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

[julcab12]

http://arxiv.org/abs/1407.0989

White hole anyone?..^^

"...intense gravity creates a horizon, but it is not an event horizon. It is locally like an horizon, but not globally. So, matter is trapped for a while, but not forever; it is called sometimes a “trapping” horizon."-CR

 

[marcus]

QG effects spark BH to WH tunneling

Fascinating paper! Thanks for posting the link.

http://arxiv.org/abs/1407.0989

White hole anyone?..^^

"...intense gravity creates a horizon, but it is not an event horizon. It is locally like an horizon, but not globally. So, matter is trapped for a while, but not forever; it is called sometimes a “trapping” horizon."-CR

http://arxiv.org/abs/1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Jul 2014)
We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum-tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon: we show that contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.
Comments: 10 pages, 5 figures

BTW I'd say the tunnel here is not to be thought of as a "wormhole" but more analogous to the radioactive decay of a nucleus where a wavefunction tunnels through a potential barrier.

 

[haael]

Isn't this just another look on black hole evaporation?

 

[marcus]

Isn't this just another look on black hole evaporation?

In what sense? That's an interesting question, so I wish you would explain your idea a little more. You may have the beginnings of a generalized idea of BH evaporation that includes this seemingly different scenario.

In the paper they explain how it is different from the conventional BH evaporation story, where all the mass is dissipated in Hawking radiation.

Realistically, if their scenario would be worked out in detail (as they intend), there would be SOME Hawking radiation. They compare that to friction or other dissipative effects in an otherwise time-reversible process.

They use the analogy of a bouncing ball: it's trajectory up is almost the same as down except for dissipative effects of air resistance, imperfect elasticity---friction-like effects.

For a first approximation, in this paper which considers a simplified case, they ignore Hawking radiation. They take that as negligible, so nothing is lost by evaporation and everything goes into the bounce. So at least in a superficial sense it looks very different from our conventional idea of BH evaporation.

 

[haael]

In the paper they explain how it is different from the conventional BH evaporation story, where all the mass is dissipated in Hawking radiation.

I may not understand the topic, but I always thought of Hawking radiation as energy tunelling from the singularity through the gravitational potential of the BH. This is also the standard popular-science explanation with pairs of virtual particles one of which falls into the horizon. Numbers also agree.

So I don't see much difference between the statements: "energy leaks via Hawking radiation" and "energy tunnels out from BH".

 

[marcus]

...
So I don't see much difference between the statements: "energy leaks via Hawking radiation" and "energy tunnels out from BH".

Thanks for explaining! I think you are generalizing the idea to be more inclusive. I don't recall any place where Haggard and Rovelli say "energy tunnels out from the BH". Can you point me to the passage?

What I understand from their paper is that it connects with the earlier papers Rovelli wrote this year with Vidotto ("planck stars") and Barrau ("planck star phenomenology") in which there is some Hawking radiation but then, much later as a distant observer would time it, an explosion.

In this simple model they are treating the final explosion as the time reverse of the initial collapse. This makes the analysis easier to do, and in a general sense that is what a WH is, a BH run in reverse, so it makes sense.

In particular, we disregard Hawking radiation. This requires a comment. A widespread assumption is that the energy of a collapsed star is going to be entirely carried away by Hawking radiation. While the theoretical evidence for Hawking radiation is strong, we do not think that the theoretical evidence for the assumption that the energy of a collapsed star is going to be entirely carried away by Hawking radiation is equally strong. After all, what other physical system do we know where a dissipative phenomenon carries away all of the energy of the system?

Hawking radiation regards the horizon and its exterior: it has no major effect on what happens inside the black hole. Here we are interested in the fate of the star after it reaches (rapidly) r = 0. We think that it is also possible to study this physics first, and consider the dissipative Hawking radiation only as a correction, in the same vein one can study the bounce of a ball on the floor first and then correct for friction and other dissipative phenomena. This is what we are going to do here.
...
What should we expect for the metric of the second part of the process, describing the exit of the matter? The answer is given by our assumption about the time reversal symmetry of the process: since the first part of the process describes the in-fall of the matter to form a black hole, the second part should describe the time reversed process: a white hole streaming out-going matter.

