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/blog/archives/000855.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.
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/blog/archives/000855.html#comments
samples from fh/Distler conversation RE: THE LQG LANDSCAPE A few comments from a newcommer. Prof Distler, you keep asking what LQG has to say on the problems and strengths of effective field theoriy. The simple answer is, little to nothing. If physics to you is synonymous to representing interactions on Fock spaces then LQG is not even wrong, it is silent and an utter failure to produce anything resembling this. The idea of LQG is of course to take “Georgis objection” serious, there is no limit in which you’d look at a free gravitational theory and trying to describe it’s degrees of freedom as free field + interaction is doomed and non-renormalizable to boot. The reaction of the LQG crowd is to point out that GR isn’t a theory on flat spacetime and that we can use the conceptional features of the theory to determine directly how it’s degrees of freedom should look, at least qualitatively. That is, we don’t try to work our way from an effective field theory and work our way towards an UV completion. Therefore the emphasize on finiteness, renormalization get’s rid of infinities in effective field theories by shoving them into the irrelevant UV degrees of freedom. If we get them directly renormalization applies in reverse, surpressing non renormalizable terms in our effective action. What imprints the fundamental degrees of freedom leave on these emergent effective theories is hard to tell and not yet understood except in some special cases. But it’s not a natural question to ask given what the theory attempts. It will eventually have to be addressed but it’s not the main focus. Now on the degrees of freedom Prof Smollin and others developed the algebras of pretty much everything coupled to gravity can be implemented. This implies conversely that contrary to what a lot of people keep saying LQG does not assume, in it’s key insights, that the metric degrees of freedom are good DoF at all scales. Change the labels and you have different DoF. There are the area and volume operators but within LQG there are also people arguing that at thePlanckscale that are misnomers and these do not have a geometrical interpretation anymore. Again, how to get effective field theory, and what the structure of these DoF proposed implies for these theories, and how all their language looks from that perspective is not known. But just looking at classical gravity it is already clear that gravity has something to say on this, because, uniquely of all the unknown interaction above 1TeV it leaves a deep imprint absolutely everywhere. In a very very different way from the other three forces we know and understand. And this is the way around Gerogi’s objection of course, crucially we know that Gravity provides a background for the other forces, you can’t even write down these kinetic terms without assuming a certain form of the metric. Whatever the degrees of freedom of Quantum Gravity are, they are leaving a very universal imprint. Conversely then to answer your questions on effective fieldtheory, we would first need to understand how the full classical, low energy theory emerges from these DoFs. Something that isn’t even completely understood for QCD for example. Basically LQG says that you are asking the wrong questions, eventually we will have to answer these, but in the meantime we have to develop a ****load of new tools to effectively work with this fantastic Spinfoam structures we stumbled upon. As for the snipe against Thiemanns work, his work is on the level of mathematical rigour. If you have an objection to his claims voice it. In the meantime you wouldn’t have needed to read the paper but just even the abstract to see that Thiemann is claiming a completely well defined theory of Yang Mills coupled to Gravity, not in flat spacetime (which is what the Clay prize is about), which would involve somehow miraculously replacing the quantum DoF in Tiemanns theory with a classical approximation of them. Even then it is of course by no means guaranteed that this will look like flat spacetime YM or if this will generate other artifacts. Posted by: fh on July 2, 2006 01:13 PM | Permalink | Reply to this =================== WELCOME *If physics to you is synonymous to representing interactions on Fock spaces then LQG is not even wrong, it is silent and an utter failure to produce anything resembling this.* Physics, to me, is not synonymous with “representing interactions on Fock spaces.” Except in the case of fermions, which, since they enter quadratically in the action, yield – upon quantization – a Fock space bundle over the configuration space of the other fields. *The idea of LQG is of course to take “Georgis objection” serious, there is no limit in which you’d look at a free gravitational theory and trying to describe it’s degrees of freedom as free field + interaction is doomed and non-renormalizable to boot.* I think you misunderstand Georgi’s objection. *That is, we don’t try to work our way from an effective field theory and work our way towards an UV completion.