# fh/Distler conversation is interesting

by marcus
Tags: conversation, fh or distler, interesting
 Astronomy Sci Advisor PF Gold P: 22,677 There was an interesting conversation at Distler's blog between the owner and fh. I thought really thought-provoking points were raised. I would like to copy some sample exerpts here. the main address for Prof Distler's blog is: http://golem.ph.utexas.edu/~distler/blog/ This particular thread, which he titled "THE LQG LANDSCAPE", is here http://golem.ph.utexas.edu/~distler/....html#comments If you go to Distler blog and want to print it out, on my printer it is from PAGE 27 ONWARDS. so you don't have to print the first 27 pages if you just want the fh/Distler discussion If you are scrolling down, It starts just before a long series of posts labeled WELCOME because when fh came in (at 2 July 1:13 PM mytime) he said "A few comments from a newcomer. Prof Distler you,..." and Distler was perhaps pleased by the courtesy of being called Prof or by the thoughtfulness of the post, and he replied immediately and, with cordial affability, he labeled his reply WELCOME. In this way the whole subsequent branch or subthread of discussion got labeled RE:WELCOME. So that makes it easier to find if you are scrolling down looking for it. there are now over a hundred comments in all, so it helps to know what one is looking for.
 Astronomy Sci Advisor PF Gold P: 22,677 I will try to show by sample exerpts why this is an interesting discussion. I think it is because it has an historical scope that highlights different QG approaches (within the general LQG family) including some work by Thiemann in the 1990s and some work of Freidel which is not yet published. And also some research and ideas of John Baez are mentioned which he has mentioned here at PF. So these different ideas are brought out (which people can easily confuse if they have an overly simple idea of the directions in QG research) and they are spotlighted in a moderately courteous ADVERSARIAL Question/Answer probing discussion. This actually turns out to be a GOOD VENUE because ideas are being tested out under a little bit of fire----which doesn't always hurt. Having tension brings out different aspects. So I have to say that it seemed very worth while to me. I am not sure how to exerpt the thread so as to get samples of some of the good comments. I guess I will pick out some of those that raise issues that it might be interesting to discuss here, if people want to. The full thread of comments at Prof Distler's blog is here: http://golem.ph.utexas.edu/~distler/....html#comments
Astronomy
PF Gold
P: 22,677

## fh/Distler conversation is interesting

From the comments at Prof Distler's blog
Picking up from that last post by Urs

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RE: WELCOME

If quantization of 2d gravity gives different results than everything else (including the lattice = dynamical triangulation) and if even the harmonic oscillator comes out wrong, how can you just go on?

Both examples you mention live in CQL. According to Lee Smolin, most practitioners of LQG have abandoned that and moved from CQL to spin foams.

It seems to be me that hence the question is if there is a spin foam model for d-dimensional gravity, in particular for d>3 , that we could throw all these questions at.

Posted by: urs on July 4, 2006 10:54 AM
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RE: A NEW KIND OF SCIENCE?

“Do you really want to assert that LQG is a “new kind of Science”, which cannot be discussed using conventional scientific criteria?”

I want to assert that science (or more precisely fundamental physics) is not equal to standard QFT.

“You can’t “generalize” something that you can’t reproduce.”

See the work by Freidel (this is getting repetitive and tiresome) Spin Foams can reproduce ordinary flat spacetime QFT.

Also way to go to clip the context, the sentences preceding what you make bold are:

“It is an old speculation in physics that, once the gravitational field is successfully quantized, it should serve as the natural regulator of infrared and ultraviolet singularities that plague quantum field theories in a background metric. We demonstrate that this idea is implemented in a precise sense within the framework of four-dimensional canonical Lorentzian quantum gravity in the continuum.”

So we have a theory that has the Gauge group and algebra of the Standard Modell but we do not know and nowhere claim to know if and how this theory reproduces the flat spacetime limit. Though we suspect that it does we can’t prove that. The situation is the same as Lattice QFT.

Anyways, what are you suggesting? That THiemanns results are wrong? That he’s lying about what he is doing?

Posted by: fh on July 5, 2006 03:49 AM
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RE: A NEW KIND OF SCIENCE?

So we have a theory that has the Gauge group and algebra of the Standard Modell but we do not know and nowhere claim to know if and how this theory reproduces the flat spacetime limit. Though we suspect that it does we can’t prove that.

As I pointed out above, nothing of Thiemann’s analysis would have changed, had he attempted to couple an anomalous chiral gauge theory in place of the (nonanomalous) Standard Model.

That he would claim to have a consistent nonperturbative quantization of an anomalous chiral gauge theory coupled to quantum gravity ought to tell you something.

The situation is the same as Lattice QFT.

Not even close. First of all, the Lattice Gauge theorists know that they cannot currently study chiral gauge theories. Second, they have abundant numerical evidence that they can achieve the continuum limit.

Anyways, what are you suggesting? That THiemanns results are wrong?

Yes.

That he’s lying about what he is doing?

“Wild exaggeration” would be more accurate.

