Successors to string scuffle (physical assets/liabilities?)

[marcus]

Atyy I think a key issue is one you raised (actually in another thread, but applies here).

...Although a common classification of gravity theories is background and non-background independent, I believe a better classification is whether the gravitational field is fundamental or emergent. Asymptotic Safety and LQG treat the gravitational field as fundamental, while string theory and condensed matter approaches hypothesize that the gravitational field is emergent.

The question can be put more strongly, are causality and locality fundamental (not simply does a metric exist), or are they merely appearances arising from something else, an abstract algebra perhaps?

That's not for us to settle but we can, as you point out, attempt a practical classification of theories with the help of this distinction.

Is the theory constructed about a concrete mathematical representation of spacetime carrying causal and local structure, that we can plant matter fields on?

More simply, is there anything concrete in the theory that one can build matter fields on, and what is it?

I hope some other people will comment. At the moment I don't have definite ideas about this. Presumably matter fields live on a geometry, but maybe it needn't be a metric geometry. It might, I suppose, be a conformal geometry without definite scale.

This possibility was raised in 't Hooft's September 2009 talk at the Erice school. Here are the slides:
http://www.ccsem.infn.it/issp2009/professors/GtH_NoSingularities_09.ppt
Here is the corresponding paper on arxiv:
http://arxiv.org/abs/0909.3426
Quantum gravity without space-time singularities or horizons
Gerard 't Hooft
10 pages, 3 figures. Presented at the Erice Summerschool of Subnuclear Physics 2009
(Submitted on 18 Sep 2009)
"In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an "emergent" one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory. Use is made of the principle of "black hole complementarity", the notion that observers entering a black hole describe its dynamics in a way that appears to be fundamentally different from the description by an outside observer. These differences can be boiled down to conformal transformations. If we add these to our set of symmetry transformations, black holes, space-time singularities, and horizons disappear, while causality and locality may survive as important principles for quantum gravity."

So it seems to me 't Hooft sees the same important issue that you do. Even more crucial than background independence (avoidance of a prior fixed geometry) is the question of whether or not a concrete spacetime geometry that one can hope to build matter fields on exists at all, in the theory.
I was interested that 't Hooft decided to use his time at Erice this year to make this point. He raises the issue forcefully.

The slides have a lot of intuitive graphic illustration that adds to the paper. I found it helps understand the arxiv paper if you look at the slides as well.

 

[Fra]

I'll just throw my personal opinon in on one key question.

Do we know for a fact there is a force unification? If nature does not unify forces at high energies, neither should our theories

There seems to be different views of what unification means. One idea seems to replace a bunch of distinct symmetry principles by one large symmetry principle, to somehow reduce the number of free parameters or prinicples.

From my point of view, which sees no difference to treat physical process different from general scientific inference processes, information can only be acquired be means of a physical interactions - this alone is a information-theoretic basis for unification - and it's the kind of unification I expect. From this point of view I find it conceptually confused or inconsistent to treat different kinds of information in different ways. Some are treated properly, while some are treated in a realist fashion. So in my view, the information theoretic unificaiton is a requirement from consistency of reasoning if we take the scientific perspective serious and don't think that human scientists should somehow be described by a logic totoally unrelated to that describing nonbiological systems.

I think the same "inference logic" that applies to say "particle physics" must apply also to "very complex selforganised systems". I can't find any rational defense for such fundamental decomposition other than as a plain simplificaiton.

/Fredrik

 

[marcus]

to re-establish space-time as an essential frame...rather than an "emergent" one...

Judging from their most recent talks, I think that Steven Weinberg, Gerard 't Hooft, and Hermann Nicolai have come to essentially the same conclusion about priorities--what urgently needs to be done to get theoretical physics back on track. I don't suppose there has necessarily been communication about this. They just seem to be, for the present, aligned towards a common objective.

't Hooft puts this most openly and radically.

... 't Hooft's September 2009 talk at the Erice school. Here are the slides:
http://www.ccsem.infn.it/issp2009/professors/GtH_NoSingularities_09.ppt
Here is the corresponding paper on arxiv:
http://arxiv.org/abs/0909.3426
Quantum gravity without space-time singularities or horizons
Gerard 't Hooft
10 pages, 3 figures. Presented at the Erice Summerschool of Subnuclear Physics 2009
(Submitted on 18 Sep 2009)
"In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an "emergent" one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory..."

The conclusions of the Erice talk, the next to last slide, say:
==quote==
We suspect that it is not correct to dismiss
space-time as "emergent", like it is
sometimes done in Superstring theories.
Space-time can be, and should be, the
essential backbone of a theory.
==endquote==

In the same conclusions slide he also makes the intentionally provocative statement that "Scale invariance is an exact symmetry, not an approximate one!"
(One should understand that in context, which either the slides or the paper can provide.)

The annual Erice school on subnuclear physics has a long tradition. It is where, in 1976, Steven Weinberg first proposed Asymptotic Safety. Dirac lectured at Erice. 't Hooft is one of the people who guides the Erice program. In 2008 he had Renate Loll lecture at the school. I believe he thinks carefully, when he chooses the subject of his Erice talks. There is a tradition of Erice talks sometimes turning out to have historical import. And it's a nice place.

I don't think we necessarily have to take the viewpoint that this or that approach is right. I am interested in what people with vision think should be worked on. All three of them seem primarily concerned with getting physics back on track.
Steven Weinberg indicated this year that he has decided to work on Asymptotic Safe quantum gravity and cosmology, with an eye to re-establishing the Standard Model of matter on the expected new version of space-time. He expressed the opinion that that might be how the world is, rather than some Superstring emergence scenario.
Well, it is something to try, and it is in the same general direction as 't Hooft, namely working to to re-establish space-time as an essential frame.

We probably all know Hermann Nicolai is one of Europe's most respected particle theorists. He has been on the scientific organizing committee of nearly every one of the annual String conferences for the past 10 years*. He directs a branch of the Max Planck Institute concerned with unification and quantum gravity. For a long time it seemed that his research was primarily in string theory, but I think he has a broader perspective. Since 2006 he has published a series of papers with Kris Meissner that also has this same thrust, namely to re-establish space-time as an essential frame rather than (again in 't Hooft's word) dismissing it as "emergent".

The recent talks by both 't Hooft and Nicolai have in common an intense interest in scale invariance, conformal symmetry.
Here are some Nicolai links:
https://www.physicsforums.com/showthread.php?t=339154
Here's a thread about Weinberg's July 2009 talk at CERN, with links to the video and related pdf, plus some discussion of AsymSafe QG.
https://www.physicsforums.com/showthread.php?t=324841

*Strings 1999 through Strings 2010, the sole exception being 2001.

 

[Bob_for_short]

...namely working to to re-establish space-time as an essential frame.

Good! What else should we re-establish and what for? Do we have at all a suit to follow, a working model of anything or shall we first try out all our personal "postulates"?

As I said, we have two working (quite nicely) limiting cases - when sources are known. The problem is to build a self-consistent theory. How to exchange with energy-momentum without obtaining non physical results?

As I showed in "Reformulation instead of Renormalizations", we can build an interacting theory of whatever because any field theory can be reduced to (practically non-relativistic in form) Hamilton dynamics. Hamilton dynamics of non-relativistic particles is free from conceptual and mathematical difficulties, such as bare particles and constant renormalizations. For two- and more-particle systems the natural variables are the center of inertia and relative ones, i.e., the quasi-particle variables. It is easy to show that in fact it is these variables that are observable in experiment, not "personal" particle degrees of freedom. Think, for example, of the total inertia (mass, rest energy), proper frequencies, etc. We have just to recognize it as experimental and theoretical facts - we observe quasi-particle degrees of freedom of compound systems. This should serve as a flawless model to follow if we think we know what degrees of freedom (photons-electrons, quarks-gluons, whatever) have to share the energy-momentum. Any self-action approach is non-physical in principle because it introduces a positive or negative feedback that violates the conservation laws (runaway solutions are not physical). Isn't it evident?!

 

[marcus]

to re-establish space-time as an essential frame...rather than an "emergent" one.

As with any research goal stated briefly in general terms we should probably take some time to understand the context of what 't Hooft was saying.
==quote 0909.3426==
...what happens to the horizon and the space-time singularities? An answer sometimes suggested by string theorists, as well as others, is that all of space-time is just “emergent”[2][3][4]; the theory should first be formulated without space-time altogether.

Or, perhaps, time alone is an emergent concept[5]. It was argued that, at least, locality would have to be abandoned[6][7].

In this paper, however, we dismiss all such options. In particular, we insist that any satisfactory theory should have built in a strong form of causality, as well as locality, in order to explain why cause precedes effect, and why events separated at some distance from one another appear to evolve independently. For this, space-time appears to be indispensable.

Something has to give, and in this paper we claim to have found a good candidate for that: the definition of scales in space-time. It should be done in a way that differs from conventional wisdom...
==endquote==

I'm not proposing here, nor do I believe in, automatic acceptance of authority. But I think that 't Hooft continues to earn the right to have us occasionally pay special attention to what he's saying.

He thinks this message is a good one to get out to his theory colleagues & students at this time, and to it he devotes his September 2009 Erice talk together with the followup posting on arxiv which I am quoting here:
http://arxiv.org/abs/0909.3426

My comment at this point is basically that I'd like to understand this better. Does anyone want to elucidate? Why does "something have to give?"
If we want to keep the causality/locality structure embodied in spacetime---and continue to base fields on some type of concrete mathematical realization of the lightcone structure, then why are we obliged to give up something else? Why do horizons and singularities like those associated with black holes imply that "something has to give?" Maybe this seems obvious to you---please explain anyway: it will reassure me (and possibly others) that we've got it right.

