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What is the recent development of Loop Quantum Gravity 
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#91
Mar612, 01:22 PM

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At the level of detail I think what they are talking about is different. I wouldn't use the word "expansion" in the FGZ context. I think they have found a different way to look at the truncation which allows them to reconstruct a class of continuum states from a discrete one. So they "detruncate" in a sense. They go back to the continuum picture without having to take a grand limit. I think this FGZ approach is still embryonic. It might eventually augment or replace the earlier "continuum limit" part of the program. These are workinprogress areasparts of the program that are under development and could take new directions. Yesterday I listened to Marc Geiller's ILQGS talk, with the slides. It's a good talk that covers the main ideas of the FGZ paper. I'd recommend it to anyone who wants to understand their approach better. I already gave the links but will do so again: http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf http://relativity.phys.lsu.edu/ilqgs/geiller022812.wav 


#92
Mar812, 11:50 AM

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A new development in QG that hasn't been reported yet or discussed here is an initiative by the Warsaw group where they develop a systematic way to enumerate all bulk spinfoams which a given spin network boundary. This is how one calculates transition amplitudes in spinfoam QG dynamics. The boundary spin network can, for instance, be thought of as representing initial and final states of geometry and the spinfoam bulk as a process transitioning from one to the other.
The Warsaw group has introduced a new OSN (operator spin network) formalismgraphs labeled by operators instead of spin numbers. This paper just came out: http://arxiv.org/abs/1203.1530 One vertex spinfoams with the Dipole Cosmology boundary Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta (Submitted on 7 Mar 2012) We find all the spinfoams contributing in the first order of the vertex expansion to the transition amplitude of the BianchiRovelliVidotto Dipole Cosmology model. Our algorithm is general and provides spinfoams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator SpinNetwork Diagrams framework. 23 pages, 30 figures Happily enough much of the content was already presented earlier in a recorded seminar talk at ILQGS by Puchta! Having the audio and slides presentation in parallel with the paper can make it easier to understand both. ILQGS: The Feynman diagramatics for the spin foam models Jacek Puchta SLIDES: http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf AUDIO: http://relativity.phys.lsu.edu/ilqgs/puchta092011.wav (alternative audio http://relativity.phys.lsu.edu/ilqgs/puchta092011.aif ) Nominally the recorded seminar talk was based on this paper, but there is considerable overlap with what just appeared: http://arxiv.org/abs/1107.5185 Feynman diagrammatic approach to spin foams Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta (Submitted on 26 Jul 2011) "The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper. 36 pages, 23 figures 


