Black hole fireworks: quantum-gravity effects outside the horizon spar

[Demystifier]

Anyway, even I look very critical to some of the Rovelli's recent ideas, I actually like very much his idea of the Planck star. In fact, at the moment I am working on a simple model inspired by this idea, but without a bounce. Here I do not want to uncover any details, but they will be available when (and if) I work out all the details and write a paper.

The idea and some details are available now:
https://www.physicsforums.com/threa...ics-area-laws-lqg.376399/page-14#post-5112227

 

[marcus]

Thanks! It's an interesting idea to have a change of phase into a "crystal" phase.
http://arxiv.org/abs/1505.04088
Gravitational crystal inside the black hole
H. Nikolic
(Submitted on 15 May 2015)
Crystals, as quantum objects typically much larger than their lattice spacing, are a counterexample to a frequent prejudice that quantum effects should not be pronounced at macroscopic distances. We propose that the Einstein theory of gravity only describes a fluid phase and that a phase transition of crystallization can occur under extreme conditions such as those inside the black hole. Such a crystal phase with lattice spacing of the order of the Planck length offers a natural mechanism for pronounced quantum-gravity effects at distances much larger than the Planck length. A resolution of the black-hole information paradox is proposed, according to which all information is stored in a crystal-phase remnant with size and mass much above the Planck scale.
6 pages

I think that important advances in physics occasionally have something at the "philosophical" or "conceptual" level, that makes them different.
Whether or not this will turn out to be successful, it has this very interesting new perspective:

"We propose that the Einstein theory of gravity only describes a fluid phase and that a phase transition of crystallization can occur under extreme conditions such as those inside the black hole. Such a crystal phase with lattice spacing of the order of the Planck length offers a natural mechanism for pronounced quantum-gravity effects at distances much larger than the Planck length."

So what about evaporation?

This Nicolic idea reminds me of the 1995 Jacobson idea of the "Einstein equation of state" describing the collective behavior of little things we can't see. Now the new idea is that these little things can form a crystal (and require a new equation to describe their behavior in the new phase).
I would like to see a reaction to this paper by Ted Jacobson.

I like this idea very much (without myself having any ability to judge if it could or could not be right). It is even more than usually entertaining, if it is possible for physics ideas to be considered entertaining.

 

[marcus]

About evaporation, the Nicolic paper says:
==quote==
Equating (7) with (9) and using (5), (6), (8) and (10), we obtain

(α/3)r_core³ = (1 + η)(M₀² − M²⁾,

(11)which describes how r_core increases as the black-hole mass M decreases due to Hawking radiation. As the black-hole radius R = 2M shrinks, the crystal radius rcore grows.

This black-hole shrinking and crystal growth is a continuous process which lasts as long as Hawking radiation is created at the horizon at R = 2M. This happens as long as general relativity is valid at r ≥ R. However, at some point r_core becomes equal to R, at which point general relativity ceases to be valid at the horizon. At this point there is no reason to expect any further creation of Hawking radiation, so the process of crystal growth stops at that point. At this critical point we have r_core = R = 2M, so (11) gives

(8α/3)M³ = (1 + η)(M₀² − M²⁾.

(12)

which is a cubic equation for M.
==endquote==

What about the long-term stability of this crystal remnant of geometric collapse? What happens if it bumps something?

 

[marcus]

For a long time I have suspected that the Croatian name
Hrvoje
could be pronounced "Harvey".

Would you mind being called Harvey? It is a familiar name in English, deriving from the French name Herve which was brought to England by the Normans. It is a virtual certainty that Bertrand Russell had some classmates at Cambridge named Harvey.

 

[wabbit]

Doesn't seem very close, : https://www.howtopronounce.com/hrvoje/

 

[marcus]

Doesn't seem very close, : https://www.howtopronounce.com/hrvoje/

Hear-vosch
to rhyme with EAR-POCHE
where by poche is meant the French word for pocket or pouch.

Wabbit thanks for looking up the pronunciation. Indeed it doesn't seem close. But I suppose still might be etymologically related to the French (originally Breton) word hervé

The wikipedia etymology of the French name Hervé is "iron-bright"

 

[Demystifier]

So what about evaporation?

Normally, Hawking radiation is created outside of the black hole, near the horizon. Hence, as long as crystal is inside the black hole, the evaporation still takes place. The role of crystal core inside the black hole is to absorb the information created by Hawking radiation, which otherwise could not be absorbed by the black hole due to the Bekenstein bound valid for the fluid phase (described by standard GR).

However, due to evaporation the horizon shrinks, while the crystal core grows. So at some point the core and the horizon become of an equal size, and evaporation stops at that point. Thus at this point we have a remnant (with mass and size much above the Planck scale) which stores all the information. In this way the information paradox is resolved.

