Firewalls, Singularities, and the Unibabe Question

[marcus]

The fuss over "firewalls" is basically a challenge to the assumption that watching a black hole form and evaporate, the observer at infinity sees unitary evolution. If you think the BH singularity might be resolved in a bounce, you don't make that assumption (as Lee explains in the following). On the other hand if you do make that assumption you arrive at one or more unattractive or paradoxical conclusions.
===quote===
Scott Aaronson says:
August 23, 2013 at 9:32 pm
...While I’m obviously far from an expert, where I think I part ways from you and Unruh is on the following. We’re pretty sure black holes have an entropy, which goes like the area of the event horizon in Planck units. We’re pretty sure that, from an external observer’s perspective, infalling stuff gets “pancaked” on the event horizon and scrambled beyond recognition, never making it through to the interior. Finally, we’re pretty sure that the external observer ultimately sees the black hole evaporate, through Hawking radiation that emerges (appears to emerge?) from the horizon. To me, these facts would seem like an intolerable coincidence, if the black hole didn’t have microstates—”stored,” one wants to imagine, on or near the event horizon—and if the Hawking radiation didn’t carry away the information about those microstates...
...
...
Lee Smolin says:
August 24, 2013 at 7:18 am
Dear Scott,

Thanks, but either I don’t understand your argument or else it is circular. What do you suppose happens to the singularity as well as to the quantum state of the star whose collapse formed the black hole in the first place? If the singularity is eliminated then the Hilbert space in the future is a direct product of a factor spanned by observables which describe degrees of freedom to the future of where the singularity would have been and a factor spanned by observables external to the horizon. The evolution onto this product can be assumed to be unitary but (I feel silly telling you this) it cannot be when restricted to either of its factors. Hence the observer at infinity describes a density matrix gotten by tracing out the degrees of freedom in the baby universe inaccessible to them.

Isn’t this a completely reasonable option, especially because it avoids the otherwise paradoxical implications of the firewall argument?

The pancake is a non-sequitur: why does it matter what information does or doesn’t get to infinity or when, if infinity is not the only place information goes to? So to refer to it seems to assume what you are claiming to demonstrate.

Many thanks,
Lee
==endquote==
http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159261
and
http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159264

 

[marcus]

Pointed use of the term reductio ad absurdum where you draw out the consequences of some position until they are seen to be absurd:

http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159253
==quote Lee==
...
The lesson in my opinion is that the key issue in quantum black holes and the information problem is not at the horizon, it is at the singularity. It is unreasonable to expect any new physics at horizons where the curvatures are small, but necessary to find new physics at the approach to singularities. The focus on the firewall problem is in my view a consequence of insufficient appreciation of this point. It can be seen as a reductio for the assumption that the problem can be resolved without investigating how quantum gravity effects eliminate the singularity and taking on board the consequences of the resulting evolution to the future of where the classical singularity would have been.
==endquote==

 

[marcus]

Here's the current KITP firewall kerfluffle:
http://online.kitp.ucsb.edu/online/fuzzorfire_m13/

Don Marolf explained at the start of his talk that in spite of the fact that his talk was titled
"The Case for Firewalls" the firewall picture was not something he believed.

It is instead something to study and think about, a way in fact to find out if we are making unjustified assumptions, to realize that we don't yet have a good understanding of BH information problem.

He explained that his assignment was to kick off the discussion the first day of the workshop, thus the title.

The talk by Bill Unruh struck me as definitely to be recommended. Enjoyed it very much.
http://online.kitp.ucsb.edu/online/fuzzorfire_m13/unruh/
Ted Jacobson played the game Marolf proposed very seriously, avoiding any suggestion that info might leak out via whatever process occurred instead of a singularity. Good--although quite a bit more technical and less intuitive than Unruh's.
http://online.kitp.ucsb.edu/online/fuzzorfire_m13/jacobson/

 

[Ben Niehoff]

I just came back from this Fuzz or Fire workshop, I thought the talks were very interesting. Bill Unruh definitely took a traditional GR point of view, as did Bob Wald.

However, in regards to the Aaronson / Smolin exchange in the OP: If black holes evaporate, then all the information does have to radiate out to infinity. Or I guess, I am pretty sure this is what we mean by "evaporate".

