Planck Stars: Carlo Rovelli & Francesca Vidotto

[marcus]

I was just going to post this on the Loop&allied QG bibliography and found MTd2 beat me to it by barely a minute:

http://arxiv.org/abs/1404.5821

Planck star phenomenology

Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
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.

It's 5 pages, with 4 figures.

 

[marcus]

Barrau and Rovelli have an interesting calculation of the range at which a PBH explosion can be detected, say with a reasonable size (1 square meter) detector in orbit.

The GRB explosion is powerful, but it is carried by a comparatively small number of high-energy photons, not enough are likely to hit the finite area of the detector to register, if the explosion is more than a few hundred LY away.

Check out their calculation. they also recalculated what the initial mass has to be in order for the thing to explode at present, digging in more detail into the evaporation process, and using a numerical integration.

0.61 billion metric tons. So the mnemonic rhyme has to say "Point six" for 0.6, or words to that effect.

Planck star, you dark rebounder,
how long before you burst?
"I'm almost done!"

Deep time in you goes slowly,
what mass were you at first?
"Point six billion tons!"
_____________

 

[Ken G]

(if I may presume to improve on your fine poem, how do you feel about "doth founder" instead of "goes slowly" to create an even deeper rhyming connection between the verses?)

 

[marcus]

(if I may presume to improve on your fine poem, how do you feel about "doth founder" instead of "goes slowly" to create an even deeper rhyming connection between the verses?)

"Founder" is an excellent suggestion! Let's try this:


Planck star, you dark rebounder,
what mass were you at first?
"Point six billion tons."

Deep time in you does founder,
how long before you burst?
"I'm almost done!"



________

 

[Ken G]

Yes, I like that reversal in the order, it gives a punchline in the end and still serves to help you recall the putative mass of these potential denizens of the otherwise dark corners of space. (Does "deeply" fit the meter better?)

 

[marcus]

Yes, I like that reversal in the order, it gives a punchline in the end and still serves to help you recall the putative mass of these potential denizens of the otherwise dark corners of space. (Does "deeply" fit the meter better?)

I think you are right about the fourth line being metrically just slightly awkward. Let's try this slight change in that line, so the stress comes naturally on the second syllable "time" just as it does on the second syllable of the first line "star".Planck star, you dark rebounder,
what mass were you at first?
"Point six billion tons."

Deep time, in you, must founder.
How long before you burst?
"I'm almost done!"Let's let it sit like that for a while and get used to it before we try further changes. You've already helped improve the little rhyme quite a lot. I'd rather not make changes too fast. Let's look at it again tomorrow.

BTW the shadow pop song structure I'm hearing as a kind framework is Righteous Brothers "Unchained melody". It's on YouTube. Was popular in the mid-Sixties.
The third line in the RB original is:
"a LONG lonely time" and we are saying
"point SIX billion tons" as a partial echo with their stress pattern

The sixth line in the RB original is:
"are you still MI-I-I-NE?!" and we are saying
"I'm almost DONE!" again partially echoing the stress pattern.

Here's a 1965 recording of the Righteous Brothers song I'm referring to for stress pattern:

 

[marcus]

For people who have already taken a look at 1401.6562, and 1404.5821, I'm repeat some things I found interesting and/or surprising.

One thing I found interesting is the detection range that Barrau Rovelli calculate in 1404.5821 (the phenomenology paper) namely with an assumed square meter detector surface a range of only 200 light years!

This is for an explosion wattage which is, for a brief interval like one second, roughly 100 times the wattage of the sun. Namely the "mc²" energy equivalent of 0.4 billion tons mass*, delivered in, say, one second. If that power were presented in the usual starlight spectrum---the UV-visible-IR range---it would be detectable much farther off than 200 light years. What they point out in 1404.5821, that I didn't think of earlier myself[/color] is that since the power is presented in very high energy photons there are way fewer photons.