At first this seems surprising. What does a white hole have to do with the real universe? But further reflection shows that this is reasonable: if quantum gravity corrects the singularity yielding a region where the classical Einstein equations and the standard energy conditions do not hold, then the process of formation of a black hole does not end in a singularity but continues into the future. Whatever emerges from such a region is then something that, if continued from the future backwards, would equally end in a past singularity. Therefore it must be a white hole. A white hole solution does not describe something completely unphysical as often declared: instead it is possible that it simply describes the portion of spacetime that emerges from quantum regions, in the same manner in which a black hole solution describes the portion of spacetime that evolves into a quantum region. Thus our main hypothesis is that there is a time symmetric process where a star collapses gravitationally and then bounces out. This is impossible in classical general relativity, because once collapsed a star can never exit its horizon. Not so if we allow for quantum gravitational corrections...

I think the tunneling they are talking about is not in any simple sense the tunneling of energy (as I see it) but rather the tunneling of a quantum wave function from a collapsing state to an expanding one. You could say it is a tunneling that happens in configuration space---loosely speaking a tunneling of amplitude, rather than of particles or energy.

 

[marcus]

http://arxiv.org/abs/1407.0989

White hole anyone?..^^

"...intense gravity creates a horizon, but it is not an event horizon. It is locally like an horizon, but not globally. So, matter is trapped for a while, but not forever; it is called sometimes a “trapping” horizon."-CR

Julcab, I just remembered you were the person that pointed me to the YouTube of that talk by Rovelli on the "Zero-th principle of thermodynamics" at this year's FQXi conference

https://www.physicsforums.com/showthread.php?t=751199

Fun talk! Should be interesting to see the thermodynamics worked out for this BH bounce idea (with its B-to-W tunneling)

By coincidence that talk on thermal time, and the zero-th principle, was based on another Haggard Rovelli paper:
http://arxiv.org/abs/1302.0724
Are they going to bring those two idea-threads together, connecting to the BH bounce? They must be

Some of the puzzling over black holes seems to come from how the event horizon is defined (which requires knowing the complete future history of the universe out to script-eye infinity) and the way it is taken so seriously (as something you absolutely never never never get out of!)

Part too may come from the def. of entropy as missing or hidden information, inaccessible degrees of freedom, hilbertspace dimensions, micro-vs-macro, things which don't affect you because you're so coarse you don't interact with them. It's like, a thief comes in the night and steals your Quantum Mechanics textbook and puts it in a box somewhere, you don't know where. You think that information is lost to you forever! and the entropy around you has greatly increased! But then your Uncle Hal shows up, who knows how to pick locks with a bent paperclip. He happens upon a box in the grass by the side of the road, and he picks the lock, and your textbook jumps out! The fingerprints found on the lid of the box are those of Sir Arthur Eddington, who collapses in abject humiliation.

 

[julcab12]

Fun talk! Should be interesting to see the thermodynamics worked out for this BH bounce idea (with its B-to-W tunneling)

By coincidence that talk on thermal time, and the zero-th principle, was based on another Haggard Rovelli paper:
http://arxiv.org/abs/1302.0724
Are they going to bring those two idea-threads together, connecting to the BH bounce? They must be

Some of the puzzling over black holes seems to come from how the event horizon is defined (which requires knowing the complete future history of the universe out to script-eye infinity) and the way it is taken so seriously (something you absolutely never get out of!)

^^. It's fun imagining that scenario. I think so too. I was looking at both papers. It's based on the same line of thinking only this time CR included a standard quantum phenomena -a particle can go where classically it could not go. So black-hole metric can tunnel into a white-hole metric and can be viewed as external metric of the process at superslow speed from far away. Quite in line also on event horizons – and relativistic expulsion jets in association with accretion events where particle decay as it approached speed of light so what's left is strong force interaction in some sort of dimensionless binding mass eventually retained within the black hole. Pretty neat indeed!

 

[marcus]

I wanted to get a rough idea of what primordial black holes there might be, that you could expect to be exploding (something to observe to test the theory) so I looked at:
http://en.wikipedia.org/wiki/Primordial_black_hole

According to the Big Bang Model, during the first few moments after the Big Bang, pressure and temperature were extremely high. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe, a primordial black hole would be stable, persisting to the present.

