Holographic Dualities and the Potential Misdirection of String Theory

[negru]

This

Finally, it could be true that:
Our theory of black holes is not correct, they don't exist as we currently conceive them
Entropy for the black hole replacement in the new theory scales as r^3

and also why AS can't be right.

 

[inflector]

Well, it is only the last of four items. I don't consider it the most likely that's why it is last.

It remains nevertheless a possibility. If someone were to propose a specific theory at some time in the future that had this trait then it would be incumbent upon the authors of that theory to address the issues with respect to QFT that you have shown.

As Tom pointed out earlier:

In GR a black hole is a curvature singularity with an event horizon hiding the singularity from the outside oberver. One can calculate its temperature and entropy using classical GR with a semiclassical approx. for QFT. In LQC the curvature singularity is replaced by a mathematically well-behaved region w/o any singularity, whereas in full LQG the event horizon enclosing the singularity seems to stay intact. So the interior which no longer contains a singularity is still hidden from the outside observer.

So we have different theories predicting different ideas about what a black hole is. My earlier point that this means that we can't necessarily rule out behaviors which are predicted by GR stands. Let any given theory of quantum gravity deal with black holes as it may. As long as it is internally consistent, deals with the correspondence principle and can explain more than GR and quantum mechanics alone, it will find some success.

As to AS? I don't know enough about it specifically to offer even a modest assessment.

The issue of whether or not gravity is emergent or fundamental is orthogonal to the issue of black hole entropy being a function of area. That was the real focus of this thread. Black hole entropy is why holographic dualities are of interest because they solve the problem in one plausible way.

There are other ways that can address black hole entropy without relying on holographic dualities and some of these ways include the idea that gravity is fundamental.

 

[tom.stoer]

So we have different theories predicting different ideas about what a black hole is.

No, not really.

We know tat GR breaks down at or near the singularity and has to be replaced by something which is mathematically consistent.

LQG seems to do the job, unfortunately it is too complicated to solve all equations and one has to use approximations. LQC is (in a certain sense - please don't mess up my argument with minor details) such an aproximation. It predicts the elimination of the singularity but is unfortunately not able to talk about entropy of the gravitational degrees of freedom because it reduces the theory to only a finite number of them - which means that it has nothjing to say about entropy.It's a kind of toy model for full LQG - but a rather interesting one as it discusses exactly the same limiting cases as the spherically symmetric Schwarzsschild black hole and the spherically symmetric spherically symmetric scenarios.

Full LQG is too complicated to discuss the full dynamics, but one is already able to derive some quite interesting results, namely entropy via nearly exact counting of states, and the quantum geometry of the horizon.

So we do not have three different theories, but theories which are related to each other by taking the classical limit (GR from LQG) and by symmetry reduction (LQC from LQG).

Please note that in LQG the horizon degrees of freedom emerge automatically and need not be introduced by hand. Therefore the holoraphic principle is somehow emergent as soon as one studies a boundary between too regions of spacetime.

 

[inflector]

Thanks for the clarification tom.

 

[atyy]

This


and also why AS can't be right.

I understand your argument about the scaling of entropy, and agree that it is unclear how AS will solve it. Nonetheless, it is not clear that AS cannot solve it.

What do you make of this statement http://golem.ph.utexas.edu/~distler/blog/archives/001585.html ?"In any case, the existence of a “quantum” conformal symmetry in quantum gravity is compatible with there being a nontrivial dimensionful scale in the theory, so I don’t see a-priori why it’s incompatible with black holes."

A second question is, is it clear in AdS/CFT, that if one runs the renormalization flow from high energies to low, that there isn't an IR fixed point in the full theory that would be a UV fixed point in gravity?

 

[tom.stoer]

For me the question is if AdS/CFT can be translated to the real world which is not AdS.