================
I'm considering the notion that we could evaluate our halfdozen rival approaches in terms of four ad hoc criteria:
1. Does the approach come to terms with renormalization and the running of couplings. (Which, as Weinberg observed, might give a natural explanation of inflation.)
2. Does it have spontaneous dimensional reduction, as discussed by Carlip. 4d down to 2d at small scale.
3. Does it have a concrete mathematical realization of spacetime--that gives meaning to causality/locality and you can define fields on.
4. Could it be adapted so as to fill the bill for Nicolai (and it sounds like 't Hooft would like this too.) Could it acquire conformal symmetry in the limit.

 

[tom.stoer]

Let's put it that way: we know that the combination of usual calculational tools with spacetime manifold leads to diverging = physically meaningless results. This applies both to classical physics (GR) and quantum field theories. So there are two options:

1) Let spacetime stay a four dim. manifold but change the way how to put matter on top in a fundamental way yet to be discovered.
2) Change the way how to represent spacetime

1) was the program followed by string theory over some decades; OK, spacetime was 10 dm., but that was the only difference.
2) is e.g. the idea of LQG and non-comm.geometry.

It's hard to say what an emerging spacetime means. Is spacetime emerging from spin networks? Or are spin networks only a different representation of spacetime? (they already have to right symmetry).

 

[Fra]

Here is a very basic comment from me, that sits at the bottom leve of the LQG type of constructs.

2) Change the way how to represent spacetime
...
It's hard to say what an emerging spacetime means.

My expectation is that the kind of programs that are likely to be most successful is those where spacetime and matter are required to emerge simultanesouly, rather than trying to patch a matter model ontop of a spacetime model without loosing the coherence of the constructions.

If you take the machian idea that spacetime is only a matter of relations between objects rather than anything absolute. One is naturally lead to question the ontological status of these relations. Rovelli has said that there are similarly (this is to me required by consistency of reasoning, once you start to walk in this direction) no absolute relations, but only RELATIVE relations; and the only way for systems to COMPARE their "establised/observed relations" is by interacting with each other (communicating).

So far I am totally with Rovelli.

I associate here "systems" with "matter system". So the observers communicating their subjectively establised relations to their environments, are simply the matter-matter interactions.

But then Rovelli conjectures that standard QM, describes this interaction. This is not satisfactory for me, and it's at this point, my lack of confidence in the rest of the construction is rooted.

IF we can rework it from here (which is what I like to do), I think the original relational kind of spirit of Rovelli can be continued even more coherently.

Somehow I think expectations from such reconstruction must reconstruct spacetime and matter at the same time, because matter is somehow the physical basis where the subjective relations to it's environment is encoded.

I don't know in detail all the details of the variations of LQG style reasoning, but I did start to read rovelli's book and his papers some years ago and was put off by these kinds of what I consider to be serious flaws in the reasoning.

/Fredrik

 

[tom.stoer]

... that the kind of programs that are likely to be most successful is those where spacetime and matter are required to emerge simultanesouly, rather than trying to patch a matter model ontop of a spacetime model without loosing the coherence of the constructions.

Yes, that's my feeling as well.
I liked strings in the sense that they were heading towards unifying matter and gravity both emerging from the fundamental vibrating string, but unfortunately they could not get rid of the spacetime background.
I think LQG is a step forward in the sense that they incorporate the lessons from GR (background independence, diff.-invariance) but as I said, emerging matter is still only wishful thinking (braiding is a good idea but as far as I can see not very actively studied).
Non-comm. geometry seems to do something like what we have in mind, but I am not an expert (just started to study some review papers).

... but I did start to read rovelli's book and his papers some years ago and was put off by these kinds of what I consider to be serious flaws in the reasoning.

I wouldn't agree. Writing his book Rovelli had a clear strategy, namely to describe in detail and in a rigorius manner the current knowledge about quantum gravity / quantized geometry. He left out unification - not accidentally but simply because he wanted to focus on gravity.
LQG is still not fully developed; it seems that the spin foam approach can answer questions regarding the classical limit and smooth 4-dim. spacetime, but there are still some puzzles regarding canonical quantization, Imirzi parameter, cosmological constant etc.
As soon as they have patched together all these pieces, they are definately able to start with the next step in their program, namely to understand if LQG allows not only to put matter on top, but to let matter emerge from geometry. In addition there is a different research area waiting to attract attention, namely if it's possible to quantize SUGRA according to the LQG methods and if one can understand if matter and gravity can be harmonized in that way.

I am pretty sure that Rovelly, Smolin, Ashtekar and others are well aware that LQG (as of today) is not the final answer, but maybe it's simply too early (even for them!) to attack these questions now and to skip the more basic ones. In addition they have to chec if they are still on the right track (LQC is just doing that: try to solve a simpler models and check for consistency and possibly experimental verification).

Do you think that Heisenberg, Pauli, Dirac, Schrödinger could have been successful w/o the early attempts (Bohr, Sommerfeld, ...)? Do you think that one could have skipped the 20 years between 1905 and 1925? I don't think so.

Tom

 

[Fra]

I wouldn't agree. Writing his book Rovelli had a clear strategy, namely to describe in detail and in a rigorius manner the current knowledge about quantum gravity / quantized geometry. He left out unification - not accidentally but simply because he wanted to focus on gravity.

Maybe my comments came out harsh. I like Rovelli, and parts of his writings and papers are great, no doubt about that. I also think he wrote a nice book, which contains great perspectives on the problem of QG in a reasonably neutral way in the intro parts.

But when I got that book, and before I actually knew anything about LQG, I already had my own questions, and my own sort of loose ideas. And I got an early vision of a possible interpretation of his spin networks, and I started to read up on his work to see if his ideas, could be the answers to my questions - I found out it wasn't, but maybe they could be, but it's the same as with string theory. If you rework a theory enough, would you still call it by it's old name?

Even from my very personal view, I can see possible reworkings of both string theory and LQG that might satisfy me, but then the reworkings would be significant and then one certainly starts to wonder what "characterises" say string theory? Would anything a string theorist come up with be called string theory? would string theory were the strings are emergent rather than fundamental be farily called string theory?

Now of course, it really doesn't matter what we call it though, but I can do nothing but to try to pose and answer my own questions, and the starting point provided by rovelli in the subset of his work I've seen is not convincing me.

My reason for reading rovelli was basically, could this research give my a jump start on my journey? As far as I understand it, it can't, because it suggests jumping in a direction that I can not defend. I have to defend my own actions, even if authorative people suggests otherwise.

I think I've pointed in previous threads on the key principles of construction of rovelli's reasoning as well as the string theory reasoning that I do not like. I think at least I have rational objections, but both programs contains great things as well of course. But the principles that are used to define the framework of LQG, or the framework of string theory are important and if I find reasons to doubt some of the principles and see how it could be done differently it wouldn't be rational of me to buy into them just because they are advocade by famous and intelligent researchers. But it would be equally irrational to not look at what they have done and try to learn! I have done that, I take what I like and leave what I don't like :)

/Fredrik

 

[tom.stoer]

Frederik,

I understand and appreciate your reasoning, but I didn't want to repeat the discussion from the other thread :-) That's the only reason why I didn't follow your ideas here.

Thomas

 

[marcus]

... So there are two options:

1) Let spacetime stay a four dim. manifold but change the way how to put matter on top in a fundamental way yet to be discovered.
2) Change the way how to represent spacetime

1) was the program followed by string theory over some decades; OK, spacetime was 10 dm., but that was the only difference.
2) is e.g. the idea of LQG and non-comm.geometry.

It's hard to say what an emerging spacetime means. Is spacetime emerging from spin networks? Or are spin networks only a different representation of spacetime? (they already have to right symmetry).

You hit the sensitive issue. The word "emergent" is used two different ways in this context.
One way is you don't give up trying to represent spacetime. Spacetime still exists in your model----there is still some mathematical object that you can point to and say it represents space time, only it may not be a smooth metric manifold.

In that first way you can say that spacetime is described by more fundamental degrees of freedom but it is still represesented, and the familiar smooth manifold is an epiphenomenon that arises or emerges from it.

In this way "emergence" is only a small step---rising from an underlying microscopic description to a more smoothed out macro. It is merely a "zooming out" emergence, like you zoom the camera.

In the other way, spacetime does not even exist in the model. The ground of existence is way off in some other department. This is a kind of unintuitive radical emergence, like in the movie "Matrix" or like in a holographic projection. Perhaps my analogies or my description of this way are bad because I don't really grasp this kind of radical emergence. I think 't Hooft is pulling away from this.
I think he is not arguing against the other. So this word, because of a double usage, could be obstructing clarity.

 

[tom.stoer]

You hit the sensitive issue. The word "emergent" is used two different ways in this context.

What do you mean exactly by "two different ways"?

 

[marcus]

What do you mean exactly by "two different ways"?

Thanks for the comment. I edited my post to explain what I meant.

As a reminder (mostly for my own benefit) there are some possible criteria to use in evaluating these 4D QG approaches.

...we could evaluate our halfdozen rival approaches in terms of four ad hoc criteria:
1. Does the approach come to terms with renormalization and the running of couplings. (Which, as Weinberg observed, might give a natural explanation of inflation.)
2. Does it have spontaneous dimensional reduction, as discussed by Carlip. 4d down to 2d at small scale.
3. Does it have a concrete mathematical realization of spacetime--that gives meaning to causality/locality and you can define fields on.
4. Could it be adapted so as to fill the bill for Nicolai (and it sounds like 't Hooft would like this too.) Could it acquire conformal symmetry in the limit.

In both the talks by Nicolai and by 't Hooft, conformal symmetry played a big role, so as a resource here is Sam's PF tutorial thread on Conformal Symmetry
https://www.physicsforums.com/showthread.php?t=172461

 

[marcus]

Readers might want to refer back to some of the interesting posts in this thread such as:

Nicolai wrote a viewpoint on how insights from string theory contributed to evidence consistent perturbative finiteness of N=8 SUGRA... http://physics.aps.org/articles/v2/70...

The article by Nicolai that Atyy links to is fascinating. Recommended. Role of string there is not as a physics theory about nature but as a set of mathematical techniques (the strings are eventually shrunk down to points, after they have greatly facilitated a calculation.)

