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List of plenary talks at Loops '07 conference

  1. Apr 6, 2007 #1

    marcus

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    Registration deadline for Loops '07 is coming up
    deadline for early registration is 25 April
    late registration in May.

    Looks like most splendid Loops conference yet (even includes string as a small auxilliary sub-department :smile: See Rozali's talk further down the page)
    TOPICS COVERED
    * Foundational questions of quantum gravity
    * Loop quantum gravity
    * Spin foam models
    * Dynamical triangulations
    * Causal sets
    * String theory
    * Cosmology related to quantum gravity
    * Phenomenology of quantum gravity

    http://www.matmor.unam.mx/eventos/loops07/index.html
    ==quote from provisional program for the June 2007 conference==
    Program

    At this time the program is in an early planning stage. Below is the list of plenary speakers with titles of talks. Abstracts can be displayed by clicking on the title of the respective talk or by going to the abstracts page. When more detailed information on the program is available it will be posted on this site.

    Jan Ambjørn

    Abhay Ashtekar "LQG: Lessons from models"

    Martin Bojowald "Loop quantum cosmology and effective theory"

    John F. Donoghue "Effective field theory and quantum general relativity"

    Klaus Fredenhagen

    Lucien Hardy "The causaloid formalism: a tentative framework for quantum gravity"

    James B. Hartle "Generalizing Quantum Mechanics for Quantum Spacetime"

    Sabine Hossenfelder "Phenomenological Quantum Gravity"

    Bei-Lok B. Hu "Stochasticity and Nonlocality in Quantum-Classical and Micro-Macro Interfaces"

    Fotini Markopoulou

    Daniele Oriti

    Alejandro Perez

    Jorge Pullin "Uniform discretizations and spherically symmetric loop quantum gravity"

    Michael Reisenberger

    Martin Reuter "Asymptotically safe quantum gravity and cosmology"

    David Rideout

    Carlo Rovelli

    Moshe Rozali "Background Independence in String Theory"

    Lee Smolin

    Rafael Sorkin*

    Artem Starodubtsev "Some physical results from spinfoam models"

    Jörg Teschner*

    Thomas Thiemann
    ==endquote==

    Abstracts of most of the talks with titles shown above are already available at the website. I have printed them out here as well
    https://www.physicsforums.com/showthread.php?p=1294958#post1294958
     
    Last edited: Apr 6, 2007
  2. jcsd
  3. Apr 20, 2007 #2

    marcus

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    The list of plenary speakers at Loops '07 has been made more definite and some abstracts have been added. The plenary talks page was updated 18 April as follows:
    ==quote==
    Abstracts of plenary talks
    Abhay Ashtekar: LQG: Lessons from models

    In the last couple of years, several simple but physically interesting models were solved. Solutions led to concrete and detailed realizations of a number of ideas that have been heuristically expected for decades. There were also some surprises. These analyses suggest viewpoints and strategies for full quantum gravity. I will summarize some of them in broad terms.
    ===

    Martin Bojowald: Loop quantum cosmology and effective theory

    Several models have been defined to study the broad framework of loop quantum gravity. We present a common perspective for cosmology where configurations are close to being isotropic. Effective techniques are then used to illustrate important features of the semiclassical limit and to show examples of correction terms to Einstein's equation. Applications include corrections to the Newton potential and to cosmological perturbation equations relevant for the CMB power spectrum.
    ===

    John F. Donoghue: Effective field theory and quantum general relativity

    I will review the foundations and recent progress in the use of effective field theory to elucidate the quantum predictions of general relativity.
    ===

    Lucien Hardy: The causaloid formalism: a tentative framework for quantum gravity

    General Relativity is a deterministic theory with non-fixed causal structure. Quantum Theory is an inherently probabilistic theory with fixed causal structure. A theory of Quantum Gravity must reduce, in appropriate limits, to General Relativity and Quantum Theory. It seems likely, therefore, that Quantum Gravity will inherit the radical aspects of these two less fundamental theories – namely that it will be a probabilistic theory with non-fixed causal structure. In this talk I will present the causaloid formalism which is a general framework for such theories. I will show how Quantum Theory can be formulated in the causaloid framework and provide tentative results for how it may be possible to formulate General Relativity in the framework. Finally, I will discuss the issue of formulating a theory of Quantum Gravity using the causaloid formalism.
    ===

    James Hartle: Generalizing Quantum Mechanics for Quantum Spacetime

    Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has been generalized as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This talk will review a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. In this generalization, states of fields on spacelike surfaces and their unitary evolution are emergent properties appropriate when spacetime geometry behaves approximately classically. The principles of generalized quantum theory would allow for further generalization that would be necessary were spacetime not fundamental. Emergent spacetime phenomena are discussed in general and illustrated with the examples of the classical spacetime geometries with large spacelike surfaces that emerge from the `no-boundary' wave function of the universe. These must be Lorentzian with one, and only one, time direction. The question will be raised as to whether quantum mechanics itself is emergent.
    ===