If we are to suppose that black holes toss all the incoming information into a baby universe which is inaccessible to the outside observer, I don't think we've solved any problems. Philosophically speaking, this is a bit of an empty statement. First of all, it is inherently untestable, because the baby universe is inaccessible to observers in this universe. Second, it is tautological that a mixed state can be purified by coupling to an appropriate external system, so saying "The time evolution is unitary if you throw extra universes into the Hilbert space so as to make it unitary" doesn't really get you anything.

 

[Ben Niehoff]

By the way, if you're watching the videos, I'm the guy sitting to the left of the projector with a laptop. And Iosif Bena mentions me at the beginning of his talk.

 

[marcus]

I'll look for you! I think I just saw your laptop near the start of Jacobson's talk.

Jacobson at the start of his talk:"...almost nobody believes that the firewall is plausible...
so we can hope to learn something about quantum gravity and ads/cft just by reconciling..."

Jacobson says that Don Marolf's recent argument (e.g. in the paper "Holography without

Strings") that it simply arises from diffeomorphism invariance had persuaded him to take boundary unitarity seriously.

 

[atyy]

During Marolf's talk or Bena's talk? Left, looking from the audience? Second row?

 

[Ben Niehoff]

Yes, that's my laptop near the beginning of Jacobson's talk. I'm sitting behind Bob Wald and next to Lenny Susskind (who doesn't appear in that video, I think).

Atyy, in the main conference room I took the same chair every time. I think I'm in all of the first week's videos. I won't be there for the second week, though.

I wasn't aware my bald spot was that bad already. I think that's the main thing I learned from this workshop.

 

[Haelfix]

I don't understand Smolin's argument at all, it doesn't make sense. As far as I can see, the argument for firewalls, also applies along the Rindler wedge. You draw the exact same picture, and run the exact same arguments.

If it doesn't apply on the Rindler wedge (and I am aware there are papers that say that it doesn't) it is for very subtle reasons and has almost nothing to do with any statement regarding the singularity (which everyone agrees logically dissappears in a full theory of quantum gravity), the case for firewalls is very strictly confined to horizon, near horizon and stretched horizon degrees of freedom, that are presumably entangled with late time observers.

 

[atyy]

Ben Niehoff, seems you needed the holographic version of the workshop to learn that fact.

 

[atyy]

I don't understand Smolin's argument at all, it doesn't make sense. As far as I can see, the argument for firewalls, also applies along the Rindler wedge. You draw the exact same picture, and run the exact same arguments.

If it doesn't apply on the Rindler wedge (and I am aware there are papers that say that it doesn't) it is for very subtle reasons and has almost nothing to do with any statement regarding the singularity (which everyone agrees logically dissappears in a full theory of quantum gravity), the case for firewalls is very strictly confined to horizon, near horizon and stretched horizon degrees of freedom, that are presumably entangled with late time observers.

So it's not generally accepted that the firewall arguments don't apply to the Rindler wedge? I'd assumed they can't if the firewall argument is to make sense at all, since we're pretty sure there's no firewall in Minkowski spacetime.

 

[Haelfix]

Most people expect that firewalls don't exist physically (even if they don't apply in flat space, we might already be past an astrophysical horizon yet observed no excited modes), its just very hard to evade the argument. Logically the argument seems to also hold (the same cartoon) in Rindler space, hence its an important toy model to use and understand (its actually central to the ER-EPR picture). At this time, it seems like we have to throw out one of four cherished principles or find a flaw in the argument.

I see that Lee likes to give up unitarity for the outside observer by postulating a baby universe. That actually works, b/c it does throw out one of the assumptions of the AMPS paper, but it is not without its own problems.

 

[marcus]

...

I see that Lee likes to give up unitarity for the outside observer by postulating a baby universe. That actually works, b/c it does throw out one of the assumptions of the AMPS paper, but it is not without its own problems.

Seems straightforward enough to me. What could those "own problems" be? It keeps small scale unitarity. You just have to put up with mixed states vis à vis an astrophysical black hole.

