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The future of LQG

  1. Jun 30, 2012 #1

    julian

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    This is what an oldey in LQG thinks: http://arxiv.org/pdf/1201.4598.pdfs [Broken] - page 27.

    It's great to see such confidence expressed in something that has for so long been put to the side because of the popularity in string theory.

    What most interests you and why? Do you have a different opinion to Ashtekar?
     
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  3. Jun 30, 2012 #2

    marcus

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    http://arxiv.org/abs/1201.4598
    http://arxiv.org/pdf/1201.4598.pdf

    I think those are wise words. Loop is currently moving into the limelight and getting a larger share of researchers' attention (probably for good reason).
    The blue text is what you quoted in your original post. Amen to that, I say! Good advice for the young people. He's an elder statesman in the gravity and quantum geometry research community, and was giving the opening talk at last year's Zakopane school, primarily for young researchers just getting into LQG.
     
    Last edited: Jun 30, 2012
  4. Jun 30, 2012 #3

    marcus

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    If I put together Ashtekar's words and what you said in your post what I get is 3 main points:

    1. LQG now carries sufficient weight for us to "take the basic ideas seriously and continue to develop them by attacking the hard conceptual and technical open issues."

    2. The list of these conceptual/technical issues "is long enough to keep young researchers busy and happy for quite a while!"

    3. As you originally asked, but I would put in the plural: What do you think are the most important directions?
     
  5. Jun 30, 2012 #4

    julian

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    Having a background in condensed matter so I'm especially interested in deriving an effective field theory and whether it would contain emergent degrees of freedom that could correspond to matter coupled to classical GR. I know that Smolin et al have been working on a preon type model based on q-deformed LQG type theory in which micro causality is made explicit...I have been wondering whether unification could arise in general LQG.

    Plus I like how Smolin gives an explanation for dark energy in terms a small amount on non-local contaminate to semi-classical states. One question I have is how to incorporate this into the effective dynamics of LQC or extensions of.

    Off course there is the huge hurdle of proving your model has the correct semiclassical limit. This not only involves a verification of the dynamical laws in their quasi differential form but also the construction of a suitable complete set of observables that have small quantum fluctionas with respect to specific semi-classical states. This latter problem - according to Thiemann - is on a same footing as proving confinement in QCD from first principles - now there is something for youngsters to work on!!.
     
    Last edited: Jun 30, 2012
  6. Jun 30, 2012 #5

    julian

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    Plural. What are the most important questions is better.
     
  7. Jun 30, 2012 #6

    julian

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    Something else I'm interested in is Rovelli's attempts to formulate statistical thermal physics in a timeless context.

    Relating to this, how to understand our usual experience of time and it's flow, comming from not having a complete knowledge of the system.
     
    Last edited: Jun 30, 2012
  8. Jun 30, 2012 #7

    marcus

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    Julian, i noticed that on the "Our picks" MIP poll both you and Nonlinearity voted for this paper of Gielen and Wise:


    http://arxiv.org/abs/1206.0658
    Linking Covariant and Canonical General Relativity via Local Observers
    Steffen Gielen, Derek K. Wise
    (Submitted on 4 Jun 2012)
    Hamiltonian gravity, relying on arbitrary choices of "space," can obscure spacetime symmetries. We present an alternative, manifestly spacetime covariant formulation that nonetheless distinguishes between "spatial" and "temporal" variables. The key is viewing dynamical fields from the perspective of a field of observers -- a unit timelike vector field that also transforms under local Lorentz transformations. On one hand, all fields are spacetime fields, covariant under spacetime symmeties. On the other, when the observer field is normal to a spatial foliation, the fields automatically fall into Hamiltonian form, recovering the Ashtekar formulation. We argue this provides a bridge between Ashtekar variables and covariant phase space methods. We also outline a framework where the 'space of observers' is fundamental, and spacetime geometry itself may be observer-dependent.
    8 pages

    I didn't vote for it but I think it's a very interesting idea.

    I think you voted for two or three other things, out of the 20, and I can see a degree of correspondence between what you suggest here are good directions for research to go in and your choices in the poll.
     
  9. Jul 1, 2012 #8

    julian

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    I am quite interested in covariance formulation and spin foams. Espicially those derived using the Master constraint. I've writen up some of the calculations.
     
    Last edited: Jul 1, 2012
  10. Jul 1, 2012 #9

    atyy

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    "But it would be a mistake if a significant fraction of the community focuses on constructing new models every few months, making a first stab and then passing on to the next model."