* Do you rely on divine revelation for all of the particle physics degrees of freedom from 100 GeV up to 10 19 GeV? Or do you suppose that information will be revealed in a blinding flash of insight, once you properly understand “octopi”? *And this is the way around Gerogi’s objection of course, crucially we know that Gravity provides a background for the other forces, you can’t even write down these kinetic terms without assuming a certain form of the metric.* I don’t see how that addresses, let alone is “the way around” Georgi’s objection. *Conversely then to answer your questions on effective fieldtheory, we would first need to understand how the full classical, low energy theory emerges from these DoFs. Something that isn’t even completely understood for QCD for example.* And coupling to gravity is supposed to make understanding that easier? *In the meantime you wouldn’t have needed to read the paper but just even the abstract to see that Thiemann is claiming a completely well defined theory of Yang Mills coupled to Gravity, not in flat spacetime (which is what the Clay prize is about), which would involve somehow miraculously replacing the quantum DoF in Tiemanns theory with a classical approximation of them.* Well, … 1. Nothing about Thiemann’s constructions relied on the peculiar U(1 ) hypercharge assignments of the fields in the Standard Model. Nor would it have changed in the slightest, had one omitted the right-handed (SU(2 )-singlet) up-quark from the theory. In other words, it would have applied equally well to an anomalous chiral gauge theory which, I continue to insist, cannot be turned into a well-defined quantum theory, with – or without – coupling to quantum gravity. 2. Whether in flat space, or coupled to quantum gravity, if one has indeed written down a complete quantization of Yang Mills, which is “entirely non-perturbatively defined and second quantized” then it contains all the relevant information (confinement and the existence of a mass gap) necessary to collect the Clay Prize. But, of course, both you and Lee, and most everyone else in the LQG 'biz take refuge in the plaint that “We don’t know how semiclassical spacetimes emerge.” to avoid having to make any contact with actual physics (anything, even in principle, measurable). The difficulties that LQG have in this regard are both a telling symptom that something deep may be wrong and largely irrelevant to the kind of questions I’m interested in. First of all, in any theory, including quantum gravity, the asymptotics of the fields are not fluctuating degrees of freedom, but rather represent superselection sectors in the theory. Thus, it is possible to speak of “asymptotically-flat spacetimes” (with fixed ADM mass, possibly zero), even if those spacetimes are in no sense semiclassical. And many interesting questions about quantum gravity can be phrased in terms of the physics of such asymptotically-flat spacetimes, regardless of whether they are dominated by any semiclassical configurations. [The same words can be said about asymptotically-AdS spacetimes. And there, string theory has very powerful, nonperturbative, and “background-independent” things to say.] But this is a whole 'nother topic, deserving of a post in itself. Perhaps I will get around to doing that sometime. Posted by: Jacques Distler on July 2, 2006 02:27 PM ============= RE: WELCOME *Whether in flat space, or coupled to quantum gravity, if one has indeed written down a complete quantization of Yang Mills, which is “entirely non-perturbatively defined and second quantized” then it contains all the relevant information (confinement and the existence of a mass gap) necessary to collect the Clay Prize.* First, isn’t Thiemann’s construction on a spin-foam model with no clear connection to any kind of space-time? Second, if Thiemann qualifies for the conditions of the Clay prize, would not the lattice guys also qualify? Posted by: Arun on July 4, 2006 02:12 AM ============== RE: WELCOME *First, isn’t Thiemann’s construction on a spin-foam model…* No, it’s in the canonical “CQL” framework. *Second, if Thiemann qualifies for the conditions of the Clay prize, would not the lattice guys also qualify?* The Lattice guys don’t claim to have proved anything. (Though, for pure Yang-Mills, they do have numerical results, believed to be accurate to within a few percent.) Posted by: Jacques Distler on July 4, 2006 02:25 AM =================== RE: WELCOME Really, if you are honestly interessted in discussing LQG and it’s failures and successes (rather then winning a shouting match) it is indispensible that you a) take the time to carefully read at least the abstracts, b) don’t assume that anything is claimed beyond what the abstract says, c) stop assuming everyone else is stupid and d) accept that LQG is approaching Quantum Field Theory from a completely different perspective, and with (in parts) completely different goals. In particular it does not have the goal to single out a theory of everything, it is developing a new and more general class of QFTs. Posted by: fh on July 4, 2006 07:15 AM ================== RE: WELCOME * LQG is approaching Quantum Field Theory from a completely different perspective, and with (in parts) completely different goals.