#93
Mar1012, 11:44 AM

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Karmerlo's original question, starting the thread, was about recent Loop developments. My take on that is based on the idea that this is a fast moving field and that at each stage there should be a clear definite testable formulation. In 2010 we got a new version of Loop that culminated in the Zakopane 2011 version ( http://arxiv.org/abs/1102.3660 ). Now I'm looking ahead to see what the 2013 formulation might be like.
This is a quiet period now when IMHO people are getting new thoughts in order. I expect quantum relativists to construct a QG which resolves the cosmo ("bang") singularity, is testable with early universe data, and recovers a good approximation of usual GR. As long we don't have SEVERAL alternatives that do this, and therefore do not need to choose between them, I am not going to quibble about the "pedigree" or quantization ritual that was used to arrive at the theory. That comes later when we have more than one satisfactory alternative. This is simply my (pragmatic) philosophyother people of course assume other intellectual stances. So thinking ahead to Loops 2013 (to be held at Perimeter Institute) what is the most important paper we should now be looking at? What has the seeds of a new formulation? I think it is page 5 of the January 2012 paper of Bonzom and Smerlak. Merely an observant bystander's nonprofessional guessbut maybe sometimes that's OK to offer. In the excerpt that follows I used curly brackets to distinguish the moduli space {M} from the ordinary manifold M, while the authors used a special font. ==quote from page 5 of http://arxiv.org/1201.4996 == Relation to the loop formalism. The above method naturally gives rise to the loop quantization of BF theory. In the loop approach, one quantizes before restricting to flat gauge fields. Given an embedded, closed graph γ, cylindrical wave functions are functions of the Wilson lines along the lines of γ. For each graph there is a Hilbert space whose measure is given by the Haar measure of G on each line, ∏_{e} dg_{e}. The Hilbert spaces of two different graphs are orthogonal. The standard gauge symmetry requires invariance under Gtranslation on the source and end nodes of the lines. Heuristically, the transition amplitudes in the continuum (7) suggest that they can be formulated in the loop approach by taking as boundary states cylindrical functions restricted to the moduli space {M}, the torsion still providing the measure. Assume M has two disconnected boundaries N_{1},N_{2}, with two closed, embedded graphs γ_{1}, γ_{2} associated with two cylindrical functions Ψ_{γ1} , Ψ_{γ2} . The transition is regularized by choosing a cell decomposition K of M such that γ_{1},γ_{2} are included into the 1skeleton. The ungaugefixed transition amplitude reads ⟨Ψ_{γ2}Z_{BF}Ψ_{γ1}⟩=∫∏_{e}dg_{e} Ψ^{∗}_{γ2}(g_{e})Ψ_{γ1}(g_{e})∏_{f}δ(H_{f}). As the shift symmetry does not act on Wilson lines, the process of the previous section applies. The wavefunctions are evaluated on {M} because there are no fluctuations around flat connections, yielding [eqn (31)]: ⟨Ψ_{γ2} Z'_{BF}Ψ_{γ1}⟩ = Ʃ_{[φ]∈M}Ψ^{∗}_{γ2}([φ]) Tor[φ] Ψ_{γ1}([φ]). Finally, the regulator K can be removed thanks to the topological invariance of the torsion, which makes the continuum limit result into the above formula. Let us mention an outcome of this result: the loop quantization of the BF model does not distinguish knottings of the graphs γ_{1,2}. Conclusion. We have performed a topological quantization of discrete BF theory, proving its equivalence to the usual quantization in the continuum. This result solves several open problems of the field and extends previous results obtained in dimension 3 to arbitrary dimensions: (1) transition amplitudes are finite, answering the issue of bubble divergences [11, 28]; (2) the gauge symmetries in the discrete setting exist, generalizing [11, 12], and (3) they can be gaugefixed to derive the loop quantization, generalizing [13]; (4) as a result, one gets a topological invariant, which proves that the classical gauge symmetries are correctly promoted to the quantum level. The crucial steps of our quantization require to take into account cells of all dimensions in the cell complex, and not just its 2skeleton like in the “spinfoam quantization”. A challenge for future investigations is to find a representation of (31) as a statesum, as is done in the latter approach. The last issue we mentioned in the introduction is the major difficulty in quantum gravity: understanding the quantum version of diffeomorphisminvariance. It is wellknown that diffeomorphisminvariance in the BF model is contained within its shift symmetry [20]. Hence the substance of general relativity is to break the topological invariance while preserving diffeomorphisminvariance. Spinfoam models for quantum gravity are very much in line with this idea, as they start by quantizing BF theory and then introduce some breaking of the shift symmetry to restore the local degrees of freedom. It is also known that discrete models of gravity generically break diffeomorphisminvariance [17]. Showing that it is restored in the continuum limit (after some coarsegraining, or summing over spinfoams appropriately) is one of the main programs in the spinfoam approach. Now that the shift symmetry is correctly controlled in the discrete setting, we feel that the noose is tightening around diffeomorphisms. ==endquote== 


#94
Mar1012, 12:52 PM

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#95
Mar1012, 01:22 PM

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My guess is that this is the new spinfoam. The old 2skeleton approach was started back in the late 1990s by Reisenberger and Rovelli. It is over 20 years old and probably needs to be updated. Of course I could be wrong



