Planck Stars: Carlo Rovelli & Francesca Vidotto

[Paulibus]

Intuition is partly shaped by experience that is limited by the scale of human activities and
perceptions. Nature, by contrast, operates on scales of time and space that suits Her, not us. This
scale stretches way beyond ours, so to us the bounce-determining h seems tiny and the limiting
speed c huge. But speculations like yours, Marcus, along the lines of “Nature does not like to be
pinned down too precisely” may be insights that lead towards an understanding of what underpins
the values of such constants. Sad that the factors that determine their numerical values are still
mysterious; physicists generally shy away from discussing this elephant in the
physics-comprehension room.

Einstein, as quoted by Wilczek in your nice simple reference (it’s too narrowband for video here)
seems to have favoured a ‘bootstrap’ set-up in which ‘Nature is so constituted that it is possible
logically to lay down such strongly determined laws that within these laws only rationally
completely determined constants occur (not constants, therefore, whose numerical value could be
changed without destroying the theory)’.

I liked this, and your friend’s comments.

 

[marcus]

Intuition is partly shaped by experience that is limited by the scale of human activities and
perceptions. Nature, by contrast, operates on scales of time and space that suits Her, not us. This
scale stretches way beyond ours, so to us the bounce-determining h seems tiny and the limiting
speed c huge. But speculations like yours, Marcus, along the lines of “Nature does not like to be
pinned down too precisely” may be insights that lead towards an understanding of what underpins
the values of such constants. Sad that the factors that determine their numerical values are still
mysterious; physicists generally shy away from discussing this elephant in the
physics-comprehension room.

Einstein, as quoted by Wilczek in your nice simple reference (it’s too narrowband for video here)
seems to have favoured a ‘bootstrap’ set-up in which ‘Nature is so constituted that it is possible
logically to lay down such strongly determined laws that within these laws only rationally
completely determined constants occur (not constants, therefore, whose numerical value could be
changed without destroying the theory)’.

I liked this, and your friend’s comments.

A lot of exciting issues here! I just recalled an interesting paper by Gambini and Pullin (several actually from around 2004-2005). You know that there is no one preferred TIME there are MANY TIMES
threading thru the process that is spacetime. For us on the outside the lifespan of a stellar mass Planck star is in the trillions of years. But for the star it is brief: the bounce occurs in an instant. So there are all these timeS occurring at different rates, bundled together in the overall process of spacetime.

So there is no time without some real clock, no "abstract" time is meaningful. There are only correlations amongst real processes and some of those processes we designate to be clocks. (the "partial observables" idea).

So what becomes of the idea of UNITARITY?

And if a stellar mass Planck star blows up after 100 trillion years does it really deliver back to us the information that originally fell into it? Or has that information faded, and become utterly irrelevant, over that long period of time? So maybe the information is lost after all?

I was thinking about these matters this morning and I recalled those Gambini&Pullin papers:
==quote from earlier post==
http://arxiv.org/abs/gr-qc/0501027
...
I am glad to see they are following up on their argument about decoherence (which would make the BH information paradox unobservable)

they have constructed a discrete quantum gravity which, I believe allows them to be more precise about the decoherence----which however was established in an earlier paper using a thought experiment with optimal quantum clocks

Fundamental decoherence in quantum gravity
Rodolfo Gambini, Rafael Porto, Jorge Pullin
6 pages, to appear in the proceedings of DICE 2004 (Piombino, Italy)


"A recently introduced discrete formalism allows to solve the problem of time in quantum gravity in a relational manner. Quantum mechanics formulated with a relational time is not exactly unitary and implies a fundamental mechanism for decoherence of quantum states. The mechanism is strong enough to render the black hole information puzzle unobservable."

----a brief exerpt from the conclusions section at the end---

Summarizing, we have shown that unitarity in quantum mechanics only holds when describing the theory in terms of a perfect idealized clocks. If one uses realistic clocks loss of unitarity is introduced. We have estimated a minimum level of loss of unitarity based on constructing the most accurate clocks possible. The loss of unitarity is universal, affecting all physical phenomena. We have shown that although the effect is very small, it may be important enough to avoid the black hole information puzzle.

