View Poll Results: Which of these potential string successors seem most promising?  
CDT  2  18.18%  
AsymSafe  3  27.27%  
Horava QG  1  9.09%  
new look Loop  6  54.55%  
Hamber Regge  1  9.09%  
Wen's condensed matter analog  1  9.09%  
Multiple Choice Poll. Voters: 11. You may not vote on this poll 
Successors to string scuffle (physical assets/liabilities?)by marcus Tags: physical, scuffle, string, successors 

#91
Oct309, 10:21 PM

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PF Gold
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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 any one 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} ~ 10^{19} 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 CPproblem, 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 Xray/γray 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 nuMSM 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://wwwspires.slac.stanford.edu/...tecount%28d%29 Here's are those publications listed most recent first, to get an idea of his recent work: http://wwwspires.slac.stanford.edu/...ENCE=ds%28d%29 



#92
Oct409, 05:05 AM

P: 100

voted.




#93
Oct609, 07:36 PM

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PF Gold
P: 22,803

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 MeissnerNicolai 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 (10^{19} 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. neutrino physics not left wide open of courseM&N model employs seesaw 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 matterI don't recall Nicolai addressing this, will check free parametershalf 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. 



#94
Oct609, 11:55 PM

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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 nonmetric 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 IRlike fixed point, as Freidel, Gurau and Oriti hope for? http://arxiv.org/abs/0905.3772 



#95
Oct709, 01:32 AM

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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 lowenergy 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 lowenergy 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 rederivation 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 npoint 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 angleyou 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. 



#96
Oct709, 03:10 AM

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Here's a link to the Haelfix post I was quoting back a couple, right before Atyy's post:
http://physicsforums.com/showthread....75#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/p...=1.3%20Nicolai Slides http://www.ift.uni.wroc.pl/~plancksc.../3Nicolai.pdf 



#97
Oct709, 11:32 PM

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P: 8,006

"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 “scaleinvariant gravity” [5, 4]. ... 4. M. Shaposhnikov and D. Zenhausern, Quantum scale invariance, cosmological constant and hierarchy problem, Phys. Lett. B 671 (2009) 162166 [arXiv:0809.3406 [hepth]]; Scale invariance, unimodular gravity and dark energy, Phys. Lett. B 671 (2009) 187192 [arXiv:0809.3395 [hepth]]. 5. M.E. Shaposhnikov and I.I. Tkachev, Quantum scale invariance on the lattice, 5 pages [arXiv:0811.1967 [hepth]]." 



#98
Oct709, 11:53 PM

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PF Gold
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Shaposhnikov! You found another reference to his scaleless 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 g_{0}^{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 nonperturbatively 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 nonrenormalizability of the EinsteinHilbert theory is cured by adding higherorder 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 latticetheory 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 “scaleinvariant 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== 



#99
Oct2809, 10:07 PM

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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 uptodate list of presentations that can serve to introduce the various competing approaches. Here is an update of an earlier list:
CDT Loll at the Planck Scale conference is topsbest available single lecture on the subject. http://www.ift.uni.wroc.pl/~rdurka/p...tul=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/ad.../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 LoopFoam QG that Rovelli gave at Strings 2008: http://cdsweb.cern.ch/record/1121957?ln=en http://indico.cern.ch/getFile.py/acc...s&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...wski102009.pdf http://relativity.phys.lsu.edu/ilqgs...wski102009.wav more information if needed at http://relativity.phys.lsu.edu/ilqgs/ and the corresponding recent arxiv paper http://arxiv.org/abs/0909.0939 "SpinFoams 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/pr...rs/DixonI.ppt 1 September: http://www.ccsem.infn.it/issp2009/pr...s/DixonII.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 XiaoGang 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 Gravity Some 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 wellknown 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 XiaoGang Wen ('Sabah) Thanks to all who have responded so far! 



#100
Nov3009, 12:36 PM

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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 



#101
Dec109, 04:54 AM

P: 63





#102
Dec1309, 07:39 PM

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In retrospect, a lot of what we were discussing in this thread is actually part of the
nofrills unification trend in fundamental physics! One of the things we already discussed a lot in this thread is Nicolai's unification gambit, which is kind of the archetypical nofrills proposal. He expounded it in that Wroclaw Planck Scale talk that in the other thread I said would be the "manifesto" of the movement if it is a movement. And also remember that nearly the top thing in the poll, in this thread, turned out to be ASYMPTOTIC SAFE GRAVITY! And that fact about gravityits apparent UV fixed pointhas provided kind of a backbone for the nofrills approach to get off the ground. Asymsafe gravity means a number of different thingstakes part in several different approaches. I realize now, more that when I set up the poll, that it is not just one thing. BTW Garrett, who is in touch with the pro theory scene, responded to the poll and chose Asymptotic Safety and the category with most interest/potential. Things like that can serve as straws in the wind for us. 



#103
Dec1709, 05:46 PM

P: 1,925

I should had voted for Asymptotic Safety. It is just simple and beautiful. After knowing that, I have no confidence at all at anything supersymmetric beyond the level of providing qualitative toy models.




#104
Dec1709, 07:40 PM

PF Gold
P: 2,432

He says it looks like it could underpin either a string or loop interpretation so has the promise of being more basic. It is background dependent of course. But despite being a GR guy, he does not think this is a dealbreaker in practice. Is there a reason why it is not registering on the "hotness" meter here? 



#105
Dec1709, 11:37 PM

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PF Gold
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Interesting question Apeiron. Something that helped me personally decide was watching Ted Jacobson sum up the Perimeter conference on Horava gravity.
http://pirsa.org/09110066/ Remarkable performance. Jacobson decided to attend the conference as a good way to learn about HorGrav, he had not done research in it, and IMHO probably will not. Naturally he was not intending to present a paper! However he's an old QG hand and highly respected, so (seeing that he was there) the Perimeter organizers asked him to give the summary talk at the end. Jacobson showed deep insight and a light touch. In my view he demolished HorGrav without showing the slightest desire to do soin a gentle, casual, offhand way. That was midNovember 2009. Do you have anything more recent than that from Matt Visser? The Horava fad trajectory has been fastmoving. I'd be curious to know what a distinterested commenter might have to say in the present timeframe. It's certainly of academic interest, in the sense of being new visible and accessibleif someone wants to write a paper investigating some hypothetical case, some aspect, they can get in quick, write the paper, and get out without too much trouble. So yeah, Matt Visser's description of a mid2009 "feeding frenzy" is quite apt. HorGrav was a perfect setup for a feeding frenzythis regardless of it's eventual prospects. 



#106
Dec1809, 02:39 AM

PF Gold
P: 2,432

Visser's comments are pretty recent. I was having lunch with him today after he presented his own summary of Horava for a local GR conference.
He is still very positive, saying it is the cutest thing he has seen in 10 years and more promising than the other lines mentioned here. Despite his very clear presentation of Horava, I still don't really have an intuitive understanding of what it is about though. He gave a thumbs up to Volovik, thumbs down to Nottale, lots of other interesting comments about this and that. 



#107
Dec1809, 08:30 AM

P: 1,925

DO YOU KNOW MATT VISSER PERSONALLY!?!?!?



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