For more details see the paper.

EDIT: Now I have seen that you already quoted the relevant part of the paper.

 

[Demystifier]

What about the long-term stability of this crystal remnant of geometric collapse? What happens if it bumps something?

I believe this question cannot be answered without knowing the nature of the more fundamental degrees of freedom (for which GR is only a long-distance effective theory).

 

[Demystifier]

For a long time I have suspected that the Croatian name
Hrvoje
could be pronounced "Harvey".

Would you mind being called Harvey? It is a familiar name in English, deriving from the French name Herve which was brought to England by the Normans. It is a virtual certainty that Bertrand Russell had some classmates at Cambridge named Harvey.

Doesn't seem very close, : https://www.howtopronounce.com/hrvoje/

The best pronounce I can find is the one by the google translate. (The one on the right is better.)
https://translate.google.com/#hr/es/Hrvoje

 

[Demystifier]

This Nicolic idea reminds me of the 1995 Jacobson idea of the "Einstein equation of state" describing the collective behavior of little things we can't see.

Speaking of Jacobson, have you seen his paper that appeared today?
http://lanl.arxiv.org/abs/1505.04753

 

[Haelfix]

That paper is just a writeup of a talk he gave at KITP a few weeks ago, which should be available online. It's essentially a combination of his original idea from 20 years ago and the more modern version due to Van Raamsdonk (which uses Holography and the Ryu-Takanayagi theorem).

 

[martinbn]

There is an assumption that at the core the geometry is minkowskian, how reasonable is this assumption? Also what happens in the case of a Kerr black hole?

 

[Demystifier]

There is an assumption that at the core the geometry is minkowskian, how reasonable is this assumption?

Do you mean why is it flat (which is what a physicist is likely to ask), or why is it not Euclidean (which is what a mathematican is more likely to ask)?

Also what happens in the case of a Kerr black hole?

Why do you expect that this case might be significantly different?

Sorry for answering with questions, but I need to know what kind of answer do you expect.

 

[martinbn]

Do you mean why is it flat (which is what a physicist is likely ask), or why is it not Euclidean (which is what a mathematican is more likely to ask)?

The first, why flat?

Why do you expect that this case might be significantly different?

The singularity has a very different nature, for one it is not space-like. Is the core going to be ring like?

 

[Demystifier]

The first, why flat?

So you think like a physicist, that's good. It is known that properties of a moving fluid in flat spacetime can be described by an effective curved metric. But crystals in flat spacetime do not share this feature - their effective metric is still flat. By analogy, one expects the same for a gravitational crystal.

The singularity has a very different nature, for one it is not space-like. Is the core going to be ring like?

Since there is no spherical symmetry in this case, I cannot answer these questions with my simple model. A more profound model would be needed for that purpose. But my first guess is that the core might indeed be ring like.

 

[wabbit]

The best pronounce I can find is the one by the google translate. (The one on the right is better.)
https://translate.google.com/#hr/es/Hrvoje

Interesting - the one I saw said approximately "hirvoge", but actually it's more like "hirvoy" - not so far from "harvey" actually, or "hervé" for that matter, I suspect marcus is right, these may all share a common origin. Are there links between croatian and celtic languages?

 

[Demystifier]

Are there links between croatian and celtic languages?

Not to my knowledge.

 

[martinbn]

So you think like a physicist, that's good. It is known that properties of a moving fluid in flat spacetime can be described by an effective curved metric. But crystals in flat spacetime do not share this feature - their effective metric is still flat. By analogy, one expects the same for a gravitational crystal.

That seems very counter intuitive to me. If the space-time is not empty i.e. the stress-energy tensor is not zero then it cannot be flat (not even Ricci flat). Of course you say that you depart from GR in that region as not reliable, but flat space-time just seems unexpected (of course it might be the case). I don't understand the anlogy with crystals, probably should look into it before asking questions, but you say crystals in flat spacetime have effective metric that is also flat. What about crystals on curved spacetime?

Since there is no spherical symmetry in this case, I cannot answer these questions with my simple model. A more profound model would be needed for that purpose. But my first guess is that the core might indeed be ring like.

Fair enough.

 

[wabbit]

As I understand it, in the Planck star model, the mass of the black hole is concentrated in a small volume of Planckian density - actually, this seems generic since anything should reach the central region quickly in proper time, and that region should not have more than Planck density, though it may be surrounded by a transition region of slightly lower density.

So speculatively, I wonder if the fraction of the mass you describe as crystallized might be either
- An inner core
- A crust
The latter might mesh nicely with its role as a recipient for information landing on the core:)

Would either be somewhat compatible with what you describe? If not, where is the rest of the mass, the uncrystallized part? Or am I using too naive a notion of mass here, after all I don't know how the mass of that crystal as quoted relates to its effective gravitating mass?