---end quote---
==endquote from earlier post==

 

[marcus]

Paulibus, when I said "trillions" of years that was something of an understatement

I see from the Planck star paper that the lifespan of a Planck star goes as the CUBE of the initial mass.

And a 0.6 billion metric ton mass implies a lifespan of about 14 billion years (current expansion age).

So that's 0.6e12 kilograms, in google calculator notation. So I can take the mass of an astrophysical BH as 3 solar, and put this into google:

(3*mass of sun/(.6e12 kg))^3

it comes out to be a huge number. That's how many times the current expansion age the lifespan is. It is an inconceivably long time. So the Gambini Pullin statute of limitations on information would surely have taken effect and every shred of relevance have faded.

 

[marcus]

A couple of posts back, I gave a pointer to one of Gambini Pullin's Fundamental Decoherence papers. As it happens that paper was not among their most highly cited on the topic, so I want to give a more complete biblio

...there are all these timeS occurring at different rates, bundled together in the overall process of spacetime.
...there is no time without some real clock, no "abstract" time is meaningful. There are only correlations amongst real processes and some of those processes we designate to be clocks. (the "partial observables" idea).

So what becomes of the idea of UNITARITY?
And if a stellar mass Planck star blows up after [umpteen ] trillion years does it really deliver back to us the information that originally fell into it? Or has that information faded, and become utterly irrelevant, over that long period of time? So maybe the information is lost after all?
...

http://arxiv.org/abs/gr-qc/0501027
...
Fundamental decoherence in quantum gravity
Rodolfo Gambini, Rafael Porto, Jorge Pullin
6 pages, to appear in the proceedings of DICE 2004 (Piombino, Italy)
"A recently introduced discrete formalism allows to solve the problem of time in quantum gravity in a relational manner. Quantum mechanics formulated with a relational time is not exactly unitary and implies a fundamental mechanism for decoherence of quantum states. The mechanism is strong enough to render the black hole information puzzle unobservable[/B]."

----from the conclusions section---
Summarizing, we have shown that unitarity in quantum mechanics only holds when describing the theory in terms of a perfect idealized clocks. If one uses realistic clocks loss of unitarity is introduced. We have estimated a minimum level of loss of unitarity based on constructing the most accurate clocks possible. The loss of unitarity is universal, affecting all physical phenomena. We have shown that although the effect is very small, it may be important enough to avoid the black hole information puzzle.
---end quote---

Here's a more complete listing of the papers on this:

http://inspirehep.net/record/645205 47 cites (A Relational solution to the problem of time in quantum mechanics and quantum gravity induces a fundamental mechanism for quantum decoherence)
http://inspirehep.net/record/653376 38 cites (Realistic clocks, universal decoherence and the black hole information paradox)
http://inspirehep.net/record/674573 12 cites (Fundamental decoherence in quantum gravity)
http://inspirehep.net/record/712912 38 cites (Fundamental decoherence from quantum gravity: A Pedagogical review)
http://inspirehep.net/record/735013 25 cites (Relational physics with real rods and clocks and the measurement problem of quantum mechanics)

Incidental info: B.L. Hu at U Maryland(Ted Jacobson's department) has reviewed Fundamental Decoh. papers by G&P and also by a number of other authors. So he gives a broader picture of the literature on this, not just referring to Gambini and Pullin. However in the following he focuses in large part on their work:
http://inspirehep.net/record/781938 (Intrinsic and Fundamental Decoherence: Issues and Problems)
http://inspirehep.net/author/profile/B.L.Hu.1 (profile of Bei Lok Hu)
http://inspirehep.net/author/profile/J.A.Pullin.1 (profile Jorge Pullin)
http://inspirehep.net/author/profile/R.Gambini.1 (profile Rodolfo Gambini)

 

[marcus]

In my (non-expert) view the most interesting of the Gambini&Pullin "fundamental decoherence" papers from the Planck Star perspective is the JUNE 2004 one
hep-th/0406260 titled "Realistic clocks…"

It is really interesting, they explain why the most accurate possible clock (over the long haul) IS a black hole! If you try to make a more ordinary clock (like a light pulse bouncing between two mirrors) more and more precise over longer and longer intervals you get something so massive that it collapses to hole anyway!

