Reformulation of Loop gravity in progress, comment?

[marcus]

No, spin foams are not "new" in the Jacobson sense...

By your personal interpretation of what "Jacobson sense" means.
You seem to want to control the meanings of words like "Rovellian" and perhaps you will be talking about the true meaning of "Jacobsonian".

Tom often objects that one DOESN'T actually get spinfoam dynamics by a canonical Dirac quantization of GR equation and the relation between the approaches isn't clear.

And on the other hand you now seem to be complaining that one actually DOES get spinfoam dynamics by some kind of (rigorous conventional I suppose) quantization and therefore the spinfoam degrees of freedom are not "new in the true Jacobsonian sense". Or some such thing.

All this breathless quibbling about who said what when in which refined "sense". Why not just relax and see what a few exceptionally creative lucky people make of it?

 

[marcus]

...

Since we just turned a page, I'll copy post #19 as a reminder of what the reformulation topic-of-the-thread is about. It's interesting that things are in flux because we are now effectively in the runup to Loops 2013 which will be held at Perimeter Institute in about one year's time.

====quote post #19==
The reformulation of Loop now being explored is complex, and some parts seem still tentative.
I see three main initiatives:

A. Immirzi-less BH entropy.
Bianchi and others find S = A/4. The coefficient of area no longer depends on Immirzi parameter γ. So gamma is unclamped. arxiv:1204.5122 arxiv:1205.5325

B. un-Diracly quantizing GR.
Jacobson proposed a new goal. Find the correct quantum "molecules" of spacetime geometry for which Einstein's GR equation is the thermodynamic equation of state.
It could turn out that the Spinfoam description of geometric evolution already provides the correct degrees of freedom, and GR is simply the equation of state of spinfoam.
So that instead of quantizing GR Diracly, one has quantized it un-Diracly.
arxiv:1204.6349 arxiv:1205.5529

C. The sign of the tetrad--could one detect a region of "antispacetime"?
One possible crude picture of spacetime geometry is that of a partially coherent swarm of tetrads. Like flocking birds or shoals of fish, these tetrads tend to be oriented coherently with their neighbors. But in principle, divisions might occur: there could appear patches with opposite orientation. The set-up described in the May paper "Discrete Symmetries in Covariant LQG" arxiv:1205.0733 allows for this to happen. The usual Holst action is modified in a significant way---by introducing the sign of the tetrad, a symbol s which can be +1, 0, or -1 depending on the sign of the determinant of the tetrad.
Since fermions couple to the tetrad, phase can evolve in either of two senses and a double slit experiment can in principle detect reversed geometry by a shift of the interference pattern.
==endquote==

 

[atyy]

By your personal interpretation of what "Jacobson sense" means.
You seem to want to control the meanings of words like "Rovellian" and perhaps you will be talking about the true meaning of "Jacobsonian".

Tom often objects that one DOESN'T actually get spinfoam dynamics by a canonical Dirac quantization of GR equation and the relation between the approaches isn't clear.

And on the other hand you now seem to be complaining that one actually DOES get spinfoam dynamics by some kind of (rigorous conventional I suppose) quantization and therefore the spinfoam degrees of freedom are not "new in the true Jacobsonian sense". Or some such thing.

All this breathless quibbling about who said what when in which refined "sense". Why not just relax and see what a few exceptionally creative lucky people make of it?

Rovelli's programme has not yet been shown to succeed. My point, and Tom's, I think, is that if it succeeds, then the covariant quantization will be equivalent to a canonical quantization.

 

[marcus]

Rovelli's programme has not yet been shown to succeed. My point, and Tom's, I think, is that if it succeeds, then the covariant quantization will be equivalent to a canonical quantization.

That sounds reasonable, there is a Loop research community and a Loop program which involves a number of DIFFERENT approaches and versions. Bianchi has laid out several different ones. Etera Livine has some great ideas. Engle has too. Rovelli's view is obviously in flux. Lewandowski and Ashtekar are clearly major players in the program. That's only the beginning of a list

So there is a Loop program. That is something real and it may or may not succeed. And if it succeeds it MAY OR MAY NOT contain a background independent QFT that was derived by some preconceived "quantization" method which you have in mind. So that is all real enough and makes sense.

But sometimes you sound as if you actually believe there is a definite permanently fixed "Rovellian" approach to Loop QG. And you go on about how this conflicts with what Engle says or what Jacobson says etc etc. This sounds peculiar to me. When you talk about the "Rovellian" this or that as if you knew of some permanent definite approach it does not seem based on reality.

As far as I can see, Loop is rapidly evolving and advancing on several fronts and seems to change every two or three years. So far it has been up to Ashtekar and Rovelli to present a coherent in-a-manner-of-speaking "OFFICIAL" version every 2 or 3 years. In any given year they are the ones normally asked to supply the principle review paper and give the overview conference talk. With Ashtekar concentrating on the cosmology side.

However the lineup could change. Younger people could be invited to start filling these roles. And this year Jerzy Lewandowski is doing a great job reviewing organizing representing the program.
Also Jorge Pullin. A program has to have leaders and if there is rapid progress then every 2 or 3 years you need an official redefinition or reformulation. But who and how it's done can change. We will see how it shapes up at Loops 2013.

I don't known enough to even begin to give you a complete accurate portrait, of course.
But obviously back in 2011 a defining role was played by Rovelli's Zakopane lectures
arxiv 1102.3660 and May presentation at the Madrid Loops conference. Now we can expect something new and we can wonder what shape will it take this time? What will take the place of arxiv 1102.3660 when people gather for Loops 2013 at Perimeter. Who will give the main overview? What new work will stand out? It could be several peoples' work.

 

[atyy]

That sounds reasonable, there is a Loop research community and a Loop program which involves a number of DIFFERENT approaches and versions. Bianchi has laid out several different ones. Etera Livine has some great ideas. Engle has too. Rovelli's view is obviously in flux. Lewandowski and Ashtekar are clearly major players in the program. That's only the beginning of a list

So there is a Loop program. That is something real and it may or may not succeed. And if it succeeds it MAY OR MAY NOT contain a background independent QFT that was derived by some preconceived "quantization" method which you have in mind. So that is all real enough and makes sense.

But sometimes you sound as if you actually believe there is a definite permanently fixed "Rovellian" approach to Loop QG. And you go on about how this conflicts with what Engle says or what Jacobson says etc etc. This sounds peculiar to me. When you talk about the "Rovellian" this or that as if you knew of some permanent definite approach it does not seem based on reality.

Oh, I usually mean very specific statements of X are in conflict with very specific statements of Y. I never mean all statements of X are in conflict with all statements of Y, which would be absurd. In this case, it has to do with the possibility of canonically quantizing GR, even in the UV. Rovelli does seem to alternate between two views. Sometimes he does seem to indicate that one could have a successful spin foam quantization which does not meet up with the canonical formalism. But I think the he mostly approaches spin foams as a way to meet up with canonical quantization. You can trace this line of thinking quite consistently over a period of more than 10 years, including the latest paper about anti-spacetime:

http://arxiv.org/abs/gr-qc/9806121 (bottom of p1)
"Here, we complete the translation of canonical loop quantum gravity into covariant spacetime form initiated in [6]. The “quantum gravity Feynman graphs” are two-dimensional colored branched surfaces, and the theory takes the form of a “spin foam model” ..."

http://arxiv.org/abs/0708.1236 (abstract)
"... providing a solution to the problem of connecting the covariant SO(4) spinfoam formalism with the canonical SO(3) spin-network one. ..."

http://arxiv.org/abs/0711.0146 (abstract)
"These results establish a bridge between canonical loop quantum gravity and the spinfoam formalism in four dimensions."

http://arxiv.org/abs/1205.0733
p2: "In canonical loop gravity one works in the time gauge and chooses a linear combination of the connection and its Hodge dual as a canonical variable. The corresponding conjugate momentum is the Ashtekar electric field Eai, but (confusingly) one finds two different expressions for this field in the literature ... The two expressions differ by the sign s and can be derived from S' and S", respectively."

footnote 5: "... we know from canonical loop quantum gravity that links with j = 0 can be erased from the spin-network. ..."