* But can we agree at least that it should reproduce well understood results of ‘conventional’ QFT? 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? Posted by: wolfgang on July 4, 2006 08:45 AM RE: WELCOME Yes, and everybody keeps saying that, it should make contact with ordinary QFT, Freidel showed that in 2+1 it does. But at the same time it’s fundamentally different from ordinary QFT. So making this contact is a nontrivial task. Soon we will hopefully see Freidels work extended to 4D, getting an ordinary field theory out of defects in an otherwise topological one. So we are close to knowing that the types of QFTs LQG hqs produced are an honest non-trivial generalization of the framework of flat space QFT. If this framework is powerfull enough to capture Quantum Gravity nobody knows. Helling’s objections are a red herring, too. There is only one bit of quantization that differs drastically from ordinary quantization, the implementation of diffeomorphisms, and that is protected by the LOST theorem. (the rest is unusual, but clearly connects to standard methods) Posted by: fh on July 4, 2006 10:35 AM ================== ... [skip] ... RE: WELCOME *But yes for 3+1 it would be a Spinfoam that reproduces the flat spacetime results, not a cannonical quantization.* I am still trying to make somebody tell me which spin foam model that is, precisely. All that I am aware of is John Baez et al.’s work on soliton-like strings coupled to a gerbe, as in the followup of this. I understand that it is hoped that in 4 dimensions this will give rise to a theory with string-like defects that would be describeable by a spin foam model. Is that the spin foam model you have in mind? Has it already been constructed? Does it really describe gravity? Posted by: urs on July 5, 2006 06:09 AM ============== RE: WELCOME Hi Urs, this is unpublished, it’s not the Perez+Baez Stringy Spinfoam, It’s a whole new Spinfoam model by Baratin+Freidel, apparently quite unlike the ones we considered for something gravity like. Baez talks about it in this thread: https://www.physicsforums.com/showthread.php?t=123902 This is powerfull to me because it suggests that Spinfoams are a much more powerfull formalism then anyone could have assumed a priori. There has been a long standing idea in Gravity research going back all the way to Riemanns introduction of differential geometry that topology of space and matter are tightly interwoven, this actually makes the correspondence explicit. (My own ramblings:) Trying to quantize gravity forced people to come up with ideas like Spinnetworks and Spinfoams. I think these days a lot of people working on them would not neccessarily think that this is a direct quantization of Gravity but that these structures are powerfull enough to stand on their own and that, for various physical reasons, we might be well guided to mistrust the naive interpretation as a quantized geometry all the way down to the planck length. There might be a geometry limit as well as a QFT limit and so on, but the geometric notions become just as meaningless as the flat spacetime QFT ones for the fundamental excitations. There is no way to regard E^{2} as the physical area, and there is no S-Matrix at this level, Really just what you would expect from taking GR and QM serious. Posted by: fh on July 5, 2006 07:38 AM ==================== [I HAVE HIGHLIGHTED WHERE THERE SEEMED SOME NEW VISION CAME THRU] notice this was in response to Urs question, so Urs replies back. ==================== RE: WELCOME *[…] this is unpublished […]* Ok, thanks. So it seems to me that for the present discussion to lead anywhere, we would have to wait for that particular spin foam model to be published and then try to figure out the issue of coupling to chiral matter in that particular model. Posted by: urs on July 5, 2006 09:26 AM =================== RE: WELCOME *It’s a whole new Spinfoam model by Baratin+Freidel, apparently quite unlike the ones we considered for something gravity like. Baez talks about it in this thread: https://www.physicsforums.com/showthread.php?t=123902 * Great, thanks. Let’s see. What John Baez announces is not a spin foam model that reproduces gravity, but that *This spin foam model is thus a candidate for the G -> 0 limit of any spin foam model of quantum gravity and matter!