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## the future of LQG

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.
 Quote by marcus 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?
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

 Recognitions: Science Advisor 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
 Recognitions: Gold Member Science Advisor 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.
 Recognitions: Gold Member Science Advisor In case anyone else might be interested in this way of looking at the future of LQG (thru the eyes of an expert organizing the Loop session of an international conference) here are some links: The Marcel Grossmann triennial conference being held this week in Stockholm (MG13). There are 1021 registered participants. http://www.icra.it/mg/mg13/parallel_sessions.htm Here is an over 5 hour session organized by Lewandowski (the first of his two sessions). Click on talk titles to see brief summaries of the talks. QG!A http://ntsrvg9-5.icra.it/mg13/FMPro?...&-Max=50&-Find Here are two roughly 4 and 1/2 hour sessions organized and chaired by Jorge Pullin (with Param Singh) QG4A http://ntsrvg9-5.icra.it/mg13/FMPro?...&-Max=50&-Find QG4B http://ntsrvg9-5.icra.it/mg13/FMPro?...&-Max=50&-Find The Stockholm MG13 conference covers a wide range, not just Quantum Gravity but also EXPERIMENTAL, NUMERICAL, OBSERVATIONAL, AND THEORETICAL General Relativity and extensions, Extreme Gravity, Astrophysics, Cosmology, Fields on curved, and including several kinds of alternate approaches and modifications of standard theory that researchers are currently trying. BTW there is also a 4 and 1/2 hour parallel session explicitly devoted to String papers! Chaired by Henningson--here's a link: http://ntsrvg9-5.icra.it/mg13/FMPro?...&-Max=50&-Find The next big international meeting focused on these topics will be the General Relativity and Gravitation triennial conference (GR 20) which will be held in Warsaw July 8 - 13, 2013. Lewandowski will be the lead organizer. He is also handling the Loop lectures at the Erlangen QG School this October. So we will get several opportunities to see how he views the future of LQG.
 well, a bit of optimism is not bad ...Calculating these constants for the EPRL/FK vertex amplitude appears to be a diﬃcult problem, but the solution must exist... http://arxiv.org/pdf/1101.3294v4.pdf

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 Quote by audioloop well, a bit of optimism is not bad ...Calculating these constants for the EPRL/FK vertex amplitude appears to be a diﬃcult problem, but the solution must exist... http://arxiv.org/pdf/1101.3294v4.pdf
And Jerzy Lewandowski seems to confirm your interest in the work of Aleksandar Mikovic.
Mikovic is giving a 20 minute talk in Lewandowski's second session (Thursday 5 July) at the MG13 conference:
http://ntsrvg9-5.icra.it/mg13/FMPro?...&-Max=50&-Find

Jerzy's two sessions are both titled "Loop Quantum Gravity, Quantum Geometry, Spin Foams". I posted the link to the first one in my previous post. You might be interested to see what he has chosen for the lineup of talks. Click on the talk titles to get an abstract summary of any that are of interest.

BTW I think the business of simply calculating the vertex amplitudes (with the existing vertex formulas) has been taken care of. There are still outstanding questions to work on, though, having to do with the "asymptotics" of the vertex amplitudes---their limiting behavior for large quantum numbers j---their large scale limit behavior. It's one of many topics newcomers to the field can choose from, to work on.

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Please let me know where you think that ...
 Quote by marcus Some look real to me---major going concerns.
... I am right ...

...and where you think that ...
 Quote by marcus Others seem based on your own ingrained preconceptions ..., others just distractions, ...
... I am wrong

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 Quote by tom.stoer 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
How do you see the holographic principle? I too think it must factor in, but the way it occurs in string theory, it seems also to be tied up with unification, whereas canonical LQG seems anti-unification. Because of string theory, I tend to think that maybe the holographic principle is more fundamental, and so I tend to think canonical LQG will not work out. Do you think there's a way for holography and canonical LQG to work together?

 Recognitions: Gold Member Science Advisor Hi Tom, all I want to do is characterize our different viewpoints at the level of generality. I don't think you consider me an expert so my detailed comment wouldn't be useful to you. I see the suggested directions you offer as a "mixed bag". As I said: "...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== That could apply to Sundance approach or Asselmeyer approach. Not much going on with them now, just a few people--but either could "suddenly jump up in importance." I don't think we can know the future and I DON'T MAKE BETS. Basically I just watch the professional research community. Where my perspective differs markedly from yours, and where I possibly might benefit you by giving an alternate point of view, is IF AND WHEN I notice a trend that you might have overlooked. I've noticed that Thiemann and the Erlangen group seem to be getting away from strict Dirac quantization. And I think that fits with Hartle "Decoherent Histories" reformulation of standard QM. Which I think has a lot going for it. Fundamentally more valid than the split version of QM that prevailed in Dirac's day. So I think your use of language above is questionable where you say "their equivalence" and "H rigorously constructed". This is just IMHO but I suspect you may eventually have to give up on the strict Dirac canonical, just as I already see happening in the Erlangen group. If Hartle DH is right, it's not valid in a fundamental sense, just "effective" or "emergent" in certain regimes. To recapitulate: ==quote== 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. ==endquote==

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 Quote by marcus So I think your use of language above is questionable where you say "their equivalence" and "H rigorously constructed". This is just IMHO but I suspect you may eventually have to give up on the strict Dirac canonical, just as I already see happening in the Erlangen group. If Hartle DH is right, it's not valid in a fundamental sense, just "effective" or "emergent" in certain regimes.
It is still strict canonical - just not Dirac quantization of a gauge theory via constraints. And it is not new.

http://arxiv.org/abs/0711.0119: "There are two major approaches to the canonical quantisation of such theories. ... The advantage of the Dirac apporoach is that the unreduced phase space of non observables is typically a smooth (Banach) manifold so that the algebra of non – observables is sufficiently simple and representations thereof are easy to construct. Its disadvantage is that one has to deal with spurious degrees of freedom which is the possible source of ambiguities and anomalies in the gauge symmetry algebra. The advantage of the reduced phase space approach is that one never has to care about kinematical Hilbert space representations. However, its disadvantage is that the reduced phase space typically no longer is a smooth manifold turning the induced algebra of observables so difficult that representations thereof are hard to find."