    Sabine Hossenfelder: Phenomenological Quantum Gravity

    The search for a satisfying theory that unifies general relativity with quantum field theory is surely one of the major tasks for physicists in the 21st century. During the last decade, the phenomenology of quantum gravity and string theory has been examined from various points of view, opening new perspectives and testable predictions. I will give a short introduction into these effective models which allow to extend the standard model and include the expected effects of the underlying fundamental theory. I will talk about models with extra dimensions, models with a minimal length scale and those with a deformation of Lorentz invariance. The focus is on observable consequences, such as black hole and graviton production and modifications of standard-model cross-sections.
    ===

    Daniele Oriti: Group field theory: spacetime from quantum discreteness to an emergent continuum

    Group field theories are non-local quantum field theories on group manifolds, and a generalization of matrix models. Having been first introduced in the context of simplicial quantum gravity, have gained attention as being potentially of much interest in the context of loop quantum gravity and spin foam models. After a brief introduction to the group field theory formalism, I review some of the results already obtained in this approach. I will then try to offer a new perspective on how group field theories should be interpreted and used towards a complete theory of quantum gravity. In particular, I will argue that group field theories can represent on the one hand a common unifying framework for loop quantum gravity, spin foam models and simplicial approaches, like quantum Regge calculus and dynamical triangulations, and on the other hand a consistent microscopic description of spacetime considered as a condensed matter system. From this, a novel approach to the issues of the emergence of the continuum and of General Relativity as an effective description of spacetime, in this approximation, is proposed. Finally, I will briefly report on some recent results and work in progress inspired by and supporting this new perspective.
    ===

    Jorge Pullin: Uniform discretizations and spherically symmetric loop quantum gravity

    We discuss the application of the uniform discretization procedure to handle the dynamics of loop quantum gravity with spherical symmetry.
    ===

    Michael Reisenberger: Canonical gravity with free null initial data

    Free (unconstrained) gravitational initial data variables are known for initial hypersurfaces consisting of two intersecting null hypersurfaces. Recently the Poisson bracket on functions of such data has been obtained. This opens the prospect of a constraint free canonical formulation of general relativity. The reasons for looking for such a formulation, and some of its features will be discussed.

    Martin Reuter: Asymptotically safe quantum gravity and cosmology

    The basic ideas and main results of the asymptotic safety scenario in Quantum Einstein Gravity (QEG) are reviewed and possible implications for the cosmology of the early universe are discussed.

    Moshe Rozali: Background Independence in String Theory

    I review several notions of background independence in quantum gravity: full or partial, manifest or not etc., and discuss the evidence for and against taking the notion seriously as a guide for research. I review the current status in string theory with regard to this question and discuss the extent to which existing non-perturbative formulations of string theory are background independent.

    John Stachel: Projective and Conformal Structures in General Relativity

    Our goal is to contribute to the development of a background-independent, non-perturbative approach to quantization of the gravitational field based on the conformal and projective structures. Physically the conformal structure is determined by the local behavior of null wave-fronts and rays; and the projective structure by the local behavior of freely falling massive test particles, respectively. In general relativity, these structures may be taken as fundamental, and the pseudo-Riemannian metric and affine connection derived from them. Various initial value problems in GR may be reformulated on the basis of the conformal-projective breakup-- in particular, null-initial value problems and the 2+2 decomposition of the field equations-- with the aim of investigating how best to isolate the two degrees of freedom of the gravitational field, a question of crucial importance for their quantization. The quantum of action sets limits on the co-measurability of various physically measurable quantities, and thereby determines their commutation relations. Hence, the co-measurability of quantities derived from the conformal and projective structures, such as the conformal 2-structure, will be analyzed as a heuristic guide to their quantization. We have already derived the connection and metric from a Palatini-type variational principle utilizing the conformal and projective structures, and will explore this and various other variational principles that could form the starting point for a Feynman type quantization of the gravitational field.
    ===

    Artem Starodubtsev: Some physical results from spinfoam models

    Given the known mathematical fact that a spinfoam is a Feynman diagram the data relevant for particle scattering amplitudes could be identified in it.

    ==endquote==
     
    Last edited: Apr 21, 2007
  4. Apr 21, 2007 #3
    Hey Marcus,
    Since you seem to be an insider, how does the list of who gets invited to talk gets chosen, for Loops 07?

    Is there any possibility Xiao-Gang Wen could be invited to Loops 07?

    Recent advances in condensed matter theory have revealed that new and exotic phases of matter can exist in spin models (or more precisely, local bosonic models) via a simple physical mechanism, known as "string-net condensation." These new phases of matter have the unusual property that their collective excitations are gauge bosons and fermions. In some cases, the collective excitations can behave just like the photons, electrons, gluons, and quarks in our vacuum. This suggests that photons, electrons, and other elementary particles may have a unified origin -- string-net condensation in our vacuum. In addition, the string-net picture indicates how to make artificial photons, artificial electrons, and artificial quarks and gluons in condensed matter systems.

    A local quantum bosonic model on a lattice is constructed whose low energy excitations are gravitons described by linearized Einstein action. Thus the bosonic model is a quantum theory of gravity, at least at the linear level. We find that the compactification and the discretization of metric tenor are crucial in obtaining a quantum theory of gravity.

