Black Holes & Firewalls: Recent Papers

[elfmotat]

So, does anyone have any thoughts on the papers recently published by Almheiri/Marolf/Polchinski/Sully (http://arxiv.org/pdf/1207.3123.pdf) and Susskind (http://arxiv.org/pdf/1207.4090.pdf)?

 

[bcrowell]

Interesting stuff!

I've only skimmed the two papers, and lack the background needed in order to understand them deeply (even though the authors have made admirable efforts to make their writing accessible to a non-specialist).

My gut reaction is to question whether there is, even in principle, any way to test the existence of a firewall against experiment, if it exists only behind an event horizon. If not, then this is really philosophy and not science. Maybe it's testable if you have access to a black hole that you are able to observe through the process of evaporation...? On the other hand, it would be interesting if there was theoretical evidence that quantum gravity effects can manifest themselves at scales far bigger than the Planck scale, because that would suggest that q.g. could be testable with foreseeable technology.

 

[tom.stoer]

both LQG and string theory (fuzzball proposal) suggest that QG effects are relevant not at Planck scale but at horizon scale!

 

[Physics Monkey]

This is certainly a provocative set of thoughts, but I'm skeptical of their interpretation.

One thing I was thinking about was the eternal ads black where I don't see any sign of a firewall. Granted the physical setup is a bit different, but its not clear how essential that is for their argument.

 

[lfighter]

Actually I prefer Bousso's statement that the cloning paradox and entanglement paradox don't exist at all(http://arxiv.org/abs/1207.5192). No observer can see both of the qubits, so it does not contradict no cloning principle. This is the 'observer complementarity'.

 

[yenchin]

Bousso gave a talk at Strings 2012 on this topic, the slides are available on the conference website [direct link here]. The video is also available.

 

[Finbar]

I think that Bousso's response to these papers helps clarify the beautiful subtleties of BHC.

In particular that inconsistencies between what two observers see but cannot communicate with each other are not physically inconsistent since they have no operational meaning. Instead they are complementary.

 

[Demystifier]

In quantum mechanics, complementarity can be expressed in a clear and simple mathematical way, as the fact that the same state can be expanded in different bases, corresponding to eigenstates of different observables. The nice thing about this is that the state itself, viewed as an object which does not depend on the choice of basis, does NOT DEPEND ON THE OBSERVER.

However, as far as I know, nothing similar exists for black hole complementarity. As far as this is the case, I cannot take black hole complementarity seriously.

 

[Finbar]

In quantum mechanics, complementarity can be expressed in a clear and simple mathematical way, as the fact that the same state can be expanded in different bases, corresponding to eigenstates of different observables. The nice thing about this is that the state itself, viewed as an object which does not depend on the choice of basis, does NOT DEPEND ON THE OBSERVER.

However, as far as I know, nothing similar exists for black hole complementarity. As far as this is the case, I cannot take black hole complementarity seriously.

I share your reservations. BHC is not standard QM. But I think what QM taught us is that nature can be very weird to the point that there almost seems to be a contradiction (light is both a wave and particle). But it was found that in-fact it can be this way without leading to any paradoxes.

BHC is inspired by complementarity(in QM) but should not be confused with it. The point of BHC is to remove the paradoxes of black hole physics. To do this it makes the rather drastic step of saying that the states become observer dependent with huge inconsistencies between the two observers. The crux being that aslong as these two observers cannot communicate then there is no paradox. At leasts this is my understanding.



Not sure I buy it. But I remain open minded.

 

[Demystifier]

I think it is extremely important in physics to have a description which does NOT depend on the observer. I have already explained what it is in QM. In classical GR we also have it - a covariant and even a coordinate-independent formalism. In particular, in classical GR the universe behind the horizon exists irrespective of the observer.

Otherwise, it seems to me that ANY PARADOX in any inconsistent theory can be "resolved" by introducing a new kind of "complementarity". For example, if some inconsistent theory predicts that x=1 AND (not OR) x=2, you can always say it is consistent because it corresponds to two complementary observers who use the theory in two different ways.

 

[jarod765]

To Demystifier:

A theory of physics, with complementarity, should not allow inconsistencies for *each* observer i.e. if one is making a measurement in a specific reference frame, then there should be no inconsistencies in the laws of physics. However this does not mean that the same measurements, performed in a different reference frame, will yield the exact same results as those in the original frame. This is one of the principles of complementarity.