The photons are comparatively speaking so sparse that beyond a certain range (like 200 LY) they might be so spread out that they entirely miss the finite area detector. If you assume a larger detector you get a longer detection range of course---with 4 square meters the range doubles to 400 LY. But the detector has to be up outside the atmosphere so there are reasonable cost limits on what area one wants to assume is practical.

BTW what this seems to imply is that one couldn't rule out primordial Planck stars as a significant fraction of dark matter, merely because then we would be seeing lots and lots of them. Since we would only detect the explosions if they are within 200 light years there could be a substantial amount of dark mass out there in the form of the ASTEROID-MASS objects which would not show up in LENSING SEARCHES.
Of course primordial PS might NOT constitute a significant fraction of dark mass, but it looks like we cannot rule it out so easily. One needs to search for nearby gamma ray bursts with predicted photon energy around 10 MeV (wavelengths around a tenth picometer) and get some STATISTICS before one can put constraints on the relevance to dark matter.

That detection range limit only applies to seeing INDIVIDUAL EVENTS. They also start investigating the implied effect of Planck star model primordial black hole explosions on the gamma ray BACKGROUND.
That means integrating the diffuse radiation that one would expect from more distant, earlier, younger explosions as well: "hotter" radiation because from lower mass smaller size primordial BH that exploded earlier e.g. in distant galaxies, and on the other hand redshifted, e.g. z=2 or z=3. The redshifting and the "hotter" partially cancel each other. So Barrau Rovelli also look at the phenomenological consequences to the gamma background as distinct from what one expects in the individual GRB events department.

*the final mass is the initial mass divided by sqrt(2), so final is about 70% of initial. For primordial Planck stars exploding at present time, initial is .6 billion tons, so final is about 0.4 billion tons.
Their more precise figure is 0.43, see eqn 2.7.

http://arxiv.org/abs/1401.6562
http://arxiv.org/abs/1404.5821

 

[Paulibus]

In my post #72 I found the root cause of the bounce difficult to fit into my primitive background
of Physics. I’m still having difficulty with this, by Rovelli and Vidotto:

http://arxiv.org/abs/1401.6562, p.1, paraphrased ...The bounce is due to a quantum-gravitational repulsion which originates from the Heisenberg
uncertainty, and is akin to the "force" that keeps an electron from falling into the nucleus...

This introduction to the key point of the whole ‘bounce’ scheme is on p. 1 of the Rovelli-Vidotto
paper.

But I’d still like to have help to better understand how this "quantum-gravitational repulsion"
arises. In fact I wonder if the bounce is not better related to the Pauli Exclusion principle, rather
than to Heisenberg’s uncertainty principle?

The reason that two particles (say Fermions; particles with half-integral spin, like electrons) can’t
occupy the same state is that electrons are all identical, so when their wave functions overlap they
can’t be kept track of as individuals. Indeed there is a

.. deep physical connection between transformation of states under spatial
rotations (like spin?).. and the statistics of many-particle systems...

that leads to failure
for repeatedly deploying a quantum-mechanical ‘creation’ operator to place more than one
electron in a state labelled with the same quantum numbers.

Happily for us, a lot follows from the quantum quirk of being able to sort fundamental particles
into Bosons and Fermions. This distinction prevents electrons from all collapsing into a common
ground state. Via short-range repulsion it stabilizes structures with electrons, like atoms and
metals. Indeed it enables the Periodic table and let's electricity flow easily along metal wires into
our houses. Great stuff. But in this case without a shattering bounce.

Perhaps some more esoteric quantum-mechanical quirk related to the Pauli principle could helps
to violently rip apart collapsing Planck stars and so avoid a singularity?

...I want us to be able to contemplate something more quantitative...
...I get the impression that central people like Ashtekar do not themselves have an intuitive
understanding of why the cosmological bounce has turned out to be such a robust feature of their
model. They have acted as if they were surprised when it surfaced in 2001 and still cautious about
it in 2006. But that shouldn't make US give up on getting an intuitive sense of why it happens.

. Agreed.