See quote from Lee in the OP

 

[Haelfix]

Having a baby universe is essentially identical to having information lost. It means that you can store an arbitrary large amount of entropy in an arbitrarily small region (and this plays havok with effective field theories), its completely unclear why you also wouldn't generically violate unitarity for any theory order by order in perturbation theory by simply pair-creating virtual black hole loops, and these will generically and badly diverge in the infrared.

It also implies that you have to give up conservation laws (if information is lost in the observable physical world, then you actually run into problems with the quantum version of Noethers theorems)

All of this also typically runs afoul of the superposition principle as well.

The point is, giving up observer unitarity runs very afoul of quantum mechanics, and is really very radical (it requires very detailed conspiracies in the microphysics to make all these violations small and unobservable)

 

[marcus]

its completely unclear why you also wouldn't generically violate unitarity for any theory order by order in perturbation theory by simply pair-creating virtual black hole loops, and these will generically and badly diverge in the infrared.
...

Thanks Haelfix,
Your objections here remind me of those offered by "wolfgang".

==quote==
wolfgang says:
August 24, 2013 at 9:20 am
@Lee
...
One problem of simply allowing a non-unitary evolution in the exterior is that
particle physics becomes non-unitary as soon as you (have to) include virtual black holes.

Lee Smolin says:
August 24, 2013 at 10:01 am
Wolfgang,

That is not a convincing argument and it is partly addressed in the paper I mentioned. The basic point is that there is no reason one has to include contributions from “virtual black holes.” When one looks at it carefully it becomes not at all clear what would be meant by that in a well defined background independent formulation of quantum gravity. The intuition that any process should have large or even divergent contributions from “virtual black holes” is based on an incorrect use of effective field theory, as discussed in section 4 of the paper with Hossenfelder I mentioned above.

Another reason is that there is no reason to think that horizons make sufficient sense in terms of quantum geometry at the Planck scale to give meaning to the semiclassical intuition of a virtual or Plank scale black hole. If quantum geometry is discrete at Planck scales then there are no horizons, curvatures or singularities at those scales and no way to give meaning to a Planck scale black hole. There is no contradiction in believing that quantum gravity is simply unitary at small scales while real astrophysical black holes create baby universes.

Thanks,

Lee
==endquote==
http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159265
The paper Lee cited is arXiv:0901.3156

 

[jimgraber]

To reduce it to a slogan: Give up Black Holes, Save Unitarity
Baby Universes were mentioned, but I think the idea of replacing black holes deserves much more serious study than it has received. The problems discussed at the recent “Black Holes Complementarity Fuzz or Fire?” workshop emphasize only some of the serious problems with the black hole concept. I realize the workshop was predicated on acceptance of the black hole concept as well as the general accuracy of the Hawking radiation paradigm, but neither of these ideas has a strong experimental support. Gravitationally collapsed objects similar but not identical to black holes could alleviate many issues if they were unitary and did not involve either horizons or singularities.

 

[marcus]

To reduce it to a slogan: Give up Black Holes, Save Unitarity
Baby Universes were mentioned, but I think the idea of replacing black holes deserves much more serious study than it has received. The problems discussed at the recent “Black Holes Complementarity Fuzz or Fire?” workshop emphasize only some of the serious problems with the black hole concept. I realize the workshop was predicated on acceptance of the black hole concept as well as the general accuracy of the Hawking radiation paradigm, but neither of these ideas has a strong experimental support. Gravitationally collapsed objects similar but not identical to black holes could alleviate many issues if they were unitary and did not involve either horizons or singularities.

Hi Jim,
The part from "I realize the workshop was predicated..." onwards was quoted by Scott Aaronson earlier, as if you posted in the Woit thread. But I could not find your post there, so it may have somehow gotten erased.