    Yet this is what the mainstream seems to be. EPRL seems already dead.

    "Thus there is ample evidence that the subject is now sufficiently mature to have applications to other areas. In these explorations, it is important to focus on problems that other communities consider as important in their areas. In my view, this ‘outward bound’ spirit is the second pillar on which further development of the field will rest."

    I believe this is a better line of thought - that pure LQG will fail - but the mathematics of LQG will be relevant to string theory.
     
    Last edited: Jul 1, 2012
  11. Jul 1, 2012 #10

    marcus

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    EPRL I would say started one of the most vital lines of development. It seems totally wacky to call it "dead" :biggrin:
    Two of the papers on the MIP poll (one by Engle one by Rovelli) have directly to do with that line of development.
    Of course it is steadily evolving, that is what a live theory does. When people discuss it they need to address the current version, which last year was characterized by the Zakopane Lectures (which this paper by Ashtekar was in effect introducing).
    Next year it will almost certainly have been slightly modified*. It's an evolutionary process.

    *See the papers by Engle and by Rovelli for ideas of how that might go.
     
    Last edited: Jul 1, 2012
  12. Jul 1, 2012 #11

    atyy

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    It's those two papers that I think make EPRL dead. It's fair to consider them lines of development, but at the same time they seem to be proposals for new models, because the old model was unsatisfactory. The new models appear unsatisfactory too, so they may be pointing towards a profusion of new models that Ashtekar was hoping against.
     
  13. Jul 1, 2012 #12

    julian

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    I think it was Ashtekar who stated that the Master constraint is the most promising way to derive the corect spinfoam formulism. So even if EPRL were wrong the Master constraint might still provide correction away from it. I dont know?
     
  14. Jul 1, 2012 #13

    atyy

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    I'm not sure about the Master constraint, but one thing that is nice about the new Rovelli and Wilson-Ewing paper is that the new models are due to trying to make contact with the canonical formalism, eg. where they note "The corresponding conjugate momentum is the Ashtekar electric field ... but (confusingly) one finds two different expressions for this field in the literature [7, 8] ... The two expressions differ by the sign s and can be derived from S′ and S′′, respectively."

    Another paper that goes in the direction of profusion of models to the point where I think maybe they are really stuck is the Giesel and Thiemann paper where even basics like the physical and kinematical Hilbert spaces being different are up in the air!

    BTW, since you have a condensed matter background, have you seen http://arxiv.org/abs/0907.2994 and http://arxiv.org/abs/1106.1082 which note links between LQG, condensed matter and string theory?
     
    Last edited: Jul 1, 2012
  15. Jul 1, 2012 #14

    tom.stoer

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    EPRL is not dead, but wrong. That means it sufferes from wrong constraint structure, phase space and therefore wrong quantization. That does not mean that SFs are dead. One has to find a consistent coinstraint algebra and its quantization.

    Canonical LQG is not dead, either, but it is still inknown how to quantize the Hamiltonian constraint. There is a related issue, namely the step-by-step implementation of the constraints: Gauss - Diff - Hamiltonian; maybe it's this stepwise approach which is wrong. If one can fix Diff + H at once this may be a way out, but if it's even the very first step i.e. Gauss (which results in the kinematical Hilbert space) then we are really in trouble.
     
  16. Jul 1, 2012 #15

    marcus

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    In line with what you are saying I would also extend the idea to General Rel and Quantum Mechanics. GR and QM are not dead either, but simply wrong. They have their obvious problems and one has to find improved formulation. That is how physics goes. :biggrin:
     
  17. Jul 1, 2012 #16

    atyy

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    But they make correct predictions. LQG must make a correct prediction beyond GR as an effective QFT. At this stage, I would certainly say that is not even the goal. The more limited goal is to be a UV completion for GR. There may be more than one possible completion, and experiment would have to decide between them. At this stage, LQG is not even a candidate completion, and on grounds decided by the LQG programme itself.
     
  18. Jul 1, 2012 #17

    tom.stoer

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    No!

    :devil:

    Think about QM with wrong commutators, e.g. by using non-cartesian coordinates and not taking into account the Jacobians; or think about the PI of QCD with ∂αAα=0 and 'neglecting' the Fadeev-Popov ghosts; that's the type of error they currently make in SF models ... The problem is not that they make unreasonable physical assumptions but that they do not solve / impose all constraints correctly. In QCD w/o Fadeev-Popov ghosts you get wrong amplitudes w/o.
     