==page 3, right after equation (5)==
"We therefore see that when time reaches the evaporation time T = T_max, the density matrix element vanishes, i.e. the state has decohered completely. Therefore there is no information puzzle to be contended with."
==endquote==

==page 1 second column==
The fundamental accuracy with which one can measure a time Tmax is therefore determined by the lifetime of the black hole and is given by
δT ∼ t_P (T_max/t_P)^1/3 (1)
where t_P is Planck’s time and from now on we choose units where h ̄ = c = 1 .
In order to do quantum mechanics with realistic clocks, one has to include the clock as part of the system under study. A suitable construction has been proposed by Page and Wootters [4] and a recent reanalysis is present in the paper by Dolby [5]. What one does it to compute probabilities for quantities of the system under study conditional on the quantities describing the clock taking given values. If the clock behaves semiclassically, the resulting probabilities satisfy approximately a Schrödinger equation. However, since the clock can never behave entirely classically, there will be corrections, at least if one wishes to recover Schrödinger’s equation at a leading order [6]. We have estimated the type of corrections in reference [7] in the context of a discrete theory [8] but the construction can also be applied to the continuum case. In particular, the corrections imply that the quantum states do not evolve unitarily. Notice that the argument is based on ordinary (unitary) quantum mechanics, we are just recasting the theory in terms of a realistic clocks and this is the root of the loss of unitarity. The magnitude of the loss of unitarity is characterized by a function with units of time that is associated with how accurate the clock one considers is with respect to an ideal classical clock.
We briefly recount the derivation of the decoherence formula from reference [7]. We consider a system described by a variable X and a clock described by a variable T . Both variables are treated quantum mechanically and evolve according to Schr ̈odinger’s theory with respect to an ideal time t. We can start the system in an optimal quantum state for the clock, in which the probability density for the variable T has the shape of a Dirac delta…
==endquote==
[7] R. Gambini, R. A. Porto and J. Pullin, Class. Quant. Grav. 21, L51 (2004) [arXiv:gr-qc/0305098];
New J. Phys. 6, 45 (2004) [arXiv:gr-qc/0402118].
The latter reference is to the "Relational solution to the problem of time…" article mentioned in previous post.

A followup essay by G&P (second prize in one of the FQXi essay contests)
http://arxiv.org/abs/0903.1859

 

[Chronos]

I remain disappointed with the lack of observational support for minimum scale parameters. Perhaps our observational tests still lack adequate signal to noise ratios.

 

[marcus]

Hi Chronos, thanks for your interest. I just realized that Aurelien Barrau (co-author of the most recent Planck Star paper) just gave an International LQG Seminar talk in which he pointed out the observational interest for LQG of the Planck Star Gamma Ray Burst prediction, and showed a plot of the predicted gamma ray spectrum.
Barrau's slides:
http://relativity.phys.lsu.edu/ilqgs/barrau042914.pdf
Audio of joint talk by Agullo Barrau Mena
http://relativity.phys.lsu.edu/ilqgs/agullobarraumena042914.wav

Since we turned a page I've neglected to give a link to the Barrau Rovelli Planck Star paper. As you may recall, since it's essential to the main topic here, Barrau Rovelli predicted the GAMMA RAY SPECTRUM of a Planck Star burst that would be currently observable according to their model. See Figures 3 and 4 of their paper.