 

[Ken G]

That's an interesting point about the exclusion principle, it suggests that a black hole made of a distinguishable mixture of Fermions, like different quarks, or even of bosons, like gluons or photons, would have very different "quantum bounce" properties than one made up of identical fermions. That's a problem, it seems to me, because if one wants to use the bounce to solve the information paradox, and if the bounce spits out a bunch of gamma rays (which are bosons), how can it retain the information of what kinds of particles went in there in the first place? Or put differently, if they are "primordial" black holes, they might have been there "from the beginning", and so it might not be well defined what kinds of particles created them, one might only know the mass of the primordial black hole. That can't be if the bounce needs to know if they are bosonic or fermionic. Perhaps one simply expects them to form from the quark-gluon plasma, but the quarks are fermions and the gluons are bosons, so wouldn't they need to know the relative proportion to know how the bounce happens?

 

[Paulibus]

I should warn that my temerity in suggesting that a Rovelli-Vidotto remark is wrong (their writing that the gravitational-quantum bounce is caused by the Uncertainty Principle) is based on my pretty shaky understanding of the whole 'bouncing loop-quantum-gravity' scenario.

My further suggestion that one should look rather to the Exclusion Principle for intuitive understanding of such a 'bounce' scenario should therefore be taken with a pinch of salt.

But the Exclusion Principle is known to be such an effective condensation or collapse preventer, while the Uncertainty Principle hardly fits this needed role, so it's faute de mieux!

 

[marcus]

Hi Paulibus, I've been slow to respond. It's a good question. there should be a path of INTUITIVE reasoning from some basic principle such as HUP (or some other if not that) to, say, the discreteness of the LQG area operator---a minimum positive area eigenvalue.

After that it seems intuitive---curvature is reciprocal of area. A minimum positive area means a maximum curvature (so collapse can't go all the way to classical singularity). Also you may remember the paper where Rovelli and Vidotto showed there is a maximum ACCELERATION in Lqg and argued that the BH singularity is thereby avoided. that was also based on the discrete areas spectrum as I recall.

I went back to the January Planck Star paper http://arxiv.org/abs/1401.6562 to see what their reference was. One of their references, [17], was to http://arxiv.org/abs/1310.8654 which I want to check out. It might help.
==quote http://arxiv.org/abs/1401.6562 page 1==
For instance, a collapsing spatially-compact universe bounces back into an expanding one. The bounce is due to a quantum-gravitational repulsion which originates from the Heisenberg uncertainty, and is akin to the “force” that keeps an electron from falling into the nucleus [16]. The bounce does not happen when the universe is of Planckian size, as was previously expected; it happens when the matter energy density reaches the Planck density [17].
==endquote==
The two references are to
http://arxiv.org/abs/gr-qc/0612104
http://arxiv.org/abs/1310.8654

In the latter paper (October 2013) I see the Heisenberg uncertainty principle appearing on page 2 in a discussion of bounce dynamics. But I can't give you an intuitive account, as yet. I don't understand this well enough yet. I see the simple Friedman universe being modeled using Heisenberg dynamics and a CONJUGATE pair of variables (in the classical development) which become operators in the quantum version. I see a Heisenberg dynamics equation using the commutator of these operators. I see the HUP applied to the conjugate pair "c" and "p" where p is associated with the scale factor---the "size" of the universe in the Friedmann cosmic model. And where c is conjugate to that. Maybe analogous to how momentum is conjugate to position---could "c" be a rate that the scale factor is changing? Could "c" be related to energy density? I keep seeing an HUP-like expression involving ΔcΔp or the operator version of that, with the tildes. I don't understand this well enough to discuss it. Anyway maybe some intuition can be dug out of the October 2013 paper (by Rovelli and Wilson-Ewing) or else out of the earlier December 2006 paper that was referred to also.