... it is through such (less than) altruistic threads at this forum that I have any touch with alternative theories during my otherwise string-ridden grad school :)

Thanks The value of reporting is in howevermuch objectivity, and let the (less than) altruism chips fall where they may. If you mean favorism then altruism shouldn't enter as an issue.

... a shift from pure string theory into more applied areas like condensed matter/atomic physics ...
... string theory had something to say about real world applications, ... get out of the esoteric and difficult subject matter of quantum gravity (which as a rule was overpopulated with little to no tangible rewards).

Again the "real world applications" is as a set of mathematical techniques, not as a fundamental theory of nature. Like the application to QCD which Atyy mentioned, and the application to the study of superconductivity we heard about this year. Facilitating calculations at much larger scale, e.g. "condensed matter/atomic physics."

Here "pure" string seems to refer to the program of unification and [string] quantum gravity. Failed or stalled, so a way out is needed for both the researchers and the departments which have hired them. Redirection into the use of string mathematical techniques (to non-unification ends) is one way out.

... it is in the air to make a scientific guess what is a more promising and realistic...

Yes the problem of the QG succession, what will take up the slack in fundamental physics, remains an exciting problem.

 

[marcus]

Back around 20 October 2007, Christine wonderfully gave us some excerpts of a piece by Hermann Nicolai that was published in Nature. It illustrates something Haelfix just referred to (a couple of years later.)

Keep in mind that Nicolai is a leader in the string community, but avoids favoritism. He also does strong research outside of the string program and is a clear-eyed critic of string short-comings. Here (and in the recent piece Atyy linked) he warmly praises where he sees a useful string success. Views like this are potentially valuable information.

==exerpts from 18 October 2007 issue of Nature==
String theory: Back to basics

Long touted as a theory of everything, it seems that string theory may at last succeed as a theory of something very specific — the interactions of particles under the strong nuclear force.

Whether string theory can live up to its claim of being a ‘theory of everything’, and whether it will ever produce a falsifiable prediction as such, remain hotly debated questions. Meanwhile, developments in a quieter side-alley[1–8] indicate that the theory might be about to deliver something of its original promise: helping us to understand the physics of interactions mediated by the strong nuclear force. String theory was born in the 1960s, (...)

But initial attempts to describe the forces between the quarks, and why they form the bound states they do, failed miserably. So particle physicists started casting around for other ways of attacking the problem. In 1968, the Italian theoretician Gabriele Veneziano made a brilliant guess [9] and wrote down a concrete mathematical expression, the Veneziano amplitude, that explained some important features of high-energy scattering. But his formula could not be understood in terms of point-like particles; instead, it required the existence of extended objects — strings. (...)

The arrival in the early 1970s of quantum chromodynamics (QCD), the quantum-field theory of the strong interaction, dealt the final blow to these early attempts to understand nuclear physics in terms of string theory. But, unfortunately, QCD is incredibly complex. (...) In this ‘perturbative’ regime, we understand (at least in principle) how to work with QCD. But for the strong coupling that occurs over larger distances, one has to resort to computer-simulation techniques, known as lattice QCD. (...)

The new approach that revives the link to string theory first suggested itself in 1998, when Juan Martín Maldacena conjectured[12] a link between a close relative of QCD and a ‘superstring’ living in a ten-dimensional curved space-time. (...) The Maldacena conjecture raised a lot of interest, but seemed for a long time to be quantitatively unverifiable. (...)

Help came from an entirely unexpected direction. Following a prescient observation[13], the spectrum of the N = 4 theory has been found[1,2] to be equivalently described by a quantum-mechanical spin chain of a type discovered by Hans Bethe in 1931 when modelling certain metallic systems. (...) Indeed, even though the mathematical description of the duality on the string-theory side is completely different from that on the condensed-matter side, a very similar, exactly solvable structure has been identified here as well[3–5]. Puzzling out the details of the exact solution is currently an active field of research. (...)

Just recently, Beisert, Eden and Staudacher[8] have extracted the analogue of this observable on the field-theory side, and have been able to write down an equation valid at any strength of the coupling. Since then, work has established that their ‘BES equation’ does indeed seem, for the first time, to offer a means of reformulating theories such as QCD as string theories. Much still needs to be learned from this one exactly solvable case. There is justifiable hope that this solution will teach us how to go back to the physically relevant case of QCD and finally arrive at the long-sought dual description by a string theory. It may even take us closer to realizing the quantum-field theorist’s ultimate dream, unfulfilled for more than 50 years: completely understanding an interacting relativistic quantum-field theory in the four space-time dimensions that we are familiar with. Progress towards this goal can be judged independently of loftier attempts to use strings in the construction of a theory of everything.

Hermann Nicolai is at the Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Mühlenberg 1, D-14476 Potsdam, Germany.

1. Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0303, 013 (2003).

2. Beisert, N., Kristjansen, C. & Staudacher, M. Nucl. Phys. B 664, 131–184 (2003).

3. Bena, I., Polchinski, J. & Roiban, R. Phys. Rev. D 69, 046002 (2004).

4. Kazakov, V. A., Marshakov, A., Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0405, 024 (2004).

5. Arutyunov, G., Frolov, S. & Staudacher, M. J. High Energy Phys. 0410, 016 (2004).

6. Gubser, S. S., Klebanov, I. R. & Polyakov, A. M. Nucl. Phys. B 636, 99–114 (2002).

7. Frolov, S. & Tseytlin, A. A. J. High Energy Phys. 0206, 007 (2002).

8. Beisert, N., Eden, B. & Staudacher, M. J. Stat. Mech. P01021 (2007).

9. Veneziano, G. Nuovo Cimento 57A, 190 (1968).

10. Ramond, P. Phys. Rev. D 3, 2415–2418 (1971).

11. Neveu, A. & Schwarz, J. H. Nucl. Phys. B 31, 86–112 (1971).

12. Maldacena, J. M. Adv. Theor. Math. Phys. 2, 231–252 (1998).

13. Lipatov, L. N. preprint available at www.arxiv.org/abs/hep-th/9311037 (1993).

14. Zaanen, J. Nature 448, 1000–1001 (2007).

NATURE|Vol 449|18 October 2007NEWS & VIEWS
==endquote==

Most of Nicolai's references are to papers already several years old. There is one 2007 paper that plays a pivotal role in what he has to say, the BES. I will put it here for convenience of anyone who wants to check it out as well:

http://arxiv.org/abs/hep-th/0610251
Transcendentality and Crossing
Niklas Beisert, Burkhard Eden, Matthias Staudacher
31 pages
(Submitted on 23 Oct 2006 (v1), last revised 14 Nov 2006 (this version, v2))

"We discuss possible phase factors for the S-matrix of planar N=4 gauge theory, leading to modifications at four-loop order as compared to an earlier proposal. While these result in a four-loop breakdown of perturbative BMN-scaling, Kotikov-Lipatov transcendentality in the universal scaling function for large-spin twist operators may be preserved. One particularly natural choice, unique up to one constant, modifies the overall contribution of all terms containing odd zeta functions in the earlier proposed scaling function based on a trivial phase. Excitingly, we present evidence that this choice is non-perturbatively related to a recently conjectured crossing-symmetric phase factor for perturbative string theory on AdS5xS5 once the constant is fixed to a particular value. Our proposal, if true, might therefore resolve the long-standing AdS/CFT discrepancies between gauge and string theory."

Incidentally, the byline says two authors at AEI (Beisert and Staudacher) where Nicolai is director and one (Eden) at 't Hooft's Utrecht institute. AEI and Utrecht are like Perimeter Institute in being strong in non-string QG as well as string research. They are places where string and non-string QG researchers work in neighboring offices, chat in the coffeeroom and can easily attend each other's seminars. One group is not frozen out by the other. Grad students have a choice. That is not how it typically is in the US.

 

[arivero]

N=8 SUGRA. But it is a compactification of D=10 or D=11 SUGRA, isn't it? And it is related to a limit of string theory. So yes it could be a good candidate, but not really an alternative to strings. I would say it is a hint about what way string theory should focus on

 

[marcus]

I would add SUGRA to the original poll, morally it is done. We now have 2 votes for N=8 SUGRA, earlier Tom Stoer said that, and now you, Arivero. We can count up now. Seven people have responded so far (six on the original poll plus Arivero with a "write in" vote) as follows:

4 for Loop (marcus, MTd2, tom.stoer, SW VandeCarr)
2 for AsymSafe (marcus, william donnelly)
2 for SUGRA (arivero, tom.stoer)
1 for CDT (marcus)
1 for Regge (marcus)
1 for Xiao-Gang Wen ('Sabah)

No votes for Horava.

====
Arivero, here is the earlier discussion of supergravity that came up in this QG succession thread:

Marcus,

why not considering supergravity?

- there are indications that it could be renormalizable
- there are versions possibly rich enough to contain the standard model
- SUGRA is a theory on its own and does not necessarily need strings

Tom, let's add SUGRA to the list of contenders. I keep seeing Kelly Stelle's name on speaker lists, might he serve as a persuasive advocate? Is there an introduction/overview to recommend?

 

[tom.stoer]

N=8 SUGRA. But it is a compactification of D=10 or D=11 SUGRA, isn't it? And it is related to a limit of string theory ...

I am not so sure about that.

It seems that duality in string theory relies basically on large-N limit considerations. So perhaps it's exactly the other way round: not ordinary gauge fields are (low-energy) limits of strings, instead string theory may be the large-N limit auf certain gauge theories. If this is true, SUGRA may very well be an own candidate w/o the requirements to derive it from strings.

One question: which SUGRAs are the limits of certain string theory and which are not?

 

[Bob_for_short]

Seven people have responded so far (six on the original poll plus Arivero with a "write in" vote) as follows:

4 for Loop (marcus, MTd2, tom.stoer, SW VandeCarr)
2 for AsymSafe (marcus, william donnelly)
2 for SUGRA (arivero, tom.stoer)
1 for CDT (marcus)
1 for Regge (marcus)
1 for Xiao-Gang Wen ('Sabah)
0 for Horava QG.
1 for VK quasi-particle divergence-less construction (Bob_for_short)

I added my not yet developed approach.