#96
Mar1112, 01:50 PM

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Matteo Smerlak's PhD thesis is a useful source of background for the 6page BonzomSmerlak letter.
I should give the latter's abstractdidn't do that yet. http://arxiv.org/abs/1201.4996 Gauge symmetries in spinfoam gravity: the case for "cellular quantization" Valentin Bonzom, Matteo Smerlak (Submitted on 24 Jan 2012) The spinfoam approach to quantum gravity rests on a "quantization" of BF theory using 2complexes and group representations. We explain why, in dimension three and higher, this "spinfoam quantization" must be amended to be made consistent with the gauge symmetries of discrete BF theory. We discuss a suitable generalization, called "cellular quantization", which (1) is finite, (2) produces a topological invariant, (3) matches with the properties of the continuum BF theory, (4) corresponds to its loop quantization. These results significantly clarify the foundations  and limitations  of the spinfoam formalism, and open the path to understanding, in a discrete setting, the symmetrybreaking which reduces BF theory to gravity. 6 pages A concise excerpt from page 1: ==quote 1201.4996== The purpose of this letter is to argue that there is a good reason for this: when dealing with 2complexes only, as in the spinfoam formalism, there is no shift symmetry. To identify this symmetry, one must instead resort to an extension of the spinfoam formalism including higherdimensional cells. This realization paves the way to what we call cellular quantization. This cellular quantization solves problems 1 to 4, and sheds interesting new light on problem 5. The letter is organized as follows. We start by reviewing the basic properties of the continuum BF theory, emphasizing its gauge symmetries and relationship to analytic torsion. We then describe the “spinfoam quantization” of BF theory, as described e.g. in Baez’s reference paper [5]. We show how to identify the gauge symmetries in a discrete setting and perform a quantization which does preserve the topological features of the continuum theory. Finally we establish that this cellular quantization corresponds to the loop canonical quantization. ==endquote== Problems 1 through 5, mentioned in the above excerpt, are as follows: ==quote== 1. Bubble divergences. The original PRO [PonzanoReggeOguri] partition functions are in general divergent. How should one regularize them? 2. Topological invariance. The PRO partition functions are formally invariant under changes of triangulations, up to divergent factors. How can one turn them into finite topological invariants? 3. Relationship to the canonical theory. The connection between the PonzanoRegge model and loop quantum gravity in 3 dimensions was established in [13]. Can this connection be extended to 4 dimensions and higher? 4. Relationship to the continuum theory. BF theory was quantized in the continuum in [21, 22], and was showed to be related to the RaySinger torsion. Are the PRO models similarly related to torsion? (See [14] for a positive answer in certain threedimensional cases.) 5. Diffeomorphism symmetry. Both the continuum BF action and the EinsteinHilbert action are diffeomorphisminvariant. What is the fate of this symmetry in the PRO models? ==endquote== For completeness, here is the abstract of Smerlak's thesis. It doesn't overlap in results, but shares some conceptstherefore is helpful in part simply because it is longer (over 100 pages instead of only 6) and more deliberate. Goes thru some definitions in a less condensed way. http://arxiv.org/abs/1201.4874 Divergences in spinfoam quantum gravity Matteo Smerlak (Submitted on 23 Jan 2012) In this thesis we study the flat model, the main buidling block for the spinfoam approach to quantum gravity, with an emphasis on its divergences. Besides a personal introduction to the problem of quantum gravity, the manuscript consists in two part. In the first one, we establish an exact powercounting formula for the bubble divergences of the flat model, using tools from discrete gauge theory and twisted cohomology. In the second one, we address the issue of spinfoam continuum limit, both from the lattice field theory and the group field theory perspectives. In particular, we put forward a new proof of the Borel summability of the BoulatovFreidelLouapre model, with an improved control over the largespin scaling behaviour. We conclude with an outlook of the renormalization program in spinfoam quantum gravity. 113 pages. PhD thesis, introduction and conclusion in French, main text in English. Paper by Ileana NaishGuzman and John Barrett cited on page 4 ref [14] in connection with the discrete exterior derivative on a cell complex. http://arxiv.org/abs/0803.3319 Similarly cited was [26] an earlier paper by Bonzom and Smerlak http://arxiv.org/abs/1103.3961 Additional webstuff about de Rham complex http://en.wikipedia.org/wiki/De_Rham_cohomology http://www.vttoth.com/CMS/pahysicsn...erhamcomplex 