Furthermore, one should not be so hesitant in discussing observer dependent theories. For example, in quantum gravity it is impossible to define a preferred vacuum state and therefore different observers will see drastically different physics (see Unruh radiation).

 

[tom.stoer]

But there is a possibility to construct a map between the different vacuum states via a Bogoliubov transformation. So whenever I can introduce a class of states with an explicit map I wouldn't call that theory observer-dependent (of course the theory allows to extract observer dependent physics)

 

[jarod765]

Yes it is true that there is a mapping via Bogoliubov transformations between two different observer frames. But this is also the case for BHC. Any information carried into the black hole in a falling frame should in principle be mapped to states in the Hawking cloud.

I guess we should really say that interpretation of the physics is observer dependent. This is of course true in BHC and in the non-unique vacua of quantum gravity.

 

[tom.stoer]

Sorry to say that but if that is all what there is to say then it's rather trivial!

We know this since Galilei invariance of Newtonian physics. I mean a Bogoljubov transformation between vacuum states of QFT on curved background is more complicated that just x' = x-vt, but conceptionally is identical. You have the same underlying physics, but observer dependend observations.

Do I miss something?

EDIT:

There is a interpretation problem regarding the "reality" of the observed particles.

 

[jarod765]

In some sense BHC is a statement that there is only observer dependent observations. What makes BHC interesting is that one cannot do quantum mechanics in a *global* space-time picture. That is to say one cannot count independent degrees of freedom (DOF) throughout all of space-time.


This is different from standard field theory where one assigns to each point a DOF everywhere in space.

 

[Finbar]

. I mean a Bogoljubov transformation between vacuum states of QFT on curved background is more complicated that just x' = x-vt, but conceptionally is identical.

Isn't a Bogoljubov transformation conceptually the same as a transformation between non-inertial observers rather than intertial ones?



But I agree with Jarod I think BHC is conceptually different from this.

 

[tom.stoer]

Isn't a Bogoljubov transformation conceptually the same as a transformation between non-inertial observers rather than intertial ones?

with x' = x-vt I didn't want to say that the Bogoljubav trf. applies to inertial frames. I only wanted to claim that observations are frame dependent and that the Unruh effect is nothing else but an effect due to this frame-dependence. The big difference is that it acts on the Fock space and "creates particle states from the vacuum".

But as said it's slightly more complicated than that: there is an interpretation problem regarding the "reality" of the observed particles i.e. regarding a real event"; and there seems to be a lack of "global definition" of states or d.o.f.

 

[Finbar]

with x' = x-vt I didn't want to say that the Bogoljubav trf. applies to inertial frames. I only wanted to claim that observations are frame dependent and that the Unruh effect is nothing else but an effect due to this frame-dependence. The big difference is that it acts on the Fock space and "creates particle states from the vacuum".

But as said it's slightly more complicated than that: there is an interpretation problem regarding the "reality" of the observed particles i.e. regarding a real event"; and there seems to be a lack of "global definition" of states or d.o.f.

Yes but if the observer measures anything but the Minkowski vacuum this implies that there is some force (be it gravity or any other force) acting on them right? This is rather different from an inertial transformation. Also to measure the Unruh effect each observer must detect particles. So each observer is conducting an experiment. But these experiments are different! So i am not so sure that it's simply frame dependence.

 

[MTd2]

Susskind withdrew his paper

http://arxiv.org/abs/1207.4090

 

[Demystifier]

Furthermore, one should not be so hesitant in discussing observer dependent theories. For example, in quantum gravity it is impossible to define a preferred vacuum state and therefore different observers will see drastically different physics (see Unruh radiation).

In the case of Unruh effect, all observers agree that the state is |0_Minkowski>. In Susskind black hole complementarity there is no such a universal object on which all observers agree.

 

[Finbar]

In the case of Unruh effect, all observers agree that the state is |0_Minkowski>. In Susskind black hole complementarity there is no such a universal object on which all observers agree.

Wouldn't Susskind argue that what is true of black hole horizons is true also of Rindler horizons in Minkowski space? So all observers do not have to agree.