    We give an example of a purely bosonic model -- a rotor model on the 3D cubic lattice -- whose low energy excitations behave like massless U(1) gauge bosons and massless Dirac fermions. This model can be viewed as a ``quantum ether'': a medium that gives rise to both photons and electrons. It illustrates a general mechanism for the emergence of gauge bosons and fermions known as ``string-net condensation.'' Other, more complex, string-net condensed models can have excitations that behave like gluons, quarks and other particles in the standard model. This suggests that photons, electrons and other elementary particles may have a unified origin: string-net condensation in our vacuum.

     
  5. Apr 21, 2007 #4

    marcus

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    I'm the opposite of an insider:biggrin: but thanks for the compliment! I'm a bystander---like the people on the sidewalk gawking at the riggers putting up a building.

    Why couldn't X-G Wen just go to the conference and deliver a paper? They would surely love to have him show up!
    Judging by Loops '05, in a conference like this there is space for about 60 - 80 talks, of which only 20-some are "invited", and the rest are called "contributed".

    There are usually the invited plenary talks in the MORNING. This conference has around 22 invited speakers. they will somehow have to schedule them, which will use up about half the time (most likely the mornings)

    then the remaining time (e.g. afternoons and evenings) they run 2 or more PARALLEL SESSIONS. So at any given time, say 2PM, you have a choice of several different talks, and you pick one of them to go to.

    If X-G Wen would just register for the conference, they would quickly assign him a good spot in one of the parallel sessions. He is well known! Then he would give a talk and probably it would attract a big audience, in one of the afternoon sessions. If his talk was a hit, then next year, or next time they have a conference, the organizer committee (and their advisor committee) might well decide to ask Wen to give a plenary talk.

    Of course if you feel strongly about it you could write email to someone on the "scientific advisory committee" and urge that they invite Wen to give plenary talk THIS year. But they already have 22 confirmed plenary speakers and that really is a lot to squeeze into a week of morning sessions.
    I don't think it is a good idea to urge that, at this point, because plenary session time is at a premium. During the plenary session you cannot, by definition, be running parallel sessions. So every halfhour that you commit to plenary means two or more halfhour talks that you cannot be having in the parallel sessions.
    Plenary session time is costly in terms of how many other talks you can schedule. So I would not urge for Wen to give one of those talks---which they already have enough. Better he should just give one of the ordinary talks. Maybe he will! We have not seen the list of other speakers yet. There will be at least 50 others and so far we have no idea who they are.
     
    Last edited: Apr 21, 2007
  6. Apr 21, 2007 #5

    marcus

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    Well Dan, I just checked the poster and I my initial guess was wrong. I thought they would be having 3 parallel sessions, but they say they will only have two.
    "Format: The conference will consist of four plenary talks each morning from Monday 25 to Saturday 30 June. After lunch two parallel sessions with contributed talks will take place Monday 25 to Friday 29 June. An exception is the afternoon of Wednesday 27 June which is free. We plan to offer social events for this day, including an excursion to the beautiful town of Patzcuaro."

    they seem to have maybe one morning spot left uncommitted because they say they are running 24 morning talks. Of course some might be WELCOME talks or Concluding talks by the hosts. My hunch is the morning is now full.

    And also they say they will only have TWO parallel sessions for the "contributed talks" and that will be just be the afternoon of Monday Tuesday Thursday Friday.
    So that is 4 days x 2 sessions x maybe 5 talks in an afternoon session so that is only room for maybe 40 contributed talks. So I was off some.
    =======================

    BTW I think it is absolutely fantastic to have Lucien Hardy, and Jim Hartle, and Martin Reuter AS WELL AS so many more familiar top quantum gravity people.

    The mexican organizers have gotten a majorly heavyweight batting lineup.

    and they also have a fine choice of young people helping to give the plenary talks (in many fields only the prominent senior people do that but here they have Dan Oriti and Bee Hossenfelder and so on). the more i inspect the more intelligent it looks, how they are assembling Loops '07
     
    Last edited: Apr 21, 2007
  7. Apr 21, 2007 #6
    Hi,
    So who should I email so they can contact Wen? His lattice graviton model is quantum gravity.

    Thanks
     
    Last edited: Apr 21, 2007
  8. Apr 21, 2007 #7

    marcus

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    It's obvious, isn't it? If you are interested, email X-G himself and ask if he is planning to attend the conference.

    He may already have plans to go in June and deliver a paper! In case he doesn't know about the conference, give him a link to the announcement.
    http://www.matmor.unam.mx/eventos/loops07/index.html

    If he hasn't already registered, and you think he should go and give a talk, then remind him to register soon, because the deadlines are 25 April and then 30 May (for late registration).

    The lineup for the morning talks looks like it is already decided. So I would think it is counterproductive to be talking about that, at this late date.
     
    Last edited: Apr 21, 2007
  9. Apr 21, 2007 #8
    Oh I can email him but his spam mail filter will treat my yahoo email as spam, probably. Still I can try.
     
  10. Apr 21, 2007 #9

    marcus

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    I just checked the "Participants" list and there are already 86 people signed up, including a couple we know here at PF Beyond forum
    F-h and
    KEA

    It looks like this is going to be a popular conference (by quantum geometry- quantum gravity standards) because many people have hurried to register.