#97
Mar1912, 01:38 PM

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A major international conference like the triennial Marcel Grossmann meeting can give a snapshot of "recent development".
Here is the brief statement summing up the situation from the chair of one of the LQG sessions at MG13 (Stockholm July 2012): http://www.icra.it/MG/mg13/par_sessi...tm#lewandowski Jerzy LEWANDOWSKILewandowski heads the LQG group at Warsaw (one of a handful of leading groups: Marseille, Perimeter, PennState, Warsaw, LSU, Erlangen, AEI...) I gather that another triennial meeting, the International Conference on General Relativity and Gravitation, will have its 2013 venue in Warsaw. Lewandowski will doubtless be the main organizer of GR20. It should have an interesting lineup in QG: background independent quantum geometry. The next biennial Loops conference, Loops 2013, will be held at Perimeter. One way to gauge progress and follow developments is to keep an eye on the topics featured in the programmes of the main conferences as they take shape. ======================== Along the same lines, we already know the LQG talks to be presented at the April 2012 meeting of the APS (American Physical Society). I listed links and abstracts here: http://physicsforums.com/showthread....64#post3784064 and here: http://physicsforums.com/showthread....86#post3788486 Loop is in course of achieving parity with String, visibilitywise at major conferences. One can get an idea of which recent directions and results in Loop research are considered important by seeing what the main conference talks, especially those invited by the organizers, are about. 


#98
Mar2312, 01:10 PM

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Another snapshot of the current definition of Loop and the problems to be worked on will be Rovelli's upcoming (23 April) talk at the Princeton Institute of Advanced Studies.
http://www.princeton.edu/physics/eve...ent.xml?id=347 High Energy Theory Seminar  IAS  Carlo Rovelli, AixMarseille University, France  Loop quantum Gravity: Recent Results and Open Problems Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit. Location: Bloomberg Lecture Hall Date/Time: 04/23/12 at 2:30  3:30 pm ============================ My comment. This talk may be substantially similar to the one at Perimeter on 4 April, which I believe will subsequently be available video online. I imagine there might be an eventual writeup covering the same material. Loop is fast moving and is reformulated from time to time. It has been having small "revolutions" on roughly a 35 year basis, so Rovelli's wording should be noted "present the current definition...results that have been proven...what I think is still missing..." "Tentative" here I think means an attempt: to be tested by observationto be put on trial in other words. Any theory can only be tentative until its predictions are tested and either confirmed or not. Over the past few years it's been mainly up to Rovelli to give a clear precise definition of the current Loop theory, write the survey papers, list the open problems. So these two talks will serve as a significant landmark. Will it be essentially a restatement of the February 2011 formulation (Zakopane lectures http://arxiv.org/abs/1102.3660 ) or will there be some new features? Here's the PIRSA link for the Wednesday 4 April one at Perimeter. http://pirsa.org/12040059 