Anyway in QM all observers don't have to agree on what the state is. If one observer preforms a measurement of one observable then the state will be an eigenstate of the observable. But if the other observer makes a measures a different observable then for her it will be in an eigenstate of a different observable.

Now if they both measure the same observable they have to agree on the eigen state. Horizon complementarity seems to give this up provided there is no way for the two observers can comunicate.

 

[Demystifier]

Anyway in QM all observers don't have to agree on what the state is. If one observer preforms a measurement of one observable then the state will be an eigenstate of the observable. But if the other observer makes a measures a different observable then for her it will be in an eigenstate of a different observable.

That is not true. In QM it is not possible to measure simultaneously two different (mutually non-commuting) observables, not even if the measurements are performed by different observers. For example, if one observer measures momentum at time t and obtains the value p, there is no way that another observer could get a measurement result which is not compatible with the fact that momentum at time t is equal to p.

This fact is what makes complementarity in ordinary QM consistent. Unfortunately, it seems that nothing similar exists for black hole complementarity.

 

[Demystifier]

Susskind withdrew his paper

http://arxiv.org/abs/1207.4090

Currently it is quite popular to withdraw papers on that issue. Harlow also withdrew it:
http://xxx.lanl.gov/abs/1207.6243

 

[Finbar]

That is not true. In QM it is not possible to measure simultaneously two different (mutually non-commuting) observables, not even if the measurements are performed by different observers. For example, if one observer measures momentum at time t and obtains the value p, there is no way that another observer could get a measurement result which is not compatible with the fact that momentum at time t is equal to p.

This fact is what makes complementarity in ordinary QM consistent. Unfortunately, it seems that nothing similar exists for black hole complementarity.

You're right, but I didn't say that they had to measure the observables simultaneously.
(However I concede I was far from clear.)


In relativity there is no observer independent notion of simultaneous events.
I think this an important point. For some observer (i.e. some world line) we have to pick a time slicing over which states evolve. If for the observer outside the black hole we pick a time slicing which remains within her causal diamond then ordinary QM applies without any contradictions. If we take the in-falling observer then for her she can choose a time slicing within her causal diamond and again we have consistent QM. Only if we try to do QM on a time-slicing which is not within a causal diamond does it breakdown. But since such a time slicing would lead to states that no observer could attempt to measure it is meaningless.

A quantum state should always correspond to some observers knowledge of the physical system. As long as we stick to this horizon complementarity says that consistent unitary QM applies. Could be wrong, could be right. But I think the idea is a compelling one.

 

[Demystifier]

You're right, but I didn't say that they had to measure the observables simultaneously.
(However I concede I was far from clear.)


In relativity there is no observer independent notion of simultaneous events.
I think this an important point. For some observer (i.e. some world line) we have to pick a time slicing over which states evolve. If for the observer outside the black hole we pick a time slicing which remains within her causal diamond then ordinary QM applies without any contradictions. If we take the in-falling observer then for her she can choose a time slicing within her causal diamond and again we have consistent QM. Only if we try to do QM on a time-slicing which is not within a causal diamond does it breakdown. But since such a time slicing would lead to states that no observer could attempt to measure it is meaningless.

A quantum state should always correspond to some observers knowledge of the physical system. As long as we stick to this horizon complementarity says that consistent unitary QM applies. Could be wrong, could be right. But I think the idea is a compelling one.

I would accept it if one could translate it into an observer-free language. For example, one can introduce Tomonaga-Schwinger formalism, in which time evolution Psi(t) (with this or that time t) is generalized to Psi[Sigma], which is a functional of an arbitrary spacelike hypersurface Sigma. If one could find SINGLE functional Psi[Sigma] that contains both complementary views of a black hole just by taking different Sigma in the same Psi[Sigma], then I would accept it.

 

[Finbar]

I would accept it if one could translate it into an observer-free language. For example, one can introduce Tomonaga-Schwinger formalism, in which time evolution Psi(t) (with this or that time t) is generalized to Psi[Sigma], which is a functional of an arbitrary spacelike hypersurface Sigma. If one could find SINGLE functional Psi[Sigma] that contains both complementary views of a black hole just by taking different Sigma in the same Psi[Sigma], then I would accept it.

Ok so it's really about causality.

