BeGroMaS: gravity was renormalizable after all, so why all the fuss?

[Careful]

We do. String theory. In particular, AdS/CFT, which unfortuantely has already been falsified. But maybe studying it will give us some ideas. And maybe the rest of string theory will also turn out to be coherent.

Well as I said once, ST scores 6,5 on my personal scale and LQG 3 on 10, that is. Still, I find 6,5 to be too meager.

But some ideas of string theory are certainly worthwhile, but the strings themselves are not in my opinion. But as my stringy friends tell me, ST is moving away from this picture slowely, no ?

 

[Careful]

Ever looked at lattice gauge theories? Perhaps you do not care b/c it's conceptually boring - but they can calculate observables :-)

Yes, I know that, but as far as I know, it stops at the level of the lattice, no ? :!) I may be wrong here, I am not a specialist in this kind of approach.

 

[atyy]

Well as I said once, ST scores 6,5 on my personal scale and LQG 3 on 10, that is. Still, I find 6,5 to be too meager.

But some ideas of string theory are certainly worthwhile, but the strings themselves are not in my opinion. But as my stringy friends tell me, ST is moving away from this picture slowely, no ?

You had better be using a log scale there!

 

[Careful]

You had better be using a log scale there!

One can never give too few points to the opponent, can one ?

 

[Careful]

It's great, I hope it was not too long ago, so maybe you will have time to tell entire communities what the final answer to high energy physics is. Until then, you may want to check your dictionary for "humility".

Ok, I apologize for having said that, although I think it is a correct statement, I should perhaps have kept my opinion for myself.

Careful

 

[PAllen]

We do. String theory. In particular, AdS/CFT, which unfortuantely has already been falsified. But maybe studying it will give us some ideas. And maybe the rest of string theory will also turn out to be coherent.

Hey, can you tell me about this? I thought this was a significant achievement of the string program, and never heard it was in doubt.

Thanks.

 

[atyy]

Hey, can you tell me about this? I thought this was a significant achievement of the string program, and never heard it was in doubt.

Thanks.

I mean that although AdS/CFT is almost certainly a coherent theory of quantum gravity, it doesn't seem to give rise to cosmologies that match observations.

 

[tom.stoer]

But isn't the point that in QG, we are looking for all theories that are internally consistent. Consequently, the statement is of course mathematical in nature. Whether the physics is correct is decided by comparison with observations. The Wilson-Weinberg viewpoint is very simple, either it is consistent with a UV fixed point, like QCD, or new degrees of freedom must be introduced, like string theory. There are theories known where the fixed point is non-Gaussian, the only question is whether gravity without additional degrees of freedom is such a theory.

I'll try to come back to the original topic, so this could be a good starting point.

The main problems in QG (including AS approch) seem to be
- renormalizibility
- non-perturbative approach (related)
- background independence

If this can be achieved consistently then one has a theory of QG that is viable theoretically. If the latter can be addressed in the AS approach is still unclear to me; I have to go though all the details.

Now, assume that the above mentioned steps have been completed, then we are in a rather strange situation: we have the interactions SM + QG(AS) valid up to Planck scale (or perhaps beyond), but we know that the SM still has some problems. Perhaps the different interactions influence each other such that e.g. QED becomes soft (asymptotoically free) instead of having a Landau pole. Fine. But the remaining question is about unification (not directly relatated to QG).

The picture then is
- no need for SUSY and strings
- no idea how to bring these interactions together, no common principles

 

[PAllen]

I'll try to come back to the original topic, so this could be a good starting point.

The main problems in QG (including AS approch) seem to be
- renormalizibility
- non-perturbative approach (related)
- background independence

If this can be achieved consistently then one has a theory of QG that is viable theoretically. If the latter can be addressed in the AS approach is still unclear to me; I have to go though all the details.

Now, assume that the above mentioned steps have been completed, then we are in a rather strange situation: we have the interactions SM + QG(AS) valid up to Planck scale (or perhaps beyond), but we know that the SM still has some problems. Perhaps the different interactions influence each other such that e.g. QED becomes soft (asymptotoically free) instead of having a Landau pole. Fine. But the remaining question is about unification (not directly relatated to QG).

The picture then is
- no need for SUSY and strings
- no idea how to bring these interactions together, no common principles

You can use dark matter plus easier unification as a motivation for SUSY orthogonal to QG. This synergy has made me believe in the likelihood of SUSY independent of what happens with the string program.

 

[tom.stoer]

Agreed - but I guess one must face the idea that this is peraps the final truth.

 

[atyy]

Furthermore, can one show that AS remains when whatever matter is added in?