Posted by: Jacques Distler on July 5, 2006 10:47 AM
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RE: A NEW KIND OF SCIENCE?

You can tell that they are wrong from reading the abstract?

While demonstrating amply that you have not attempted to understand what he is claiming or trying to do?

While it is clear that there is a different use of language as well, resulting from the different perspectives, that you fail to acknowledge?

I’m sorry but this is a pure (and not the first in this thread) ad hominem. You have not pointed out ANYTHING about Thiemanns paper that is wrong.

You’re free to claim that the quantization of SM+GR THiemann is producing is unphysical for various reasons (including the fact that it apparently doesn’t know about anomalies), you can not claim thaat he doesn’t have one.

Posted by: fh on July 5, 2006 11:46 AM
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RE: A NEW KIND OF SCIENCE?

The fact that he claims to be able to quantize an anomalous gauge theory, merely by coupling it to quantum gravity, is, indeed sufficient grounds to be confident that he is wrong.

Yes, I could carefully work through his paper to find the precise point where he goes astray. But what would that gain?

True-believers, like yourself, would dismiss it as a mean-spirited attack. And people like Lee would say, "It doesn’t matter, because we’ve all moved on to study spin foams, anyway.”

The US Patent Office does not accept applications for Perpetuum Mobile, without a working model. Similarly, I am not interested in entertaining any purported quantizations of chiral gauge theories, without a detailed explanation of how anomalies are realized.

Posted by: Jacques Distler on July 5, 2006 12:08 PM

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RE: THE LQG LANDSCAPE

Dear MoveOn,

There are lots of finite theories that have running effective coupling constants: take any lattice QFT and choose the lattice spacing short enough to accomodate the present limits on uv breaking of Lorentz invariance.

Dear Jacques and others,

Yes, the results together amount to a demonstration of the existence of a new kind of QFT which has no background metric but is diffeomorphism invariant. Moreover, there are “semiclassical states” when gravity is coupled to matter fields, such that expansion around them reproduces at long wavelength a cutoff version of the matter QFT.

I understand your skepticism as I was also trained as a conventional QFT, but at some point you have to decide to take the chance that we are neither dishonest nor stupid and investigate whether these claims have merit. To do this there is no alternative but to study the books and papers. We do take great pains to be honest about open questions, but we also have to insist when questions are not open. These claims are not new (Thiemann’s paper in question is nine years old) and they have been thoroughly checked and examined by a community of smart, critically minded people. If your interest is, as I hope it is, in good faith-that is if this is science and not a debating club-you might consider taking the time to study the papers and understand exactly what the claims are and how they are demonstrated.

I would urge that this is only fair. I only expressed criticisms of string theory after having learned the basics, taught a graduate course in the subject from the standard textbooks, and published a dozen technical papers in the subject.

I promise you that if you do the same you will understand that everything we have claimed is true, as we have stated them. But I do not see how more repetition of those claims can help if you are implacably hostile to the possibility that they are right.

Thanks,

Lee

Posted by: Lee Smolin on July 5, 2006 10:43 AM

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 P: 859 Thanks, Marcus. I can't read Distler's blog on my computer here! The conversation seems to be a mash of several conversations at once, blog style, but you have clearly identified the interesting part. Distler et al don't seem to realise that, although Baratin-Freidel is indeed only a $G$ --> 0 theory, people have been thinking, at least conceptually, about the (category theory) stuff that goes beyond that for quite a while now. For example, how many times do I have to tell people that if you want to understand the mass gap (and confinement) you need to study Gordon-Power-Street and find the parity cube and understand how the 5 faces representing the pentagon of monoidal structures are split by the sixth?
 P: 859 The next task, I think, is to collect papers on deformed Poincare type structures and understand them in tricategorical terms.
 Sci Advisor P: 1,652 The whole discussion is super bizarre, I completely emphasize with Distler and company, b/c atm im now confused what the LQG people are trying to say. On one hand they seem to be claiming they can embed the algebra of the SM into their framework consistently and singularity free. Of course we will object to this by saying, pick any choice of low energy physics entailing a choice metric (say flat space), you better *hope* your theory captures anomalies in this limit. It is super bizarre and completely counter to everything we've ever learned to expect that the anomolous currents are not in the higher energy theory but rather restored in the low energy modes. But now I don't know anymore, maybe thats not what they're saying.
Astronomy
PF Gold
P: 22,677
 Quote by Kea The next task, I think, is to collect papers on deformed Poincare type structures and understand them in tricategorical terms.
you must be getting better

you sound stronger than before this hospital-bout of whatever

I take it as a good sign that you didnt even bother with the habitual smilie signature. you just said the task you were thinking. you could be right. (Urs found some 3-group did he not?)
P: 859
 Quote by marcus Urs found some 3-group did he not?
Marcus

The String 3-groups are no doubt important, but stop thinking gauge theories! The best thing about Baratin-Freidel is that they find the particular gauge fixing that they need is forced on them by the topological considerations, which are clearly more fundamental. This means approaching the full theory from a topological point of view. People like John Barrett have been saying this for years. Take a look at the Barrett papers referenced in Baratin-Freidel.