So let's say that we have a path integral representation of the theory and that it is a functional Z[Sigma_i,Sigma_f] of the initial and final space-like hyper surfaces Sigma_i and Sigma_f. One may want these to be arbitrary. The restriction that horizon complementarity puts on Sigma_i and Sigma_f is that they must lie in the intersection of the causal future of some point p and the causal past some other point q. This is what is known as the causal diamond associated to p and q.

(See this paper for a better explanation
http://arxiv.org/pdf/hep-th/0010252v2.pdf )

The point of this restriction is that it does not rule out any experiment that can be preformed by any physical(causal) experiment.

So QM is not defined if we try to define states that aren't in some causal diamond. But this is fine since no experiment could ever measure such a state.

 

[MTd2]

http://arxiv.org/abs/1208.2026

Is Alice burning or fuzzing?

Borun D. Chowdhury, Andrea Puhm
(Submitted on 9 Aug 2012)
Recently, Almheiri, Marolf, Polchinski and Sully have suggested a Gedanken experiment to test black hole complementarity. They claim that the postulates of black hole complimentarily are mutually inconsistent and choose to give up the "absence of drama" for an in-falling observer. According to them, at least after Page time, the black hole is shielded by a firewall. This has generated some controversy. In our opinion a lot of this is caused by confusions stemming from an observer-centric language. In this letter we formulate the AMPS's Gedanken experiment in the decoherence picture of quantum mechanics without invoking any sentient beings. While we find that the objections raised by advocates of observer complimentarily are irrelevant, an interesting picture emerges when we take into account objections from the advocates of fuzzballs. We find that low energy wave packets "burn up" like AMPS claim while high energy wavepackets do not. This is consistent with Mathur's recent proposal of approximate complementarity for high energy quanta. Within the fuzzball proposal AMPS's firewall fits in nicely as the thermal bath that low energy in-coming quanta perceive.

http://lanl.arxiv.org/abs/1208.2005

Comments on black holes I: The possibility of complementarity

Samir D. Mathur, David Turton
(Submitted on 9 Aug 2012)
We comment on a recent paper of Almheiri, Marolf, Polchinski and Sully who argue against black hole complementarity based on the claim that an infalling observer `burns' as he approaches the horizon. We show that in fact measurements made by an infalling observer outside the horizon are statistically identical for the cases of vacuum at the horizon and radiation emerging from a stretched horizon. This forces us to follow the dynamics all the way to the horizon, where we need to know the details of Planck scale physics. We note that in string theory the fuzzball structure of microstates does not give any place to `continue through' this Planck regime. AMPS argue that interactions near the horizon preclude traditional complementarity. But the conjecture of `fuzzball complementarity' works in the opposite way: the infalling quantum is absorbed by the fuzzball surface, and it is the resulting dynamics that is conjectured to admit a complementary description.

 

[Finbar]

Lenny's back!


Singularities, Firewalls, and Complementarity

Leonard Susskind
(Submitted on 16 Aug 2012)
Almheiri, Marolf, Polchinski, and Sully, recently claimed that once a black hole has radiated more than half its initial entropy (the Page time), the horizon is replaced by a "firewall" at which infalling observers burn up, in apparent violation of the equivalence principle and the postulates of black hole complementarity. In this paper I review the arguments for firewalls, and give a slightly different interpretation of them. According to this interpretation the horizon has standard properties, but the singularity is non-standard. The growing entanglement of the black hole with Hawking radiation causes the singularity to migrate toward the horizon, and eventually intersect it at the page time. The resulting collision of the singularity with the horizon leads to the firewall. Complementarity applies to the horizon and not to the singular firewall.
Almheiri, Marolf, Polchinski, and Sully conjecture that firewalls form much earlier then the Page time; namely at the scrambling time. I argue that there is no reason to believe this generalization, and good reason to think it is wrong.
For most of this paper I will assume that the firewall argument is correct. In the last section before the conclusion I will describe reasons for having reservations.

 

[MTd2]

Here is the link!

http://arxiv.org/abs/1208.3445

 

[valentin mano]

the event horizon is where the time stops and nothing goes further.if a singularity existed,it could not be detected,because no gravitation could be emitted by the black hole.That is what Susskind and Howking forgot.