Lee and Sabine Hossenfelder present what they are discussing in much more conservative terms
http://arxiv.org/abs/0901.3156
Don't get rid of black holes. KEEP black holes but think about what could be happening at the singularity.
IOW make the least possible disruption in existing theory that will resolve the info puzzle.
Conservative solutions to the black hole information problem
Sabine Hossenfelder, Lee Smolin
(Submitted on 20 Jan 2009)
We review the different options for resolution of the black hole loss of information problem. We classify them first into radical options, which require a quantum theory of gravity which has large deviations from semi-classical physics on macroscopic scales, such as non-locality or endowing horizons with special properties not seen in the semi-classical approximation, and conservative options, which do not need such help. Among the conservative options, we argue that restoring unitary evolution relies on elimination of singularities. We argue that this should hold also in the AdS/CFT correspondence.
25 pages, 7 figures

They examine a number of alternatives for a conservative resolution of this type, not just what I was calling "unibabes" (baby universes). Argue pros and cons and so on.

 

[marcus]

...But I could not find your post there, so it may have somehow gotten erased...

Jim,
I see it now, right before Scott's.

 

[Drakkith]

Good lord I thought this was spam when I saw the title, Marcus!

 

[marcus]

Good lord I thought this was spam when I saw the title, Marcus!

For shame, Drakkith! You were just letting your imagination run wild

 

[Drakkith]

For shame, Drakkith! You were just letting your imagination run wild

Psh! I cut the leash on that guy long ago! Let him run free!

 

[marcus]

Since we've turned a page, I'll bring forward an update of the OP.
The fuss over "firewalls" is basically a challenge to the assumption that watching a black hole form and evaporate, the observer at infinity sees unitary evolution. If you think the BH singularity might be resolved in a bounce, you don't make that assumption (as Lee explains in the following). On the other hand if you do make that assumption you arrive at one or more unattractive or paradoxical conclusions.
===quote===
Scott Aaronson says:
August 23, 2013 at 9:32 pm
...While I’m obviously far from an expert, where I think I part ways from you and Unruh is on the following. We’re pretty sure black holes have an entropy, which goes like the area of the event horizon in Planck units. We’re pretty sure that, from an external observer’s perspective, infalling stuff gets “pancaked” on the event horizon and scrambled beyond recognition, never making it through to the interior. Finally, we’re pretty sure that the external observer ultimately sees the black hole evaporate, through Hawking radiation that emerges (appears to emerge?) from the horizon. To me, these facts would seem like an intolerable coincidence, if the black hole didn’t have microstates—”stored,” one wants to imagine, on or near the event horizon—and if the Hawking radiation didn’t carry away the information about those microstates...
...
...
Lee Smolin says:
August 24, 2013 at 7:18 am
Dear Scott,

Thanks, but either I don’t understand your argument or else it is circular. What do you suppose happens to the singularity as well as to the quantum state of the star whose collapse formed the black hole in the first place? If the singularity is eliminated then the Hilbert space in the future is a direct product of a factor spanned by observables which describe degrees of freedom to the future of where the singularity would have been and a factor spanned by observables external to the horizon. The evolution onto this product can be assumed to be unitary but (I feel silly telling you this) it cannot be when restricted to either of its factors. Hence the observer at infinity describes a density matrix gotten by tracing out the degrees of freedom in the baby universe inaccessible to them.

Isn’t this a completely reasonable option, especially because it avoids the otherwise paradoxical implications of the firewall argument?

The pancake is a non-sequitur: why does it matter what information does or doesn’t get to infinity or when, if infinity is not the only place information goes to? So to refer to it seems to assume what you are claiming to demonstrate.

Many thanks,
Lee
==endquote==
http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159261
and
http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159264

Here's the current KITP firewall kerfluffle:
http://online.kitp.ucsb.edu/online/fuzzorfire_m13/

Don Marolf explained at the start of his talk that in spite of the fact that his talk was titled
"The Case for Firewalls" the firewall picture was not something he believed.

It is instead something to study and think about, a way in fact to find out if we are making unjustified assumptions, to realize that we don't yet have a good understanding of BH information problem.

He explained that his assignment was to kick off the discussion the first day of the workshop, thus the title.

The talk by Bill Unruh struck me as definitely to be recommended. Enjoyed it very much.
http://online.kitp.ucsb.edu/online/fuzzorfire_m13/unruh/
Ted Jacobson played the game Marolf proposed very seriously, avoiding any suggestion that info might leak out via whatever process occurred instead of a singularity. Good--although quite a bit more technical and less intuitive than Unruh's.
http://online.kitp.ucsb.edu/online/fuzzorfire_m13/jacobson/

 

[Ben Niehoff]

Personally I think the singularity is a red herring, and any resolution of the problem will require excising the entire interior. But this is based on the point of view that the black hole really does evaporate, and radiates its information to infinity. If this is the case, then it is the event horizon itself that causes the problem.