Vladimir Kalitvianski.

 

[atyy]

Since we don't know the future, aren't you making an assumption (a big one, at that) when you even say 'string successors'?
I think the poll should have included an option like 'a new string' to successors to current string scuffle, if the motive is purely altruistic. If the motive is less than altruistic, then I don't have anything to add.

But I should also say that it is through such (less than) altruistic threads at this forum that I have any touch with alternative theories during my otherwise string-ridden grad school :)

What do you think "new string" might be? I know Xiao-Gang Wen in his talks contrasts his approach with "old strings", leaving open the interpretation that there isn't a philosophical difference between the condensed matter approaches and "new string". My impression of what's cool is the link between strings and condensed matter through gauge/gravity duality, which maybe even extends to non-relativistic theories. The matrix theories are also interesting, but I don't know if those can be linked to condensed matter, except in the broadest sense of gravity being emergent.

 

[arivero]

I am not so sure about that.

To me, the point is that in D=4 SUGRA you must do more guesswork that in a D=10 or D=11 SUGRA where the gauge fields come via Kaluza Klein. For instance, the coupling constants in the original, pre Randall-Sundrum, theory were to depend basically of the quotient between gravity scale and compactification scale.

Of course the big problem of D=11 is chiral fermions. And the big problem of D=10 is that unbroken standard model does not fit there. Given that in Nature the standard model gauge group is broken, I have never understood why it was a so big objection.

question: which SUGRAs are the limits of certain string theory and which are not?

I think that the whole point of "type I" and "type II" is that it was meant to agree with the same labels for SUGRA.

 

[marcus]

Christine Dantas (one of us registering our opinions/judgments/guesses) has a QG-related blog you might like to check out. http://egregium.wordpress.com/ . So far eight people have responded!: seven on the original poll plus Arivero with a "write in" vote for SUGRA.

5 for Loop (Christine, MTd2, tom.stoer, SW VandeCarr, marcus)
2 for AsymSafe (william donnelly, marcus)
2 for SUGRA (arivero, tom.stoer)
1 for CDT (marcus)
1 for Regge (marcus)
1 for Xiao-Gang Wen ('Sabah)

No votes for Horava.

The idea here was to limit the poll to distinct well-known on-going 4D QG research programs which clearly have some chance of taking up slack resulting from current loss of focus and interest in the stringy QG&unification program.
Each of these contending programs has, or should have, a conspicuous representative who can define and make the case for it. So far I don't have an online presentation of the supergravity program---can anyone suggest one, or give a link?
It should be acknowledged that string math, instead of an attempt to describe nature at a fundamental level, can be constructively viewed as a bag of innovative mathematical techniques that is finding application e.g. in nuclear physics, condensed matter, even a recent highly publicized description of superconductivity! String math techniques are being applied to aid in a variety of calculations. This gives researchers a possible way out of the QG&U program into useful and satisfying career paths, and benefits their departments.
This was mentioned in an earlier post, but is not what we are focusing on here.
I'll fetch the links given earlier of some representative talks.

==adapted from earlier post==
...Some of these approaches may either teach us something valuable or continue into a successful development. And of course some may not. In any case for now I want to choose one single strong advocacy for each approach (wherever a current presentation is available). Here is an up-date of the earlier list:

CDT
Loll at the Planck Scale conference is tops--best available single lecture on the subject.
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-4.flv&tytul=1.4%20Loll
Video: "Causal Dynamical Triangulations and the Quest for Quantum Gravity"

AsymSafe
The last 12 minutes of Weinberg's CERN talk can't be beat.
http://cdsweb.cern.ch/record/1188567/
Video: "The Quantum Theory of Fields: Effective or Fundamental?"
To save time jump to minute 58.

Horava QG
A video lecture by Horava himself. Fixed camera though. We may get something better after the November conference.
http://online.itp.ucsb.edu/online/adscmt_m09/horava/rm/flash.html
Video: "Quantum Gravity with Anisotropic Scaling"

new look Loop
Waiting for the Corfu School talks to be posted online.
For the time being here's Rovelli's talk at Strings 2008.
http://cdsweb.cern.ch/record/1121957?ln=en
http://indico.cern.ch/getFile.py/access?contribId=30&resId=0&materialId=slides&confId=21917

As a placeholder for SUGRA
PPT slides from Lance Dixon's Erice 2009 talks:
31 August: http://www.ccsem.infn.it/issp2009/professors/Dixon-I.ppt
1 September: http://www.ccsem.infn.it/issp2009/professors/Dixon-II.ppt

Hamber Regge QG
Hamber does a great job on PIRSA
http://pirsa.org/09050006/
Video: "Quantum Gravitation and the Renormalization Group"

Condensed matter approach (à la Wen)
In my view, Fotini M. makes the most persuasive presentation. This is not Wen exactly, but same general idea.
http://pirsa.org/09030018/
Video: "Quantum Graphity: a Model of the Emergence of Locality in Quantum Gravity"

However Atyy has suggested a 2008 PIRSA video of Xiao-Gang Wen. So let me put that link up too.
http://pirsa.org/08110003/
Video: "The Emergence of Photons, Electrons, and Gravitons from Quantum Qbit Systems"

=======endquote=======

We still don't have a video talk by a strong 4D SUGRA advocate. I think the person I want may be Lance Dixon, but have only found powerpoint slides from his August 31-September 1 Erice talks.

 

[MTd2]

I think Horava gravity is a new kind of gravity, for now. The concept of causality in light cone breaks down near singularities, space-time becomes euclidean and gravity repulsive. Dark Matter, being a kind of defect in space time, is not Lorentz invariant, but doesn't violate causality. The concept is similar to why inflation did not violate causality, that is, there is no limit to the "speed" of expansion of space. But in Horava Gravity, it happens as a non expansive deformation of space time, like a gravitational soliton. Note that particles that cross this potential pit do respect General Relativity and Special Relativity, so that dark matter is felt by them as an invisible normal matter.

I don't know if I am right, I wish someone could correct me.

 

[atyy]

It should be acknowledged that string math, instead of an attempt to describe nature at a fundamental level, can be constructively viewed as a bag of innovative mathematical techniques that is finding application e.g. in nuclear physics, condensed matter, even a recent highly publicized description of superconductivity! String math techniques are being applied to aid in a variety of calculations. This gives researchers a possible way out of the QG&U program into useful and satisfying career paths, and benefits their departments.
This was mentioned in an earlier post, but is not what we are focusing on here.

Condensed matter approach (à la Wen)
In my view, Fotini M. makes the most persuasive presentation. This is not Wen exactly, but same general idea.
http://pirsa.org/09030018/
Video: "Quantum Graphity: a Model of the Emergence of Locality in Quantum Gravity"

However Atyy has suggested a 2008 PIRSA video of Xiao-Gang Wen. So let me put that link up too.
http://pirsa.org/08110003/
Video: "The Emergence of Photons, Electrons, and Gravitons from Quantum Qbit Systems"

Where does string math like the AdS/CFT correspondence come from (perhaps not historically, but in hindsight)? Horowitz and Polchinksi http://arxiv.org/abs/gr-qc/0602037: "The AdS/CFT duality is a close analog to the phenomenon of emergent gauge symmetry (e.g. D’Adda et al., 1978, and Baskaran & Anderson, 1988)."

Konopka, Markopoulou and Severini's quantum graphity http://arxiv.org/abs/0801.0861 draws inspiration from Wen, and where does Wen's emergent photons and electrons come from? http://arxiv.org/abs/hep-th/0302201: "The local boson models studied here are just a few examples among a long list of local boson models[8, 28, 29, 31–33, 35, 37–47] that contain emerging fermions and gauge fields." [38] is D'Adda et al 1978, [28] is Baskaran and Anderson 1988.

Drawing too strong a distinction between fundamental/not fundamental seems very contrary to the whole emergent viewpoint, especially where the math has shown cool links to exist.

 

[MTd2]

BTW, these 2 papers uploaded in the last few days are of fundamental importance to Horava Gravity:

http://arxiv.org/abs/0909.5405
Particle Kinematics in Horava-Lifgarbagez Gravity

Dario Capasso, Alexios P. Polychronakos
(Submitted on 29 Sep 2009)
We study the deformed kinematics of point particles in the Horava theory of gravity. This is achieved by considering particles as the optical limit of fields with a generalized Klein-Gordon action. We derive the deformed geodesic equation and study in detail the cases of flat and spherically symmetric (Schwarzschild-like) spacetimes. As the theory is not invariant under local Lorenz transformations, deviations from standard kinematics become evident even for flat manifolds, supporting superluminal as well as massive luminal particles. These deviations from standard behavior could be used for experimental tests of this modified theory of gravity.http://arxiv.org/abs/0909.4833
Notes on Matter in Horava-Lifgarbagez Gravity

Takao Suyama
(Submitted on 26 Sep 2009)
We investigate the dynamics of a scalar field governed by the Lifgarbagez-type action which should appear naturally in Horava-Lifgarbagez gravity. The wave of the scalar field may propagate with any speed without an upper bound. To preserve the causality, the action cannot have a generic form. Due to the superluminal propagation, a formation of a singularity may cause the breakdown of the predictability of the theory. To check whether such a catastrophe could occur in Horava-Lifgarbagez gravity, we investigate the dynamics of a dust. It turns out that the dust does not collapse completely to form a singularity in a generic situation, but expands again after it attains a maximum energy density.

 

[marcus]

...we could evaluate our halfdozen rival approaches in terms of four ad hoc criteria:
1. Does the approach come to terms with renormalization and the running of couplings. (Which, as Weinberg observed, might give a natural explanation of inflation.)
2. Does it have spontaneous dimensional reduction, as discussed by Carlip. 4d down to 2d at small scale.
3. Does it have a concrete mathematical realization of spacetime--that gives meaning to causality/locality and you can define fields on.
4. Could it be adapted so as to fill the bill for Nicolai (and it sounds like 't Hooft would like this too.) Could it acquire conformal symmetry in the limit.