 

[tom.stoer]

But I think the he mostly approaches spin foams as a way to meet up with canonical quantization.

I think this is what the objective of the whole community - find a mathematical consistent and physically reasonable quantization. They use different approaches - canonical, covariant canonical, spin foams, group field, ... - not b/c these different approaches are mutually exclusive but complementary views, just like in ordinary quantum mechanics.

 

[marcus]

I think this is what the objective of the whole community - find a mathematical consistent and physically reasonable quantization. They use different approaches - canonical, covariant canonical, spin foams, group field, ... - not b/c these different approaches are mutually exclusive but complementary views, just like in ordinary quantum mechanics.

Good point! Different approaches can indeed complement each other and help to deepen and fill out the understanding. I remember Eugenio Bianchi saying this same thing--he has developed/worked on several alternate formulations of Loop gravity--they can improve or supplement each other. I don't recall his exact words.

BTW this just came out today. It has to do with the topic I called "The Sign of the Tetrad" (the possibility of having regions of spacetime geometry where the phase of a fermion rotates in reverse).

http://arxiv.org/abs/1206.3903
How to detect an anti-spacetime
Marios Christodoulou, Aldo Riello, Carlo Rovelli
(Submitted on 18 Jun 2012)
Is it possible, in principle, to measure the sign of the Lapse? We show that fermion dynamics distinguishes spacetimes having the same metric but different tetrads, for instance a Lapse with opposite sign. This sign might be a physical quantity not captured by the metric. We discuss its possible role in quantum gravity.
6 pages, 8 figures. Article awarded with an "Honorable Mention" from the 2012 Gravity Foundation Award.

 

[marcus]

==quote post #52==
B. un-Diracly quantizing GR.
Jacobson proposed a new goal. Find the correct quantum "molecules" of spacetime geometry for which Einstein's GR equation is the thermodynamic equation of state.
It could turn out that the Spinfoam description of geometric evolution already provides the correct degrees of freedom, and GR is simply the equation of state of spinfoam.
So that instead of quantizing GR Diracly, one has quantized it un-Diracly.
arxiv:1204.6349 arxiv:1205.5529
==endquote==

Regarding this general theme I should mention recent work by Thomas Thiemann and the group at Erlangen. Abstracts of several paper are given here
https://www.physicsforums.com/showthread.php?p=3964712#post3964712
with some comment. They seem to be exploring paths to a kinda-sorta Hamiltonian-style quantization without being constrained to a strictly Dirac format. If someone has a different interpretation of what's happening in those 3 new papers, please share it. I'd be interested to know how you see it. I like Derek Wise and Steffen Gielen's paper that uses the concept of a field of observers (straight out of standard cosmology).

In this same connection we should also look at a paper by a German PhD student David Schroeren, now at Marseille. He makes what seems to me creative and effective use of some ideas of Gell-Mann, Hartle, and others. See Hartle's 1993 Les Houches account http://arxiv.org/abs/gr-qc/9304006 .
As described there by Hartle an important motivation was to restructure Quantum Mechanics so that it would be more suitable for Cosmology (where there is no separate Observer, since the System is the whole universe.) Obviously Quantum Theory must be reformulated if it is going to be applied to the whole universe, and when reformulated it might in fact be GENERALLY BETTER and turn out to be useful for other applications besides Cosmology.

So we get proposals with names like "decoherent histories" QM and "consistent histories" QM with some slightly different formalism. Now Schroeren has tried applying these heretical ideas about Quantumtheory to Spinfoams.
It leads to a different kind of quantization of General Relativity, so I list this paper too.
http://arxiv.org/abs/1206.4553
Decoherent Histories of Spin Networks
David P.B. Schroeren
(Submitted on 20 Jun 2012)
The decoherent histories formalism, developed by Griffiths, Gell-Mann, and Hartle is a general framework in which to formulate a timeless, 'generalised' quantum theory and extract predictions from it. Recent advances in spin foam models allow for loop gravity to be cast in this framework. In this paper, I propose a decoherence functional for loop gravity and interpret existing results as showing that coarse grained histories follow quasiclassical trajectories in the appropriate limit.
13 pages

 

[marcus]

Decoherent Histories (DH) quantum mechanics looks interesting. I think the most active proponent is James Hartle (UC Santa Barbara)

Other authors are Murray Gell-Mann and Robert Griffiths, but I think of it primarily as "Hartle-QM"

It is a definition of QM that depends less heavily on the Observer making Measurements with a classical instrument. There is no essential split of the universe into a quantum system and a classical observer.

It is a "path integral" or Histories approach. The basic mathematical objects are PARTITIONS of all possible histories.

A partition is a collection of disjoint subsets whose union is the whole. Generally a partition involves many subsets, but a simple example could be a partition into just two:
"the ball went into the hole" versus "the ball did not go into the hole"

Partitions of all possible histories can represent things that we might care about, which matter to us, or which we might want to risk betting on, like whether the flight will land safely in Seattle or a certain flip will flop or a bridge not break. We may want to know which set of histories the world is in whether or not we are classical creatures and whether or not we are making measurements at the moment.

"the moon is there" versus "the moon is not there" has an approximate welldefined probability even when no one is looking. The set of histories in which it is there has high probability.

So Hartle-QM frees quantum mechanics from a kind of ontological dependence. One can invoke approximate probabilities of the subsets in a partition when the partition is sufficiently decoherent
(almost by definition) and a key part of Hartle-QM is formalizing when partitions are sufficiently unambiguous in this sense.

I'd like to see Hartle-QM applied to Spinfoam QG. I'll be interested to see the outcome.
I'll bring over some links.
Hartle Gell-Mann 2011 paper: http://arxiv.org/abs/1106.0767
Hartle 2008: http://arxiv.org/abs/0801.0688 (appendix A especially helpful)
Hartle 2006: http://arxiv.org/abs/gr-qc/0602013 (generalizing QM for quantum spacetime)

 

[marcus]

In just a week from tomorrow, on Tuesday 10 July, Fay Dowker is going to talk about something which I think is important to the development of Loop gravity. It will go into the PIRSA online video archive. I for one am certainly going to watch the talk.