It has been proposed that primordial black holes, specifically those forming in the mass range of 10¹⁴ kg to 10²³ kg,[1] could be a candidate for dark matter. This is due to the possibility that at this low mass they would behave as expected of other particle candidates for dark matter. Being within the typical mass range of asteroids, this excludes those black holes too small to persist until our era and those too large to explain gravitational lensing observations.

I'm wondering what the lifetimes would be in that mass range. If there are any, even if they are far too infrequent to account for a significant part of the dark matter it might still be possible to observe their final explosions.

I did a rough calculation using formulas from the Haggard Rovelli paper that showed that 10²³ kg was too massive to be exploding at this stage in history. The lifespan of a primordial BH that massive (according to them) would be too long. But somewhere in that range there might be PBH whose explosions could be observed.

Some of their analysis is done from the standpoint of an observer near but outside, at radius 7/6 of schwarzsch r.

the lifespan HE perceives would, I guess, be shorter than the lifespan perceived from infinity---by a factor of
sqrt(1 - 6/7) = sqrt(1/7) = 0.378

So if we calculate the lifespan as seen from the near-outside guy we still have to multiply by sqrt(7) = 2.65 to get what it is at a distance. But it's the same order of magnitude.

Since the lifespan (for the near-outside guy) goes as the square of the mass if you want to adjust for that factor of 2.65 you would adjust the mass by sqrt (2.65) = 1.63

 

[marcus]

OK I don't completely understand and am not sure about some of the terms in the equations in the Haggard Rovelli paper, but they suggest assuming b=1 making the factor k in equation (42) equal to 0.0595*, so I will calculate the mass of a PBH which has its finale at the present day. And I may have to change this later. I comes out remarkably close to 10²³ kg. (like about 1/3 of the mass of the planet mercury?)

0.76 x 10²³ kg.

A PBH with that initial mass would still have most of its mass at its finale. The Hawking lifespan of something that massive is way longer than the current expansion age. So it wouldn't have evaporated very much, and that gives an idea of the size of the final burst.

If you want to check the calculation, what I did was take the current age of 13.8 billion years and divide by 2.65 to get the age which the near-outside guy sees.
And put that in for tau τ on the lefthand side of equation (42)
and solve for mass m.

Anybody who checks this let me know of any misconceptions/errors you find I've made.

It's curious thinking of a PBH that massive. Of course a BH of that mass is REALLY TINY and at the moment the U was presumably breeding PBH it was very dense, so perhaps it's conceivable such things were formed.

Aurelien Barrau and Julien Grain have a 2004 paper on PBH, I haven't looked at it but it might be helpful

*for the definition of k see equation (18) and following.

FWIW a recent not-very-helpful paper on the putative mass spectrum of PBH:
http://arxiv.org/abs/1405.7023

 

[marcus]

The Haggard Rovelli paper got some comments from Steve Giddings and others (Giddings is acknowledged.)
And the story was picked up by Nature News.
http://www.nature.com/news/quantum-bounce-could-make-black-holes-explode-1.15573
"Quantum Bounce Could Make Black Holes Explode" article by Ron Cowen.

Cowen included reactions by others. Helps put the idea in perspective. Huffington Post republished he Nature article--it generated a fair amount of interest.

==quote Nature News==
…
If the new work sheds any light on this black-hole information paradox, “it would be important”, says theoretical physicist Steven Giddings of the University of California, Santa Barbara. “Understanding how information escapes from a black hole is the key question for the quantum mechanics of black holes, and possibly for quantum gravity itself.”

The authors acknowledge that some of the conclusions in their paper have yet to be fleshed out with detailed calculations. Other physicists, including Joseph Polchinski of the University of California, Santa Barbara, also worry that the scenario involves quantum effects that are unrealistically large.

Theoretical physicist Donald Marolf of the University of California, Santa Barbara, cautions that the quantum bounce could violate one of the most fundamental principles of physics: that entropy, a measure of the amount of disorder in a system, can increase but can never decrease. He says that the outgoing material from the white hole, initially packed into a small region, would seem to have a smaller entropy than the black hole itself. Rovelli and Haggard maintain that in their scenario entropy would not decrease.