Atyy spotted a 1979 paper of Smolin which suggests that a version asymptotic safe QG could exhibit the asymptotic conformal symmetry required by Nicolai---criterion #4.

Such an approach, only sketched in Smolin's paper of 30 years back, if it could be implemented and found consistent, would meet all four criteria listed here. In that case, here is how the four questions would be answered.

1. Does the approach come to terms with renormalization and the running of couplings?
Yes in fact it is based on that, as an asymsafe QG approach.

2. Does it have spontaneous dimensional reduction, as discussed by Carlip?
One would suppose yes, since this feature has been shown for asymsafe QG. But it would have to be checked for this particular version.

3. Does it have a concrete mathematical realization of 4D spacetime?
Yes it establishes spacetime as an essential frame. Asymsafe QG is a basically a form of General Relativity quantized with running couplings. So it lives on a 4D differential manifold where, however, weird stuff is allowed to happen if you zoom into very very small scale.
Everything we know and love can still live on the differential manifold as usual. So it is nice and straightforward about that.

4. Could it be adapted so as to fill the bill for Nicolai? Could it acquire conformal symmetry in the limit?
YES! Happily enough the approach has the asymptotic conformal feature that it seems several people are currently interested in. Meissner and Nicolai could run their extreme minimalist version of the Standard Model all the way out to Planck scale on this 4D quantum continuum.

However the M&N standard model extension makes falsifiable predictions. It predicts something which Nicolai says is within reach of the LHC at design energy to rule out.

So one would have a falsifiable package of an SM version build on an AS spacetime, which predicts things about particle mass signatures that can be presently falsified if they aren't true.

That's a hypothetical case, assuming that the AsymSafe QG version that Atyy fished up from 1979 could actually be consistently worked out and all the parts fit. I'm happy. I was wondering about that.

So now we could evaluate all halfdozen 4D QG approaches on our poll, in terms of the same four criteria---the same four questions.

In case anyone wants to look some of this up, here is a thread Atyy started about that 1979 paper:
https://www.physicsforums.com/showthread.php?t=341577
Here is an online PDF of the paper itself:
http://ccdb4fs.kek.jp/cgi-bin/img/allpdf?197909044
Here is that Nicolai talk where he explains why he wants a QG with asymptotic conformal symmetry.
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-3.flv&tytul=1.3%20Nicolai
He wants it so as to complete the construction of a minimalist Standard Model able to go to Planck scale without breaking down, which incidentally could be wrong. Nature might tell us she didn't like it but nice try. Such testability is all to the good. Seems like a worthwhile undertaking.

 

[atyy]

4. Could it be adapted so as to fill the bill for Nicolai? Could it acquire conformal symmetry in the limit?
YES! Happily enough the approach has the asymptotic conformal feature that it seems several people are currently interested in. Meissner and Nicolai could run their extreme minimalist version of the Standard Model all the way out to Planck scale on this 4D quantum continuum.

Hmmm, seems Nicolai doesn't believe in Asymptotic Safety since here he's trying to get conformal symmetry out of non-conformal gravity http://arxiv.org/abs/0907.3298 . I think you need to send Meissner, Nicolai and Weinberg a heads up.

 

[marcus]

Hmmm, seems Nicolai doesn't believe in Asymptotic Safety since here he's trying to get conformal symmetry out of non-conformal gravity http://arxiv.org/abs/0907.3298 . I think you need to send Meissner, Nicolai and Weinberg a heads up.

You and I have just been reading the same paper. I've been reading mainly that one for the past hour or so. I didn't understand something that was said in the other thread. Beta functions going to zero is of course a condition of it being a fixed point but I don't see how this implies that the Lagrangian becomes conformally symmetric at the UV fixed point. There is a coupling to matter and that would, I believe, introduce a scale. But in any case conformal at the UV end is not what we are looking for, is it? What Nicolai is talking about is a "flat space limit", not a UV limit.
That is, a limit as kappa, the coupling constant, goes to zero. I'll get the quote from the paper we were reading. Page 15 right at the end:
"The main conjecture put forward in this paper can therefore be summarized as follows: the hierarchy problem can conceivably be solved via ‘anomalous’ logarithmic quantum corrections in a UV finite theory of quantum gravity, if the latter admits a flat space limit which is classically conformally invariant. The mass spectrum and pattern of couplings observed in elementary particle physics could then have their origin in quantum gravity."

He's talking about a UV finite theory of QG. And he wants that theory to have a (low energy, not UV, I think) flat space limit as kappa -> 0 which is "classically conformally invariant".
It's possible I'm just being dense. Still struggling with this.

I know you're kidding about sending Meissner a heads-up, but he is a younger guy, less eminent than his coauthor, and I speculate might be kind enough to answer an email question. I'm not ready to ask for help yet. It is certain that Nicolai, probably Meissner too, know the whole story about the Asymptotic Safety program, which has been led by Reuter at Mainz for the past 10 years. There must be some obstacle to just taking over the AS version of QG. It seems to be UV finite, let's suppose it is, but most likely lacks the desired conformal symmetry in the flat space limit. That's where some modification would be necessary.

Here's a quote from page 2:
" Einstein’s theory (with SM-like matter couplings) is certainly not conformally invariant due to the presence of the dimensionful coupling κ = M_P^-1 , and it is therefore far from evident how a classically conformal Lagrangian might arise out of such a theory at low energies. "

Seems clear he's looking for conformality in a low energy limit, not a UV limit as was being discussed in that other thread. I didn't realize this earlier.

 

[Haelfix]

A UV finite theory (particle physics terminology) essentially means that it is conformal quantum mechanically. It loosely means there is no cutoff dependence and no renormalization is in fact possible. The best known example is N=4 Super Yang Mills and string theory in D = 2.

The observation is that the standard model is classically conformally invariant up to terms arising from the electroweak symmetry breaking (mass terms and the like). So the idea is you want to have a high energy UV finite (eg conformal) theory and then spontaneously break it, and it would then be natural to use a CW mechanism to explain the mass terms in electroweak symmetry breaking (eg quantum mechanics breaks the classical invariance eg its anomalous). So you want the mass terms to be invisible in the classical theory, and instead arise as quantum fluctuations or condensates, or something like that... Lots of model building possibilities.

The problem with that scenario (as explained in the paper above) is that quantum gravity has to appear at some stage, and vanilla GR gravity is most assuredly not conformal. So typically and historically all attempts have been based on replacing the EH action with Weyl gravity or its generalizations and proceed from there. But unfortunately that has a host of problems (ghosts and things like that) and it typically does not lead to the low energy limit that you are interested in.

So, Nicolai asks a different question. Instead of using a conformal theory like Weyl or (Weyl)^2 gravity, under what circumstances can you get IR (eg standard model) classical conformal invariance from a nonconformal high energy gravity theory. So he takes a Supergravity theory (which is not conformally invariant), and tries to get N = 4 superyang mills as a limit. This is of course stronger than he wants (b/c N =4 SYM is both classically AND quantum mechanically conformally invariant).

Hope this clears up the confusion.

 

[marcus]

Excellent, this clears up some of the confusion.

 

[atyy]

It is certain that Nicolai, probably Meissner too, know the whole story about the Asymptotic Safety program, which has been led by Reuter at Mainz for the past 10 years. There must be some obstacle to just taking over the AS version of QG. It seems to be UV finite, let's suppose it is, but most likely lacks the desired conformal symmetry in the flat space limit. That's where some modification would be necessary.

Well, maybe they just don't think AS is likely. If it is, gravity would be conformal at high energies. I was actually thinking of it not so much from Meissner and Nicolai's point of view, but more from if AS is true, does that mean it's predictive at the Planck scale? I would say no, because it must still couple to matter, and the standard model will break down before the Planck scale due to the Landau pole that Nicolai mentions in his talk. So if AS works, then they will need to correct the standard model to work above the Landau pole. I think most such constructions involve supersymmetry, which would lead to SUGRA, which suggests string theory. So maybe Meissner and Nicolai's adjustment of the standard model, although not UV complete, since it has only classical conformal invariance, not quantum conformal invariance, would work at a high enough energy to make AS predictive.

 

[marcus]

BTW, in his conference talk Nicolai said that he and Meissner are working on their own QG to see if they can get a quantum gravity that has the desired low energy behavior--the right flat classical limit.
I'm looking forward to seeing what they come up with! I imagine you may be curious too.

There were some things in what you said that I didn't fully understand.

Well, maybe they just don't think AS is likely.

Likely or unlikely in what sense? I believe the issue for AS is whether or not gravity has a UV fixed point in nature (with finite dimensional critical surface). Evidence is building up that it does. Why would M&N suppose that that this is unlikely? It doesn't have a direct bearing on their proposal. Agnosticism I could see. In his conference talk Nicolai said they were taking an "agnostic" attitude toward the various QG developments.
Maybe that is what you meant.

If it is, gravity would be conformal at high energies...

I don't understand this comment. Whether or not it is true, it seems to me that M&N are not interested in conformality at high energies. It doesn't seem relevant to the low energy behavior they are looking for.

 

[atyy]

I don't understand this comment. Whether or not it is true, it seems to me that M&N are not interested in conformality at high energies. It doesn't seem relevant to the low energy behavior they are looking for.

Well, my impression was that M&N talk about three scales: low - medium - high. In the first set of papers, they talk about the medium and low scales, where the problem is that the standard model fails at medium scales, and propose a solution which has classical conformal invariance and works at the medium scale. This problem with this solution is that the high scale is believed not to have classical conformal invariance due to gravity, so why would the medium scale have it? The second set of papers tries to solve the second problem. I was thinking maybe the second problem doesn't exist if gravity is conformally invariant at high energies. So M&N do care about conformal invariance at high energy - or at least the lack of it, since that is teh setup for why the second problem exists. However, I think I misunderstood the relationship between AS and conformal gravity in my earlier comment, since I think gravity only approaches conformal invariance in AS, and at a fourth scale: infinite energy.