PIRSA:12070001
Title: The Path Integral Interpretation of Quantum Mechanics
Speaker(s): Fay Dowker - Imperial College
Abstract: In 1932 Dirac wrote that the lagrangian approach to classical mechanics was probably more fundamental than the hamiltonian approach because the former is relativistically invariant whereas the latter is "essentially nonrelativistic". In quantum theory the hamiltonian approach leads to canonincal quantisation, Hilbert space, operators and the textbook rules for state vector "collapse", which are all indeed more or less divorced from the spacetime nature of the physical world as revealed by relativity. The "essentially relativistic" lagrangian approach on the other hand leads to the path integral, as shown by Dirac in 1932 and developed by Feynman. I will show how the interpretation of quantum mechanics in a path integral framework is based directly on events in spacetime and show that it leads to a second "fork in the road" depending on whether it is necessary for probabilities to play a fundamental role in the theory.
Date: 10/07/2012 - 3:30 pm
Series: Quantum Foundations
Location: Time Rm
URL: http://pirsa.org/12070001/
================

Basically I think this goes back to Jim Hartle's talk to the 2005 Solvay Conference (on the "Quantum Structure of Space and Time"). The talk was written up and posted in early 2006. I'll get the abstract:
http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
(Submitted on 2 Feb 2006)
Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has changed 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 paper reviews a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. This generalization is constructed within the framework of generalized quantum theory. This is a minimal set of principles for quantum theory abstracted from the modern quantum mechanics of closed systems, most generally the universe. 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 allow for the further generalization that would be necessary were spacetime not fundamental...
31 pages. 4 figures.

To paraphrase, states and evolution of fields defined on spacelike surfaces are ONLY appropriate as math idealizations when geometry behaves APPROXIMATELY CLASSICALLY. In more general situations such idealizations are NOT appropriate.
They are, as Dowker put it, "more or less divorced from the spacetime nature of the physical world".

 

[marcus]

Ultimately if you think of Loop gravity as based on a fixed set of discrete points or a smooth manifold "continuum" then you aren't likely to understand the line of future progress I'm talking about.

The Dirac canonical quantization applied to GR leads to a lot of paraphernalia which it is NOT appropriate to assume (Hartle suggests) unless spacetime geometry is behaving in approximately classical manner. It's a picture that only "emerges" under specific tame circumstances.
That goes for approaches using EMBEDDED spin networks as well. They need a manifold--i.e. extra baggage.

Conversely the spinfoam dynamics approach, by now familiar to everybody, does not involve extra baggage--in particular, no manifold. It is based on what Dowker's abstract seems to be talking about: events. Related combinatorially. No infinite sets, just a finite web of facts/predictions, depending entirely on the history!

So we'll see. We'll watch the video of Dowker's talk and see if it fits with and extends what Hartle had to say to the 2005 Solvay Conference.

Dowker may steer the talk in the Causal Sets direction but that's all right. Loop and Causal Sets share foundation roots--to some extent a common rationale. Eventually "Quantum Foundations" considerations are going to influence the development of Loop gravity--indeed they may already have influenced it to a considerable extent.
================================
It may help clarify the issues if I paste in a short summary of how I see Hartle's "Decoherent Histories" (DH) version of quantum mechanics. This was originally a post in the "Loop future" thread:


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.

Sets in a partition represent things you might like to know or to predict. 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.
...[That gets you to a nice effective halfway station. Since it's not fundamental, why not make life easy and assume some dust? Going further down that road brings you to Histories=Foams]

 

[tom.stoer]

The Dirac canonical quantization applied to GR ... That goes for approaches using EMBEDDED spin networks as well. They need a manifold--i.e. extra baggage.

Conversely the spinfoam dynamics approach, by now familiar to everybody, does not involve extra baggage--in particular, no manifold.

I think this is misleading.

I agree that spin networks are constructed from a manifold and that one get's rid of the manifold during quantization, constraint fixing an "projecting" to phys. d.o.f. = spin networks. Therefore in some sense spin networks have (or had) this extra baggage (historically).

But I do not agree that SFs do not have this extra baggage. They are constructed using the same ideas as spin networks; the only difference is that one switched from networks to foames rather late. There is no conceptual difference between spin networks and spins. It's a matter of taste whether you postulate a kinematical Hilbert space and a Hamiltonian or whether you postulate vertex amplitudes and PI measures.

Spin foams and spin networks share the same weakness; historically they are rooted in a picture using a manifold - and their derivation is by no means complete. Not deriving but postulating them has a different weak point, namely guessing ;-)

Nevertheless I agree that the main weak points could be that one is simply quantizing the wrong degrees of freedom (just like QFT applied to Navier-Stokes equations). These are essentially two weak points
1) wrong d.o.f.
2) quantization (which is never unique)

 

[marcus]

...
I agree that spin networks are constructed from a manifold...

But I do not agree that SFs do not have this extra baggage. They are constructed using the same ideas as spin networks; ...

Sorry, you misunderstood. In the modern treatment spin networks are NOT constructed from a manifold. It used to be the case that spin networks were EMBEDDED.
When I say "embedded spin network" I mean to older object.

When I simply say "spin network" it is a combinatorial object, as per the standard Loop source paper. It does not have the extra baggage.

As you say, SF are constructed using the same ideas. Therefore they do NOT have manifolds or other extra baggage.

It seems you understood completely opposite from what I intended.
I must try to write more clearly.

 

[tom.stoer]

marcus, it's not fair to say that non-embeded networks are not constructed from a manifold; yes, they are combinatorial objects, but nevertheless they share many features with the embedded one; they are not completely bagge-free, even uif this baggage may be deeply hidden.

The first baggage I see is SU(2) or SO(3); why not SU(7) ?

 

[marcus]

marcus, it's not fair to say that non-embeded networks are not constructed from a manifold; yes, they are combinatorial objects, but...

I am just talking about the facts. The standard LQG formulation is http://arxiv.org/abs/1102.3660 ("Zakopane lectures") and in that paper the theory is developed using non-embedded networks and foams. No manifold representing spacetime continuum.

We both recognize this.

HISTORICALLY much of this grew out of work with similar structures EMBEDDED in a manifold.

So let's make a clean break. We recognize that the theory is now defined with combinatorial objects that represent geometric information. Measurements, predictions, hypothetical measurements, events of one sort or another.

There is no continuum in the theory, all we have is relationships among geometric info.

Now you ask "What about the Lie groups? What about SU(2)?"

Well I'm no authority--I can only tell you how I personally understand it. The choice of Lie group, for me, says something about the kinds of measurements that are being made at various points in the network.

We are trying to DESCRIBE Nature and how she responds to geometric measurement and how her geometry evolves. We think manifolds are unrealistic so we throw them out. Now we have a web of measurements (areas volumes angles...). We pick the best Lie group that describes the symmetries of measurement as we experience them. We pick the group that works best.

That's just how I personally understand it. So then the graph Hilbert space automatically comes out to be the square integrable ("L₂") functions on a product of as many copies of the group G as there are links in the graph( G^L)
Some redundancy has to be factored out but basically that's the graph Hilbert space.

You're surely familiar with this--I'm sure you've read the Zako lectures paper.

I suppose using SU(2) is a way of noting that our world has 3D rotations. We don't go so far as to assign it a differential manifold structure, that would be adding a lot of extra. Unnecessary extra. But we do observe that a local observer can turn and tip things.
So we put in that detail about Nature---rotation.

We are painting a portrait, and SU(2) is the color of her eyes.

So the Hilbert space turns out to be L₂[SU(2)^L]

 

[tom.stoer]

SU(2) is one critical relict of 3+1 dim. spacetime; you can't explain why to use SU(2) w/o referring to 3+1 dim. spacetime.

It is not clear what happens if you start with SU(7) - as an example; it is not clear to which manifold this reduces in the semiclassical limit - or if there is convergence to a Riemann or Riemann-Cartan manifold at all - classically there is no Riemann-Cartan manifold with SU(7) structure group.

