old photons/other tiny particle stuck together by gravity ... but it seems a degenerate state of matter beyond from which light cannot escape is simple, logical, and plausible without blowing up physics as we know it.

The problem is that we do not know any force in nature that is able to stabilize this state. There are indications coming from quantized gravity (strings, loops) that gravity itself could do the job, but this is still work in progress.

 

[Bernie G]

Yes, the "relativists" say that inside the Schwarzschild radius a photon must have energy greater than mc^2 if it is not to proceed inexorably towards the center. My experience is they do not accept that other forces (such as radiation pressure) can overcome or balance the force of gravity inside the Schwarzschild radius. One possible error in this way of thinking is that just inside the Schwarzschild radius of a large black hole, the gravitational force is much weaker than that just inside the Schwarzschild radius of a small black hole. This might be settled someday if and when the effects of the merger of 2 roughly equal size black holes are observed. If there is a large radiation burst the relativists will not be able to explain it. In the meantime they will give each other praise and awards.

 

[phinds]

... In the meantime they will give each other praise and awards.

This point of view seems to me to demonstrate a totally unwarrented distain for scientists and the scientific method. Is that in fact how you feel or am I misinterpreting you?

 

[tom.stoer]

My experience is they do not accept that other forces (such as radiation pressure) can overcome or balance the force of gravity inside the Schwarzschild radius. One possible error in this way of thinking is that just inside the Schwarzschild radius of a large black hole, the gravitational force is much weaker than that just inside the Schwarzschild radius of a small black hole.

Either have to explain (e.g.) the radiation pressure in terms of the energy-momentum tensor, or you have to explain weakening of the gravitational force in terms of the gravitational field. As soon as you an derive a repusive effect based on the Einstein equations this will sound convincing. But as long as no such calculation exists we have to accept the collaps.

btw.: we know about quantum corrections of Einstein equations in certain quantum gravity models (e.g. LQC) which result in a to a "short-range repulsive core of the gravitational potential".

 

[IttyBittyBit]

According to the holographic principle, every bit of information that there is to know about a black hole is encoded on it's even horizon (albeit in a very mixed-up and highly entropic way). Given that we can never probe a black hole's interior, I was under the impression that the holographic principle implies that there is no 'inside' to a black hole. Asking what's inside a black hole is like asking what was there before the big bang. Correct me if I'm wrong.

 

[Bernie G]

If two relatively small equal size black holes merge, and IF each contains a radiation star with 80% of the radius of the Schwarzschild radius, at the point of contact of each star’s surface there will be no net gravitational force and a large radiation burst will occur. Probably each star would only have to be only 70% of the radius Schwarzschild radius for a radiation burst to occur because of the bulging effect at each surface as each black hole approaches the other. Therefore its possible to get information out of a black hole, but it would require contact with another black hole. Its true I don’t have a lot of respect how awards are given out nowadays, not just scientific awards but also in many other fields. Often its a one hand washing the other relationship. The Nobel prize has even been corrupted. But I should not have made the praise and awards comment as it is a distraction from the technical discussion.

 

[Bernie G]

I think the following figures are roughly correct: One of the sources of gamma ray bursts may be the merger of orbiting black hole - neutron star pairs in other galaxies, and perhaps about a hundred of these occur annually. If about 1% of this number is the merger of orbiting black hole - black pairs, then about one of these observable BH-BH mergers should occur annually.

 

[Bernie G]

The last sentence should have read: If about 1% of this number is the merger of orbiting black hole - black pairs, then about one of these BH-BH mergers should occur annually, and possibly are observable by a gamma ray burst.

 

[tom.stoer]

According to the holographic principle, every bit of information that there is to know about a black hole is encoded on it's even horizon (albeit in a very mixed-up and highly entropic way). Given that we can never probe a black hole's interior, I was under the impression that the holographic principle implies that there is no 'inside' to a black hole. Asking what's inside a black hole is like asking what was there before the big bang. Correct me if I'm wrong.

You must distinguish between "representation" of the information inside the BH on the EH and the interior itself. According to GR an astronaut could fall into a BH and would while crossing the horizon of a sufficiently large BH) not feel or see anything special.

 

[genneth]

You must distinguish between "representation" of the information inside the BH on the EH and the interior itself. According to GR an astronaut could fall into a BH and would while crossing the horizon of a sufficiently large BH) not feel or see anything special.

I've always thought that this might just be a pathology of classical GR.

As Ashtekar is keen to point out, the correct concept to think about wrt. horizons is that of isolated or dynamic horizon, if for no other reason than that it is possible to do Hamilotian mechanics this way (conserved Louiville form, etc.). From the point of view of an asymptotic observer, nothing ever falls through the horizon. The quantum description of the spacetime for that observer should simply not include the space inside the horizon.