I understand that this seems absurd from a classical GR point of view. The event horizon is just a smooth piece of spacetime, and the curvature there can be made arbitrarily small. There is nothing, at least locally, in classical GR to suggest that it breaks down at the horizon. Most of the arguments by those coming from classical GR amount to simply repeating this fact; however, I don't think it is under-appreciated.

The firewall argument (and an earlier argument by Mathur using subadditivity of entanglement entropy) is interesting because it shows that there are global effects that suggest GR should break down at the horizon. In hindsight, it should be no surprise that we must resort to such an argument, because the horizon is not even defined locally (strictly speaking, an observer must have access to future null infinity to talk about the locations of horizons), and so we shouldn't expect a local notion (the equivalence principle) to tell us anything useful about it.

 

[marcus]

Thanks for presenting that viewpoint! Incidental note Ben let us know he attended the first week of the workshop! See posts #5,6,7,8. He is visible sitting just to the left of the slide projector, in the videos of the talks that I've watched (e.g. Bill Unruh, Ted Jacobson, Don Marolf)

 

[marcus]

In the passage quoted in post #22 Lee is advocating what a number of people consider the most conservative solution of the BH information problem. It disrupts accepted physics as little as possible, consistent with resolving the paradox.

One simply observes that the BH singularity ("infinite" density and curvature) is widely considered unphysical and due to be replaced. And by resolving the singularity one inevitably creates a place for information to go. The Hilbertspace of states is a direct sum of states external to the BH horizon and states to the future of where the singularity was.

This reasoning applies to astrophysical black holes but not, as Lee explained, to "virtual" or "microscopic" BH. It's an important point, so I will bring forward the relevant part of an earlier post:

==quote http://www.math.columbia.edu/~woit/wordpress/?p=6208&cpage=1#comment-159265==
wolfgang says:
August 24, 2013 at 9:20 am
@Lee
...
One problem of simply allowing a non-unitary evolution in the exterior is that
particle physics becomes non-unitary as soon as you (have to) include virtual black holes.

Lee Smolin says:
August 24, 2013 at 10:01 am
Wolfgang,

That is not a convincing argument and it is partly addressed in the paper I mentioned. The basic point is that there is no reason one has to include contributions from “virtual black holes.” When one looks at it carefully it becomes not at all clear what would be meant by that in a well defined background independent formulation of quantum gravity. The intuition that any process should have large or even divergent contributions from “virtual black holes” is based on an incorrect use of effective field theory, as discussed in section 4 of the paper with Hossenfelder I mentioned above.

Another reason is that there is no reason to think that horizons make sufficient sense in terms of quantum geometry at the Planck scale to give meaning to the semiclassical intuition of a virtual or Plank scale black hole. If quantum geometry is discrete at Planck scales then there are no horizons, curvatures or singularities at those scales and no way to give meaning to a Planck scale black hole. There is no contradiction in believing that quantum gravity is simply unitary at small scales while real astrophysical black holes create baby universes.

Thanks,

Lee
==endquote==

The paper cited is http://arxiv.org/abs/0901.3156

 

[Haelfix]

I happen to be very skeptical about Smolin and Bee's paper for a number of reasons, and I don't really believe it is a standard point of view amongst experts either.

For one, it really requires that you not believe in the AdS/CFT correspondance (Smolin of course is on record as being a skeptic), which doesn't involve anything like remnants or baby universes.

Secondly, I don't really agree with the technical observations in the paper. Amongst many, the statement that Lee wrote "There is no contradiction in believing that quantum gravity is simply unitary at small scales while real astrophysical black holes create baby universes.". There is nothing anywhere in the laws of physics that forbids arbitrarily large fluctuations from occurring, so I think that there actually is a contradiction in principle. The bag of gold picture they list in their paper seems to me to involve believing a really radical departure from effective field theory (it's always the same thing here, you have to give up a really cherished principle at some stage)

Further the case against Remnants/baby universes is by now very old and established. Much of the literature from the 80s and 90s were trying to make those types of scenarios work, and there always were problems with them.