I agree that the algebraic structures of non-embedded spin networks do not contain any directly visible relict of the manifold, but besides the structure group there are others: In the canonical formulation there is the operator algebra G^a, V^a and H; at least H survives! in addition when using H there is the requirement for a global foliation like R³*T (with T being the time direction). In the SF framework there are the simplicity constraints which are understandable only when referring to a manifold structure from which the theory has been created, and of course there are vertex amplitudes which are related to some - unknown - hamiltonian H.

So yes, the relicts are deeply hidden, but they are present even for non-embedded spin networks.

 

[marcus]

SU(2) is one critical relict of 3+1 dim. spacetime; you can't explain why to use SU(2) w/o referring to 3+1 dim. spacetime.

It is not clear what happens if you start with SU(7) - as an example; it is not clear to which manifold this reduces in the semiclassical limit - or if there is convergence to a Riemann or Riemann-Cartan manifold at all - classically there is no Riemann-Cartan manifold with SU(7) structure group...

Nice comment! I think it would be interesting in a theoretical/mathematical sense for someone to explore what happens when you use some different Lie groups in the Loop setup.

I think there HAS to be some way of telling the theory about the dimensionality we live in and giving it SU(2) is a kind of minimal way.

Youi don't give it a whole differential manifold with all that extra machinery, you just tell it the rotational symmetry that belongs to our world.

For me that's very satisfying. It is a minimal way of telling the theory what dimensionality we live in. I don't expect the theory to tell me why there MUST be 3+1 dimensions to the world (although perhaps some day a theory WILL tell us that--it would be exciting, to be sure!)

==================

I've been thinking about the *embedded* issue and I wonder if we couldn't find a recent paper analogous to 1102.3660 that presents the embedded approach--so then we could have a DUAL standard. Would you like this? Then there need be no tension. At the beginning of my post I could say I am talking about purely combinatorial networks+foams as in 1102.3660 and at the beginning of your post you could say you are talking about embedded ones as in 11xx.yyyy. Maybe some recent paper by Lewandowski?

 

[marcus]

Talking about things that might figure in a reformulation of Loop gravity (showing up next year at GR20 and Loops 2013), one thing we seem to have completely overlooked is the new, hard, and potentially very important OSN line of development by Lewandowski's group.
http://arxiv.org/abs/1107.5185
This is the systematic way to do spinfoams without spinfoams. But you have to learn it like a new language. Jerzy is a mathematician's mathematician. Check it out.
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 2-cell 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

And then just recently there was the followup on this, which (of course) is included in the 2nd quarter MIP poll!

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

Note that Jerzy is bilingual---he can talk and think non-embedded Loop (purely combinatorial structures) and also the physical (non-Dirac, non-constraint) Hamiltonian approach with DUST--using (if he chooses) the older embedded structures in a manifold. Here is the talk he gave today at Stockholm MG13:

Lewandowski, Jerzy
Quantizable canonical LQG
Abstract :The canonical quantization scheme can be completed with the framework of Loop Quantum Gravity for several examples of the gravitational field coupled to matter fields. Explicitly, that has been accomplished for the generic dust, non-rotating dust, and massless scalar field. Those results will be presented and recent progress will be discussed...
================================

EDIT TO RESPOND TO NEXT POST.
Hi Tom, since I can still edit I will reply to your post here. You've got an interesting perspective that I want to think about. I can't say much at the moment. I'm intrigued. I need to think about it some more. I'd also like to understand Lewandowski OSN diagrams better---is his approach really useful? I think it is but am not completely sure yet.

Alesci, who is currently Thiemann's postdoc, has chosen to go to Warsaw this autumn, for his next postdoc fellowship. It is ridiculous for me to imagine myself young, and wonder what I would do in his place. But I think, ridiculous as it is, that I would do the same as Alesci, at this point.

 

[tom.stoer]

marcus: let me state clearly that I do believe that the non-embedded networks are the right way to go, that the manifold emerges in a semiclassical limit but that one should (and can) get rid of the manifold in order to define the theory; construction (over 25 years) and definition (in its final formulation) need not be identical. In addition I strongly believe that non-embedded spin networks are in some sense equivalent to spin foams. I think that LQG (after further reformulations) will solve quantization issues, operator algebra anomalies, second-class and simplicity constraints and Dirac quantization, PI measure etc.

Some time ago I started to think about limitations of the current approach and issues that are not addressed by LQG as of today.

I identified one central issue, namely SU(2)! SU(2) emerges from the complexification SL(2,C) is therefore deeply related with the local symmetry structure SO(3,1) of the spacetime manifold. So SU(2) spin networks and its cousins still 'know' something about the spacetime manifold Ashtekar started with. Now, 25 years later, a second central question is the semiclassical limit and the emergence of a smooth spacetime manifold in a certain regime i.e. described by a certain limit of the theory.

The simple question is this: why should the manifold of the semiclassical limit be the same kind of manifold we started with? and why should the dimensions coincide?

Related questions are: what would happen if we start with a different manifold, e.g. a manifold of different dimension? The mathematical tools are much less developed, but afaik Thiemann has done some work in this direction.
And what would happen if we start with a different group for the spin network construction, e.g. SU(7), for which no manifold with SU(7) as its 'structure group' is known? What would be the semiclassical limit of such a spin network?

Regarding quantization, derivation of theories etc. I think Wittgenstein's 'Tractatus' has something interesting to say: "… finally recognizes [my propositions] as senseless, when he has climbed out through them, on them, over them. He must so to speak throw away the ladder, after he has climbed up on it … then he sees the world rightly"

 

[marcus]

I should keep this around and think about it.
==quote==
marcus: let me state clearly that I do believe that the non-embedded networks are the right way to go, that the manifold emerges in a semiclassical limit but that one should (and can) get rid of the manifold in order to define the theory; construction (over 25 years) and definition (in its final formulation) need not be identical. In addition I strongly believe that non-embedded spin networks are in some sense equivalent to spin foams. I think that LQG (after further reformulations) will solve quantization issues, operator algebra anomalies, second-class and simplicity constraints and Dirac quantization, PI measure etc.

Some time ago I started to think about limitations of the current approach and issues that are not addressed by LQG as of today.

I identified one central issue, namely SU(2)! SU(2) emerges from the complexification SL(2,C) is therefore deeply related with the local symmetry structure SO(3,1) of the spacetime manifold. So SU(2) spin networks and its cousins still 'know' something about the spacetime manifold Ashtekar started with. Now, 25 years later, a second central question is the semiclassical limit and the emergence of a smooth spacetime manifold in a certain regime i.e. described by a certain limit of the theory.

The simple question is this: why should the manifold of the semiclassical limit be the same kind of manifold we started with? and why should the dimensions coincide?

Related questions are: what would happen if we start with a different manifold, e.g. a manifold of different dimension? The mathematical tools are much less developed, but afaik Thiemann has done some work in this direction.
And what would happen if we start with a different group for the spin network construction, e.g. SU(7), for which no manifold with SU(7) as its 'structure group' is known? What would be the semiclassical limit of such a spin network?

Regarding quantization, derivation of theories etc. I think Wittgenstein's 'Tractatus' has something interesting to say: "… finally recognizes [my propositions] as senseless, when he has climbed out through them, on them, over them. He must so to speak throw away the ladder, after he has climbed up on it … then he sees the world rightly"
==endquote==
At the moment I find myself without anything helpful to say! Only that what you are talking about is interesting.
I don't know if the following is relevant--it's been on my mind for some time. A kind of backbone of the combinatorial network+foam approach, the Zakopane dynamics as defined last year, is the "f" map from SU(2) representations to SL(2,C) representations. Do you have some insight or perspective on this map? I do not understand why something like this should turn out to be so important.