You asked if they possibly could have meant Pauli exclusion instead. I think that primarily involves Fermionic matter and here we are mainly concerned with geometry, sometimes with a scalar field as token matter. As best I can see right now, probably they really mean HUP, not Pauli exclusion. but I can't be sure! Hope to understand this better in a few days and be able to respond with a bit more competence

 

[marcus]

Since we've turned a page, I'll bring forward a quote from discovery.com that describes the basic Planck Star idea rather well, for genera audience:

, there's a rather good popular news article about the idea in "discovery.com" magazine:
http://news.discovery.com/space/could-black-holes-give-birth-to-planck-stars-140211.htm

===sample excerpt===
What goes on inside a black hole’s event horizon has actually caused a theoretical conflagration and now, two theoretical physicists have proposed a new idea that may marry quantum mechanics with gravity, extinguishing the tricky “firewall” and finding a solution to the “information paradox.”
==endquote==

==more from the discovery.com article==
…Rovelli and Vidotto looked at this problem from a different perspective. While working on models of a collapsing universe — i.e. the opposite to the Big Bang, known as the Big Crunch — they found that the fundamental quantum structure of the Universe prevents an infinitely dense singularity from forming. The collapse of the Universe therefore reaches a fundamental density, causing the universal collapse to rebound, or “bounce.”...

Say if a similar model can be used to describe a black hole?

A Planck Star Rises

If a massive star explodes as a supernova, creating a black hole in its wake, what if the superdense material that formed the black hole actually didn’t form a “singularity”? Sure, the material is unimaginably dense, but the object in the core of the black hole still has structure. Rovelli and Vidotto argue that the inward force of gravity is counteracted by the quantum structure of the Planck density.

If we were to zoom in, far beyond the size of quantum particles, it is theorized that we will reach a fundamental scale known as the Planck length. Should matter be compressed to these scales, rather than disappearing into an “infinitely dense” singularity — a solution that doesn’t make a whole lot of sense — perhaps the contraction stops at the Planck density, creating a “Planck Star” and the object rebounds, or “bounces.” From the perspective of the Planck Star, it will be a very short-lived affair; it’s collapse and bounce would occur rapidly. But to outside observers elsewhere in the Universe (i.e. us), as space-time surrounding the Planck Star is so extremely warped, time dilation makes the black hole (and the Planck Star it contains) seem static and unchanging.

Over time, as the black hole loses mass to Hawking Radiation and the Planck Star continues to expand after the rebound, the event horizon of the black hole will slowly contract, eventually reaching the surface of the Planck Star contained within. At this point, argue the researchers, all of the information the black hole ever consumed over its lifetime will be suddenly released to the Universe — solving the “information paradox.” What’s more, we should be able to detect this deluge of information.
...
...
“(Planck Stars) produce a detectable signal, of quantum gravitational origin, around the 10^-14cm wavelength,” they write. This signal could embody itself in cosmic rays of energies in the GeV range, a signal that can be easily detected by gamma-ray observatories.
==endquote==

 

[Paulibus]

Thanks for that very full reply to my muddled post on the cause of repulsion that causes the bounce. I'd not properly read the Rovelli-Wilson-Ewing paper, which explains the relevance of the Heisenberg uncertainty principle to the bounce. I'm still baffled by the c in the their uncertainty relation, which seems to be defined as a "configuration variable" in "Mathematical structure of loop quantum cosmology" by Ashtekar, Bojowald and Lewandowski (arXiv:gr-qc/0304074v4 24 Dec 2003). It's way above my head. But c must represent something physical and measurable, but I can't see quite what, yet. I'll think about your suggestions.

 

[marcus]

Thanks for that very full reply to my muddled post on the cause of repulsion that causes the bounce. I'd not properly read the Rovelli-Wilson-Ewing paper, which explains the relevance of the Heisenberg uncertainty principle to the bounce. I'm still baffled by the c in the their uncertainty relation, which seems to be defined as a "configuration variable" in "Mathematical structure of loop quantum cosmology" by Ashtekar, Bojowald and Lewandowski (arXiv:gr-qc/0304074v4 24 Dec 2003). It's way above my head. But c must represent something physical and measurable, but I can't see quite what, yet. I'll think about your suggestions.

I'm still far from being able to get my mind around the significance of the HUP and how it plays a role here. But maybe there is hope. A friend writes suggesting that I should think more about PHASE SPACE. You know in conventional dynamics of an N particle system each particle has a 3D position and a 3D momentum. So there are a pair of conjugate variables for each particle. Phase space is this large dimension vector space recording these 2N variables.