    Last edited: Jul 1, 2012
  19. Jul 1, 2012 #18

    marcus

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    I want to recall the initial ideas that Julian started us off with in the first post. This is how I boiled them down---into 3 main points.
    I don't think we need to waste time venting our personal attitudes---good-mouthing, bad-mouthing, cherrypicking and interpreting Ashtekar etc.
    The thing is HOW DO YOU SEE THE FUTURE of the Loop program?

    I don't think any of us can accurately envision the future of an active research program but I will tell you my guesses.

    Right now I'm looking thru Hartle-QM glasses (explain that later) and I see Thiemann and the Erlangen group all going in the direction of DUST. That is what his "matter reference system" means and what Gielen Wise "field of observers" means and it makes sense from a Hartle-QM perspective.

    Hartle and friends propose a reformulation of Quantum theory we can call "Histories" QM which basically says that the machinery of Dirac quantization does not exist--it is merely emergent at low energies, a convenient workable approximation to reality over a limited range. The spacelike 3D manifold does not exist in reality. To formulate QM, you need three things:
    A. Histories
    B. Partitions of histories (grouping, classifying, "coarsegraining" them)
    C. a Decoherence functional that tells you when a given partition is bettable.

    A given partition is bettable when you can assign fair odds (approximate conventional probabilities) to it, make predictions, settle bets, in other words make honest book on it.
    The Decoherence functional tells you when a partition of the histories is sufficiently uncorrelated that the probabilities will be additive---interference is small enough to be considered negligible.

    Hartle Histories QM is, I believe gaining acceptance. So it makes sense to me, in that light, that the Erlangen group should be moving away from a strict Dirac quantization and in the direction of DUST.

    None of this has to do with "right" or "wrong". It has to do with Sociology. That is, watching the glacier-slow shifts of the research community, which is basically all we can know. IMHO it is naive to pretend that we can declare what is "right" or "wrong" (except to admit that all living human theories are wrong and subject to revision). All we can do is watch the community and see where their blind instinct leads them. It is an awesome and wonderful process, but it does not obey set rules :biggrin:
     
    Last edited: Jul 1, 2012
  20. Jul 1, 2012 #19

    tom.stoer

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    I see different directions:
    - technical: reformulation in terms of spinors, twistors, group field theory, ...
    - technical: fixing the issues with quantization (both canonical and PI/SF) + their equivalence
    - dynamics! (once the SF and H are rigorously constructed)
    - boundary Hilbert spaces and holographic principle (get rid of the bulk)
    - coarse-graining / Kadanoff's renormalization group approach / certain other related limits
    - matter d.o.f., unification, possibly the Sundance Bilson-Thompson approach
    - exotic smoothness, PL manifolds, 'non-diffeomorphic defect-structure', ... relation to Asselmeyer's work
     
  21. Jul 1, 2012 #20

    marcus

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    Thanks for telling us about all these directions you see, Tom. Some look real to me---major going concerns. Others seem based on your own ingrained preconceptions of what "ought" to happen, others just distractions, or where almost no work is being done. But who knows? Some of them might suddenly jump up in importance.
    ==quote==
    I see different directions:
    - technical: reformulation in terms of spinors, twistors, group field theory, ...
    - technical: fixing the issues with quantization (both canonical and PI/SF) + their equivalence
    - dynamics! (once the SF and H are rigorously constructed)
    - boundary Hilbert spaces and holographic principle (get rid of the bulk)
    - coarse-graining / Kadanoff's renormalization group approach / certain other related limits
    - matter d.o.f., unification, possibly the Sundance Bilson-Thompson approach
    - exotic smoothness, PL manifolds, 'non-diffeomorphic defect-structure', ... relation to Asselmeyer's work
    ==endquote==

    Basically my perception is guided by people "in the business" like Lewandowski and Pullin and I'd say to anybody look at the lineup of Loop talks at Stockholm this week.
    The MG13 organizers gave Lewandowski nearly 10 hours of parallel session, and Pullin over 9 hours.
    A total of 19 hours designated for Loop gravity. These guys know the field, what's active, what their colleagues are interested in hearing about.

    It's certainly not a perfect indicator, but it can give one something outside oneself to balance one's subjective favorites and preconceptions.
     
    Last edited: Jul 1, 2012
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