Physicists Demonstrate How Information Can Escape From Black Holes based in LQG

cristo
Staff Emeritus
I'm moving this to BtSM: it's regarding black holes in LQG, so folks there will be better placed to discuss it.

The issue is whether unitarity is broken in QG, right ? And they (Ashtekar) can not be break unitarity, can they ? Maybe better

Information is Not Lost in the Evaporation of 2-dimensional Black Holes

BTW, there has been at least one proposal for a "mechanism for how information might escape from a black hole", but maybe the author did not classify it at as "plausible". Namely, that there are correlations in the supposedly purely thermal radiation emitted by the BH.

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marcus
Gold Member
Dearly Missed
thanks. I will look for more on that.
I gather it is work by Ashtekar, Taveras, and Varadarajan
on black holes in 2D
and they believe it will generalize to 4D

It says their paper will be published in Physical Review Letters. 20 May 2008

Here
http://arxiv.org/abs/0801.1811
Information is Not Lost in the Evaporation of 2-dimensional Black Holes
(Submitted on 11 Jan 2008)

Abstract: We analyze Hawking evaporation of the Callen-Giddings-Harvey-Strominger (CGHS) black holes from a quantum geometry perspective and show that information is not lost, primarily because the quantum space-time is sufficiently larger than the classical. Using suitable approximations to extract physics from quantum space-times we establish that: i)future null infinity of the quantum space-time is sufficiently long for the the past vacuum to evolve to a pure state in the future; ii) this state has a finite norm in the future Fock space; and iii) all the information comes out at future infinity; there are no remnants.

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Ashtekar's idea is not the first but rather only the latest idea about how to avoid violating unitarity. Hawking in the framework of euclidean quantum gravity and Susskind in string theory both offer solutions. Interestingly, Hawkings idea actually rests on the assumption of ads/cft. Susskind's approach involves the introduction of a concept called black hole complementarity. Ashtekar's is the least elegant of the three. However I don't find any of them convincing. My opinion is that the real solution is that somehow, black hole degrees of freedom live both on and inside the event horizon, but in a way that doesn't violate the no quantum xerox principle.

nrqed
Homework Helper
Gold Member
Ashtekar's idea is not the first but rather only the latest idea about how to avoid violating unitarity. Hawking in the framework of euclidean quantum gravity and Susskind in string theory both offer solutions. Interestingly, Hawkings idea actually rests on the assumption of ads/cft. Susskind's approach involves the introduction of a concept called black hole complementarity. Ashtekar's is the least elegant of the three. However I don't find any of them convincing. My opinion is that the real solution is that somehow, black hole degrees of freedom live both on and inside the event horizon, but in a way that doesn't violate the no quantum xerox principle.
Very interesting. I hope you will stick around and provide more details.

For now, I have a very stupid question. I am confused by one thing. I always hear people saying that quantum mechanics "preserves information" and hence is in conflict with GR because of black holes. But the measurement process in QM is not unitary (at least if we use the Copenhagen interpretation). So why do people always say that QM preserves information?

Demystifier
Gold Member
Very interesting. I hope you will stick around and provide more details.

For now, I have a very stupid question. I am confused by one thing. I always hear people saying that quantum mechanics "preserves information" and hence is in conflict with GR because of black holes. But the measurement process in QM is not unitary (at least if we use the Copenhagen interpretation). So why do people always say that QM preserves information?
Excellent point!
This is indeed a part of the information-paradox solution proposed in
http://xxx.lanl.gov/abs/0708.0729
See in particular Sec. 3.

personal reflection

I am confused by one thing. I always hear people saying that quantum mechanics "preserves information" and hence is in conflict with GR because of black holes. But the measurement process in QM is not unitary (at least if we use the Copenhagen interpretation). So why do people always say that QM preserves information?
As far as I see it, since QM doesn't model the measurement process in a context where the observer is subject to feedback. It rather models the the expected evolution between measurements relative to this "idealised observer", that as it seems have inifinite memory capacity etc.

So IMO, the unitary evolution of QM, is an expectation only IMO. Sometimes this expectation is very good! and make sense, sometimes not. That's how I personally see it. It's not fundamental to me.

The way I prefer to see this is that "lost/hidden" information may simply be indistinguishable in the general uncertainty. And the complexity of the observer must possible put a bound on the confidence in anything, thus certain things that are indistinguishable _relative to this observer_ (say a black hole) may IMO without contradiction be distinguishable relative to another obsever.

So perhaps, from the point of view of the black hole itself, the radiation is "random", and thus no distiniguishable loss. It then seems to me that by this reasoning one would expect that very small black holes are radiating "less random" than a larger one, as judged by the same observer.

I always thought the origin of alot of mess is the abstraced observer used in QM. There is no constraint HOW MUCH information it makes sense to have at once. Which does not to me, seem physically reasonable in any way. This is why I think the QM formalism needs relaxation.

/Fredrik

Isn't the information most likely to be transformed, rather than preserved?

For example, consider the gastrointestinal tract as a biophysical black hole.
Nutrients enter but are transformed either to useful biological entities or into waste.
Now in total the information is prserved, but in is difficult to piece together the original nutrient from the waste alone or from useful biological entities alone.