It seems that the Planck's hbar is the natural RESOLUTION SCALE of phase space! It indicates how fine you can grind it or how clear you can see it. It seems significant that the UNIT of hbar is length*momentum, or equivalently time*energy. which is also the separation unit in phase space!

So if n is the dimension of phase space the volume of a "phase-fuzz element" or "blur-cell" of phase space is hbarⁿ. Not sure what that means. It is the volume of a blob that your eyes can resolve into two blobs. It is interesting that, if the system has 5 particles and hence ten 3d degrees of freedom and so phase space is 30 dimensional Euclidean R³⁰ that then there should be this small volume which we can calculate by taking the 30th power of Planck constant: hbar³⁰

and the units work out, that is the right unit of volume because the unit along the axes in R³⁰ is in fact length*momentum. Not only am I not sure I know what this means, I know I am NOT sure what it means.

It seems that Nature holds the line against precision, defies being pinned down, beyond a certain point. If I understood conventional dynamics better I might be able to grasp how this defiant existential frivolity of nature could lead to a bounce. Today I shall just have lunch and not try anymore to understand her.

 

[Paulibus]

Yes, I'm familiar with resolving power, say of the human eye (about 1/10th of a mm). And of limits to resolution imposed by the wavelength of light, or electrons, used as observing probes. I suppose one can regard the lattice spacing of say, a copper crystal, as a graininess akin to a resolving power, which controls some of the measured physical properties of copper, like mechanical strength and electrical conductivity. But I'm talking here of limited 'resolution' in an observer-related sense that's quite 'real' for me.

When it comes to 'limited resolution', 'phase-fuzz element' or 'blur cell' connected with abstract human constructs like multi-particle phase space, my understanding starts to totter. But I often wonder, idly, if regular, simple-seeming, everyday space-time will eventually be revealed as grainy, in which case I'd be more comfortable with a grainy phase space.

Perhaps the abstract physics of a bouncing cosmos described in terms of a grainy phase space defines a path that leads this way, hopefully to be someday confirmed by prediction and observation. In the meantime I'll watch this space with interest and just enjoy Sunday lunch.

 

[marcus]

... In the meantime I'll watch this space with interest and just enjoy Sunday lunch.

We had a really enjoyable Sunday. It was my wife's birthday. Our son (I almost wrote sun) came over and did some redwood carpentry and told us latest tech/geek news and we watched a movie called *quartet* that Dustin Hofmann directed, about very old retired musicians at a place in English countryside. Redwood is a really nice material. Durable but soft, easy to handle, and lovely to look at as rich chocolate is to taste.

I think of you sailing one of those over-sailed planing-hull scows equipped with a trapeze. Must be an unforgettable excitement.

You mentioned that graininess of PHASE-SPACE would be more intuitive if one already had the idea of graininess of space. One does have it in LQG geometry, in a very interesting sense. GEOMETRIC MEASUREMENTS in LQG are always grainy because of the oldest most basic theorem in Loop, the discreteness of the area operator spectrum. There is a smallest positive area eigenvalue which can occur from a measurement of area. (Similar facts about volume, angle etc.)

But geometry in LQG is also grainy in another sense. This is more involved and more work to understand. You know that in 1915 GR the geometry is an *equivalence class* of metrics defined on manifolds. There is no preferred representative of the geometry. No preferred manifold or set of points, and no preferred metric ON whatever manifold happens to be be used.
Because of general covariance and background independence there is no fixed material space which could be grainy, no aether which could have grains.

Space is nothing but the gravitational field itself. Nothing but the geometry field. Nothing but the abstract equivalence class of metric-and-matter layouts. (no underlying point-set) So ontologically it is somewhat analogous to the electromagnetic field. Something one cannot see, but believes physically real, and which is experienced by matter through interactions e.g. measurements of the field by some event.