If you or someone else wanted to experiment by constructing a Zakopane-like setup but with different groups, would you need a pair of groups, and an analogous mapping between their representations? Or could this, perhaps, be avoided?

 

[tom.stoer]

It's especially the math like the f-map which makes the 3+1 stuff so special. I have to check Thiemann's papers on LQG+SUGRA etc. He states ants to study LQG in arbitary dimensions, so he must have found a way to get rid of these special properties of 3+1 dim Riemannian manifolds and SL(2,C).

 

[marcus]

I want to look now at a different direction (or perhaps it is related) that the reformulation of LQG could go, over the next year or two. This is indicated by the Gielen Wise paper on the current MIP poll.

In this paper the authors work with the concepts of "field of observers" and "space of observers". I see this as part of an historical process of the subjectification of spacetime. It is related to the research at Perimeter Institute concerning "Principle of Relative Locality". To me personally, what Gielen Wise are talking about it more interesting than "Relative Locality", and may contain it. But this may simply be arbitrary preference on my part.

Here's the latest one:
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

In this, Gielen and Wise say they have in preparation a new paper called
Lifting general relativity to observer space

So we will see how soon that one comes out and what, if any, impact it has. Last year the authors posted a couple of papers on this general subject:

http://arxiv.org/abs/1111.7195/
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise
http://arxiv.org/abs/1112.2390
The geometric role of symmetry breaking in gravity
Derek K. Wise

 

[marcus]

Some thoughts in back of mind about this: a projectile doesn't HAVE a continuous trajectory.
You can't monitor it along an infinite number of points. You just have a finite series of places you know it's been. A continuous traj. for the projectile doesn't EXIST in nature.
(you can't tell which slit it went thru unless you monitored there.)

A spacetime is like a trajectory. So spacetime does not exist in nature. The universe doesn't HAVE a continuous spacetime geometry.

There are kind of two responses that I see:
1. Hartle's new standard QM. You can partition the histories according to a finite number of factual questions. Some partitions will be sufficiently uncorrelated that you can assign odds and make bets (predictions) and settle bets. No essential role for any observer.
(Of course there is no fundamental space time either. It does not exist in the theory, except as a low energy approximation. Conventional spacetime geometry "emerges" under appropriately "tame" conditions from the more primitive Q&A of Hartle's decoherent partitions of the set of possible histories.

2. Gielen and Wise's response, which I'm still vague about. Instead of fashioning a mathematical model of spacetime geometry (which doesn't exist, that's the problem) you construct a mathematical model of the space of observers which they claim is 7 dimensional. On the face of it, it sounds strange. But I think it's probably worth having a look. In any case, I posted the links to their papers.

 

[marcus]

We know there is some interest in joining LQG with the Hartle "Histories" approach to Quantum Mechanics--at least one grad student working on this. So it makes sense to keep that in our field of vision.

In that connection, Fay Dowker is giving a talk at Perimeter on Tuesday about the Histories approach and in particular how one can recover the Hilbert state space of older QM (under certain assumptions) starting from a PoV in which Histories, not states, are fundamental.
She seems to be one of the main people developing the approach that Hartle initiated.

The talk will probably be based on two 2010 papers. I will get the links:
http://arxiv.org/abs/1002.0589
Hilbert Spaces from Path Integrals
Fay Dowker, Steven Johnston, Rafael D. Sorkin
(Submitted on 2 Feb 2010)
It is shown that a Hilbert space can be constructed for a quantum system starting from a framework in which histories are fundamental. The Decoherence Functional provides the inner product on this "History Hilbert space". It is also shown that the History Hilbert space is the standard Hilbert space in the case of non-relativistic quantum mechanics.
22 pages.

http://arxiv.org/abs/1007.2725
On extending the Quantum Measure
Fay Dowker, Steven Johnston, Sumati Surya
(Submitted on 16 Jul 2010)
We point out that a quantum system with a strongly positive quantum measure or decoherence functional gives rise to a vector valued measure whose domain is the algebra of events or physical questions. This gives an immediate handle on the question of the extension of the decoherence functional to the sigma algebra generated by this algebra of events. It is on the latter that the physical transition amplitudes directly give the decoherence functional. Since the full sigma algebra contains physically interesting questions, like the return question, extending the decoherence functional to these more general questions is important. We show that the decoherence functional, and hence the quantum measure, extends if and only if the associated vector measure does...
23 pages, 2 figures

And here is the link for the PIRSA video of next week's (10 July) seminar talk:
http://pirsa.org/12070001/
The Path Integral Interpretation of Quantum Mechanics
Speaker(s): Fay Dowker
Abstract: In 1932 Dirac wrote that the lagrangian approach to classical mechanics was probably more fundamental than the hamiltonian approach because the former is relativistically invariant whereas the latter is "essentially nonrelativistic". In quantum theory the hamiltonian approach leads to canonincal quantisation, Hilbert space, operators and the textbook rules for state vector "collapse", which are all indeed more or less divorced from the spacetime nature of the physical world as revealed by relativity. The "essentially relativistic" lagrangian approach on the other hand leads to the path integral, as shown by Dirac in 1932 and developed by Feynman. I will show how the interpretation of quantum mechanics in a path integral framework is based directly on events in spacetime and show that it leads to a second "fork in the road" depending on whether it is necessary for probabilities to play a fundamental role in the theory.
Date: 10/07/2012 - 3:30 pm

I suspect that the "second fork in the road" is where one decides whether or not to make an additional assumption (may allow one to recover effective use of the mathematical utilities of conventional state-space QM.)

 

[atyy]

@marcus, one of the things Rovelli likes to say is that time is emergent. In the histories formulation, isn't time primary?

I think Markopoulou has argued that time is primary, so maybe the histories formulation would be more compatible with her viewpoint?

Here's a Markopoulou paper that mentions "histories", but I'm not sure if it's related to Hartle's "histories": http://arxiv.org/abs/gr-qc/0703097

Ooops, from this review, I see the histories approach is Griffiths's!

 

[marcus]

In the case of Dowker's talk (and the tentative exploration of Hartle's QM that I've seen from Marseille) we are following a specific line of development. It might just be a distraction at this point to talk about Griffith's work in 1975, or Markopoulou (who means something else by "causal histories") or the Hohenberg review you linked, which does not use Hartle's terminology and is not focused on this specific line.

If anyone is interested in understanding the significance of Dowker's talk, I would suggest studying Hartle's 2006 paper, that was presented at the 23rd Solvay Conference---whose theme was "The Quantum Structure of Space and Time". Hartle's paper sets out axioms for the decoherence functional which is basic to his particular Histories approach. Of course he acknowledges Griffiths 1975 work but that's ancient history.
I've looked over a bunch of papers and this is a key one:

http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
(Submitted on 2 Feb 2006)
Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has changed 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 paper reviews a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. This generalization is constructed within the framework of generalized quantum theory. This is a minimal set of principles for quantum theory abstracted from the modern quantum mechanics of closed systems, most generally the universe. 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 allow for the further generalization that would be necessary were spacetime not fundamental...
31 pages. 4 figures.