#99
Mar2612, 09:07 AM

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The University of Vienna and Vienna Tech are holding a 5day Quantum physics + Gravity school in early September, intended for PhD students and other young researchers wanting to get into gravityrelated research.
A nice feature is how applied it is. 2 out of the 4 main lecturers are discussing applications (linked to astrophysical observation) rather than pure QG theory. http://www.coqus.at/events/summerschool2012/ The title of the School is Quantum physics meets Gravity Here's the poster: http://www.coqus.at/fileadmin/user_u...s/CoQuS_a3.pdf Here are the more applied, hardwareoriented topics that two of the lecture series will be about: "Experimental gravitation and geophysics with matterwave sensors" "Gravitational wave detection and quantum control" =============================== While I think of it, the journal SIGMA is publishing a special issue devoted to Loop gravity and cosmology assembled by a group of guest editors. They now have a dozen articles in final form having gone thru the peer review and editorial process. These could give some clues as to what the editors see as significant current developments in the field. http://www.emis.de/journals/SIGMA/LQGC.html ==quote== Papers in this Issue: Introduction to Loop Quantum Cosmology Kinjal Banerjee, Gianluca Calcagni and Mercedes MartínBenito SIGMA 8 (2012), 016, 73 pages [ abs pdf ] Learning about Quantum Gravity with a Couple of Nodes Enrique F. Borja, Iñaki Garay and Francesca Vidotto SIGMA 8 (2012), 015, 44 pages [ abs pdf ] Emergent Braided Matter of Quantum Geometry Sundance BilsonThompson, Jonathan Hackett, Louis Kauffman and Yidun Wan SIGMA 8 (2012), 014, 43 pages [ abs pdf ] Matter in Loop Quantum Gravity Ghanashyam Date and Golam Mortuza Hossain SIGMA 8 (2012), 010, 26 pages [ abs pdf ] Lessons from ToyModels for the Dynamics of Loop Quantum Gravity Valentin Bonzom and Alok Laddha SIGMA 8 (2012), 009, 50 pages [ abs pdf ] Entropy of Quantum Black Holes Romesh K. Kaul SIGMA 8 (2012), 005, 30 pages [ abs pdf ] Discretisations, Constraints and Diffeomorphisms in Quantum Gravity Benjamin Bahr, Rodolfo Gambini and Jorge Pullin SIGMA 8 (2012), 002, 29 pages [ abs pdf ] Numerical Techniques in Loop Quantum Cosmology David Brizuela, Daniel Cartin and Gaurav Khanna SIGMA 8 (2012), 001, 26 pages [ abs pdf ] Statistical Thermodynamics of Polymer Quantum Systems Guillermo ChacónAcosta, Elisa Manrique, Leonardo Dagdug and Hugo A. MoralesTécotl SIGMA 7 (2011), 110, 23 pages [ abs pdf ] The Space of Connections as the Arena for (Quantum) Gravity Steffen Gielen SIGMA 7 (2011), 104, 12 pages [ abs pdf ] Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter Merced Montesinos and Mercedes Velázquez SIGMA 7 (2011), 103, 13 pages [ abs pdf ] A LorentzCovariant Connection for Canonical Gravity Marc Geiller, Marc LachièzeRey, Karim Noui and Francesco Sardelli SIGMA 7 (2011), 083, 10 pages [ abs pdf ] ==endquote== 


#100
Mar3012, 01:11 PM

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Judging from past performance, Rovelli can be relied on for a comprehensive insightful account of the current state of development of Loop gravity and the remaining problems to be worked out. The abstract of the talk he is to give at Princeton IAS was posted earlier.
Today I see that the abstract for the Perimeter Institute colloquium talk he is giving next Wednesday (4 April) has also been posted. It's approximately the same summary description, probably much the same talk. So if things go as expected, we'll soon have an online video of an uptodate firsthand view of Loop with Perimeter audience Q&A. http://pirsa.org/12040059/ Transition Amplitudes in Quantum Gravity Speaker(s): Carlo Rovelli Abstract: The covariant formulation of loop quantum gravity has developed strongly during the last few years. I summarize the current definition of the theory and the results that have been proven. I discuss what is missing towards of the goal of defining a consistent quantum theory whose classical limit is general relativity. Date: 04/04/2012  2:00 pm For comparison here is the announcement of the Princeton talk: http://www.princeton.edu/physics/eve...ent.xml?id=347 High Energy Theory Seminar  IAS  Carlo Rovelli, AixMarseille University, France  Loop quantum Gravity: Recent Results and Open Problems Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit. Location: Bloomberg Lecture Hall Date/Time: 04/23/12 at 2:30  3:30 pm Rovelli will also be giving a series of lectures on QG during the first week of September at the Vienna "Quantum Physics meets Gravity" School that I mentioned in the preceding post: http://www.coqus.at/events/summerschool2012/ ================================= The April meeting of the American Physical Society also starts next week in Atlanta. There will be several invited and contributed Loop talks. Links and abstracts are listed here: http://physicsforums.com/showthread....64#post3784064 and here: http://physicsforums.com/showthread....86#post3788486 Bianchi and Agullo give invited talks Monday 2 April http://meetings.aps.org/Meeting/APR12/Event/170161 http://meetings.aps.org/Meeting/APR12/Event/170160 