Nonetheless, the work puts the idea of a quantum bounce on a surer footing, says Abhay Ashtekar of Pennsylvania State University in University Park, another one of the founders of loop quantum gravity. But he says that he would like to see more detailed calculations before he is convinced.
…
==endquote==

 

[marcus]

Here's an excerpt from the article suggesting some of the issues raised by the Haggard Rovelli paper:
==quote http://www.nature.com/news/quantum-bounce-could-make-black-holes-explode-1.15573 ==
…
…Many physicists, however, believe that at some stage in this process, quantum-gravity effects should take over, arresting the collapse and avoiding the infinities.
...
One of the leading approaches to merging quantum theory and gravity, pioneered by, among others, theoretical physicist Carlo Rovelli of Aix-Marseille University in France, posits that it is not just gravity but space-time itself that is quantized, woven from tiny, individual loops that cannot be subdivided any further. The loops in this ‘loop quantum gravity’ — a theoretical attempt that has yet to find experimental support — would be so tiny that to any observer space-time looks smooth and continuous. In the new work¹, Rovelli and his Aix-Marseille colleague Hal Haggard have calculated that the loop structure would halt the collapse of a black hole.

The collapsing star would reach a stage at which its inside can shrink no further, because the loops cannot be compressed into anything smaller, and in fact they would exert an outward pressure that theorists call a quantum bounce, transforming a black hole into a white hole. Rather than being shrouded by a true, eternal event horizon, the event would be concealed by a temporary 'apparent horizon', says Rovelli. (Theoretical physicist Stephen Hawking of the University of Cambridge, UK, has recently suggested that true event horizons would be incompatible with quantum physics.)

Other loop-quantum theorists have made similar calculations for cases in which it is not just a star that is collapsing but an entire universe^{2, 3}. They found that the universe could bounce back, and suggested that our own Universe’s Big Bang could in fact have been such a ‘big bounce’. Rovelli and Haggard have now shown that the quantum bounce does not require an entire universe to collapse at once. “We think this is a possible picture,” says Rovelli. “We have found that the [transformation] process can be completely contained in a limited region of space-time. Everything outside behaves following the classical Einstein equations.”

Information paradox
If black holes turn into white holes and release all of their innards out again, it could provide a solution to one of the most troublesome questions of fundamental physics. Hawking calculated in the 1970s that a black hole should emit radiation out of its event horizon, slowly losing energy and shrinking in the process until it completely disappears. This 'Hawking radiation' means that information carried by the matter that fell into the black hole would then seem to vanish forever. This would violate one of the fundamental principles of quantum theory, according to which information cannot be destroyed…
==endquote==

The Huffington Post republication of the article is convenient because it has HYPERLINKS instead of footnotes, so one can go immediately to the source e.g. the referenced article by Hawking etc.
http://www.huffingtonpost.com/2014/07/18/black-holes-white-holes-explode-_n_5597006.html

 

[marcus]

More news about the Black Hole Explosion idea. Rovelli will be presenting the results so far at the September 2014 workshop on Experimental Search for Quantum Gravity
http://www.sissa.it/app/esqg2014/

One of the main organizers of the ESQG series of workshops/conferences is Sabine Hossenfelder, who today discussed the Haggard Rovelli paper. She commented at length so here are the last few paragraphs

==quote http://backreaction.blogspot.com/2014/07/can-black-holes-bounce-to-white-holes.html ==
What one would need to do to estimate the transition probability is to work out some product of wave-functions describing the background metric close by and far away from the classical average, but nothing like this is contained in the paper. (Carlo told me though, it’s in the making.) It remains to be shown that the process of all the matter of the shell suddenly tunneling outside the horizon and expanding again is more likely to happen than the slow evaporation due to Hawking radiation which is essentially also a tunnel process (though not one of the metric, just of the matter moving in the metric background). And all this leaves aside that the state should decohere and not just happily build up quantum fluctuations for the lifetime of the universe or so.

By now I’ve probably lost most readers so let me just sum up. The space-time that Haggard and Rovelli have constructed exists as a mathematical possibility, and I do not actually doubt that the tunnel process is possible in principle, provided that they get rid of the additional energy that has appeared from somewhere (this is taken care of automatically by the time-reversal). But this alone does not tell us whether this space-time can exist as a real possibility in the sense that we do not know if this process can happen with large probability (close to one) in the time before the shell reaches the Schwarzschild radius (of the classical solution).