    There will be a lot of demand for the afternoon parallel session time---they may need to increase the number of parallel sessions from two (as planned) to three.
     
  11. Apr 27, 2007 #10

    marcus

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    Early registration is now closed, late reg is possible thru 30 May. 126 participants registered so far.
    the titles of many of the plenary talks are now listed, most with abstracts available.

    title of Lee Smolin's talk is listed.
    it looks like an exceptionally interesting conference with new ideas and approaches not represented at the earlier Loops 04 and 05 in Marseille and Potsdam.
    I have highlighted several titles which were not on the list given in the first post of this thread, and so may not have been noticed.

    ==update list of Loops '07==
    Plenary Speaker Title
    Jan Ambjørn "4d quantum gravity as a sum over histories"
    Abhay Ashtekar "LQG: Lessons from models"
    Martin Bojowald "Loop quantum cosmology and effective theory"
    John F. Donoghue "Effective field theory and quantum general relativity"
    Klaus Fredenhagen
    Lucien Hardy "The causaloid formalism: a tentative framework for quantum gravity"
    James B. Hartle "Generalizing Quantum Mechanics for Quantum Spacetime"
    Sabine Hossenfelder "Phenomenological Quantum Gravity"
    Bei-Lok B. Hu "Stochasticity and Nonlocality in Quantum-Classical and Micro-Macro Interfaces"
    Fotini Markopoulou
    Daniele Oriti "Group field theory: spacetime from quantum discreteness to an emergent continuum"
    Alejandro Perez
    Jorge Pullin "Uniform discretizations and spherically symmetric loop quantum gravity"
    Michael Reisenberger "Canonical gravity with free null initial data"
    Martin Reuter "Asymptotically safe quantum gravity and cosmology"
    David Rideout
    Carlo Rovelli
    Moshe Rozali "Background Independence in String Theory"
    Lee Smolin "Chiral excitations of quantum geometries as a possible route to unification"
    Rafael Sorkin*
    John Stachel "Projective and Conformal Structures in General Relativity"
    Artem Starodubtsev "Some physical results from spinfoam models"
    Thomas Thiemann "Elements of Loop Quantum Gravity"
    ==endquote==

    I already mentioned some PF posters who will be at the conference, two more I didn't mention are Garrett Lisi and William Donnelly. WD has a blog ( http://williamdonnelly.blogspot.com/ ) so we might get some reports there. Alejandro Satz who has the blog "Reality Conditions" will be there so we might get some reporting from him too.

    Jan Ambjorn's abstract:
    4d quantum gravity as a sum over histories

    "In this plenary talk I will review the attempts to formulate 4d quantum gravity as a sum over histories in such a way that computer simulations can be performed. I will report on computer simulations of a quantum universe with a positive cosmological constant as well as a quantum universe where test matter is included."

    I didn't yet see a preprint of any paper telling the results of computer simulations of this sort that included matter.
     
    Last edited: Apr 27, 2007
  12. May 8, 2007 #11

    Chronos

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    I'm delighted that Hardy is invited. I've been intrigued by the causaloid idea for a couple years. It's still in its infancy, but has great potential, IMO.
     
  13. May 8, 2007 #12
    Hi Marcus. It's unlikely that I'll be blogging from Morelia for a couple of reasons. The first is that maintaining a blog is a lot of work (I have no idea how you manage to be so prolific on these forums). The second is that I probably won't have a laptop. I might write some kind of summary when I get back, either way I'll let you know.
     
  14. May 8, 2007 #13

    marcus

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    Thanks, I've rarely found "live" blog reports as satisfactory as reports from when the blogger is back home and has had time to recollect and reflect and filter.

    For me, the best report from the QGQG school was what Alejandro Satz provided over the course of several days after he was back in England.
    A "live" report might have had vivid impressions but could have been confusing
    because of so much going on.

    I found your blog in the aftermath of Jerzy Kowalski-Glikman's seminar talk at Perimeter exceptionally helpful. If you have time and energy to provide anything like that from one or two of the talks in Morelia, I shall be grateful. It almost doesnt matter which talks you pick to report on.
    And if you don't have time in the days after the conference, that's fine too.
    Clearly there's not time for everything.

    Have you heard any reactions at Perimeter about the new formula spinfoam vertex proposed by Etera Livine and Simone Speziale? I think there's a chance it might be a substantial improvement over the B-C spinfoam model in general use.
     
  15. May 9, 2007 #14
    Thanks Marcus. I'm not a student at Perimeter and don't know the authors, so I haven't heard anything about it. But it's likely that they will speak about it at one of the quantum gravity meetings.
     