#101
Apr1012, 08:07 AM

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There will be a QG School in Beijing in August. I was interested to see who the lecturers are going to be:
http://physics.bnu.edu.cn/summerschool/en/index.php ==quote== The 2nd BNU International Summer School on Quantum Gravity: 1218 August 2012, Beijing Beijing Normal University (BNU), China The BNU International Summer School on Quantum Gravity is intended to provide a pedagogical introduction for graduate students and young postdocs to the main fields closely related to loop quantum gravity. Topics include: Loop quantum gravity, Loop quantum cosmology, Spin foams, Group field theory, Regge calculus Lecturers: Abhay Ashtekar (Penn State Univ, USA) Benjamin Bahr (Cambridge Univ, UK) John Barrett (Univ of Nottingham, UK) Jonathan Engle (Florida Atlantic Univ, USA) Thomas Krajewski (Univ of Provence & CPT Marseille, France) Jerzy Lewandowski (Univ of Warsaw, Poland) Etera Livine (ENS de Lyon, France) ==endquote== The University of Vienna and Vienna Tech are holding a 5day Quantum physics + Gravity school in early September, intended for PhD students and other young researchers wanting to get into gravityrelated research. http://www.coqus.at/events/summerschool2012/ The title of the School is Quantum physics meets Gravity Here's the poster: http://www.coqus.at/fileadmin/user_u...s/CoQuS_a3.pdf Lecturers: · Philippe Bouyer (University of Bordeaux, Institut d'optique and CNRS, France) · Michèle Heurs (Max Planck Institute for Gravitational Physics, Hannover, Germany) · Ulf Leonhardt (University of St. Andrews, UK) · Carlo Rovelli (Centre de Physique Theorique de Luminy, Marseille, France =============================== Update on the journal SIGMA's special issue on Loop Gravity and Cosmology, being assembled by a group of guest editors. So far they have fifteen articles in final form which have passed peer review and been posted online. I was interested to see the lineup since it gives an idea of what the editors see as significant current research directions. http://www.emis.de/journals/SIGMA/LQGC.html Here's an updated listing of the articles. They are free online. Colored Tensor Models  a Review Razvan Gurau and James P. Ryan SIGMA 8 (2012), 020, 78 pages [ abs pdf ] Intersecting Quantum Gravity with Noncommutative Geometry  a Review Johannes Aastrup and Jesper Møller Grimstrup SIGMA 8 (2012), 018, 25 pages [ abs pdf ] Relational Observables in Gravity: a Review Johannes Tambornino SIGMA 8 (2012), 017, 30 pages [ abs pdf ] Introduction to Loop Quantum Cosmology Kinjal Banerjee, Gianluca Calcagni and Mercedes MartínBenito SIGMA 8 (2012), 016, 73 pages [ abs pdf ] Learning about Quantum Gravity with a Couple of Nodes Enrique F. Borja, Iñaki Garay and Francesca Vidotto SIGMA 8 (2012), 015, 44 pages [ abs pdf ] Emergent Braided Matter of Quantum Geometry Sundance BilsonThompson, Jonathan Hackett, Louis Kauffman and Yidun Wan SIGMA 8 (2012), 014, 43 pages [ abs pdf ] Matter in Loop Quantum Gravity Ghanashyam Date and Golam Mortuza Hossain SIGMA 8 (2012), 010, 26 pages [ abs pdf ] Lessons from ToyModels for the Dynamics of Loop Quantum Gravity Valentin Bonzom and Alok Laddha SIGMA 8 (2012), 009, 50 pages [ abs pdf ] Entropy of Quantum Black Holes Romesh K. Kaul SIGMA 8 (2012), 005, 30 pages [ abs pdf ] Discretisations, Constraints and Diffeomorphisms in Quantum Gravity Benjamin Bahr, Rodolfo Gambini and Jorge Pullin SIGMA 8 (2012), 002, 29 pages [ abs pdf ] Numerical Techniques in Loop Quantum Cosmology David Brizuela, Daniel Cartin and Gaurav Khanna SIGMA 8 (2012), 001, 26 pages [ abs pdf ] Statistical Thermodynamics of Polymer Quantum Systems Guillermo ChacónAcosta, Elisa Manrique, Leonardo Dagdug and Hugo A. MoralesTécotl SIGMA 7 (2011), 110, 23 pages [ abs pdf ] The Space of Connections as the Arena for (Quantum) Gravity Steffen Gielen SIGMA 7 (2011), 104, 12 pages [ abs pdf ] Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter Merced Montesinos and Mercedes Velázquez SIGMA 7 (2011), 103, 13 pages [ abs pdf ] A LorentzCovariant Connection for Canonical Gravity Marc Geiller, Marc LachièzeRey, Karim Noui and Francesco Sardelli SIGMA 7 (2011), 083, 10 pages [ abs pdf ] 