I'll repeat the links
Barrau Rovelli (with the GRB spectrum plot):
http://arxiv.org/abs/1404.5821
Planck star phenomenology
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.
5 pages, 4 figues

The reservation here is that primordial BH might be TOO RARE and their explosions TOO SELDOM to be observed. However very short GRB are in fact observed, so it would be possible to compare the spectral information on observed bursts with the predicted spectrum to see if there are any candidates.

Wide audience coverage:
http://news.discovery.com/space/could-black-holes-give-birth-to-planck-stars-140211.htm

Original Rovelli Vidotto Planck Star paper:
https://inspirehep.net/record/1278812
http://arxiv.org/abs/arXiv:1401.6562
I see it has 4 citations already
https://inspirehep.net/record/1278812/citations

 

[marcus]

The Planck star model of black holes, which came out just recently (earlier this year) had begun appearing in the workshop/conference context.
It was featured in an hour lecture at the June 2014 SIGRAV school on quantum gravity.
http://www.centrovolta.it/sigrav2014
And in September it will again be presented for discussion at the Experimental Search for Quantum Gravity (ESQG) workshop to be held in Trieste at the International School for Advanced Studies (SISSA)
http://www.sissa.it/app/esqg2014/
"The purpose of the workshop is to bring together experimentalists, theoreticians, and phenomenologists interested in possible tests probing the quantum/discrete structure of spacetime. There will be a number of rather focussed talks discussing possible phenomenological tests of quantum gravity and proposing some new ideas in this direction."

The point is that Planck star model of black holes has definite and distinctive observational consequences---the final explosion of the BH with a power and spectrum depending on the initial mass (therefore the lifespan) and thus, in the case of primordial black holes the epoch in which they end.

I thought the lineup of ESQG participants this time was interesting, so will list them.

Stephon Alexander	Dartmouth
Giovanni Amelino-Camelia    Sapienza, Rome
Massimo Cerdonio	INFN - Padua
Astrid Eichhorn	        Perimeter Institute, Waterloo
Agnes Ferte        	Institut d'Astrophysique Spatiale
Julien Grain        	Institut d'Astrophysique Spatiale
Jonathan Granot         Open University of Israel
Giulia Gubitosi	        Sapienza, University of Rome
Brian Keating	        University of California, San Diego
John Kelley	        IMAPP, Radboud University, Nijmegen
Jerzy Kowalski-Glikman	University of Wroclaw
Joao Magueijo	        Imperial College, London
David Mattingly	        University of New Hampshire
Jakub Mielczarek	Jagiellonian University, Crakow
Jonathan Miller	        Universidad Tecnica Federico Santa Maria
Daniele Oriti	        Albert Einstein Institute
Igor Pikovski	        Vienna Center for Quantum Science and Technology
Carlo Rovelli	        Aix-Marseille University
Floyd Stecker	        NASA - Goddard Space Flight Center

 

[marcus]

As of today, Gambini and Pullin have brought out an alternate nonsingular BH-->WH model that also, like that of Rovelli et al, appears to solve problems such as the "information loss paradox" and the "firewall" worry.

The info and insides of the BH come out in a White Hole formed in a separate region rather than exploding back, in a Gammaray Burst (GRB) as in the Planck star case.

http://arxiv.org/abs/1408.3050
A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin
(Submitted on 13 Aug 2014)
We incorporate elements of the recently discovered exact solutions of the quantum constraints of loop quantum gravity for vacuum spherically symmetric space-times into the paradigm of black hole evaporation due to Ashtekar and Bojowald. The quantization of the area of the surfaces of symmetry of the solutions implies that the number of nice slices that can be fit inside the black hole is finite. The foliation eventually moves through the region where the singularity in the classical theory used to be and all the particles that fell into the black hole due to Hawking radiation emerge finally as a white hole. This yields a variant of a scenario advocated by Arkani-Hamed et al. Fluctuations in the horizon that naturally arise in the quantum space time allow radiation to emerge during the evaporation process due to stimulated emission allowing evaporation to proceed beyond Page time without reaching the maximum entanglement limit until the formation of the white hole. No firewalls nor remnants arise in this scenario.
5 pages

I think it's supportive research because it shows interest moving in the same general direction even though there is divergence in some details.