* He also conjectures that this spin foam model is equivalent to the one proposed by Louis Crane and Marni Sheppeard, which -- correct me if I am wrong -- implies that the Crane-Sheppeard model can also only be a G -> 0 -limit of quantum gravity? Is that right? I have no problem with this being just a first step toward a hoped-for spin foam model for quantum gravity. In fact, I am pretty intrigued by these theories of d-brane-like objects coupled to higher gerbes, as you can imagine. But I do wonder what we tried to talk about when we were apparently discussing the coupling of quantum gravity to chiral matter in the context of LQG. Judging from what has been said, this seems to be a topic for the future. No? Posted by: urs on July 5, 2006 09:39 AM =================== RE: WELCOME (Disclaimer, I’m a beginner, too, just studying this stuff, far from expert!) I think what you say is perfectly right. As for what we were talking about, well, there are different ways to look at this. Freidel doesn’t actually couple matter to Spin Foams, in some sense it arises from the possible topologies. Also their model IS Ponzano-Regge. On the other hand what Lee Smollin was talking about was putting matter into the lagrangian and quantizing that. For gaugefields that’s straightforward, LQG is nothing but a way to quantize gaugefields on a manifold without a metric, nothing more. Yang Mills coupled to Gravity falls under this category, Yang Mills on flat spacetime does not. The Standard Modell + GR Lagrangian defines a theory on a manifold without metric and can be done to. (Of course this classical theory doesn’t actually describe anything we see in nature, since the low energy limit of most of the standard modell does not at all look like it’s field content). Can be done means it defines an algebra of observables and a hamiltonian constraint that can be promoted in a well defined way to a quantum operator algebra. This is rigorously defined but in a way those who chide Thiemann for speaking about the Standard Modell and QFT are right, since they use these terms to think of the physics these effective theories capture and not of some abstract algebras. And we do not know if Thiemanns construction captures these physics (to the best of my knowledge). Does this actually give matter in some sense? One might suspect so, but the quantization used is very different, as pointed out here. It allows for the inclusion of virtually arbitrary lagrangians, One might conjecture that upon renormalization in the flat spacetime limit only renormalizable effective theories remain by the standard Wilson argument, and that only the non-anomalous parts of the algebra survive because what in flat spacetime is the anomaly is a lack of semi classical flat spacetime states in LQG or something. That’s of course *wild* speculation. Emphasize on wild. Posted by: fh on July 5, 2006 10:30 AM ============== RE: WELCOME BTW to answer your specific question, a Spinfoam model with Stringlike defects has been constructed by Perez/Baez: http://arxiv.org/abs/gr-qc/0605087 This is not Gravity but “only” BF theory. Posted by: fh on July 5, 2006 08:39 AM =============== 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 ==================
a few more sample posts From the comments at Prof Distler's blog http://golem.ph.utexas.edu/~distler/blog/archives/000855.html#comments Picking up from that last post by Urs =============== 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 ================== ... ... [skip] ... 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 =============== 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 ================ 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 ====================== 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 ... ... 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 =======END OF SAMPLE========
what I think is the core passage To me the whole conversation seems worth looking over, but if I was to choose what part of it I most wanted to think about it would be where Urs comes in and asks something to which fh replies. ====selected out exerpt==== RE: WELCOME *But yes for 3+1 it would be a Spinfoam that reproduces the flat spacetime results, not a cannonical quantization.* I am still trying to make somebody tell me which spin foam model that is, precisely. All that I am aware of is John Baez et al.’s work on soliton-like strings coupled to a gerbe, as in the followup of this. I understand that it is hoped that in 4 dimensions this will give rise to a theory with string-like defects that would be describeable by a spin foam model. Is that the spin foam model you have in mind? Has it already been constructed? Does it really describe gravity? Posted by: urs on July 5, 2006 06:09 AM ============== RE: WELCOME Hi Urs, this is unpublished, it’s not the Perez+Baez Stringy Spinfoam, It’s a whole new Spinfoam model by Baratin+Freidel, apparently quite unlike the ones we considered for something gravity like. Baez talks about it in this thread: https://www.physicsforums.com/showthread.php?t=123902 This is powerfull to me because it suggests that Spinfoams are a much more powerfull formalism then anyone could have assumed a priori. There has been a long standing idea in Gravity research going back all the way to Riemanns introduction of differential geometry that topology of space and matter are tightly interwoven, this actually makes the correspondence explicit. (My own ramblings:) Trying to quantize gravity forced people to come up with ideas like Spinnetworks and Spinfoams. I think these days a lot of people working on them would not neccessarily think that this is a direct quantization of Gravity but that these structures are powerfull enough to stand on their own and that, for various physical reasons, we might be well guided to mistrust the naive interpretation as a quantized geometry all the way down to the planck length. There might be a geometry