 

[Jimster41]

No doubt the paper in question is pretty radical and speculative, but that Rovelli guy...
Interesting to see Sabine Hossenfelder in the references/credits.

So the idea is that if quantum gravity is discrete it can't accommodate a "perfect" Killing Horizon. It is too stiff.
(Just read about the Killing Horizon, so it's on my mind, I'm may be using it in a really under-precise way).

So it (specific radii of curvature) would be QM-gravitation-ally "reflective" to some degree?

I know this is obsessing about a particular nagging question that I may have beat to death, but it just won't go away (honestly I hadn't thought of it in months, until I saw this thread/paper here):

• If there was a supermassive BH at the center of the Galaxy, at what r and over what time would Quantum effects be reflecting the bounce?
• If you take their model of distance and time over which QM effect would emerge, but make the in-fall continuous over the time required for the "reflection" to accumulate (like a reflected wave on the beach meeting itself on the way out) would interference patterns emerge?
• What about other massive in-falls like planets? It seem like they are saying you don't need/have a singularity or even an "event horizon" per-se'.

 

[Demystifier]

Of course you say that you depart from GR in that region as not reliable, but flat space-time just seems unexpected (of course it might be the case).

But the only reason why it looks unexpected is GR. And GR is not valid in this case. The idea, borrowed from condensed matter physics, is that in fluids the geometry only looks curved, while the true geometry is flat. In crystals, such an illusion of a curved geometry is not present; only the true flat geometry is seen.

Of course, it is not a proof that the same happens with quantum gravity, but at least it should make this possibility more intuitive and less unexpected.

 

[Demystifier]

As I understand it, in the Planck star model, the mass of the black hole is concentrated in a small volume of Planckian density - actually, this seems generic since anything should reach the central region quickly in proper time, and that region should not have more than Planck density, though it may be surrounded by a transition region of slightly lower density.

So speculatively, I wonder if the fraction of the mass you describe as crystallized might be either
- An inner core
- A crust
The latter might mesh nicely with its role as a recipient for information landing on the core:)

Would either be somewhat compatible with what you describe? If not, where is the rest of the mass, the uncrystallized part? Or am I using too naive a notion of mass here, after all I don't know how the mass of that crystal as quoted relates to its effective gravitating mass?

A portion of mass would be in the inner core. The ratio between core mass and total mass would increase with time.

 

[wabbit]

Interesting thanks. I thought of the core as being in a different phase than usual spacetime, but nothing like a crystal, more something close to totally disconnected as suggested in some models. An inner crystal is an intriguing possiblility - though it seems this still leaves open the description of the phase of the outer core - even if there is an effective GR-like metric describing its large scale behaviour as in Rovelli's papers, this can't really be a microscopic description, can it?

 

[Jimster41]

I am probably doing this way way wrong (but at least I'm trying to calculate something).

r\sim \frac { 7 }{ 6 } 2m

\tau \sim \frac { { m }^{ 2 } }{ { l }_{ p } }

The r number looks interesting for small m. I'm a bit confused how it grows with larger m. The time number is so long, it is pretty uninteresting. That said, I think this is trying to estimate the time it would take for that much mass to be reflected if it fell in as a single shell. Maybe it gets more interesting if it is treated more like a flow rather than a shell. In which case there is incoming mass and outgoing reflected mass, and the time it might take for interference between those to develop is on some other scale.

Or I am misunderstanding the whole concept.

 

[Demystifier]

even if there is an effective GR-like metric describing its large scale behaviour as in Rovelli's papers, this can't really be a microscopic description, can it?

Yes, that's the idea that curved metric is only an effective large-scale description of the fundamental dynamics. In this picture gravitons would only be quasi-particles similar to phonons, etc.

 

[Ilja]

So you think like a physicist, that's good. It is known that properties of a moving fluid in flat spacetime can be described by an effective curved metric. But crystals in flat spacetime do not share this feature - their effective metric is still flat.

As far as I know, if you compute the curvature (in the mathematical sense) of the stress tensor of a crystal, you will find out if there is inner stress, caused by crystal defects. Of course, if there are no defects, so that the crystal is ideal, then the stress is caused only by deformation, thus, a coordinate transformation. So that the curvature tensor has to be zero. But for crystals with inner stress this is different.

 

[Demystifier]

As far as I know, if you compute the curvature (in the mathematical sense) of the stress tensor of a crystal, you will find out if there is inner stress, caused by crystal defects. Of course, if there are no defects, so that the crystal is ideal, then the stress is caused only by deformation, thus, a coordinate transformation. So that the curvature tensor has to be zero. But for crystals with inner stress this is different.