I have remained skeptical, despite Carlo’s infinitely patience in explaining their argument to me. But if they are right and what they claim is correct, then this would indeed solve both the black hole information loss problem and the firewall conundrum. So stay tuned…
==endquote==

As a reminder, here's post #2 giving abstract of the Haggard Rovelli paper under discussion:

Fascinating paper! Thanks for posting the link.

http://arxiv.org/abs/1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Jul 2014)
We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum-tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon: we show that contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.
Comments: 10 pages, 5 figures

BTW I'd say the tunnel here is not to be thought of as a "wormhole" but more analogous to the radioactive decay of a nucleus where a wavefunction tunnels through a potential barrier.

 

[MTd2]

I have a different view. The cutting and past procedure is similar to the method of images in electrostatics.

For example, in Kruskal–Szekeres coordinates, if you map opposite hyperbolas, describing a particle and its "reflection", into one of the 2 other regions, you will get a particle and a mirrored in, the X coordinate, virtual image. When a particle falls, its virtual image will be closer. But, this virtual image is a measure of the transition amplitude, because of the relationship of the construction to the distance to the event horizon (the closer, the more probable is the tunneling).

When the particle gets very close to the horizon, it swaps places with the image. But notice something, the virtual image has an inverted time axis. So, when the swaps happens, the particle is repealed by the new image. Thus a white hole.

Notice that when the particle was falling it was already being repealed by the image, so, it was already been slown down. So, the image is a kind of hawking radiation, where the closer to the horizon, the stronger it becomes. Going further, in fact, if you consider this scenario from the stage of formation of the black hole, all particles that formed it are counterbalanced by their image, and the event horizon is merely equivalent a mirror.

So, as far as I see it, the black hole is a white hole at the same time, like a duality. The hawking radiation is the luminosity of the white hole.

 

[julcab12]

Notice that when the particle was falling it was already being repealed by the image, so, it was already been slown down. So, the image is a kind of hawking radiation, where the closer to the horizon, the stronger it becomes. Going further, in fact, if you consider this scenario from the stage of formation of the black hole, all particles that formed it are counterbalanced by their image, and the event horizon is merely equivalent a mirror.

So, as far as I see it, the black hole is a white hole at the same time, like a duality. The hawking radiation is the luminosity of the white hole.

.. Would it be possible to consider the mirror effect/white hole mirage to appear reversed again due to bounce?

http://cds.cern.ch/record/302830/files/9605063.pdf

 

[MTd2]

Sort of that. But it seems there's no flip between the image and particle, so, only part of the problem is considered.

 

[Demystifier]

Other physicists, including Joseph Polchinski of the University of California, Santa Barbara, also worry that the scenario involves quantum effects that are unrealistically large.

Yes, that is always a worry when you attempt to use a strong quantum effect to describe a macroscopic object.

 

[Demystifier]

It remains to be shown that the process of all the matter of the shell suddenly tunneling outside the horizon and expanding again is more likely to happen than the slow evaporation due to Hawking radiation which is essentially also a tunnel process

This seems not only essential for the proposal to work, but also highly unlikely. This is like suggesting that a sudden tunneling of a 1 kg chunk of radioactive uranium through a brick wall is more likely to happen than the slow radioactive decay of most of the atoms.

 

[marcus]

Hi Demy, people can be misled by inappropriate comparisons. Using the authors' equations you can calculate that the likely time for explosion comes long before the Hawking evaporation process would complete.

I found (and subsequently checked with Haggard) that the likely initial mass for a primordial BH that explodes in present era, around 13.8 billion years, would be 0.7-1.2 x 10²³ kilograms. Not to overstate precision, let's say simply 10²³ kg.

Now let's calculate the Hawking evaporation time for a BH of that mass:
(5120*pi*G^2*(10^23 kg)^3)/(hbar*c^4) in years = 2.67 x 10⁴⁵ years ≈ 3 x 10³⁶ billion years.

So we are comparing a Haggard Rovelli number like 13.8 with a Hawking number like 3x10³⁶
Clearly the conventional Hawking evaporation time on the order of a TRILLION TRILLION TRILLION times longer than an estimated time for BH to explode according to Haggard Rovelli.

 

[marcus]

Haggard points out that 10²³ kg is roughly the mass of the Moon.
So using their preliminary equations from the July 2014 paper one finds that a primordial BH with roughly the mass of the moon would be exploding (tunneling to white hole state of geometry) right about now. And all the less massive PBH would have already exploded.