I edited this a bit to explain in what sense M&N care about conformal invariance at high energy.

 

[marcus]

Thanks for clarifying. I think I understand better what you were driving at. Another BTW comment.
In his conference talk Nicolai made some of the most interesting observations about quantum gravity that I have ever heard from a particle theorist.

He said that a theory of QG is necessary for a reason that is seldom mentioned. It is not simply that there is this historical incompatibility between GR and QM.
It is because "the Standard Model is incomplete: neither it nor any of its extensions within relativistic QFT are expected to exist rigorously."

Therefore there is a "resulting need to embed the SM into a theory which is probably not a [Minkowski] space-time based QFT," which is "one of the strongest arguments for quantizing gravity."

In other words, if I understand correctly, no extension of SM within field theory can exist rigorously. To get a mathematically solid theory one needs to base it on a quantum spacetime, or at least on something other than Minkowski. This for him is one of the main reasons to quantize GR.

This is what Nicolai is saying in his slide #1 of the talk, what he calls his "Executive Summary" In slide #2 of the executive summary, he says something else that I thought was quite interesting.

"Conversely [the] search for quantum gravity better not ignore hints from SM about physics at large scales ([such as] renormalizability, anomaly cancellation...)"

So he is giving some advice to the QG researchers like Rovelli! It's all one big theory, so workers on one part can get ideas and take hints from what works in another part. Maybe particleers could be criticized for not taking a hint from the diffeomorphism invariance (general covariance) of GR, but also the relativistas could be faulted for not coming to terms with running coupling constants, cutoff scale dependence, and the like. Or at least not paying so much attention to the hints. I don't know if this is a correct paraphrase or if it is right. But it caught my attention.

Also at the end of his executive summary on slide #2 he says:
"What does the presumed UV finiteness of quantum gravity imply for low energy (that is, Standard Model) physics?"
That's a strange question. Maybe someone who understands what it could mean will explain some.

 

[atyy]

Another BTW comment.
In his conference talk Nicolai made some of the most interesting observations about quantum gravity that I have ever heard from a particle theorist.

He said that a theory of QG is necessary for a reason that is seldom mentioned. It is not simply that there is this historical incompatibility between GR and QM.
It is because "the Standard Model is incomplete: neither it nor any of its extensions within relativistic QFT are expected to exist rigorously."

Therefore there is a "resulting need to embed the SM into a theory which is probably not a [Minkowski] space-time based QFT," which is "one of the strongest arguments for quantizing gravity."

In other words, if I understand correctly, no extension of SM within field theory can exist rigorously. To get a mathematically solid theory one needs to base it on a quantum spacetime, or at least on something other than Minkowski. This for him is one of the main reasons to quantize GR.

This is what Nicolai is saying in his slide #1 of the talk, what he calls his "Executive Summary" In slide #2 of the executive summary, he says something else that I thought was quite interesting.

"Conversely [the] search for quantum gravity better not ignore hints from SM about physics at large scales ([such as] renormalizability, anomaly cancellation...)"

So he is giving some advice to the QG researchers like Rovelli! It's all one big theory, so workers on one part can get ideas and take hints from what works in another part. Maybe particleers could be criticized for not taking a hint from the diffeomorphism invariance (general covariance) of GR, but also the relativistas could be faulted for not coming to terms with running coupling constants, cutoff scale dependence, and the like. Or at least not paying so much attention to the hints. I don't know if this is a correct paraphrase or if it is right. But it caught my attention.

I believe Rovelli has a footnote in his book about the idea that quantum gravity must be solved together with the problem of the Landau poles in the SM - but he brings up a historical analogy where the analogous proposal was a "nice idea, but wrong". However, I believe it is a common viewpoint that there is no point solving the Landau poles in the SM without considering gravity - 't Hooft says this in http://www.phys.uu.nl/~thooft/lectures/basisqft.pdf . I think Weinberg also says it all over his QFT texts - supersymmetry implies supergravity, but no supersymmetric model is known to be UV complete.

Another fascinating argument in this direction comes from Arkani-Hamed, Motl, Nicolis and Vafa's http://arxiv.org/abs/hep-th/0601001 "If true, our conjecture shows that gravity and the other gauge forces can not be treated independently. In particular, any approach to quantum gravity that begins by treating pure gravity and is able to add arbitrary low-energy field content with any interactions is clearly excluded by our conjecture."

 

[atyy]

Some answers as to how Asymptotic Safety might still make some prediction, even though the correct theory of matter isn't known are given by Niedermeier and Reuter http://relativity.livingreviews.org/Articles/lrr-2006-5/ : "Compared to the effective field theory framework the main advantage lies not primarily in the gained energy range in which reliable computations can be made, but rather that one has a chance to properly identify ‘large’ quantum gravity effects at low energies."

I guess the other possibility is that as a fixed point is approached, there will be approximately "universal" behaviour for some quantities. I think this is why CDT folks look at the scaling behaviour of the computational results and say those suggest a fixed point - presumably of Asymptotically Safe gravity.
Litim http://arxiv.org/abs/hep-th/0503096: "Wilsonian flows play an important role in the study of universal scaling phenomena in gauge theories and gravity"
Codello et al http://arxiv.org/abs/0805.2909: "For example, the critical exponents should be universal quantities and therefore cutoff–independent."

 

[atyy]

Or maybe Weinberg is postulating that the whole SM, not just gravity, is asymptotically safe?

 

[marcus]

Atyy I think I can give some relevant perspective on this by quoting Nicolai's slide #5:

==quote Planck scale conf. talk==
The demise of relativistic quantum field theory

• With SM-like bosonic and fermionic matter, UV and IR Landau poles are generically unavoidable.

• Thus breakdown of any extension of the standard model (supersymmetric or not) that stays within the framework of relativistic quantum field theory is probably unavoidable [as it appears to be for λφ₄⁴].

• Therefore the main challenge is to delay breakdown until M_Pl where a proper theory of quantum gravity is expected to replace quantum field theory.

• How the MSSM achieves this: scalar self-couplings tied to gauge coupling λ ∝ g2 by supersymmetry, and thus controlled by gauge coupling evolution.
⇒ m_H ≤ √2m_Z
in (non-exotic variants of) MSSM.
==endquote==

What he said is there is no use trying to fix the SM. Eventually it will blow up (with its built-in Landau dynamite) but nature may have arranged so that this does not happen until Planck, when a new completely different theory would be expected to take over in any case.

So we can explore this possibility by ourselves devising an absolute Occam minimal barebones modification of SM which pushes the Landaus out past Planck scale!
And other people have already tried this delaying tactic! But Nicolai says his new way is considerably simpler.

The MSSM was one attempt, but it leads to predicting a lowest lying Higgs mass less than √2m_Z = 130 GeV. If a Higgs is not found under 130 GeV then MSSM is dead, he observed. The MSSM is already rather elaborate, with "tons of Higgs". But he says you pay a heavy price in terms of economy if you go to more exotic versions---i.e. NMSSM. "Even more Higgs!"

Nicolai's proposal (which still has some things to work out) has so to speak "one and a half Higgs"----a basic one with predicted mass of 207 GeV, plus it has a kind of shadow or "fat twin brother" roughly estimated around 477 GeV. So two really, but that's it. He says one of them (I forget which) should be very easy to produce and detect---a clear signature.

The proposal is amazingly clean and economical. There are also some other predictions or clear signatures to look for.

So I doubt that Weinberg is including the SM when he says asymptotic safety.
Weinberg, in his July talk at CERN was talking about the UV fixed point of gravity and a revival of interest in the "good old" standard model. The good old SM is not going to have a UV fixed point because it has Landau poles and will blow up. The only thing to do is see if you can delay the blow up until you are out to Planck scale where new physics in any case. I think that is the basic situation within which Weinberg would be thinking and working, just like Nicolai is. It is a mental environment that is not special to Nicolai or anyone person.

 

[atyy]

Atyy I think I can give some relevant perspective on this by quoting Nicolai's slide #5:

==quote Planck scale conf. talk==
The demise of relativistic quantum field theory

• With SM-like bosonic and fermionic matter, UV and IR Landau poles are generically unavoidable.

• Thus breakdown of any extension of the standard model (supersymmetric or not) that stays within the framework of relativistic quantum field theory is probably unavoidable [as it appears to be for λφ₄⁴].

• Therefore the main challenge is to delay breakdown until M_Pl where a proper theory of quantum gravity is expected to replace quantum field theory.

• How the MSSM achieves this: scalar self-couplings tied to gauge coupling λ ∝ g2 by supersymmetry, and thus controlled by gauge coupling evolution.
⇒ mH ≤ √2m_Z
in (non-exotic variants of) MSSM.
==endquote==

What he said is there is no use trying to fix the SM. Eventually it will blow up (with its built-in Landau dynamite) but nature may have arranged so that this does not happen until Planck, when a new completely different theory would be expected to take over in any case.

So we can explore this possibility by ourselves devising an absolute Occam minimal barebones modification of SM which pushes the Landaus out past Planck scale!

I understand Nicolai's point of view - am trying to figure out what Weinberg means by "It is just possible that the appropriate degrees of freedom at all energies are the metric and matter fields, including those of the Standard Model." http://arxiv.org/abs/0908.1964 since I'm sure Weinberg knows all the Landau poles very well, and is one of the people who's been saying the standard model and gravity are probably just effective theories. So I guess he must be saying maybe one can et round the Landau poles if the SM is asymptotically safe. I would guess that it's asymptotic safety of the SM and SM extensions that is keeping Nicolai from saying that we definitely need to go beyond 4D relativistic field theory - he just says probably.