To paraphrase, states and evolution of fields defined on spacelike surfaces are ONLY appropriate as math idealizations when geometry behaves APPROXIMATELY CLASSICALLY. In more general situations such idealizations are NOT appropriate.
They are, as Dowker puts it in her seminar talk abstract, "more or less divorced from the spacetime nature of the physical world".

 

[atyy]

So is time fundamental in Dowker's approach?

The Hartle paper has a beautiful quote: ‘Traveler, there are no paths, paths are made by walking.’ It's also interesting that David Gross delivered Hartle's talk.

 

[marcus]

So is time fundamental in Dowker's approach?

The Hartle paper has a beautiful quote: ‘Traveler, there are no paths, paths are made by walking.’ It's also interesting that David Gross delivered Hartle's talk.

There is no time. Time is made by histories
As I recall, David Gross was the chairman and main organizer of the 23rd Solvay.
He would have decided the theme "The Quantum Structure of Space and Time" and, I guess,
invited Hartle to contribute a paper. For whatever reason, Hartle was unable to make it to the conference and so the paper was presented in his stead.

If anyone is interested in watching, last year Hartle gave a talk at Perimeter on related matters. Here's a video:
http://pirsa.org/11020124/
Quantum Mechanics with Extended Probabilities
Speaker(s): James Hartle
Abstract: We present a new formulation of quantum mechanics for closed systems like the universe using an extension of familiar probability theory that incorporates negative probabilities. Probabilities must be positive for alternative histories that are the basis of settleable bets. However, quantum mechanics describes alternative histories are not the basis for settleable bets as in the two-slit experiment. These alternatives can be assigned extended probabilities that are sometimes negative. We will compare this with the decoherent (consistent) histories formulation of quantum theory. The prospects for using this formulation as a starting point for testable alternatives to quantum theory or further generalizations of it will be briefly discussed.
Date: 07/03/2011 - 11:00 am
Here's another, but not so recent:
http://pirsa.org/07090064/

 

[atyy]

In Dowker's http://arxiv.org/abs/1002.0589 section 3.2, it looks like time is fundamental for defining a history.

 

[atyy]

The Dowker talk abstract says something about a deterministic versus probabilistic formulation. Is that in her published work, or is that new?

 

[marcus]

In Dowker's http://arxiv.org/abs/1002.0589 section 3.2, it looks like time is fundamental for defining a history.

I don't see any evidence of that. At the beginning of section 3 they say plainly that they are considering a special case. And a time variable IS employed in that setup.

Throughout the paper they are building bridges and comparisons between their Histories approach and conventional QM, especially the example of a conventional non-relativistic particle moving in d-dimensional Euclid space according to a conventional clock. They are interested in showing that their theoretical framework can handle that and get the same results as the conventional one.

So in that paper they are always studying examples in which there IS time. But time does not appear in their axioms. So I think you are mistaken about it looking like it's fundamental.

 

[marcus]

...here is the link for the PIRSA video of next week's (10 July) seminar talk:
http://pirsa.org/12070001/
The Path Integral Interpretation of Quantum Mechanics
Speaker(s): Fay Dowker
Abstract: In 1932 Dirac wrote that the lagrangian approach to classical mechanics was probably more fundamental than the hamiltonian approach because the former is relativistically invariant whereas the latter is "essentially nonrelativistic". In quantum theory the hamiltonian approach leads to canonincal quantisation, Hilbert space, operators and the textbook rules for state vector "collapse", which are all indeed more or less divorced from the spacetime nature of the physical world as revealed by relativity. The "essentially relativistic" lagrangian approach on the other hand leads to the path integral, as shown by Dirac in 1932 and developed by Feynman. I will show how the interpretation of quantum mechanics in a path integral framework is based directly on events in spacetime and show that it leads to a second "fork in the road" depending on whether it is necessary for probabilities to play a fundamental role in the theory.
Date: 10/07/2012 - 3:30 pm
...

The Dowker talk abstract says something about a deterministic versus probabilistic formulation. ...

I think you are mistaken. There is no reference to "deterministic" in the abstract. I think you are probably reading too much into the abstract, or putting your own interpretation on it.

When one is constructing a non-deterministic theory one does not automatically get probabilities (numbers between zero and one satisfying certain laws). It may require additional stronger assumptions in order to make probabilities play a fundamental role.

 

[marcus]

Anyway the way I see it we are in an exciting moment for Loop gravity. There are all these developments that could feed into a reformulation that shows up as early as July 2013 with the Warsaw GR20, or at Perimeter's Loops 2013 conference.

A.Stacking Spin Networks (systematically to generate spin foams)
http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)

B.Histories
http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
(Submitted on 2 Feb 2006)
and currently
http://pirsa.org/12070001/
The Path Integral Interpretation of Quantum Mechanics
Fay Dowker
10 Jul 2013

C. Unclamping the Immirzi
http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi
(Submitted on 23 Apr 2012)

D. Using the tetrad's sign
http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 3 May 2012)

E. Thermodynamics
http://arxiv.org/abs/1204.6349
Gravitation and vacuum entanglement entropy
Ted Jacobson
(Submitted on 28 Apr 2012)
http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin
(Submitted on 24 May 2012)
http://arxiv.org/abs/1207.0505
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan
(Submitted on 2 Jul 2012)

F. Dust. Actual Hamiltonians (instead of constraints.)
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann
(Submitted on 17 Jun 2012)
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)

 

[marcus]

Fay Dowker has now given her seminar talk at Perimeter and the video version is already posted here:
http://pirsa.org/12070001

As I said, I see some 6 main ideas that could enter into Loop gravity and change how it is formulated. I can't talk probabilities as I'm just an interested observer, not an expert. These ideas are one the scene and some (or none!) might affect the theory. Because there are so many balls in the air, I boiled them down to 6 keywords to make them easier to recall.

Stacking
Histories
Unclamping
Tetrad
Thermo
Dust

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.

Unclamping the Immirzi parameter was a consequence of Bianchi's black hole entropy result S=A/4. It appears to me to have exciting and unpredictable implications for the theory.

The Tetrad's sign could be included in the classical theory upon which Loop gravity is based. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value. It's the one thing I feel sure will be prominently featured in Loops 2013 and next year's GR20 conference in Warsaw.

 

[marcus]

Dowker gave an impressive talk.
It helps if you download the slides PDF first (which takes me about 3 minutes):
http://pirsa.org/pdf/loadpdf.php?pirsa_number=12070001
Then scroll thru the slides while watching the video.
http://pirsa.org/12070001
There is a lot on the slides and their video images are not as legible as the PDF.

She presents Sorkin (and her) QUANTUM MEASURE THEORY as a rival alternative to Hartle's DECOHERENT HISTORIES. Both are proposed histories formulations of QM.
As she presents it, QMT is still being worked out. She also points to a drawback in Hartle's DH approach.

This seems compatively mild to me: it is that there are different ways of partitioning the set of all histories so you get approximate decoherence and additive probabilities. She refers to this as something you have to "struggle with" in the Hartle approach.

But the struggle seems more serious if the probability addition rule is relaxed and all you require is "preclusion" (that events with measure subjectively considered to be very small do not occur.) The preclusion approach is what she and Sorkin are working on. She gave two examples showing a grave contradiction in this approach, where you do not require additive probabilities. In these examples no single history could occur because each one was contained in a subset of measure zero. One example, starting at minute 60, was a variant of the GHZ construction--Greenberger, Horne, and Zeilinger--may be familiar to some.
(e-copy of original GHZ 1998 paper: http://arxiv.org/abs/0712.0921 "Going Beyond Bell's Theorem")

 

[marcus]

Prompted by Dowker's talk, I am trying to assess how serious the problems are with Hartle's Decoherent Histories QM.
Here are some papers by Adrian Kent discussing the problem that decoherent partitions are not unique, and (as of 1994 and 1996 according to Kent) can lead to contradictory predictions. I don't know if the problems alleged by Kent are real or if they have been fixed since then.