Old review on the possible fates of black holes (for the baby universe see p13): http://arxiv.org/abs/hep-th/9209058
Discussion on nonunitarity, loss of the superposition principle (p6) and black holes: http://arxiv.org/abs/hep-th/9410187

Slightly refined argument for the pair creation problem: S.B. Giddings and A. Strominger, Phys. Rev. D46 (1992) 627
Thermodynamic instability of remnants: http://arxiv.org/abs/hep-th/9501106

Perhaps more pertinently, In the Apologia to firewall paper, Polchinski also discusses the remnant/baby universe option: http://arxiv.org/abs/1304.6483

Anyway, it's hard to discuss without a specific proposal and statement about the interior degrees of freedom, so much of this material and discussion is at a heurestic level and is really begging for a concrete realization of the bh interior to really appreciate in full (the only one on the market right now is Mathur's fuzzball proposal).

 

[atyy]

Anyway, it's hard to discuss without a specific proposal and statement about the interior degrees of freedom, so much of this material and discussion is at a heurestic level and is really begging for a concrete realization of the bh interior to really appreciate in full (the only one on the market right now is Mathur's fuzzball proposal).

Mathur http://arxiv.org/abs/1308.2785 has argued "Probing the fuzzball at energies E ≫ kT excites collective modes of the fuzzball which can be well approximated by an ensemble average over fuzzballs, and this average is reproduced by the traditional black hole geometry." So it seems that for some objects there is no firewall.

But Raamsdonk http://arxiv.org/abs/1307.1796 seems skeptical about Mathur's argument "In [28, 29, 30], Mathur and collaborators argue that while the fuzzball picture of black hole microstates is necessary to solve the information paradox, an observer with energy E ≫ T falling into a black hole microstate may still experience smooth spacetime behind the horizon. A key part of the argument ... We now argue that this conclusion is not justified."

Any thoughts on this?

Also, why isn't the Hayden-Harlow idea that the experiments basically can't be done accepted as solving the issue?

 

[Haelfix]

I'm sure Ben can probably answer the first question better than I, both of those authors are excellent physicists with a great deal of expertise and I am not comfortable answering questions beyond my depth..

As for why Hayden-Harlow isn't accepted.

1) Some people accept it.
2) It's a little unsatisfying aesthetically speaking.
3) It relies on certain conjectures in quantum information theory that are not yet proved.
4) There are proposals to slow down the evaporation rate by considering very unusual warped throats and things like that, which seems to imply that the 'out' is geometry dependant and perhaps not universal.
5) In the Apologia for Firewall paper, http://arxiv.org/abs/1304.6483, various arguments against the proposal were advanced. I don't have anything really interesting to add.

 

[Ben Niehoff]

I don't think I understand the Harlow-Hayden argument well enough to comment.

As for Mathur's "fuzzball complementarity" proposal, I'm not sure what to think. One version of his argument, as I understand it, is that there is an exponentially-large density of states at the horizon whose energy E is close to the energy of whatever is falling in; therefore, the infall process can be gentle because of a phenomenon something like impedance matching.

On the other hand, we don't know anything yet about the actual dynamics of the fuzzball system, and I don't think arguments merely from the density of states are enough to decide whether an infalling observer experiences empty space or a hard landing. I think van Raamsdonk has a reasonable view on this.

 

[marcus]

To give some idea of how this is being viewed in the broader research community, back in July there was the triennial GR conference (over 800 people) and because it's topical, I guess, they had a special "joint session" called "D1+D2+D4" involving Loop+String+Pheno people.

Robert Wald, Abhay Ashtekar, Don Marolf, Gary Horowitz, and others.
GR20/Amaldi10

==quote==
Joint session D1, D2 and D4
The Quantum Mechanics of Black Hole Evaporation

Wednesday (10 July)

1. ROBERT WALD (20+5 MINUTES)
TITLE: Information Loss

We review the arguments in favor of loss of information in the process of black hole formation and evaporation.