limit as well as a QFT limit and so on, but the geometric notions become just as meaningless as the flat spacetime QFT ones for the fundamental excitations. There is no way to regard E^{2} as the physical area, and there is no S-Matrix at this level, Really just what you would expect from taking GR and QM serious. Posted by: fh on July 5, 2006 07:38 AM ==================== RE: WELCOME *[…] this is unpublished […]* Ok, thanks. So it seems to me that for the present discussion to lead anywhere, we would have to wait for that particular spin foam model to be published and then try to figure out the issue of coupling to chiral matter in that particular model. Posted by: urs on July 5, 2006 09:26 AM =================== RE: WELCOME *It’s a whole new Spinfoam model by Baratin+Freidel, apparently quite unlike the ones we considered for something gravity like. Baez talks about it in this thread: https://www.physicsforums.com/showthread.php?t=123902 * Great, thanks. Let’s see. What John Baez announces is not a spin foam model that reproduces gravity, but that *This spin foam model is thus a candidate for the G -> 0 limit of any spin foam model of quantum gravity and matter!* He also conjectures that this spin foam model is equivalent to the one proposed by Louis Crane and Marni Sheppeard, which -- correct me if I am wrong -- implies that the Crane-Sheppeard model can also only be a G -> 0 -limit of quantum gravity? Is that right? I have no problem with this being just a first step toward a hoped-for spin foam model for quantum gravity. In fact, I am pretty intrigued by these theories of d-brane-like objects coupled to higher gerbes, as you can imagine. But I do wonder what we tried to talk about when we were apparently discussing the coupling of quantum gravity to chiral matter in the context of LQG. Judging from what has been said, this seems to be a topic for the future. No? Posted by: urs on July 5, 2006 09:39 AM =================== RE: WELCOME (Disclaimer, I’m a beginner, too, just studying this stuff, far from expert!) I think what you say is perfectly right. As for what we were talking about, well, there are different ways to look at this. Freidel doesn’t actually couple matter to Spin Foams, in some sense it arises from the possible topologies. Also their model IS Ponzano-Regge. On the other hand what Lee Smollin was talking about was putting matter into the lagrangian and quantizing that. For gaugefields that’s straightforward, LQG is nothing but a way to quantize gaugefields on a manifold without a metric, nothing more. Yang Mills coupled to Gravity falls under this category, Yang Mills on flat spacetime does not. The Standard Modell + GR Lagrangian defines a theory on a manifold without metric and can be done to. (Of course this classical theory doesn’t actually describe anything we see in nature, since the low energy limit of most of the standard modell does not at all look like it’s field content). Can be done means it defines an algebra of observables and a hamiltonian constraint that can be promoted in a well defined way to a quantum operator algebra. This is rigorously defined but in a way those who chide Thiemann for speaking about the Standard Modell and QFT are right, since they use these terms to think of the physics these effective theories capture and not of some abstract algebras. And we do not know if Thiemanns construction captures these physics (to the best of my knowledge). Does this actually give matter in some sense? One might suspect so, but the quantization used is very different, as pointed out here. It allows for the inclusion of virtually arbitrary lagrangians, One might conjecture that upon renormalization in the flat spacetime limit only renormalizable effective theories remain by the standard Wilson argument, and that only the non-anomalous parts of the algebra survive because what in flat spacetime is the anomaly is a lack of semi classical flat spacetime states in LQG or something. That’s of course *wild* speculation. Emphasize on wild. Posted by: fh on July 5, 2006 10:30 AM ============== RE: WELCOME BTW to answer your specific question, a Spinfoam model with Stringlike defects has been constructed by Perez/Baez: http://arxiv.org/abs/gr-qc/0605087 This is not Gravity but “only” BF theory. Posted by: fh on July 5, 2006 08:39 AM =======end of shortened sample======== I am trying to narrow it down to something short enough that we could discuss it.
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 [itex]G[/itex] --> 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?
The next task, I think, is to collect papers on deformed Poincare type structures and understand them in tricategorical terms.
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.
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?)
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
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.
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.
"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.
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
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
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.