By contrast according to Hawking original evaporation time formula a PBH with roughly the mass of the moon would take on the order of trillion trillion trillion times the current age of the universe to evaporate.

It is questionable to suggest, without detailed inspection of their equations, that it is somehow intuitive such a hole might Hawking evaporate BEFORE it gets around to blowing up via the quantum geometry tunneling described by Haggard Rovelli.

The apparent likelihood, given their analysis, seems to be the direct opposite of that.

 

[marcus]

Since we've turned a page, I'll bring forward post #2 which has the abstract (Julcab initially posted the link to the paper.)

Fascinating paper! Thanks for posting the link.

http://arxiv.org/abs/1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Jul 2014)
We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum-tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon: we show that contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.
Comments: 10 pages, 5 figures

BTW I'd say the tunnel here is not to be thought of as a "wormhole" but more analogous to the radioactive decay of a nucleus where a wavefunction tunnels through a potential barrier.

Also some key passages from page 5 of the paper where they make more explicit what they mean by tunneling process:

==quote==
In particular, we disregard Hawking radiation. This requires a comment. A widespread assumption is that the energy of a collapsed star is going to be entirely carried away by Hawking radiation. While the theoretical evidence for Hawking radiation is strong, we do not think that the theoretical evidence for the assumption that the energy of a collapsed star is going to be entirely carried away by Hawking radiation is equally strong. After all, what other physical system do we know where a dissipative phenomenon carries away all of the energy of the system?

Hawking radiation regards the horizon and its exterior: it has no major effect on what happens inside the black hole. Here we are interested in the fate of the star after it reaches (rapidly) r = 0. We think that it is also possible to study this physics first, and consider the dissipative Hawking radiation only as a correction, in the same vein one can study the bounce of a ball on the floor first and then correct for friction and other dissipative phenomena. This is what we are going to do here.
...
What should we expect for the metric of the second part of the process, describing the exit of the matter? The answer is given by our assumption about the time reversal symmetry of the process: since the first part of the process describes the in-fall of the matter to form a black hole, the second part should describe the time reversed process: a white hole streaming out-going matter.

At first this seems surprising. What does a white hole have to do with the real universe? But further reflection shows that this is reasonable: if quantum gravity corrects the singularity yielding a region where the classical Einstein equations and the standard energy conditions do not hold, then the process of formation of a black hole does not end in a singularity but continues into the future. Whatever emerges from such a region is then something that, if continued from the future backwards, would equally end in a past singularity. Therefore it must be a white hole. A white hole solution does not describe something completely unphysical as often declared: instead it is possible that it simply describes the portion of spacetime that emerges from quantum regions, in the same manner in which a black hole solution describes the portion of spacetime that evolves into a quantum region. Thus our main hypothesis is that there is a time symmetric process where a star collapses gravitationally and then bounces out. This is impossible in classical general relativity, because once collapsed a star can never exit its horizon. Not so if we allow for quantum gravitational corrections...
==endquote==

As incidental comment, obviously they can ignore Hawking radiation because assuming their analysis applied to a primordial BH the time to explosion is a trillion trillion trillion times shorter than the Hawking evaporation time---the amount of Hawking evaporation that would have occurred before explosion is negligible.

 

[MTd2]

Haggard points out that 10²³ kg is roughly the mass of the Moon.

Wasn't there an estimate with the estimated mass of a primordial black hole exploding now? Does this mass is related to that other value, for example, the original mass being 10^23 and the other primordial black hole having a smaller mass after evaporating a bunch of matter. Or is this another new estimate?

 

[marcus]

Wasn't there an estimate with the estimated mass of a primordial black hole exploding now? Does this mass is related to that other value, for example, the original mass being 10^23 and the other primordial black hole having a smaller mass after evaporating a bunch of matter. Or is this another new estimate?

New estimate! I have no idea how the tension will be resolved! Rovelli will be talking about this new model of black hole in September at the ESQG conference at ISAS Trieste. I hope at least slides (or slides and audio) become available because I'm very interested to know how the difference gets resolved. The delayed bounce BH model is quite a new idea--estimates are preliminary and could still change considerably.

ESQG (Experimental Search for Quantum Gravity). It's the fourth conference in that series, and scheduled for first week of September, if I remember correctly. Only a little more than a month from now. It's relevant because they could look for signs of these explosions if they can get a handle on what to look for. So observational significance, which is a good thing to have built into your BH model.