For an in-falling observer, the horizon should shrink due to radiative loss. This should mean that there are no isolated horizons, but *only* dynamic ones. Is it known what an in-falling observer would see of that horizon, as it evaporates? Does the observer then still ever cross it? My gut feeling is that actually, no --- the quantum effects will always hide the inside of the horizon from view, so all observers will only need a description of the outside, which is known (i.e. conjectured from non-quantum GR) to have a Hamiltonian description and thus be described by a quantum theory.

If you know of literature to answer this, I'd be fascinated. My Google-fu is weak, and I have yet to find anything...

 

[tom.stoer]

Afaik LQG does not say anything else but classical GR. It provides a quantum description of isolated horizons, but I see no reason why the large scale dynamics should change.

Regarding radiative loss: afaik there is no theory which is able to predict this radiation in the quantum gravity regime; Hawking result is restricted to classical GR. Regarding time scales: you can calculate the time for an infalling observer to cross the horizon and compare it with the time for complete evaporation. You will find that the time to cross the horizon is much smaller than the evaporation time.

The asmptotic observer at infinity is of no relevance for the pure observer crossing the horizon in finite proper time.

I don't think that any theory of quantum gravity will change this picture

 

[genneth]

Regarding radiative loss: afaik there is no theory which is able to predict this radiation in the quantum gravity regime; Hawking result is restricted to classical GR. Regarding time scales: you can calculate the time for an infalling observer to cross the horizon and compare it with the time for complete evaporation. You will find that the time to cross the horizon is much smaller than the evaporation time.

That evaporation time you refer to is measured by an asymptotic observer, but the proper time of an infalling observer is clearly not --- these two are not comparable. My point is that I think (and would like to be educated) that the calculations do not exist, but my grasp of the subject is not good enough to simply go and calculate it myself, or understand why such a calculation might be hard/ill-posed.

 

[IttyBittyBit]

You must distinguish between "representation" of the information inside the BH on the EH and the interior itself. According to GR an astronaut could fall into a BH and would while crossing the horizon of a sufficiently large BH) not feel or see anything special.

At the event horizon gravity becomes as strong as all the other forces. It is by definition in the realm of quantum gravity. I don't think GR, by itself at least, is really applicable to studying it, even though the concept of black holes originally arose from GR.

For an in-falling observer, the horizon should shrink due to radiative loss. This should mean that there are no isolated horizons, but *only* dynamic ones. Is it known what an in-falling observer would see of that horizon, as it evaporates? Does the observer then still ever cross it? My gut feeling is that actually, no --- the quantum effects will always hide the inside of the horizon from view, so all observers will only need a description of the outside, which is known (i.e. conjectured from non-quantum GR) to have a Hamiltonian description and thus be described by a quantum theory.

That's actually a very interesting viewpoint, and if true it strengthens my argument.

According to this page: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/fall_in.html, the event horizon keeps receding until it's a point, and that is the precise moment you 'fall' in. By the time you're inside it, it is a 0-dimensional point without any inside. You are then promptly converted to Hawking radiation and ejected. So, if the inside of the event horizon remains forever beyond the grasp of any observer, there is no reason to think that the inside of it exists at all.

 

[tom.stoer]

That evaporation time you refer to is measured by an asymptotic observer, but the proper time of an infalling observer is clearly not

You have to transform the result accordingly

 

[tom.stoer]

At the event horizon gravity becomes as strong as all the other forces. It is by definition in the realm of quantum gravity. I don't think GR, by itself at least, is really applicable to studying it, even though the concept of black holes originally arose from GR.

Sorry to say that, but this is totally wrong.

It is not even true that gravity becomes strong at the horizon. The larger the black hole the smaller the surface gravity is. Closed to a sufficiently large black hole the surface gravity is very small, the observer feels nothing special, not even when he crosses the horizon.

There are numerous papers regarding black hole geometries (Schwarzschild, Kerr, ...), exact calculations (free falling observer, stable and unstable orbits, ...), numerical calculations (infalling matter, accretion discs, black hole merger, ...) all based on GR.

There is no indication that GR does break down and requires correction near the event horizon.

 

[Bernie G]

"It is not even true that gravity becomes strong at the horizon. The larger the black hole the smaller the surface gravity is. Close to a sufficiently large black hole the surface gravity is very small, the observer feels nothing special, not even when he crosses the horizon."

Yes. This is probably why the largest black holes "turn off" in terms of visibility. Weird.

 

[IttyBittyBit]

It is not even true that gravity becomes strong at the horizon. The larger the black hole the smaller the surface gravity is. Closed to a sufficiently large black hole the surface gravity is very small, the observer feels nothing special, not even when he crosses the horizon.

Maybe I phrased myself incorrectly. I should have said that at the event horizon, the curvature of spacetime becomes so huge that it can no longer be ignored at small scales. This is regardless of the size of the black hole. To study the event horizon, GR itself is insufficient - it doesn't predict Hawking radiation, for example, which is emitted `from' the event horizon.