  16. May 22, 2007 #15

    marcus

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    It has been almost a month since I last updated the list of plenary talks. Some more have been added, including one by Carlo Rovelli. So let's take a look at the new lineup. It says "Last updated 18 May 2007" so it is fairly fresh information. Abstracts are available for nearly every talk by clicking on the title. I have highlighted some talk titles and some names, somewhat arbitrarily, to help me think about aspects of the program:

    Plenary Speaker Title
    Jan Ambjørn "4d quantum gravity as a sum over histories"
    Abhay Ashtekar "LQG: Lessons from models"
    Martin Bojowald "Loop quantum cosmology and effective theory"
    John F. Donoghue "Effective field theory and quantum general relativity"
    Klaus Fredenhagen "General covariance in quantum field theory and the background problem in perturbative quantum gravity"
    Lucien Hardy "The causaloid formalism: a tentative framework for quantum gravity"
    James B. Hartle "Generalizing Quantum Mechanics for Quantum Spacetime"
    Sabine Hossenfelder "Phenomenological Quantum Gravity"
    Bei-Lok B. Hu "Stochasticity and Nonlocality in Quantum-Classical and Micro-Macro Interfaces"
    Fotini Markopoulou
    Daniele Oriti "Group field theory: spacetime from quantum discreteness to an emergent continuum"
    Alejandro Perez "Regulator dependence in quantum gravity and non perturbative renormalizability: possible new perspectives"
    Jorge Pullin "Uniform discretizations and spherically symmetric loop quantum gravity"
    Michael Reisenberger "Canonical gravity with free null initial data"
    Martin Reuter "Asymptotically safe quantum gravity and cosmology"
    David Rideout "Can the supercomputer provide new insights into quantum gravity?"
    Carlo Rovelli "Vertex amplitude and propagator in loop quantum gravity"
    Moshe Rozali "Background Independence in String Theory"
    Lee Smolin "Chiral excitations of quantum geometries as a possible route to unification"
    Rafael Sorkin* "Quantum reality and anhomomorphic logic"
    John Stachel "Projective and Conformal Structures in General Relativity"
    Artem Starodubtsev "Some physical results from spinfoam models"
    Thomas Thiemann "Elements of Loop Quantum Gravity"

    In a certain way Artem Starodubtsev is standing in for Laurent Freidel. Freidel won't be there and Artem has co-authored papers on obtaining the Feynman diagrams of usual QFT---in other words MATTER---from spinfoam quantum gravity. Also Jan Ambjorn will be talking about work he has done with Renate Loll, who won't be there. He gives the Simplex Gravity talk. It is worthwhile reading the abstracts and thinking about what will be brought out in some of these talks.
     
    Last edited: May 22, 2007
  17. Jun 8, 2007 #16

    marcus

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    http://www.matmor.unam.mx/eventos/loops07/index.html

    there are 158 registered participants (more even than Loops '05 at the Albert Einstein Institute, AEI, in Germany)

    the abstracts are now all available for the 20-some plenary talks

    David Rideout's talk about using supercomputers in numerical LQG
    seems particularly interesting. He posted a paper about this on arxiv in the past day or two.
    He and Brunnemann ran their work on the Peyote cluster at AEI.
    with a framework or operating system called "Cactus".
    their project used some 20,000 CPU hours on the Peyote cluster (as described on page 18
    and following in http://arxiv.org/abs/0706.0469 )

    Rovelli's talk covers things in a paper that he, Engle, Pereira posted on arxiv recently.

    One can get an idea of what many of the talks will be like by reading recent arxiv postings and comparing with the available abstracts.
     
    Last edited: Jun 8, 2007
  18. Jun 9, 2007 #17
    Artem has not been working with Freidel on deriving the QFT Feynman diagrams from spinfoams.. i think you were thinking about Aristide Baratin. Artem has been focusing on the (spinfoam) quantization of the Mac-Dowell-Mansouri action for GR (plus particles)...
     
  19. Jun 9, 2007 #18

    marcus

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    That's right, I had gotten the two mixed up. Thanks for the correction!
     
  20. Jun 9, 2007 #19

    garrett

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    One can hardly fault Marcus. The program reads:

    "Artem Starodubtsev: Some physical results from spinfoam models
    Given the known mathematical fact that a spinfoam is a Feynmann diagram the data relevant for particle scattering amplitudes could be identified in it."

    I'm happy to hear he's continuing work on MM GR -- looking forward to his talk.
     
  21. Jun 12, 2007 #20

    marcus

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    Hi Garrett!
    Yes I was looking at the abstract of Starodubtsev's talk and although he hasnt posted co-author with Freidel lately it did look as if among other things his talk would necessarily be reporting Freidel's recent work. Among the plenary talks, it seemed to come the closest. But "stand-in" was the wrong word.

    For anyone interested in how Loops '07 is shaping up:
    The day-by-day schedule of plenary talks has been finalized as of 11 June
    http://www.matmor.unam.mx/eventos/loops07/index.html
    click on "program"

    It is a six day conference and there are 4 plenary talks per day, mostly in the morning, with parallel sessions in the afternoons.
     
    Last edited: Jun 12, 2007
  22. Jun 15, 2007 #21
    For those who are interested, the full schedule including the parallel sessions has just been posted. Just follow the link Marcus posted.
     
  23. Jun 15, 2007 #22

    marcus

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    Congratulations Wm. D. on being in that line-up

    glad to see that you and another occasional PF poster, Garrett Lisi, are talking in the parallel sessions and have your abstracts listed, both quite interesting.