 

[MTd2]

Does he cite the other paper in this one?

 

[marcus]

I'll get you the links so you can look it up and tell us
http://arxiv.org/find/grp_physics/1/au:+rovelli_c/0/1/0/all/0/1

Black Hole Fireworks http://arxiv.org/abs/1407.0989
Planck Star Phenomenology http://arxiv.org/abs/1404.5821
Planck Stars http://arxiv.org/abs/1401.6562

EDIT (to reply to Demy's next post, #26):

Demystifier, thanks for the the thoughtful reply! It helps me understand, to see what they are doing from a different perspective. Since I can still edit this post, I respond here so as not to cover yours in the stack. It may get more readers that way.

I'm very interested in your take on the BH bounce model. It occurs to me that you live not so far from Trieste, and might even sometimes visit at ISAS where the QG phenomenology conference will be held in about one month from now.

 

[Demystifier]

Hi Demy, people can be misled by inappropriate comparisons. Using the authors' equations you can calculate that the likely time for explosion comes long before the Hawking evaporation process would complete.

I found (and subsequently checked with Haggard) that the likely initial mass for a primordial BH that explodes in present era, around 13.8 billion years, would be 0.7-1.2 x 10²³ kilograms. Not to overstate precision, let's say simply 10²³ kg.

Hi marcus, the point of my comparison was to explain, in simple terms, why such a result looks counterintuitive. Indeed, from experience in standard quantum mechanics, one does not expect that an object with the mass 10²³ kg is likely to tunnel within 13.8 billion years. Typically, the tunneling time for such objects is much longer. But of course, it doesn't mean that their calculation is wrong. It only means that their result is counteruintuitive and unexpected. Which makes their result, if it is correct, even more interesting.

But perhaps the result looks counterintuitive because of the word "tunneling". Indeed, their calculation does not look like a calculation of quantum tunneling at all. Instead, their calculation looks quite classical in spirit, based on a classical picture of a bounce. Their calculation looks to me as a classical theory of gravity which differs from Einstein theory at large densities, but is still a classical theory. So perhaps it is misleading to call it "tunneling". Indeed, when rephrased in such a classical language, their result looks less counterintuitive to me.

Further argument that their effect is classical in spirit is their Eq. (21), which is supposed to define the distance at which quantum effects appear. If you write this equation in physical units (with Planck constant and Newton gravitational constant different from 1), you will see that Eq. (21) does NOT depend on the value of Planck constant.

 

[Demystifier]

I'm very interested in your take on the BH bounce model. It occurs to me that you live not so far from Trieste, and might even sometimes visit at ISAS where the QG phenomenology conference will be held in about one month from now.

I don't live far from Trieste, but at the moment I don't plan to visit the mentioned conference.
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.

 

[marcus]

I hope very much you follow through with it. I would be interested to see the paper!

 

[marcus]

Since the ESQG conference (or is it more correctly called a workshop?) is essentially just a month away (1-5 September) and I've been looking to see what I could find about the program. Loop gravity research has been trending toward deducing observable effects--developing its phenomenological side, so we may see more specifically Loop talks at this ESQG conference than were at the previous 3.
http://www.sissa.it/app/esqg2014/
http://www.sissa.it/app/esqg2014/schedule.php
http://www.sissa.it/app/esqg2014/ESQG14_program.pdf

On Tuesday 2 September there are two speakers (Julien Grain, Mercedes Martin-Benito) whose talks might be Loop-related.
http://arxiv.org/find/grp_physics/1/au:+Martin_Benito/0/1/0/all/0/1
http://arxiv.org/find/grp_physics/1/au:+Grain_J/0/1/0/all/0/1

Wednesday 3 September has these four:
Chirco, Mielczarek, Rovelli, Vidotto.
http://arxiv.org/find/grp_physics/1/au:+Chirco/0/1/0/all/0/1
http://arxiv.org/find/grp_physics/1/au:+Mielczarek/0/1/0/all/0/1
Rovelli and Vidotto have written on the Planck Star black hole model, which has the potential for observable results, so it quite possible they will discuss that.
ESQG stands for Experimental Search for Quantum Gravity. But Vidotto also has research in the phenomenology of Loop bounce cosmology.