#102
May412, 01:12 AM

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We can start looking ahead to the next biennial Loops conference: Loops 2013, to be held at Perimeter Institute. I'm told that planning for the conference has already started. Loop gravity is constantly evolving as a theory and a paper suggesting a new classical action sets the stage, I think, for the 2013 version of the quantum theory.
http://arxiv.org/abs/1205.0733 Discrete Symmetries in Covariant LQG Carlo Rovelli, Edward WilsonEwing (Submitted on 3 May 2012) We study timereversal and parity on the physical manifold and in internal space in covariant loop gravity. We consider a minor modification of the Holst action which makes it transform coherently under such transformations. The classical theory is not affected but the quantum theory is slightly different. In particular, the simplicity constraints are slightly modified and this restricts orientation flips in a spinfoam to occur only across degenerate regions, thus reducing the sources of potential divergences. 8 pages The classical basis for the theory is the Holst action. A 4D manifold M equipped with internal Minkowski space M at each point plus a tetrad e (oneform valued in M) and a connection ω. The conventional Holst action: S[e,ω]=∫e^{I}Λe^{J}Λ(* + 1/γ) F_{I J} The * denotes the Hodge dual. A proposed new action S' uses the signum of det e: s = sign(det e) defined to be zero if det e = 0 and otherwise ±1. S'[e,ω]=∫e^{I}Λe^{J}Λ(s* + 1/γ) F_{I J} There is also a closely related alternative action S" discussed in the paper. It looks to me as if either S' or S", suitably quantized, takes care of the semiclassical limit of the theory. See equation (43) of the paper. Or in any case represents a major step. The problem addressed was that the original EPRL looked at in the limit exhibited both spacetime and "antispacetime" evolution. Both timeforward and timereversed evolution appeared. Otherwise everything was properly Regge as expected. Then there were papers by Yasha Neiman and by Jon Engle that studied this bidirectional time mixup. Rovelli and WilsonEwing (RWE) built on their results. So I expect this RWE paper to provide a basis for a new Loop initiative leading up to the conference about a year from now. Of course it is risky (even foolhardy) to forecast research trends. But that's what I think after reading the paper. It addresses several of the remaining problems in the theoryand it's quite interesting as well. 


#103
May412, 02:45 AM

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very interesting, but many difficulties in the spin foam approach (you what I am talking about ;) are still not addressed



#104
May412, 11:24 AM

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A basic premise of this thread is that there are plenty of welldefined problems to work on. That is what this thread is fundamentally about: current progress in solving problems, essentially by gradually redefining the theory. The story can be told, to some extent, in human terms. There are people who not only identify problems but also do something about them. Alesci focused on a problem with the Thiemann hamiltonian and proposed a new hamiltonian (able to grow volume). He is now joining Lewandowski's Warsaw group as postdoc. http://sites.google.com/site/grqcrumourmill/ Engle focused on a problem with largescale limit of the EPRL vertex and proposed a solution which prefigured the one in the "Discrete Symmetries" paper just mentioned. He is now faculty at Florida Atlantic. Ryan ("Simplicity constraints and the Immirzi parameter in discrete quantum gravity" ILQGS) has accepted a tenure track position at Morelia. http://relativity.phys.lsu.edu/ilqgs/ryan041211.pdf http://sites.google.com/site/grqcrumourmill/ At each stage there is a rather welldefined theory, which gives focus to the program, and there are opportunities for young researchers to do things which are clearly significant. 