Hartle's 2006 paper for the 23rd Solvay Conference proceedings does not cite Kent and does not seem to answer his criticisms, which is puzzling.
http://arxiv.org/abs/gr-qc/9809026
Quantum Histories
http://arxiv.org/abs/gr-qc/9808016
Consistent Sets and Contrary Inferences: Reply to Griffiths and Hartle
http://arxiv.org/abs/gr-qc/9607073
Quantum Histories and Their Implications
http://arxiv.org/abs/gr-qc/9604012
Consistent Sets Yield Contrary Inferences in Quantum Theory
http://arxiv.org/abs/gr-qc/9412067
On the Consistent Histories Approach to Quantum Mechanics

However I see that Griffiths and Hartle did reply to Kent's crit here:
http://arXiv.org/abs/gr-qc/9710025
Comment on "Consistent Sets Yield Contrary Inferences in Quantum Theory''
Robert B. Griffiths (Carnegie-Mellon University), James B. Hartle (University of California, Santa Barbara)
(Submitted on 3 Oct 1997)
In a recent paper Kent has pointed out that in consistent histories quantum theory it is possible, given initial and final states, to construct two different consistent families of histories, in each of which there is a proposition that can be inferred with probability one, and such that the projectors representing these two propositions are mutually orthogonal. In this note we stress that, according to the rules of consistent history reasoning two such propositions are not contrary in the usual logical sense namely, that one can infer that if one is true then the other is false, and both could be false. No single consistent family contains both propositions, together with the initial and final states, and hence the propositions cannot be logically compared. Consistent histories quantum theory is logically consistent, consistent with experiment as far as is known, consistent with the usual quantum predictions for measurements, and applicable to the most general physical systems. It may not be the only theory with these properties, but in our opinion, it is the most promising among present possibilities.
2 pages

 

[marcus]

Loops 2013 conference will be held July 22-26 next year at Perimeter Institute and it's interesting to try to identify now what new ideas and developments could enter into the formulation of Loop quantum geometry/gravity/cosmology that we'll see set out a year from now, at conference.
I should probably update my list of ideas I'm guessing could enter significantly into the picture. There are now 7 of them.
Stacking
Histories
Unclamping
Tetrad-sign
Thermo
Dust
Higgsflation

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Unclamping the Immirzi parameter was a consequence of Bianchi's black hole entropy result S=A/4. It appears to me to have exciting and unpredictable implications for the theory.
http://arxiv.org/abs/1204.5122

The Tetrad's sign could be included in the classical theory upon which Loop gravity is based. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

Higgs inflation in Loop cosmology is the topic of a new paper by three young researchers that just appeared and impressed me as potentially important. It's by Tom Pawlowski, a postdoc at Warsaw, and two PhD students there: Andrea Dapor and Michal Artymowski.
It puts inflation in a new light for me. So I expect some rapid development in this area:
http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 18 Jul 2012)
The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by critical matter energy density, (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
14 pages, 5 figures
I should give links to earlier papers by Bezrukov and Shaposhnikov
http://arxiv.org/abs/0710.3755 (209 cites)
The Standard Model Higgs boson as the inflaton
http://arxiv.org/abs/0904.1537 (78 cites)
Standard Model Higgs boson mass from inflation: two loop analysis
The latter was cited by ADP.

 

[marcus]

The Loops conference is biennial, every two years. The previous one, Loops 2011 was held in Madrid. Videos of many of the talks, and PDF files of slide presentations are online here:
http://www.iem.csic.es/loops11/ (click on the Scientific Program menu item)
Loops 2013 conference starts just one year from today. It will be held July 22-26 next year at Perimeter Institute.

I've been trying to visualize what main topics and new developments might figure prominently at the next Loops conference. After thinking it over for several weeks and considering various alternatives I've come around, at least for now, to the belief that Loop cosmology will stand out and show the most active development. Particularly the phenomenology side of it where e.g. you model different types of BOUNCE and different mechanisms of INFLATION and you figure out what traces to look for in the ancient light of the Cosmic Background. Among other things gravitational waves would leave their imprint (magnified by expansion) on the microwave sky.

I think other major themes are going to be
DUST
INHOMOGENEITY

The Warsaw and Erlangen people have been working hard on a straightforward Hamiltonian operator-based Loop program and they sound like they think they've succeeded. Lewandowski says we now have Quantum Gravity. So that puts a new light on the situation. It uses a matter field (uniform dust-like) but that is nothing new. Cosmology models have always had that convenience. I'm using "dust" as shorthand for the various approaches being used to recover a real physical Hamiltonian. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

And I keep seeing Loop cosmology papers with more and more non-uniformity, various "Bianchi" classes of messed up universes---bouncing nevertheless. That's a major trend in the program---generalizing LQC to remove the uniformity restriction and get increasingly realistic.
Inhomogeneity is the focus of this recent paper:
http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 5 Apr 2012)
The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB.
4 pages ICGC (2011) Goa Conference proceedings

Higgs inflation in Loop cosmology is the topic of a new paper that just appeared and impressed me as potentially important. It's by Tom Pawlowski, a postdoc at Warsaw, and two PhD students there: Andrea Dapor and Michal Artymowski.
It puts inflation in a new light for me. So I expect some rapid development in this area:
http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 18 Jul 2012)
The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by critical matter energy density, (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
14 pages, 5 figures
Here are links to earlier papers by Bezrukov and Shaposhnikov
http://arxiv.org/abs/0710.3755 (209 cites)
The Standard Model Higgs boson as the inflaton
http://arxiv.org/abs/0904.1537 (78 cites)
Standard Model Higgs boson mass from inflation: two loop analysis
The latter was cited by ADP.
====================
I should look and see what recent Loop CosmoPheno papers have been highly cited lately and that could give ideas. This post gave some search links for cite-ranked listings:

...
Here's a link that gets (desy key) LQG and LQC papers that mostly have to do with pheno, i.e. with opportunities for TESTING.
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+INFLATION+OR+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+&FORMAT=www&SEQUENCE=citecount%28d%29
This ranks by cites and gets around 108 papers.

Let's restrict to ones that appeared 2009 or later:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

This gets 59 papers. Again they are ranked by cites.
...
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+(dk+loop+space+and+(quantum+gravity+or+quantum+cosmology)+)+and+(gravitational+radiation+or+primordial+or+inflation+or+power+spectrum+or+cosmic+background)+and+date%3E2008&SEQUENCE=CITECOUNT(D)&SKIP=25
It only gets 57 of the 59 but works more reliably in some cases.