Daniele Oriti, who speaks on Friday, has a related research interest in Group Field Theory.
Lee Smolin will give the conference summary talk at the end of the day on Friday.

I didn't see a listing of the titles of the talks.
Here's a list of confirmed participants however:
http://www.sissa.it/app/esqg2014/participants.php
Name Institution
Abdeldjalil Aissasnou ----------University of Essenia of Oran (Algeria)
Stephon Alexander ----------Dartmouth
Giovanni Amelino-Camelia ----------Sapienza, Rome
Carlo Baccigalupi ----------SISSA
Leonardo Barcaroli ----------Sapienza, University of Rome
Alessio Belenchia ----------SISSA
Francesco Brighenti ----------University of Bologna - INFN
Jose Manuel Carmona ----------Universidad de Zaragoza
Gianluca Castignani ----------SISSA
Massimo Cerdonio ----------INFN - Padua
Goffredo Chirco ----------CPT, Universite' Aix-Marseille
Francesco Cianfrani ----------University of Wroclaw
Jose Luis Cortes ----------Universidad de Zaragoza
Bethan Cropp ----------SISSA
Roldao da Rocha ----------ABC Federal University, Sao Paulo
Eolo Di Casola ----------SISSA
Astrid Eichhorn ----------Perimeter Institute, Waterloo
Giampiero Esposito ----------INFN, Sezione di Napoli
Giulio Fabbian ----------SISSA
Agnes Ferte---------- Institut d'Astrophysique Spatiale
Julien Grain---------- Institut d'Astrophysique Spatiale
Jonathan Granot ----------Open University of Israel
Giulia Gubitosi ----------Sapienza, University of Rome
Kingsuk Kalita ----------Gauhati University
Brian Keating ----------University of California, San Diego
John Kelley ----------IMAPP, Radboud University, Nijmegen
Jerzy Kowalski-Glikman ----------University of Wroclaw
Marco Letizia ----------SISSA
Stefano Liberati ----------SISSA
Niccolo' Loret ----------Perimeter Institute, Waterloo
Yongge Ma ----------Beijing Normal University
Joao Magueijo ----------Imperial College, London
Francesco Marin ----------Universita' di Firenze and INFN
Francesco Marino ----------CNR-Istituto Nazionale di Ottica
Stuart Marongwe ----------McConnell College
Mercedes Martin-Benito ----------Radboud University Nijmegen
David Mattingly ----------University of New Hampshire
Anupam Mazumdar ----------Lancaster University
Jakub Mielczarek ----------Jagiellonian University, Crakow
Jonathan Miller ----------Universidad Tecnica Federico Santa Maria
Daniele Oriti ----------Albert Einstein Institute
Antonello Ortolan---------- LNL-INFN
Krishnamohan Parattu ----------The Inter-University Centre for Astronomy and Astr
Antonio Pasqua ----------University of Trieste
Roberto Percacci ----------SISSA
Antonio Pereira ----------Universidade Federal Fluminense/SISSA
Igor Pikovski ----------Vienna Center for Quantum Science and Technology
Aleksandra Piorkowska ----------University of Silesia
Giacomo Rosati ----------ITP, University of Wroclaw
Carlo Rovelli ----------Aix-Marseille University
Paola Ruggiero ----------University of Pisa/University Aix Marseille
Floyd Stecker ----------NASA - Goddard Space Flight Center
Tomasz Trzesniewski ----------University of Wroclaw
Gian Paolo Vacca ----------INFN - Bologna
Babak Vakili ----------IAU, Iran
Francesca Vidotto---------- Radboud University Nijmegen
David Vitali---------- Universita' di Camerino

 

[marcus]

This BH bounce idea seems to be coming from several different directions at the same time.

http://arxiv.org/abs/arXiv:1407.1391
Mutiny at the white-hole district
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
(Submitted on 5 Jul 2014)
The white-hole sector of Kruskal's solution is almost never used in physical applications. However, it might contain the solution to many of the problems associated with gravitational collapse and evaporation. This essay tries to draw attention to some bouncing geometries that make a democratic use of the black-and white-hole sectors. We will argue that these types of behaviour could be perfectly natural in some approaches to the next physical level beyond classical general relativity.
8 pages, 1 figure. Essay awarded a honorable mention in the 2014 Gravity Research Foundation essay competition

 

[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'.