Space IS geometry. Geometry is finally nothing but geometric measurements, interactions that occur. This points to the deeper discreteness than what I already mentioned. Geometry is a quantum field, inferred continuous but experienced by matter in discrete facts (interactions). Just as the electromagnetic field although inferred continuous is experienced in discrete quanta, in discrete PHOTON interactions.

In a quantum field theory phenomena are intermittent. They occur here and there, with this or that bit of matter, now and then.
Phenomena are discrete, continuities are inferred.

So geometry IS grainy, in response to your question, in both the simple sense that e.g. the area observable has discrete spectrum,
and what I think is a deeper sense that all quantum fields (whether they are geometry or matter) are grainy in their phenomena.

 

[MTd2]

Hey Marcus, have you seen my post on the LQG thread? Do you think LQG can describe a de sitter space like that? The curious thing it is that (despite trying to solve the non existent problem of boltzman brains), it seems that the end of the universe is like the beginning.

 

[Paulibus]

The enthusiasm you have for Loop Quantum Gravity's conclusion that geometry (and its manifestation as gravity) is grainy in the quantum sense is heartening, Marcus. Thanks for that illuminating post. I hope that such granularity will in time be revealed by the tried and tested physics cycle of prediction and observation, which distinguishes our subject's imaginings from prolific human fantasies; sample below.

Quantum phenomena seem to me to be often linked to the phenomenon of resonance, which in turn couples our familiar dimensions of time and space. Bohr's concept of atomic energy levels being linked to integral numbers of wavelengths is an example; standing waves. Maybe the Pythagorean concept of 'the music of the spheres' was prescient?

And that's enough mystic nonsense!

 

[marcus]

The enthusiasm you have for Loop Quantum Gravity's conclusion that geometry (and its manifestation as gravity) is grainy in the quantum sense is heartening, Marcus. Thanks for that illuminating post. I hope that such granularity will in time be revealed by the tried and tested physics cycle of prediction and observation, which distinguishes our subject's imaginings from prolific human fantasies; sample below.

Quantum phenomena seem to me to be often linked to the phenomenon of resonance, which in turn couples our familiar dimensions of time and space. Bohr's concept of atomic energy levels being linked to integral numbers of wavelengths is an example; standing waves. Maybe the Pythagorean concept of 'the music of the spheres' was prescient?

And that's enough mystic nonsense!

Frank Wilczek (nobel for the tricolor glue that holds quarks and stuff) wrote a nice article a while back about the engrained stubborn Pythagorishness of modern physics. He's part humanist (not all Vulcan so to speak) and has composed some decent sonnets. You might like the article.

How do you know how much is enough mystic nonsense? Isn't it true that there's never enough of the right kind?

Have you watched Rovelli's June 2013 Oxford "cosmology and quantum theory" YouTube?

Let me google rovelli cosmology relational and see if it comes up.

Yes, it is the first hit.

compared to that talk, the Wilczek piece is casual light reading but if curious google the title
"world's numerical recipe" and the author's name. Probably
wilczek world recipe would get it
Yes, just those three words.

 

[marcus]

About making sense of the bounce…that's the essential topic here: it's common to both the Planck star concept of BHs and Loop cosmology…so I may as well have another go at it.
A friend recently sent me an intuitive take on it part of which I'll quote:
==excerpt from private message==
...regarding ...heisenberg uncertainty principle,... there is a standard argument for the stability of atoms because of quantum theory. the electron cannot fall into the nucleus because HUP forbids it to be too localised without zipping away. i would see the cosmological bounce and the core of the Planck stars as possible manifestations of the very same thing. as you say, nature does not like to be pinned down too precisely. discreteness and therefore the area gap is a manifestation of the same: in the classical phase space, a system cannot be squeezed in a region smaller than hbar (hbar has the dimensions of phase space ...). so we cannot have an eigenvalue of the energy of a harmonic oscillator, or of the electron in a coulomb potential, or of the area of a region, or of the volume of a symmetric universe, so small as to require the corresponding state to be squeezed in too small a region of phase space. ...
==endquote==

 

[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