2. ARON WALL (15+5 MINUTES )
TITLE: Why is the generalized second law true?

A stationary Killing horizon is invariant under a much larger symmetry group than the spacetime it is embedded in. From this fact, it is possible to prove that the generalized entropy increases locally at each point on the horizon. After briefly describing the proof, I will speculate on the implications for the microstates of null surfaces in full quantum gravity.

3. ABHAY ASHTEKAR (20+5 MINUTES)
TITLE: Quantum Space-times and Unitarity of BH evaporation

There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.

4 DANIELE PRANZETTI (15+5 MINUTES )
TITLE: Dynamical evaporation of quantum horizons

We describe the black hole evaporation process driven by the dynamical evolution of the quantum gravitational degrees of freedom resident at the horizon, as identified by the loop quantum gravity kinematics. Using a parallel with the Brownian motion, we interpret the first law of quantum dynamical horizon in terms of a fluctuation-dissipation relation applied to this fundamental discrete structure. In this way, the horizon evolution is described in terms of relaxation to an equilibrium state balanced by the excitation of Planck scale constituents of the horizon. We investigate the final stage of the evaporation process and show how the dynamics leads to the formation of a massive remnant. Implications for the information paradox are discussed.

COFFEE BREAK5. GARY HOROWITZ (20+5 MINUTES)
TITLE: Black hole information from the viewpoint of string theory

We review the contributions that string theory has made to understanding black hole information. This includes the remarkable gauge/gravity duality and the counting of microstates of certain black holes. We also comment on more speculative ideas including fuzzballs and final state boundary conditions.

6. DON MAROLF:
TITLE:AdS/CFT, Unitary black hole evaporation, and firewalls (20+5 MINUTES)

We review arguments that black hole evaporation is unitary in AdS/CFT. As a result, the physics experienced by infalling observers at the horizon of at least sufficiently old black holes described by AdS/CFT must be dramatically different from that described by familiar field theory in a smooth spacetime.

7. KYRIAKOS PAPADODIMAS (15+5 MINUTES )
TITLE: Falling into a black hole and the information paradox in AdS/CFT

I will describe how the interior of a black hole can be reconstructed from the point of view of the dual gauge theory in the framework of the AdS/CFT correspondence. I will argue that the infalling observer does not notice anything special when crossing the horizon and that it is possible to resolve the information paradox without dramatic violations of effective field theory, in contrast to predictions by the recent fuzzball and firewall proposals.
==endquote==

 

[marcus]

I want to stress Ashetekar's assessment because it is such a straightforward, almost obvious response to current developments in BH singularity resolution (e.g. this year's Gambini Pullin paper also presented at GR20, in session D1).

He simply draws the common sense conclusion, in contrast to the complex speculation, consternation, and confusion seen in other reactions.

This is what I mean by the "firewallowing" we see at KITP/SantaBarbara actually lending support to the simple "unibabes" solution.

The BH singularity has to be resolved in ANY case (and good progress on that is being made, see G&P paper). When it is resolved (most likely another expanding section of space-time to future of where fictional singularity was) it will of course make room for degrees of freedom where info can go. So the Hilbertspace factors. (Big deal, get over it )

 

[atyy]

The abstract after Ashtekar's by Pranzetti also looks interesting, seems to be a sketch of a remnant scenario in LQG. Looks like it's based on Dynamical evaporation of quantum horizons .

There was another paper about remnants in 2D gravity by a subset of AMPS An Uneventful Horizon in Two Dimensions. Their work is based on numerical simulations by Ashtekar and colleagues. But in their discussion, Almheiri and Sully say that black hole evaporation in AdS/CFT probably doesn't have remnants.

 

[Haelfix]

As far as I know, most loop quantum gravity research is very much against the remnant scenario as well. It's very much the usual black hole thermodynamics picture, where information is radiated away to infinity and where the area is given by the entropy times some prefactor.

 

[marcus]

The abstract after Ashtekar's by Pranzetti also looks interesting,..