 

[tom.stoer]

Furthermore, can one show that AS remains when whatever matter is added in?

It hasn't been done so far.

This is another problem if you are not able to provide some kind of unified approach: You would have to check for each matter content seperately. So there should be some approach (SUGRA?) which allows you check a whole class of models.

 

[Careful]

If this can be achieved consistently then one has a theory of QG that is viable theoretically. If the latter can be addressed in the AS approach is still unclear to me; I have to go though all the details.

Of course not ! What about all other things like : (a) definition of observers (b) causality (c) locality (d) measurement problem and so on... All you have done when you solve the problems you adress is constructed a mathematical devise which allows you to compute some scattering matrix between asymptotic observers, there are no observations within the universe, no control over causality (which perturbation theory allows you to do) and so on. I think I once told you, that doing the technical excercise gives you a score of 33%, not 100%.

You see, that's the difference between me and most people, I am actually interested in *physics* and the mathematics is just a tool, not a purpose in itself.

Careful

 

[Careful]

Ever looked at lattice gauge theories? Perhaps you do not care b/c it's conceptually boring - but they can calculate observables :-)

This came only to my mind this morning, but the observables you are talking about are holonmies and field strength's I guess. But then, you never measure those things, how do you define particle in lattice approach given that you break Poincare invariance ? How do you define click of a measurement apparatus ?

 

[humanino]

Thank you Marcus for the link. How do you interpret Witten's question at the end of Weinberg's talk ? He asks whether they find conformal symmetry at the fixed point. Weinberg answers that they certainly have scale invariance, but he cannot tell for the full conformal. My interpretation is that full conformal is necessary for consistency with string theory. Horava also comments in that sense during the next talk.

 

[tom.stoer]

What about all other things like : (a) definition of observers (b) causality (c) locality (d) measurement problem and so on... All you have done when you solve the problems you adress is constructed a mathematical devise which allows you to compute some scattering matrix between asymptotic observers, there are no observations within the universe, no control over causality (which perturbation theory allows you to do) and so on.

You are too much focussed on old-fashioned QM, measurement problem, perturbative particle physics etc.

(a+d) have to be eliminated, not "solved"; I think that a holographic approach where quantum theories and boundary Hilbert spaces are defined on surfaces of observed world volumes is nice
(b) is eliminated if it's background independent (it is only a problem if you have a background according to which the theory shall be causal; that's the wrong approach)
(c) of course; but where do you see indications that it fails to be local?

Of course one has to solve these issues, but most of them are NOT physical. They are technical and belong to old-fashioned thinking based on perturbative particle physics. They are problems created by our methods, not by nature.

 

[tom.stoer]

... but the observables you are talking about are holonmies and field strength's I guess.

Wrong guess :-)

Lattice gauge theory is mostly about expectation values <X> = ∫ DU X exp iS

 

[Careful]

You are too much focussed on old-fashioned QM, measurement problem, perturbative particle physics etc.

(a+d) have to be eliminated, not "solved"; I think that a holographic approach where quantum theories and boundary Hilbert spaces are defined on surfaces of observed world volumes is nice

Of course, they have to be solved, they constitute the map between theory and interpretation. Without realistic interpretation, you have no theory. QFT does not put away these problems either, it restricts it self to asymptotic observers. If you feel this is physically realistic, please take your rocket and move to the boundary of the universe (and we will continue to discuss from thereon ).

(b) is eliminated if it's background independent (it is only a problem if you have a background according to which the theory shall be causal; that's the wrong approach)

Sorry, but you hit the ball entirely wrong here. NOBODY knows what causality means in a BI quantum theory of gravity ! In CDT for example, people have no control over causality whatsoever in contrast what the first letter may suggest to you.

(c) of course; but where do you see indications that it fails to be local?

Non locality is a generic feature of quantum gravity and cannot be restored, unless you modify QM a la 't Hooft.

They are problems created by our methods, not by nature.

Partially true, partially not ; see my first comment.

Careful

 

[Careful]

Wrong guess :-)

Lattice gauge theory is mostly about expectation values <X> = ∫ DU X exp iS

Well, I guess both are correct, Wilsonian observables are often computed in lattice theories. But even then, how do you *measure* such expectation values, what is your theory of measurement and particles ? No answer to that huh, have you ?

 

[tom.stoer]

Well, I guess both are correct, Wilsonian observables are often computed in lattice theories. But even then, how do you *measure* such expectation values, what is your theory of measurement and particles ? No answer to that huh, have you ?