 

[atyy]

So I doubt that Weinberg is including the SM when he says asymptotic safety.
Weinberg, in his July talk at CERN was talking about the UV fixed point of gravity and a revival of interest in the "good old" standard model. The good old SM is not going to have a UV fixed point because it has Landau poles and will blow up. The only thing to do is see if you can delay the blow up until you are out to Planck scale where new physics in any case. I think that is the basic situation within which Weinberg would be thinking and working, just like Nicolai is. It is a mental environment that is not special to Nicolai or anyone person.

Yes, I think in the past Weinberg only meant AS for gravity. But now that he's seriously considering it, he needs a theory of matter that works when quantum gravity kicks in at the Planck scale. It's conceivable one can have a matter effective field theory with Landau poles above the Planck scale for that purpose, though Arkani-Hamed et al's "Gravity as the weakest force" suggests maybe not. However, I think Weinberg is seriously considering to get around the Landau poles of the SM or SM extensions by having them asymptotically safe. A recent paper http://arxiv.org/abs/0901.2459 (which I found out from http://motls.blogspot.com/2009/01/hep-th-papers-on-monday.html) tries to see if the Landau poles in the Higgs sector of some models can be gotten round by asymptotic safety in those sectors - they proved that the UV fixed point doesn't exist in many of these models - but it nonetheless shows that Landau poles only suggest, not prove that the theory has no continuum limit - one must also prove that asymptotic safety doesn't exist.

 

[marcus]

I encountered (thanks to Nicolai) another player, whose paper may interest you. It is along the same lines as "It is a mental environment that is not special to Nicolai or anyone person." We both seem to be filling in the picture of that mental environment and noting other related work.
http://arxiv.org/abs/0708.3550
Is there a new physics between electroweak and Planck scales?
Mikhail Shaposhnikov
10 pages, talks given at the Workshop on Astroparticle Physics, Budapest 2007 and at the 11th Paris Cosmology Colloquium 2007.
(Submitted on 27 Aug 2007)
"We argue that there may be no intermediate particle physics energy scale between the Planck mass M_Pl ~ 10¹⁹ GeV and the electroweak scale M_W ~ 100 GeV. At the same time, the number of problems of the Standard Model (neutrino masses and oscillations, dark matter, baryon asymmetry of the Universe, strong CP-problem, gauge coupling unification, inflation) could find their solution at M_Pl or M_W . The crucial experimental predictions of this point of view are outlined."

"In this paper we describe a (hopefully) consistent scenario for physics beyond the StandardModel (SM )that does not require introduction of any new energy scale besides already known, namely the electroweak and the Planck scales, but can handle different problems of the SM mentioned in the abstract."

"This point of view, supplemented by a requirement of simplicity, has anumber of experimental predictions which can be tested, at least partially, with the use of existing accelerators and the LHC and with current and future X-ray/γ-ray telescopes."

Background on Shaposhnikov:
http://itp.epfl.ch/page58722.html
He has 232 papers on Spires, eight of which have been cited 250+ times. One cited over 1500 times. Several cited over 500 times.
He seems to have a command both of particle physics (including beyond standard) and cosmology/astrophysics. Interesting guy. Born 1956, so a bit over 50, still extremely productive. Respected but also pushing the edges.
That paper about nu-MSM from 2007 was not an isolated one. He has written several followups, including in collaboration with other authors. He was appointed director of the ITP at the Swiss Federal Ecole Poly (Lausanne) and also directs the Laboratory for Particle Physics and Cosmology. Good combination. Early universe so important to understanding high energy physics.
http://www-spires.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+SHAPOSHNIKOV+%2C+M&FORMAT=www&SEQUENCE=citecount%28d%29

Here's are those publications listed most recent first, to get an idea of his recent work:
http://www-spires.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+SHAPOSHNIKOV+%2C+M&FORMAT=www&SEQUENCE=ds%28d%29

 

[BigFairy]

voted.

 

[marcus]

Thanks adding your vote BiFa, and thanks everybody! So we are 9 in all, so far. 8 people have voted on the original poll plus Arivero registered a point after we decided to include 4D SUGRA. In order of most points the results are:
Loop 5
CDT 2
AsymSafe QG 2
4D Supergravity 2
Regge QG 1
Wen 1
Horava 0

One of the things discussed in this thread is what assets/liabilities different 4D QG approaches have that could attract researchers and give interesting prospects/promise to the approach. Like a natural inflation mechanism (e.g. AsymSafe).
Like spontaneous dimensional reduction at small scale (several, including Loop, CDT, AsymSafe).
An interesting issue is which if any approaches might be adapted to suit the Meissner-Nicolai proposal for continuing a minimalist Standard Model all the way from weak scale up to Planck.

In another thread, up in High Energy forum, Haelfix listed some questions which any proposal like that should address.
I'm not sure what proposal he was talking about but the checklist is a useful one that one could apply to any proposal which posits no new physical scales between weak (100 GeV) and Planck (10¹⁹ GeV). We should copy it down and go through one by one, to see which of these points Nicolai addressed in his talk. IIRC he addressed a good many of them, but we'll see.

Having a vast dessert between the Planck scale and the electro weak scale is minimal, but its pretty inconsistent with expectations/ history of particle physics and almost ruled out, at least without putting more structure in.
The hierarchy and naturalness problems go unresolved, the strong cp problem goes unresolved, dark matter goes unresolved, neutrino physics is left wide open, the assignments of the standard model look pretty arbitrary and ugly and no explanation for the free parameters is given, why do the anomalies between the quark and lepton sectors exactly cancel, why three generations and the weird mass hierarchies, why the seemingly random scale for electroweak symmetry breaking etc etc etc? Then you get into cosmology issues, like the antimatter-matter asymetry, CP violation and baryon number violation that's flagrantly inconsistent with the SM. Why is charge quantized? Whats going on with high energy cosmic rays? What about the ridiculously tiny observed value for the vacuum energy and so on.

hierarchy and naturalness---both addressed in detail--Nicolai shows "naturalness in the sense of 't Hooft" (the way 't Hooft originally defined the concept when introducing it)

neutrino physics not left wide open of course--M&N model employs see-saw and gives a prediction for neutrino mass

it also predicts 207 GeV for lowest mass Higgs, also something around 400 GeV for the only other Higgs (the "fat twin" Higgs)

dark matter---I don't recall Nicolai addressing this, will check

free parameters---half a dozen running couplings are determined by equation but since eqns hard to solve numerical methods were used and the running was plotted. Nicolai explained paraeters at some length to give intuition about why the various couplings ran as they did, and why they did not blow up. Plots running over the whole energy range from weak to Planck.

 

[atyy]

But what is new look Loop?

Is it secretly Asymptotic Safety with a UV fixed point which guarantees the theory is fundamentally continuous, as Bahr and Dittrich or Krasnov in non-metric mood envision?
http://arxiv.org/abs/0905.1670
http://arxiv.org/abs/0907.4064

Or could it be in cahoots with condensed matter by being fundamentally discrete with a continuum emerging due to an IR-like fixed point, as Freidel, Gurau and Oriti hope for?
http://arxiv.org/abs/0905.3772

 

[marcus]

Atyy, you may have a more crystalized vision of the alternatives than I. After reading your post again I think you probably do.
I am a bit bothered by the fact that the Corfu site seems slow to put the lectures on line. On their main page they have an item "online lectures" which if you click says "Lectures will be put on line as soon as they are available". But as far as I can tell none are yet. And that is true not just for the QG school but also for two other schools held there by the same organization earlier.

All I have to go on is Rovelli's abstract summary:
==quote==
Title: Covariant loop quantum gravity and its low-energy limit

Abstract and content (tentative)

I present a new look on Loop Quantum Gravity, aimed at giving a better grasp on its dynamics and its low-energy limit. Following the highly succesfull model of QCD, general relativity is quantized by discretizing it on a finite lattice, quantizing, and then studying the continuous limit of expectation values. The quantization can be completed, and two remarkable theorems follow. The first gives the equivalence with the kinematics of canonical Loop Quantum Gravity. This amounts to an independent re-derivation of all well known Loop Quantum gravity kinematical results. The second the equivalence of the theory with the Feynman expansion of an auxiliary field theory. Observable quantities in the discretized theory can be identified with general relativity n-point functions in appropriate regimes. The continuous limit turns out to be subtly different than that of QCD, for reasons that can be traced to the general covariance of the theory. I discuss this limit, the scaling properties of the theory, and I pose the problem of a renormalization group analysis of its large distance behavior.
==endquote==

What continuous limit? How is it taken? How does one take a continuous limit in the spinfoam context? Or is he not working with spinfoam?

We can see that he is not using canonical LQG because he says that this approach which he is using reproduces results and agrees with canonical LQG. So he is using some other approach. But is it actually just a lattice, where the size of the lattice can be taken to zero?

I don't have much to go on and I don't feel able to take jumps of imagination. So my reaction is to refrain from asking the question you just asked and go think about something else until we get some PDF from this series of lectures.
==============================

The papers you offer, as a way of "triangulating" to guess where Rovelli is, are in themselves quite interesting. The Dittrich, the Krasnov, and also the Freidel Gurau Oriti, which I had not tried to read earlier. I found the diagrams bewildering. FGO talk about generalized Feynman diagrams made of cell complexes, they do some kind of renormalization. It might actually be very good what they do, but it's just a beginning and I can't imagine that it could be already be transformed into school lectures. They were still working on a 3D toy model of it in May 2009.

Krasnov's work is again exploratory. It seems to have a renormalization angle----you termed it something like sub rosa Reuter. Maybe Loop research will eventually move in that direction. Maybe, but Krasnov's innovation is still preliminary: he has first to publish a spin foam vertex derived from his new Lagrangian.

I'm glad they are working on such things but I'm not ready to take these innovative projects seriously as yet. So in the meanwhile I'm studying Meissner and Nicolai's proposal.

You have a knack for finding really interesting papers though.