 Hello tom - you make very interesting points. But can I just mention one thing, to do with your comment on the area operator and it's spectrum not corresponding to an observable ...(http://arxiv.org/pdf/gr-qc/9806079.pdf): "The operator $A (\Sigma)$ is invariant under SU(2) gauge transformations, but not under three or four dimensional diffeomorphisms. Therefore, strictly speaking it is not an observable of the theory, and we cannot directly give its spectrum physical meaning. The failure of $A (\Sigma)$ to be diff-invariant is a consequence of the fact that the area of an abstract surface defined in terms of coordinates is not a diff invariant concept. In fact, physical measurable areas in general relativity correspond to surfaces defined by physical degrees of freedom, for instance matter (the area of the surface this table) or the gravitational field itself (the area of an event horizon). However, it is reasonable to expect that the fully gauge invariant operator corresponding to a physically defined area (say defined by matter) has precisely the same mathematical form as the non gauge invariant operator studied here. The reason is that one can use matter degrees of freedom to gauge-fix the diffeomorphisms – so that a non-diff-invariant quantity in pure gravity corresponds to a diff-invariant quantity in a gravity+matter theory. This expectation has been confirmed explicitly in a number of cases [see refs]" ...I dont know how many people on the forum are from the UK, but there is this BBC pop science programme called "Horizon". There is this physicist called Brian Cox and he has presented a couple of these programmes (Bizarrely he was also a member of the pop band called D'ream who had a hit with the song "things can only get better" which may have relevance here). Horizon did a programme called "how long is a piece of string" in which they ended saying you cant measure the length to aribrary accurancy b/c the photons required would induce a black hole. LQG gets around this cus the backreaction of matter on the grav field is taken into account - I kinda understand this. Anyway I wrote an email to Rovelli (c.c. Brian Cox) saying this might be a good strting point for another programme introducing LQG to the general public. Rovelli wrote back saying he would be very interested in this. Brian Cox just ingored me. I dont like Brian Cox - and other physicists have agreed with me ;).
 Can I mention that researchers in LQG have themselves stated that they dont think LQG is the final answer...it's all part of the fun.
 Recognitions: Gold Member Science Advisor Tom, your post #28 provides a nice summary of topics to consider in discussing "the future of LQG". It's something worth thinking about because LQG has moved much more into the limelight recently. I recently looked back at the program of the 2003 MG 10 conference and compared the relative attention paid to String and Loop then versus now. When you compare that program to the one for 2012 MG 13 you see something like a 4-to-one ratio in both cases only it has flipped around. So there is a lot more research interest focused on Loop now, a lot of people want the program to succeed and want to see what it offers for cosmology (and for extreme gravity as well, assume). There is quite a difference in our perspectives on "the future of LQG" and I realized just now that much of the difference simply has to do with timescale. By my standards you are looking FAR OUT into the longterm future---all sorts of things could develop say on a 10year timespan. I am focusing primarily on near term. I would like to be able to envisage the research emphasis at the Loops 2013 conference at Perimeter Institute next year. And I would like to envisage what some of the parallel sessions will be like at the Warsaw GR 20 conference July 8-12, 2013, just one year from now. Naturally I'd like to be able to anticipate developments on beyond that, but first I want to be able to check my perceptions of what the trends and directions are---soon---within a twelve-month. So I can see if I'm wrong and need to correct my perceptions. So my picture of "the Loop future" differs from yours (largely I think because of the timescale and the desire to be able to check by watching how the research community behaves). I want to try to sketch what I see happening in the next post or two. A lot of it has to do with the fact that almost nothing has happened with "Master Constraint" or with any other kind of Dirac (constraint) quantization for such a long time. And the fact that I see the Erlangen people getting into Spinfoam and Dust-Hamiltonian, or what Thiemann calls "physical" Hamiltonian. Sometimes a paper will deal with both, it doesn't seem especially hard to bridge across there. Comparing 2003 with 2012: MG10 http://www.cbpf.br/mg10/WelcomeNew.html MG13 http://www.icra.it/mg/mg13/parallel_sessions.htm
 w.r.t Dirac observables...there are certain quanities that can be measured (partial observables) but which are not Dirac observables. What GR predicts is the relationship (complete observables) between these measurable quantities - http://arxiv.org/pdf/gr-qc/0110035.pdf.

Hello atty

 Quote by atyy How do you see the holographic principle? I too think it must factor in, but the way it occurs in string theory, it seems also to be tied up with unification, whereas canonical LQG seems anti-unification. Because of string theory, I tend to think that maybe the holographic principle is more fundamental, and so I tend to think canonical LQG will not work out. Do you think there's a way for holography and canonical LQG to work together?
I'm also interested in the holographic principle but how does it arise in string theory? I just read the other day in a review by Ashtekar that in the AdS/CFT conjecture the curled up extra dimensions are n-spheres with a radius the same order of the cosmological length!