There is no "theory of measurement". That's misguided thinking due to "the measurement problem". One has to overcome this differently; developping a "theory of measurement" within the QM framework is certainly wrong.

But I think that has nothing to do with QG.

 

[MTd2]

Benedetti Groh Machado Saueressig have (what will probably turn out to be) a landmark paper where they show the Renormalization Group Flow treatment of gravity is background independent.

Isn`t it a good reason for a huge fuss? Besides, they come up to a method in which humans are no longer required, just a computer.

 

[Careful]

There is no "theory of measurement". That's misguided thinking due to "the measurement problem". One has to overcome this differently; developping a "theory of measurement" within the QM framework is certainly wrong.

But I think that has nothing to do with QG.

Why do you say such silly things ? There are different measurement problems. The one you are talking about is a slim version of the one I am interested in. All you say is the following:
(a) all I can calulate are correlation functions
(b) I have no procedure for measuring these correlation functions within the universe
(c) all I can do is calculate S-matrix using LSZ type formulae
Therefore, I have no realistic theory of observation.

My conclusion : hence, your theory is wrong since it directly contradicts experience.

End of story; the only one who is misguided here is you.

Careful

 

[marcus]

Here is a Steven Weinberg talk on the topic. It is basically his baby.
https://mediamatrix.tamu.edu/streams/327756/PHYS_Strings_2010_3-18-10C

If anyone tries this link and can't get the video, please let me know. My guess is that Weinberg is pleased with this paper by BGMS. We may hear something about that.

Thank you Marcus for the link. How do you interpret Witten's question at the end of Weinberg's talk ? He asks whether they find conformal symmetry at the fixed point. Weinberg answers that they certainly have scale invariance, but he cannot tell for the full conformal. My interpretation is that full conformal is necessary for consistency with string theory. Horava also comments in that sense during the next talk.

I'm glad you found it interesting! Unfortunately I don't have much useful to add to your interpretation of the question Witten asked, and of Weinberg's answer.
I think the question of scale invariance (and possible conformal symmetry) at the UV fixed point (if it exists) is of general interest. I have seen it raised in other contexts besides string.

Benedetti Groh Machado Saueressig have (what will probably turn out to be) a landmark paper where they show the Renormalization Group Flow treatment of gravity is background independent.
...
http://arxiv.org/abs/1012.3081
The Universal RG Machine
Dario Benedetti, Kai Groh, Pedro F. Machado, Frank Saueressig
38 pages
(Submitted on 14 Dec 2010)
...

Isn`t it a good reason for a huge fuss? Besides, they come up to a method in which humans are no longer required, just a computer.

Yes! The paper by S. MacGroBen is indeed reason for a HUGE fuss! It substantially enables the program sketched out by Steven Weinberg at a CERN conference on around 6 July 2009. Basically unification based on quantized gravity and the "good old standard model".

The fuss I was talking about, which may have been a useless 30 year diversion, is the fuss that was made based on the assumption that Einstein gravity is inherently non-renormalizable and some radical break with GR is required.

 

[tom.stoer]

(a) all I can calulate are correlation functions
(b) I have no procedure for measuring these correlation functions within the universe
(c) all I can do is calculate S-matrix using LSZ type formulae
Therefore, I have no realistic theory of observation.

rearding a) where did I say that?
rearding b) where did I say that? how does a simple theory like Newtonian mechanics tell you how to measure things?
rearding c) where did I say that?

 

[atyy]

The fuss I was talking about, which may have been a useless 30 year diversion, is the fuss that was made based on the assumption that Einstein gravity is inherently non-renormalizable and some radical break with GR is required.

Exactly, LQG will be a failure.

String will still be interesting, because theories fail even if they are renormalizable.

 

[marcus]

Exactly, LQG will be a failure.
...

You sound quite sure of that.

 

[atyy]

You sound quite sure of that.

Let me say more carefully - the poor conceptual foundations of LQG will be shown up.

LQG will survive as GFT, but all the heuristics in Rovelli's Quantum Gravity book will be discarded.

 

[MTd2]

Exactly, LQG will be a failure.

String will still be interesting, because theories fail even if they are renormalizable.

So, both of them will fail?

 

[atyy]

So, both of them will fail?

For example, there could be a UV fixed point of pure gravity, but we are off the critical surface. I believe that even in this case, although AS will not describe reality perfectly, the fixed point will have some influence on nearby trajectories and thus be physically visible.

At the same time, it will imply that gravity is emergent. I would be interested to know if the existence of AS is compatible (or provably not) with string theory in this fashion.

 

[MTd2]

I don`t see how that relates to LQG.