 

[marcus]

Another Planck star-related paper that appeared recently is the "Black Hole Fireworks" paper by Haggard and Rovelli. I'll get the abstract. This paper has been discussed in a separate thread started by JulCab, but it fits closely with the topic here.
https://www.physicsforums.com/showthread.php?t=760516
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.
10 pages, 5 figures

Next month there will be the fourth workshop on the Experimental Search for Quantum Gravity (ESQG) at the ISAS Trieste.
The schedule, with titles of talks, is now online. http://www.sissa.it/app/esqg2014/schedule.php Rovelli's talk will be:
==quote==
Carlo Rovelli (Aix-Marseille University)
10:30, Wed 3rd Sep 2014
Planck Stars

I describe a new suggestion for measurable quantum gravity effects: the bounce of a primordial Planck star.
==endquote==

 

[marcus]

There are a fair number of seminar, workshop or conference TALKS being given about the Planck star (slo-mo rebound to GRB) model of black hole.
On 5 June Eugenio Bianchi gave an hour talk at the 2014 SIGRAV (Trapping horizons and the Planck star)http://www.centrovolta.it/sigrav2014/Schedule.pdf
On 6 June at SIGRAV, Rovelli gave a second hour lecture on the Planck star model.

Today 20 August, Hal Haggard is giving a Planck star related talk at UC Berkeley based on this paper:
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.
10 pages, 5 figures

Next month Rovelli will give two talks. the first at International School of Advanced Studies:
http://www.sissa.it/app/esqg2014/schedule.php
==quote==
Carlo Rovelli (Aix-Marseille University)
10:30, Wed 3rd Sep 2014
Planck Stars

I describe a new suggestion for measurable quantum gravity effects: the bounce of a primordial Planck star.
==endquote==
The second at University of Rome-Sapienza http://ctcqg2014.relativerest.org/plenary-talks/

The fifth scheduled talk I know of will be 14 October at the online International LQG Seminar.
http://relativity.phys.lsu.edu/ilqgs/
We can expect to be able to follow the slides as we listen online.
slides PDF:
http://relativity.phys.lsu.edu/ilqgs/rovelli101414.pdf
audio:
http://relativity.phys.lsu.edu/ilqgs/rovelli101414.wav

 

[marcus]

Gambini and Pullin just posted a BH bounce paper that is remarkably similar in some respects to the Haggard Rovelli one just mentioned. E.g. it has a collapsing null shell, the black hole is in a sense partly made of light, as in the toy model case Haggard Rovelli use. Both papers use a collapsing shell of light, and make the BH-->WH transition. However G&R have the WH open in new spacetime region. H&R have the BH CONVERT to WH and its contents come bursting out right here in its initial location. I wonder how important that difference really is.
http://arxiv.org/abs/1408.4635
Quantum shells in a quantum space-time
Rodolfo Gambini, Jorge Pullin
(Submitted on 20 Aug 2014)
We study the quantum motion of null shells in the quantum space-time of a black hole in loop quantum gravity. We treat the shells as test fields and use an effective dynamics for the propagation equations. The shells propagate through the region where the singularity was present in the classical black hole space-time, but is absent in the quantum space-time, eventually emerging through a white hole to a new asymptotic region of the quantum space-time. The profiles of the shells get distorted due to the quantum fluctuations in the Planckian region that replaces the singularity. The evolution of the shells is unitary throughout the whole process.
5 pages, 3 figures

 

[marcus]