Do you know a reference where more details can be found?

 

[Ilja]

Sorry, no, this was too long time ago.

 

[Andrekosmos]

McVittie spacetimes represent an embedding of the Schwarzschild field in isotropic cosmological backgrounds. Depending on the scale factor of the background, and the presence or otherwise of a cosmological constant, the resulting spacetime may contain black and white hole horizons, as well as other interesting boundary features. Together, these points indicate that with a background Λ-CDM model with Λ > 0, McVittie spacetimes can indeed model the boundary of a black hole horizon and a white hole horizon in expanding universes.

http://arxiv.org/abs/1408.0044

Very interesting lectures including the quantum mechanical underpinning of black holes, white holes/ wormholes (Lenoard susskind):





(NEW - Cole Miller)
(NEW - Gary Gibbons)

 

[no-ir]

I have a question regarding the gravitational crystal proposal. My first impression was that, while it is a very nice idea, it was a bit philosophical (i.e. how would you test it?), since the gravitational crystal would be hidden behind a horizon (by definition), until the black hole evaporated; which for larger black holes is effectively never. But then I remembered primordial black holes:

- Would the gravitational crystal proposal offer an explanation for the (lack of) observation of primordial black hole "explosions" (explosions due to rapid Hawking evaporation of very light black holes), by imposing a lower limit on the black hole mass before the evaporation stopped (when the horizon met the crystal surface)?

- Could such gravitational crystal remnants of (as much as is possible, evaporated) primordial black holes be candidates for dark matter?

I did not yet run any calculations, since this is not my area of physics, but it seemed an interesting idea to me ... (One really should do some quick numerics though, to find if it is even plausible. And to check if there are any known facts that would discount such ideas immediately (like the ~spherical distribution of dark matter around elliptical galaxies, for example; probably not, though?) ...)

Just an idea. :)

P.S. Another reason why I really liked the idea that GR is just an effective theory for the "liquid" phase of the "fundamental degrees of freedom" (not of "space-time", since that is a valid name only in the context of GR) is because I've recently been reading the book by X.-G. Wen:

Wen, Xiao-Gang. "Quantum Field Theory of Many-body Systems: From the Origin of Sound to an Origin of Light and Electrons." Published in the United States by Oxford University Press Inc., New York, 2004. ISBN 019853094. 1 (2004).

where he talks about emergence of (simple, beautiful, universal) low-energy effective theories from completely different (possibly ugly, non-universal) microscopic theories, which reminded me that it is not so far stretched that GR could just be an effective theory where the space is still relatively flat, not necessarily inside black holes (where a "phase transition" (a fair name for the effect?) of the more fundamental degrees of freedom could conceivably occur, as is pointed out in the article).

The above mentioned book's introduction talks about emergence particularly in the context of recent developments in solid state physics (the last ~30 year), where it was realized, that Landau symmetry-breaking is not the only way for a system to undergo a "phase transition", but that there also exist so called "quantum orders" (with eg. topological phase transitions [in fractional quantum Hall, spin ices, spin liquids, etc.]) that break no symmetries, but nonetheless lead to different states of matter with different low-energy excitations.

In particular, it was realized that one could produce effective (collective) low-energy physics of gapless (massless) gauge bosons and gapless anti-commuting fermions from purely bosonic local degrees of freedom (something that was thought impossible in Landau's symmetry breaking picture, which could produce only scalar gapless bosons (so the book says)). The book proposes a unifying description of such emergence in terms of string-net (as opposed to particle) condensation (just another way to think of collective states of a complex system in terms of string nets; not necessarily connected to string theory, as far as I can tell(?)). As an application/manifestation of the principle the author also constructed an explicit "ugly bosonic spin model on a square lattice" that had effective low energy U(1) x SU(3) interactions with emergent "photons", "electrons", "quarks" and "gluons" a while back in the article:

Wen, Xiao-Gang. "Quantum order from string-net condensations and the origin of light and massless fermions." Physical Review D 68.6 (2003): 065003.

Anyway, it is a very inspiring book, by a very original physicist, and I would really recommend its introduction, at least. To me it was very illuminating (it discusses things with a much wider scope than is usual for such books, with implications far beyond solid state physics).

P.P.S. I mean to write a better summary of the introduction but no matter what I did I wrote just longer and longer version of the above text, still not doing the book any justice. So the above should do (it's still too long, I know). I recommend the book version nonetheless (the first chapter is a quick read), since I only skimmed the surface above.

P.P.P.S. A very non-serious comment: a "gravitational crystal inside a black hole" kind of reminds me of the plot of the movie Interstellar (if you've seen it I think you would understand). ;)