    I am blown away by how interesting the parallel session talks are, totally amazed. the plenary talks are more about what I know of already and expected.

    the parallel talks, many of them to judge by the abstracts, hit at the heart of major outstanding problems. they seem on the whole to be a higher-impact bunch of talks than we got in the parallel session of Loops '05 Potsdam, which were already pretty strong, again judging by online video and slides
     
  24. Jun 15, 2007 #23

    marcus

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    just a sample, apologies to presenters not included

    ===exerpts from the contributed talks abstracts posted today===
    Emanuele Alesci: Graviton propagator: the non diagonal terms
    Some components of the graviton propagator have recently been computed in the context of background independent loop quantum gravity. We extend these results to the non-diagonal terms of the propagator. ...
    ...
    Benjamin Bahr: Gauge-invariant coherent states for LQG

    The usual complexifier coherent states provide a tool for investigating the semiclassical limit of LQG. However, these states are purely kinematical, i.e. do not satisfy any of the constraints. But if one wants to work on the gauge- or Diff-invariant level, coherent states that satisfy the Gauss- and/or the Diff-constraint are desirable. In this talk I will, as a first step into this direction, describe the projection of the complexifier coherent states on the gauge-invariant Hilbert space and demonstrate their semiclassical properties. This will show that these gauge-invariant coherent states are the appropriate tool for addressing semiclassical issues in the gauge-invariant sector.

    Eugenio Bianchi: Large scale correlations in spinfoam models for quantum gravity

    In the loop approach to quantum gravity, we study large scale correlations of geometric operators on a semiclassical state peaked on a flat geometry. The correlations found can be compared to the two- and three-point functions computed in perturbative quantum gravity on a flat background. The calculations are done using the Riemannian Barrett-Crane model and involve the boundary amplitude formalism. Perturbative area-Regge-calculus plays a key role as an intermediate step in the comparison between correlations computed from the spinfoam model and from perturbative quantum gravity. The calculation presented is to be considered as a first step towards an effective field theoretical description of the semiclassical regime of loop quantum gravity. The presence of large scale correlation is a key test for spinfoam models for quantum gravity, and the details of the calculation provide constraints and hints for possible improvements of the Barrett-Crane model.

    Sundance Bilson-Thompson: Braids, loops, and the emergence of the standard model

    We will describe how the particles of the standard model may be formed of composite structures. When these structures are represented as braids, features such as parity, charge conjugation, hypercharge, and weak interactions emerge naturally. This raises the prospect that braided network states in LQG contain emergent particle states which do not need to be put in.

    ...
    Wade Cherrington: Numerical Spin Foam Computation of Pure Yang-Mills Theory

    We present a Metropolis algorithm for the computation of dual Yang-Mills theory using spin foam configurations, and describe its implementation for pure Yang-Mills in three dimensions and gauge group SU(2). We report results that are in agreement with conventional computations for a range of coupling constants, and discuss the improvements that will be necessary in treating the weak coupling limit. This work provides evidence that spin foam computations can successfully reproduce known physics while offering a compelling, gauge-invariant picture for the evolution of the physical degrees of freedom. We conclude with an outlook for the application of these spin foam algorithms to matter-coupled spin foam quantum gravity, and to other sectors of the Standard Model.

    Dan Christensen: Computations in spin foams
    ...

    Jacobo Díaz-Polo: Black hole radiation spectrum in Loop Quantum Gravity
    ...
    Bianca Dittrich: A Gauge Invariant Treatment of Perturbations in Cosmology

    Based on the framework of partial and complete observables we introduce a novel perturbative scheme for perturbations around symmetry reduced sectors of general relativity. This scheme allows the calculation of perturbations in a gauge invariant manner to an arbitrary high order. In particular we can consider backreaction effects. Applications to Loop Quantum Cosmoloy will be discussed.

    William Donnelly: Entanglement Entropy in Loop Quantum Gravity

    It has been suggested that the black hole entropy arises from entanglement between fields inside and outside the black hole horizon. We apply this proposal to loop quantum gravity by computing the entanglement entropy of the spin network states. We find that the entanglement entropy is finite, extensive over the horizon, and agrees with the asymptotic value previously computed for isolated horizons. We suggest possible future implications for the classical limit of loop quantum gravity.
    ...

    Jonathan Engle: Relating LQC to LQG: Embeddings, their properties, and questions

    In this talk, I raise and discuss conceptual questions that arise when relating loop quantum cosmology (LQC) to loop quantum gravity (LQG). I then go on to present two proposals for interpreting LQC states in the context of full LQG: The first is an earlier embedding proposed by Bojowald and Kastrup, and the second is a new embedding using holomorphic representations. Consistency with the existing LQC quantization is reviewed, and a prescription for extending these embeddings to the gauge and diffeomorphism invariant levels is considered.

    Winston J. Fairbairn: 3d spinfoam quantum gravity: matter as a phase of the group field theory

    An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models by Freidel and Livine. In this talk, I will show how this relates to group field theories and generalized matrix models. In a first part, I will explain how the effective field theory can be recast as a matrix model where couplings between matrices of different sizes can occur. In a second part, I will provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, I will show how to generate the dynamics of the effective field theory. I will identify a particular case which leads to the action of Freidel and Livine for a massive field living in a flat non-commutative space-time. The most general solutions lead to field theories with non-linear redefinitions of the momentum which I will propose to interpret as living on curved space-times. I will conclude by discussing the possible extension to four-dimensional spinfoam models.

