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

  1. Jul 4, 2014 #1
    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
     
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  3. Jul 4, 2014 #2

    marcus

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    QG effects spark BH to WH tunneling

    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.
     
  4. Jul 5, 2014 #3
    Isn't this just another look on black hole evaporation?
     
  5. Jul 5, 2014 #4

    marcus

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    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.
     
  6. Jul 5, 2014 #5
    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".
     
  7. Jul 5, 2014 #6

    marcus

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    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.

    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.
     
  8. Jul 5, 2014 #7

    marcus

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    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 :smile:

    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. :smile:
     
    Last edited by a moderator: Sep 25, 2014
  9. Jul 5, 2014 #8
    ^^. 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 whats left is strong force interaction in some sort of dimensionless binding mass eventually retained within the blackhole. Pretty neat indeed!
     
  10. Jul 6, 2014 #9

    marcus

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    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
    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
     
    Last edited: Jul 6, 2014
  11. Jul 6, 2014 #10

    marcus

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    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
     
    Last edited: Jul 6, 2014
  12. Jul 25, 2014 #11

    marcus

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    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==
     
    Last edited: Jul 25, 2014
  13. Jul 25, 2014 #12

    marcus

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    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 universe2, 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
     
    Last edited: Jul 25, 2014
  14. Jul 25, 2014 #13

    marcus

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    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:
     
    Last edited: Jul 25, 2014
  15. Jul 27, 2014 #14

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    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.
     
  16. Jul 27, 2014 #15
    .. Would it be possible to consider the mirror effect/whitehole mirage to appear reversed again due to bounce?

    http://cds.cern.ch/record/302830/files/9605063.pdf
     
  17. Jul 27, 2014 #16

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    Sort of that. But it seems there's no flip between the image and particle, so, only part of the problem is considered.
     
  18. Jul 28, 2014 #17

    Demystifier

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    Yes, that is always a worry when you attempt to use a strong quantum effect to describe a macroscopic object.
     
  19. Jul 28, 2014 #18

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    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.
     
  20. Jul 28, 2014 #19

    marcus

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    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.
     
    Last edited: Jul 28, 2014
  21. Jul 28, 2014 #20

    marcus

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    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.
     
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