Habitual
 Astronomy Sci Advisor PF Gold P: 22,677 aaawwwww I thought it was a new face, the serious *absence* of the expected smilie news: http://arxiv.org/abs/gr-qc/0607014 Particles as Wilson lines of gravitational field L. Freidel, J. Kowalski--Glikman, A. Starodubtsev 19 pages "Since the work of Mac-Dowell-Mansouri it is well known that gravity can be written as a gauge theory for the de Sitter group. In this paper we consider the coupling of this theory to the simplest gauge invariant observables that is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce exactly the dynamics of relativistic particles coupled to gravity, the gauge charges carried by Wilson lines being the mass and spin of the particles. Insertion of Wilson lines breaks in a controlled manner the diffeomorphism symmetry of the theory and the gauge degree of freedom are transmuted to particles degree of freedom." It looks to be a way that matter can arise from the geometry of spacetime. Several people were talking about this.
 PF Gold P: 2,883 A big part of the Distler thread rounds about the "cube of theories", ie the different things we get depending of when h, l_planck, and 1/c are zero or non zero. We expect that QFT regulated with gravity is not QFT anymore, and Poincare invariant QFT is recovered in the limit when the regulator, l_plank, goes to zero. Which, if h is non zero, implies G-->0. And so on. On other hand I do not like the freedom to move l_planck because traditionally it is related to GUT scale, and then to the other coupling constants. One can define gravity as the energy scale where the unbroken coupling via Z0 becomes smaller than the coupling via photons.
 P: 271 "Of course we will object to this by saying, pick any choice of low energy physics entailing a choice metric (say flat space), you better *hope* your theory captures anomalies in this limit. It is super bizarre and completely counter to everything we've ever learned to expect that the anomolous currents are not in the higher energy theory but rather restored in the low energy modes." As I said at some point (and I did get carried away), one is free to claim that the quantization employed is unphysical, it certainly looks extremely strange from a particle physicists PoV, but one can not claim that it's wrong, doesn't exist, is stupid, the people working in it are exagerating, or that they aren't aware of this strangeness. There appears to be a complete unwillingness to acceptt these strange resulty and take them as an indication that something non trivial is going on. Instead, without even examining them, they are rejected. This is frustrating, since I am just a student of LQG at the moment I see month for month more how these things work and even though I don't have a good understanding on a detailed level it becomes increasingly clear how much the critzism misses the point.
 PF Gold P: 2,883 An additional note: the "damping" of QFT infinities due to gravitational background is, as f-h tells in the discussion, an old theme. A article I had noted to read sometime, and to follow citations, is Infinity suppression in gravity modified quantum electrodynamics by Isham, Salam and Strathdee
P: 1,667
 Quote by arivero An additional note: the "damping" of QFT infinities due to gravitational background is, as f-h tells in the discussion, an old theme. A article I had noted to read sometime, and to follow citations, is Infinity suppression in gravity modified quantum electrodynamics by Isham, Salam and Strathdee
Hi,

I guess they use the perturbative approach around the Minkowski background (I guess they could also choose Schwarzschild or so) both for the gravitational field as well as for the Maxwell field no? That sounds ok to me since it is well known that *classical* gravity tames the coulomb self energy up to the compton scale of elementary particles - at the cost of a gravitational singularity though (so you have to introduce a cutoff no matter what...). But this trio seems to have written lots of interesting papers in that time : another one is the influence of f gravity on gravitational collaps'' (1973). You can pick these for free on the trieste preprint site http://users.ictp.it/~pub_off/np/preprints.html, actually the one you referred to is http://streaming.ictp.trieste.it/preprints/P/70/131.pdf

Careful
 Sci Advisor P: 1,652 FH, we are not attacking LQG, but the results do seem extraordinary and unusual and hence we do what we do with all new theories. We are skeptical. Anomalies are topologically robust and nontrivial, hence if they are not present in the high energy theory, you need to either change your topology in the lower energy theory (which I take it is forbidden in LQG at this time), or it means you've missed it somehow in the construction (or haven't added a suitable space somewhere when adding matter). I just don't see how theres anyway around that... Hence my confusion
 P: 271 LQG is unusual. The anomaly could show up in an unusual way, one possibility is the absence of sollutions corresponding to a semiclassical background. What I want to say is that there are (again unusual) ways in which the effects known from low energy effective fieldtheory can show up in the theory. Do they? We don't know (and as far as I can see this was made clear from the beginning), Smolin emphasized that this is an important point to investigate. But you can not dismiss LQG because these effects don't show up in the same way as in background effective field theory, and the fact that they are not explicit doesn't mean Thiemanns work is wrong or is not actually doing what he is claiming to do, that is, give a rigorous LQG quantization of SM+GR Lagrangean. Nowhere is it claimed that this is physical, and to me it's not clear at all that this is a good strating route to begin with.

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