    Rodolfo Gambini: Relational physics with real rods and clocks

    General relativity being a covariant theory, one expects that the evolution of quantum gravity systems will be described in relational terms. The use of real clocks and measuring rods in quantum mechanics implies a natural loss of unitarity and entanglement. More precisely, even if one assumes that the complete system, including measuring devices, precisely obeys the laws of quantum mechanics when described in terms of an ideal time, the description of the system in terms of real clocks is not unitary. The loss of entanglement is related with a similar phenomenon when real spatial measurements are taken into account. I discuss the implications of these effects for the measurement problem in quantum mechanics.
    ...

    Kristina Giesel: Algebraic Quantum Gravity

    We introduce a new top down approach to canonical quantum gravity, called Algebraic Quantum Gravity (AQG): The quantum kinematics of AQG is determined by an abstract *-algebra generated by a countable set of elementary operators labelled by an algebraic graph. The quantum dynamics of AQG is governed by a single Master Constraint operator. While AQG is inspired by Loop Quantum Gravity (LQG), it differs from it because in AQG there is fundamentally no topology or differential structure. The missing information about the topology and differential structure of the spacetime manifold as well as about the background metric to be approximated is supplied by coherent states. Given such data, the corresponding coherent state defines a sector in the Hilbert space of AQG which can be identified with a usual QFT on the given manifold and background. In particular, by means of the introduction of semiclassical perturbation theory for AQG (and LQG) we can show that AQG admits a semiclassical limit whose infinitesimal gauge symmetry agrees with that of General Relativity.
    ...
    Jonathan Hackett: Translations in Braided Ribbon Networks

    I will introduce concepts from Braided Ribbon Networks leading to the concept of microlocality and isolated substructures. From this I will demonstrate the general ability to translate isolated substructures through Braided Ribbon Networks.

    Joe Henson: Large-scale information from dynamical triangulations

    A new procedure for coarse-graining dynamical triangulations is presented. The procedure provides a meaning for the relevant value of observables when 'probing at large scales', e.g. the average scalar curvature. The scheme may also be useful as a starting point for a new type of renormalisation procedure, suitable for CDT quantum gravity. Random Delaunay triangulations have previously been used to produce discritisations of continuous Euclidean manifolds, and the coarse-graining scheme is an extension of this idea, using random simplicial complexes produced from a dynamical triangulation. In order for the coarse-graining process to be useful, it should preserve the properties of the original dynamical triangulation that are relevant when probing at large scales. Some arguments, and evidence from simulations, are presented for this. Hopefully there will also be some results from 3D CDT simulations.

    Mikhail Kagan: Effective constraints of loop quantum gravity

    Within a perturbative cosmological regime of loop quantum gravity corrections to effective constraints are computed. This takes into account all inhomogeneous degrees of freedom relevant for scalar metric modes around flat space and results in explicit expressions for modified coefficients and of higher order terms. It also illustrates the role of different scales determining the relative magnitude of corrections. Our results demonstrate that loop quantum gravity has the correct classical limit, at least in its sector of cosmological perturbations around flat space, in the sense of perturbative effective theory.

    Igor Khavkine: q-deformed spin foams for Riemannian quantum gravity

    We numerically study Barrett-Crane models of Riemannian quantum gravity. We have extended the existing numerical techniques to handle q-deformed models and arbitrary space-time triangulations. We present and interpret expectation values of a few selected observables for each model, including a spin-spin correlation function which gives insight into the behaviour of the models. We find the surprising result that, as the deformation parameter q goes to 1 through roots of unity, the limit is discontinuous. Joint work with Dan Christensen.
    ...
    Jerzy Lewandowski: Self-adjointness in the LQC models

    Mathematical properties of the quantum constraint and quantum Hamiltonian operators of LQC are studied. Self-adjointness is proven, spectra are characterized. The exact results are compared with the numerical ones.

    Garrett Lisi: Deferential Geometry

    The message from gravity is clear: our universe is geometric. But gravity does not exist alone -- it interacts with the whole zoo of standard model fields. A unification of these fields requires the gravitational connection to be absorbed as part of a larger connection, along with the intimately intertwined Higgs field and frame of GR. By extending this connection further, one generation of fermions may be derived as BRST ghosts, and the dynamics described by a modified BF theory. This description meshes well with recent approaches quantizing gravity perturbatively around a topological theory. But where could such a big, messy connection come from? As it turns out, precisely this connection matches the geometry of the largest exceptional Lie group -- giving exactly three complete generations of fermions, linked by triality! In this fully unified picture, gravity and all fields of the standard model are described by the pure geometry of a group manifold, with no strings attached.
    ...

    Roberto Pereira: The loop-quantum-gravity vertex-amplitude

    This talk is complementary to the one given in plenary by Carlo Rovelli.