#105
May1912, 05:24 PM

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PF Gold
P: 23,274

Here's an indication of interests in the broader community of Mathematical Physics. Some of the speakers' names will be familiar to people at BtSM forum especially those who have been following developments in Group Field Theory/Loopandallied QG research.
==quote== The XXIX International Colloquium on GroupTheoretical Methods in Physics August 2026, 2012, Chern Institute of Mathematics, Tianjin, China Plenary Speakers (Titles and Abstracts) Akito Arima (Musashi Gakuen, Japan) Abhay Ashtekar (Pennsylvania State University, USA) Murray Batchelor (Australian National University, Australia) Edward Corrigan (University of York, UK) MoLin Ge (Nankai University, China) Razvan Gurau (Perimeter Institute for Theoretical Physics, Canada) (Hermann Weyl Prize Awardee) Alden Mead (University of Minnesota, USA) (Wigner Medal Awardee) Jerzy Lewandowski (University of Warsaw, Poland) Jun Murakami (Waseda University, Japan) Daniel Treille (CERN, Switzerland) (Public Lecturer) Vincent Rivasseau (University ParisSud XI, France) Zhenghan Wang (Microsoft Station Q, USA) The time allocated for each plenary talk is 60 minutes (50 min presentation + 10 min questions). ==endquote== This time the biennial GroupTheoryinPhysics Conference will have 12 parallel sessions, of which #8 is "Loop Quantum Gravity", and #9 is "Group Field Theory for Quantum Gravity". I'm hoping that the titles of the talks (plenary 60 minute and parallel 30 minute) will be posted soon and that we can get from them some idea of how particular foci of interest are shaping up. http://www.nim.nankai.edu.cn/activit...nouncement.pdf http://www.cim.nankai.edu.cn/activit...0820/index.htm 


#106
May2312, 05:52 PM

Astronomy
Sci Advisor
PF Gold
P: 23,274

A week from today there will be two Loop talks at Perimeter, which we can expect to have available as online PIRSA video.
http://www.perimeterinstitute.ca/en/...xt=1338177600/ The first, at 2PM, will be the PI Colloquium talk, given by Eugenio Bianchi. Here's the Pirsa link: http://pirsa.org/12050053 Then at 4PM there will be a QG seminar talk by Hal Haggard. Haggard got his PhD in Loop QG from UC Berkeley last year and has been teaching courses in the Physics department at Berkeley for the past couple of semesters. Here is the abstract and Pirsa link for his talk: http://pirsa.org/12050084 Pentahedral Volume, Chaos, and Quantum Gravity Speaker(s): Hal Haggard Abstract: The space of convex polyhedra can be given a dynamical structure. Exploiting this dynamics we have performed a BohrSommerfeld quantization of the volume of a tetrahedral grain of space, which is in excellent agreement with loop gravity. Here we present investigations of the volume of a 5faced convex polyhedron. We give for the first time a constructive method for finding these polyhedra given their face areas and normals to the faces and find an explicit formula for the volume. This results in new information about cylindrical consistency in loop gravity and a couple of surprises about polyhedra. In particular, we are interested in discovering whether the evolution generated by this volume is chaotic or integrable as this will impact the interpretation of the spin network basis in loop gravity. Date: 30/05/2012  4:00 pm Later this summer, Haggard will be one of the invited speakers at the "Groups 29" (Group Theoretical Methods in Physics) conference mentioned in the previous post. He then takes up a 2year postdoc fellowship at Marseille starting in September. Part of the prior work referred to in the abstract is research he coauthored with Bianchi. 


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