Pranzetti is a postdoc and his paper (the talk was based on) is interesting. But for a wise broad-overview perspective, I want to stress what Ashtekar had to say. It was primarily about unibabe "enlargement", not what you'd normally consider a compact remnant.
... arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time. (quoted earlier)

And also there's a beautifully-written paper by Jacobson which precisely identifies the point of tension in the "firewall"situation.
==quote Jacobson December 2012==
Even if the AdS/CFT argument held only in that specific setting, it would be enough to consider the AMPS question. But in fact Marolf [5] has argued that the essential reason for the AdS/CFT result carries over more generally to any diffeomorphism invariant theory with an asymptotic region in which an algebra of observables can be defined. His point is that in such a theory, the Hamiltonian is a surface integral in the asymptotic region, which I will call “the boundary”. More precisely, the Hamiltonian also contains a volume integral of combinations of the diffeomorphism constraints, but those act trivially1 on any physical state in the Hilbert space (according to Dirac quantization of a constrained system). Hence the algebra of boundary observables evolves unitarily in time into itself, and this means that no boundary information can ever be lost. In the asymptotically flat case, the boundary algebra would ...
==endquote==
I cannot recommend this paper too highly.

arxiv.org/abs/1212.6944
Boundary unitarity without firewalls
Ted Jacobson
(Submitted on 31 Dec 2012)
Both AdS/CFT duality and more general reasoning from quantum gravity point to a rich collection of boundary observables that always evolve unitarily. The physical quantum gravity states described by these observables must be solutions of the spatial diffeomorphism and Wheeler-deWitt constraints, which implies that the state space does not factorize into a tensor product of localized degrees of freedom. The recent "firewall" argument that unitarity of black hole S-matrix implies the presence of a highly excited quantum state near the horizon is based on such a factorization, hence is not applicable in quantum gravity.
7 pages

Topology change in the bulk can break the monogamous relation of bulk to the previous "asymptotic region" and cause the appearance of a new component of the "asymptotic region".

This happens as soon as a hole forms leading to re-expanding unibabe space-time.

In nature of course we already have a complex boundary because we already have many astrophysical black holes. In reality the boundary cannot be a single connected piece as in the simplified approximate AdS/CFT. picture. One should never have imagined the real world was so simple that it had a single unique connected asymptotic region. Instead, down thru every astro BH there is an expanding unibabe with its own separate component of the "boundary".

This December Jacobson paper is exquisitely logical in the clean way it identifies the assumption that is causing the "firewall" tension. And it also shows the importance of Don Marolf's generalization of the AdS/CFT idea to any diffy invariant setup with "asymptotic region in which an algebra of observables can be defined."

 

[marcus]

In post #30 I quoted abstract of talk Ashtekar gave at GR20 in the joint session with Wald, Marolf, Horowitz etc specifically on BH evaporation (the "firewall" absurdity and what this means for AdS/CFT). And I mentioned that his talk reflected the "enlargement" (ie. unibabes) picture currently coming out more and more unambiguously from LQG research--for instance in recent paper by two prominent people: Gambini and Pullin. I should give a link to that paper. Pullin presented it in the main Loops session at GR20.

I want to stress Ashetekar's assessment because it is such a straightforward, almost obvious response to current developments in BH singularity resolution (e.g. this year's Gambini Pullin paper also presented at GR20, in session D1).

He simply draws the common sense conclusion, in contrast to the complex speculation, consternation, and confusion seen in other reactions.

This is what I mean by the "firewallowing" we see at KITP/SantaBarbara actually lending support to the simple "unibabes" solution.

The BH singularity has to be resolved in ANY case (and good progress on that is being made, see G&P paper). When it is resolved (most likely another expanding section of space-time to future of where fictional singularity was) it will of course make room for degrees of freedom where info can go. So the Hilbertspace factors. (Big deal, get over it )

arxiv.org/abs/1302.5265
Loop quantization of the Schwarzschild black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 21 Feb 2013 (v1), last revised 10 May 2013 (this version, v2))
We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can construct explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. New observables living in the bulk appear at the quantum level (analogous to spin in quantum mechanics) that are not present at the classical level and are associated with the discrete nature of the spin network states of loop quantum gravity. The resulting quantum space-times resolve the singularity present in the classical theory inside black holes.
4 pages, published in Physical Review Letters 110, 211301 (2013)