    Spinfoam theories are hoped to provide the dynamics of non-perturbative loop quantum gravity. But a number of their features remain elusive. The best studied one --the euclidean Barrett-Crane model-- does not have the boundary state space needed for this, and there are recent indications that, consequently, it may fail to yield the correct low-energy n-point functions. These difficulties can be traced to the SO(4) -> SU(2) gauge fixing and the way certain second class constraints are imposed, arguably incorrectly, strongly. We present an alternative model, that can be derived as a bona fide quantization of a Regge discretization of euclidean general relativity, and where the constraints are imposed weakly. Its state space is a natural subspace of the SO(4) spin-network space and matches the SO(3) hamiltonian spin network space. The model provides a long sought SO(4)-covariant vertex amplitude for loop quantum gravity.

    Andrew Randono: A New Perspective on Covariant Canonical Gravity

    By avoiding the Legendre transform we show that the canonical constraint algebra can be computed without primary constraints, and without gauge fixing. The diffeomorphism and Lorentz algebras are realized exactly, and the Hamiltonian constraint gains extra degrees of freedom. The full constraint algebra is a deformation of the four dimensional de Sitter algebra together with three dimensional diffeomorphisms. We discuss the implications of this formalism for the quantum theory, and in particular for the existence of the Kodama state.
    ...
    ...
    Parampreet Singh: Exactly Solvable LQC: New Insights on some old questions

    Under very mild assumptions we are led to an exactly solvable model in LQC that can be rigorously compared to the Wheeler-DeWitt theory. We show that LQC and Wheeler-DeWitt can be constructed from the solutions of a 1+1 dimensional Klein-Gordon theory. Like any exactly solvable model the non-triviality lies in the mapping between the simple common description and the physical theories. Working with a complete set of solutions we provide an explicit formula for the evolution of the volume operator -- the only non-trivial Dirac observable in the theory and show that:
    (1) Quantum Bounce is not restricted to semi-classical states in the simple models studied so far,
    (2) There is a precise sense in which Wheeler-DeWitt theory is not a continuum limit of LQC and
    (3) LQC is a fundamentally discrete theory.
    ...

    Johannes Tambornino: Taming observables in GR: A perturbative approach
    Separating true physical degrees of freedom from pure gauge degrees of freedom and therefore calculating physical observables is a complicated issue in theories with a highly non-trivial constraint-algebra such as General Relativity. In this talk I will present a recent perturbative approach which can be used to construct gauge-invariant observables around symmetry-reduced sectors of GR to arbitrary high order. Using this approach one can embed field theory on a fixed background into a fully general covariant setting and can take gravitational corrections into account. Further on one can apply this perturbation scheme to a cosmological setting and shed new light on the question of backreaction effects of inhomogeneities onto the dynamical evolution of homogeneous and isotropic variables.
    ...
    Daniel Terno: Entropic laws with bulk and boundary states
    ...
    Kevin Vandersloot: Dynamics of Schwarzschild interior with improved loop quantum cosmology

    We discuss dynamics of the Schwarzschild interior using an effective semi-classical description of the loop quantization. We will consider the effects of an improved loop quantization using techniques from loop quantum cosmology.

    Yidun Wan: Propagation and interactions of emergent braid states in quantum gravity

    The study of particle-like excitations of quantum gravitational fields in loop quantum gravity is extended to the case of four valent graphs and the corresponding natural evolution moves based on the dual Pachner moves. This makes the results applicable to spin foam models. We find that some braids propagate on the networks and they can interact with each other, by joining and splitting. The chirality of the braid states determines the motion and the interactions, in that left handed states only propagate to the left, and vise versa.

    Joshua Willis: Recent Work on Computing Lorentzian Spin Foams

    I will review recent work on computing Lorentzian 10J symbols and related functions.
    ===end of sample===
     
    Last edited: Jun 15, 2007
  25. Jun 18, 2007 #24

    john baez

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    Garrett Lisi's talk

    I think people are going to have fun listening to http://deferentialgeometry.org/inde... Yang-Mills theory]] Discussion [[Loops 07]]" at Loops 07. It's about MacDowell-Mansouri gravity and the Standard Model. The really fun part is how the exceptional Lie group E8 enters the game,and how triality gets related to the 3 generations of Standard Model fermions. I'm not sure this theory will work, but it's darn interesting. He came to UCR last week and explained some of it to me and my student John Huerta, who plans to work on grand unified theories.
     
    Last edited by a moderator: Apr 22, 2017
  26. Jun 19, 2007 #25

    john baez

    User Avatar
    Science Advisor
    Gold Member

    Garrett Lisi's talk

    I think people are going to have fun listening to http://deferentialgeometry.org/inde... Yang-Mills theory]] Discussion [[Loops 07]]" at Loops 07. It's about MacDowell-Mansouri gravity and the Standard Model. The really fun part is how the exceptional Lie group E8 enters the game,and how triality gets related to the 3 generations of Standard Model fermions. I'm not sure this theory will work, but it's darn interesting. He came to UCR last week and explained some of it to me and my student John Huerta, who plans to work on grand unified theories.
     
    Last edited by a moderator: Apr 22, 2017
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