 

[marcus]

Here's a link to the Haelfix post I was quoting back a couple, right before Atyy's post:
https://www.physicsforums.com/showthread.php?p=2379775#post2379775
It has a useful list of issues which we can check to see if they are addressed
by Nicolai in his Planck Scale talk.
Video
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-3.flv&tytul=1.3%20Nicolai
Slides
http://www.ift.uni.wroc.pl/~planckscale/lectures/1-Monday/3-Nicolai.pdf

 

[atyy]

I encountered (thanks to Nicolai) another player, whose paper may interest you. It is along the same lines as "It is a mental environment that is not special to Nicolai or anyone person." We both seem to be filling in the picture of that mental environment and noting other related work.
http://arxiv.org/abs/0708.3550
Is there a new physics between electroweak and Planck scales?
Mikhail Shaposhnikov
10 pages, talks given at the Workshop on Astroparticle Physics, Budapest 2007 and at the 11th Paris Cosmology Colloquium 2007.
(Submitted on 27 Aug 2007)
"We argue that there may be no intermediate particle physics energy scale between the Planck mass M_Pl ~ 10¹⁹ GeV and the electroweak scale M_W ~ 100 GeV. At the same time, the number of problems of the Standard Model (neutrino masses and oscillations, dark matter, baryon asymmetry of the Universe, strong CP-problem, gauge coupling unification, inflation) could find their solution at M_Pl or M_W . The crucial experimental predictions of this point of view are outlined."

"In this paper we describe a (hopefully) consistent scenario for physics beyond the StandardModel (SM )that does not require introduction of any new energy scale besides already known, namely the electroweak and the Planck scales, but can handle different problems of the SM mentioned in the abstract."

"This point of view, supplemented by a requirement of simplicity, has anumber of experimental predictions which can be tested, at least partially, with the use of existing accelerators and the LHC and with current and future X-ray/γ-ray telescopes."

Background on Shaposhnikov:
http://itp.epfl.ch/page58722.html
He has 232 papers on Spires, eight of which have been cited 250+ times. One cited over 1500 times. Several cited over 500 times.
He seems to have a command both of particle physics (including beyond standard) and cosmology/astrophysics. Interesting guy. Born 1956, so a bit over 50, still extremely productive. Respected but also pushing the edges.
That paper about nu-MSM from 2007 was not an isolated one. He has written several followups, including in collaboration with other authors. He was appointed director of the ITP at the Swiss Federal Ecole Poly (Lausanne) and also directs the Laboratory for Particle Physics and Cosmology. Good combination. Early universe so important to understanding high energy physics.
http://www-spires.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+SHAPOSHNIKOV+%2C+M&FORMAT=www&SEQUENCE=citecount%28d%29

Here's are those publications listed most recent first, to get an idea of his recent work:
http://www-spires.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+SHAPOSHNIKOV+%2C+M&FORMAT=www&SEQUENCE=ds%28d%29

An interesting issue is which if any approaches might be adapted to suit the Meissner-Nicolai proposal for continuing a minimalist Standard Model all the way from weak scale up to Planck.

Ambjorn et al http://arxiv.org/abs/0906.3947

"It should be mentioned that the asymptotic safety picture is not the only suggestion for a continuum quantum theory of gravity using only “conventional” ideas of quantum field theory. ... The other model goes by the name of “scale-invariant gravity” [5, 4]. ...

4. M. Shaposhnikov and D. Zenhausern, Quantum scale invariance, cosmological constant and hierarchy problem, Phys. Lett. B 671 (2009) 162-166 [arXiv:0809.3406 [hep-th]]; Scale invariance, unimodular gravity and dark energy, Phys. Lett. B 671 (2009) 187-192 [arXiv:0809.3395 [hep-th]].

5. M.E. Shaposhnikov and I.I. Tkachev, Quantum scale invariance on the lattice, 5 pages [arXiv:0811.1967 [hep-th]]."

 

[marcus]

Shaposhnikov! You found another reference to his scale-less gravity coming from another direction.
This time the reference is in a CDT paper by Renate Loll's group
"Quantum gravity as sum over spacetimes"
For convenience, I repeat the link you gave http://arxiv.org/abs/0906.3947
To save people trouble, if they want to follow, I will quote a longer excerpt from pages 5 and 6 of the paper
==quote Loll et al==
Thus we have created a picture where the underlying lattice spacing goes to zero while the physical mass (or the correlation length measured in physical length units, not in lattice spacings) is kept fixed. This is the standard Wilsonian scenario for obtaining the continuum (Euclidean) quantum field theory associated with the critical point g₀^c of a second order phase transition ...

We would like to apply a similar approach to quantum gravity, and thus obtain a new way to investigate if quantum gravity can be defined non-perturbatively as a quantum field theory. The predictions from such a theory could then be compared with the renormalization group predictions related to the asymptotic safety picture described above. It should be mentioned that the asymptotic safety picture is not the only suggestion for a continuum quantum theory of gravity using only “conventional” ideas of quantum field theory. Very recently two other scenarios have been suggested. One is called Lifgarbagez gravity [3] and is a theory where the non-renormalizability of the Einstein-Hilbert theory is cured by adding higher-order spatial derivatives in a way somewhat similar to what Lifgarbagez did many years ago in statistical models.

In fact, the setup of the theory has some resemblance with the lattice-theory setup of “Causal Dynamical Triangulations (CDT)” , to be described below, since a time foliation is assumed and the infrared limit is that of GR. However, contrary to Lifgarbagez gravity, we do not attempt to put in higher spatial derivatives in the lattice theory. However, when a continuum limit in the lattice theory is taken in a specific way which is not entirely symmetric in space and time one cannot rule out that higher spatial derivatives can play a role.

The other model goes by the name of “scale-invariant gravity” [5, 4]. It modifies gravity into a renormalizable theory by introducing a scalar degree of freedom in addition to the transverse gravitational degrees of freedom. Also this model has interesting features not incompatible with the results of computer simulations using the CDT lattice model.
==endquote==

 

[marcus]

One purpose of this thread is to choose a strong advocate for each approach (wherever a current presentation is available) and put the spotlight on a short up-to-date list of presentations that can serve to introduce the various competing approaches. Here is an up-date of an earlier list:

CDT
Loll at the Planck Scale conference is tops--best available single lecture on the subject.
http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php?plik=http://panoramix.ift.uni.wroc.pl/~planckscale/video/Day1/1-4.flv&tytul=1.4%20Loll
Video: "Causal Dynamical Triangulations and the Quest for Quantum Gravity"

AsymSafe
The last 12 minutes of Weinberg's CERN talk is the best video.
http://cdsweb.cern.ch/record/1188567/
Video: "The Quantum Theory of Fields: Effective or Fundamental?"
To save time jump to minute 58.
Percacci has an excellent paper, that just appeared. I wanted online video, but this is so good I'll list it with the others:
http://arxiv.org/abs/0910.5167
"Gravity from a Particle Physicist's perspective"

Horava QG
A video lecture by Horava himself. Fixed camera though. We may get something better after the November conference.
http://online.itp.ucsb.edu/online/adscmt_m09/horava/rm/flash.html
Video: "Quantum Gravity with Anisotropic Scaling"

new look Loop
For the time being there is no 2009 online video that can serve. Here is the intro to Loop-Foam QG that Rovelli gave at Strings 2008:
http://cdsweb.cern.ch/record/1121957?ln=en
http://indico.cern.ch/getFile.py/access?contribId=30&resId=0&materialId=slides&confId=21917
For a recent overview we have an online audio+slides talk by Lewandowski that is very good:
http://relativity.phys.lsu.edu/ilqgs/lewandowski102009.pdf
http://relativity.phys.lsu.edu/ilqgs/lewandowski102009.wav
more information if needed at http://relativity.phys.lsu.edu/ilqgs/
and the corresponding recent arxiv paper http://arxiv.org/abs/0909.0939
"Spin-Foams for All Loop Quantum Gravity

4D SUGRA No online video introduction available, but here are Powerpoint slides from Lance Dixon's two talks at Erice 2009:
31 August: http://www.ccsem.infn.it/issp2009/professors/Dixon-I.ppt
1 September: http://www.ccsem.infn.it/issp2009/professors/Dixon-II.ppt

Hamber Regge QG
Hamber does a great job on PIRSA
http://pirsa.org/09050006/
Video: "Quantum Gravitation and the Renormalization Group"

Condensed matter approach
Atyy has suggested a 2008 PIRSA video of Xiao-Gang Wen.
http://pirsa.org/08110003/
Video: "The Emergence of Photons, Electrons, and Gravitons from Quantum Qbit Systems"
I think Fotini Markopoulou makes a strong presentation along what seem to be similar lines.
http://pirsa.org/09030018/
Video: "Quantum Graphity: a Model of the Emergence of Locality in Quantum GravitySome of these approaches may teach us something valuable, probably will in fact, and of course some may not. The idea here was to limit the poll to active well-known 4D QG research programs which can take up some of the slack resulting from current loss of focus and interest in the string unification program.
===================

So far nine people have responded!: eight on the original poll plus Arivero with a "write in" vote for SUGRA.

5 for Loop (Christine, MTd2, tom.stoer, SW VandeCarr, marcus)
2 for AsymSafe (william donnelly, marcus)
2 for SUGRA (arivero, tom.stoer)
2 for CDT (marcus, BigF)
1 for Regge (marcus)
1 for Xiao-Gang Wen ('Sabah)

Thanks to all who have responded so far!

 

[marcus]

Has the picture changed?

Have any new approaches that you think are important come to light in the past 3 months?

We now have papers and conference talks by Percacci, Weinberg, Thiemann, Krasnov, Rivasseau that hadn't appeared when this thread was started. Does the paper by Stephon Alexander that appeared on arxiv yesterday change the picture?

Does anyone think the "Strand" model should be added to our list of contenders?

===================
So far 10 of us have responded: 9 have registered choices on the poll as posted, plus arivero who contributed a "write in" vote for Sugra.

Asymptotic Safe QG has gained favor, as compared with a month or two ago. The count is now:

Loop 5
AsymSafe 3
Sugra 2
CDT 2
Regge 1
Wen 1