This week is the ESQG (Experimental Search for Quantum Gravity) meeting at ISAS Trieste.
I believe several Planck star related talks are scheduled. Eugenio Bianchi may give a talk based on this paper (I need to confirm this, just a possibility)

http://arxiv.org/abs/1409.0144
Entanglement entropy production in gravitational collapse: covariant regularization and solvable models
Eugenio Bianchi, Tommaso De Lorenzo, Matteo Smerlak
(Submitted on 30 Aug 2014)
We study the dynamics of vacuum entanglement in the process of gravitational collapse and subsequent black hole evaporation. In the first part of the paper, we introduce a covariant regularization of entanglement entropy tailored to curved spacetimes; this regularization allows us to propose precise definitions for the concepts of black hole "exterior entropy" and "radiation entropy." For a Vaidya model of collapse we find results consistent with the standard thermodynamic properties of Hawking radiation. In the second part of the paper, we compute the vacuum entanglement entropy of various spherically-symmetric spacetimes of interest, including the nonsingular black hole model of Bardeen, Hayward, Frolov and Rovelli-Vidotto and the "black hole fireworks" model of Haggard-Rovelli. We discuss specifically the role of event and trapping horizons in connection with the behavior of the radiation entropy at future null infinity. We observe in particular that (i) in the presence of an event horizon the radiation entropy diverges at the end of the evaporation process, (ii) in models of nonsingular evaporation (with a trapped region but no event horizon) the generalized second law holds only at early times and is violated in the "purifying" phase, (iii) at late times the radiation entropy can become negative (i.e. the radiation can be less correlated than the vacuum) before going back to zero leading to an up-down-up behavior for the Page curve of a unitarily evaporating black hole.
35 pages, 14 figures

Its an exciting possibility. The Planck star model of BH has the potential of providing observational input---slow motion rebound terminating in a very brief GRB (gamma ray burst) with the characteristics of the burst (wavelength, brightness) to some extent predictable.

I'll get a link to the ESQG program and check to see if this might be featured.
http://www.sissa.it/app/esqg2014/
http://www.sissa.it/app/esqg2014/schedule.php
Well I see two Planck star talks, both on Wednesday 3 September, by Rovelli and by Vidotto. So they may refer to this work. But I don't see Eugenio on the program. I do see however that they have added several names to the speakers list since the last time I looked. 35 speakers are now listed:
http://www.sissa.it/app/esqg2014/speakers.php

 

[marcus]

I mentioned Vidotto's ESQG talk in the previous post. The slides PDF has now been posted.
The slides are remarkably clear and well-organized, with good graphs and diagrams. One can almost read the slides as a stand-alone exposition of the ideas.
Here is the overall ESQG schedule with links to slides pdf.
http://www.sissa.it/app/esqg2014/schedule.php

and here is the link to slides PDF for Vidotto's talk on BH bounce producing observable GRB explosions
http://www.sissa.it/app/esqg2014/slides/Vidotto_Trieste_2014.pdf

Francesca Vidotto (Radboud University Nijmegen)
14:30, Wed 3rd Sep 2014
What can we learn from Loop Quantum Cosmology? The case of Planck Stars
Loop Quantum Cosmology suggests that cosmological singularities are generically resolved by quantum effects. This can be understood at the effective level as the appearance of a repulsive force in the deep quantum-gravity regime. A similar mechanism should take place in the interior of black holes, whose singularity would then be replaced by a core of Planckian energy density. Such Planck Star provides a remnant which can help avoid the information paradox. Furthermore, if the evaporation ends with an explosive event, the Planck star could provide a precise astrophysical signal. Using the current models for primordial black holes and the bounds given by dark-matter abundance, this signal could be compatible with a specific kind of gamma rays, that we have already observed.

 

[Demystifier]

A new firework paper:
http://lanl.arxiv.org/abs/1409.1837

 

[marcus]

Another new fireworks paper :^)

http://arxiv.org/abs/1409.4031
Fast Radio Bursts and White Hole Signals
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto
(Submitted on 14 Sep 2014)
We estimate the size of a primordial black hole exploding today via a white hole transition, and the power in the resulting explosion, using a simple model. We point out that Fast Radio Bursts, strong signals with millisecond duration, probably extragalactic and having unknown source, have wavelength not far from the expected size of the exploding hole. We also discuss the possible higher energy components of the signal.
5 pages