For sure Agullo Ashtekar Nelson is going to figure in conference and also Artymowski Dapor Pawlowski, I would expect. But those haven't been out long enough to be cited much, so they would not necessarily be spotted in this kind of listing. Let's see what Loop CosmoPheno papers are picked out, however:

Links don't always remain active. So I will copy off a few items from the top of the list.
1) Cosmological footprints of loop quantum gravity.
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) . Feb 2009. (Published Feb 27, 2009). 7pp.
Published in Phys.Rev.Lett.102:081301,2009.
e-Print: arXiv:0902.0145 [gr-qc] (46)

3) Loop quantum cosmology and slow roll inflation.
Abhay Ashtekar, David Sloan, (Penn State U.) . Dec 2009. 8pp.
Published in Phys.Lett.B694:108-112,2010.
e-Print: arXiv:0912.4093 [gr-qc] (32)

4) Possible observational effects of loop quantum cosmology.
Jakub Mielczarek, (Jagiellonian U., Astron. Observ. & LPSC, Grenoble) . Aug 2009. (Published Mar 15, 2010). 11pp.
Published in Phys.Rev.D81:063503,2010.
e-Print: arXiv:0908.4329 [gr-qc] (26)

5) Big Bounce and inhomogeneities.
David Brizuela, Guillermo A.D Mena Marugan, Tomasz Pawlowski, (Madrid, Inst. Estructura Materia) . Feb 2009. 4pp.
Published in Class.Quant.Grav.27:052001,2010.
e-Print: arXiv:0902.0697 [gr-qc] (21)

6) Inflation in loop quantum cosmology: dynamics and spectrum of gravitational waves.
Jakub Mielczarek, (Jagiellonian U.) , Thomas Cailleteau, (LPSC, Grenoble) , Julien Grain, (Paris, Inst. Astrophys.) , Aurelien Barrau, (LPSC, Grenoble) . Mar 2010. (Published May 15, 2010). 11pp.
Published in Phys.Rev.D81:104049,2010.
e-Print: arXiv:1003.4660 [gr-qc] (21)

7) Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation.
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) , A. Gorecki, (LPSC, Grenoble) . Apr 2009. (Published Apr 2009). 15pp.
Published in Phys.Rev.D79:084015,2009.
e-Print: arXiv:0902.3605 [gr-qc] (20)

8) Observational constraints on loop quantum cosmology.
Martin Bojowald, (Penn State U.) , Gianluca Calcagni, (Potsdam, Max Planck Inst.) , Shinji Tsujikawa, (Tokyo U. of Sci.) . IGC-11-1-1, AEI-2011-004, Jan 2011. (Published Nov 18, 2011). 4pp.
Published in Phys.Rev.Lett.107:211302,2011.
e-Print: arXiv:1101.5391 [astro-ph.CO] (20)

9) Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology.
Masahiro Shimano, Tomohiro Harada, (Rikkyo U.) . Sep 2009. (Published Sep 15, 2009). 17pp.
Published in Phys.Rev.D80:063538,2009.
e-Print: arXiv:0909.0334 [gr-qc] (19)

10) Fully LQC-corrected propagation of gravitational waves during slow-roll inflation.
J. Grain, (Paris, Inst. Astrophys.) , T. Cailleteau, A. Barrau, A. Gorecki, (LPSC, Grenoble) . Oct 2009. (Published Jan 15, 2010). 9pp.
Published in Phys.Rev.D81:024040,2010.
e-Print: arXiv:0910.2892 [gr-qc] (17)

11) Inhomogeneous Loop Quantum Cosmology: Hybrid Quantization of the Gowdy Model.
L.J. Garay, (Madrid U. & Madrid, Inst. Estructura Materia) , M. Martin-Benito, G.A. Mena Marugan, (Madrid, Inst. Estructura Materia) . May 2010. (Published Aug 15, 2010). 16pp.
Published in Phys.Rev.D82:044048,2010.
e-Print: arXiv:1005.5654 [gr-qc] (17)

12) Tensor power spectrum with holonomy corrections in LQC.
Jakub Mielczarek, (Jagiellonian U.) . Feb 2009. (Published Feb 2009). 13pp.
Published in Phys.Rev.D79:123520,2009.
e-Print: arXiv:0902.2490 [gr-qc] (16)

13) Inflationary observables in loop quantum cosmology.
Martin Bojowald, (Penn State U.) , Gianluca Calcagni, (Potsdam, Max Planck Inst.) . AEI-2010-161, IGC-10-11-1, Nov 2010. 40pp.
Published in JCAP 1103:032,2011.
e-Print: arXiv:1011.2779 [gr-qc] (16)

14) Observational test of inflation in loop quantum cosmology.
Martin Bojowald, (Penn State U.) , Gianluca Calcagni, (Potsdam, Max Planck Inst.) , Shinji Tsujikawa, (Tokyo U. of Sci.) . AEI-2011-042, Jul 2011. 37pp.
Published in JCAP 1111:046,2011.
e-Print: arXiv:1107.1540 [gr-qc] (15)

15) Non-singular inflationary universe from polymer matter.
Golam Mortuza Hossain, Viqar Husain, Sanjeev S. Seahra, (New Brunswick U.) . Jun 2009. (Published Jan 15, 2010). 4pp.
Published in Phys.Rev.D81:024005,2010.
e-Print: arXiv:0906.2798 [astro-ph.CO] (13)

16) Constraints on standard and non-standard early Universe models from CMB B-mode polarization.
Yin-Zhe Ma, (Cambridge U., KICC & Cambridge U., Inst. of Astron.) , Wen Zhao, (Cardiff U.) , Michael L. Brown, (Cambridge U., KICC & Cambridge U., Inst. of Astron. & Cambridge U.) . Jul 2010. 41pp.
Published in JCAP 1010:007,2010.
e-Print: arXiv:1007.2396 [astro-ph.CO] (11)

17) Loop Quantum Cosmology corrections on gravity waves produced during primordial inflation.
J. Grain, (Paris, Inst. Astrophys.) . Nov 2009. 9pp.
To appear in the proceedings of INVISIBLE UNIVERSE INTERNATIONAL CONFERENCE: Toward a new cosmological paradigm, Paris, France, 29 Jun - 3 Jul 2009.
Published in AIP Conf.Proc.1241:600-608,2010.
e-Print: arXiv:0911.1625 [gr-qc] (9)

18) Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters.
Julien Grain, (Paris, Inst. Astrophys. & Orsay, LAL) , Aurelien Barrau, Thomas Cailleteau, (LPSC, Grenoble) , Jakub Mielczarek, (Jagiellonian U.) . Nov 2010. (Published Dec 15, 2010). 12pp.
Published in Phys.Rev.D82:123520,2010.
e-Print: arXiv:1011.1811 [astro-ph.CO] (9)

19) Warm inflationary model in loop quantum cosmology.
Ramon Herrera, (Rio de Janeiro, Pont. U. Catol.) . Jun 2010. (Published Jun 15, 2010). 15pp.
Published in Phys.Rev.D81:123511,2010.
e-Print: arXiv:1006.1299 [astro-ph.CO] (8)

20) The Big Bang and the Quantum.
Abhay Ashtekar, (Penn State U.) . May 2010. 18pp.
Plenary talk at INVISIBLE UNIVERSE INTERNATIONAL CONFERENCE: Toward a new cosmological paradigm, Paris, France, 29 Jun - 3 Jul 2009.
Published in AIP Conf.Proc.1241:109-121,2010.
e-Print: arXiv:1005.5491 [gr-qc] (7)

21) On the measure problem in slow roll inflation and loop quantum cosmology.
Alejandro Corichi, (UNAM, Morelia, Inst. Math. & Penn State U.) , Asieh Karami, (IFM-UMSNH, Michoacan & UNAM, Morelia, Inst. Math.) . Nov 2010. (Published May 15, 2011). 12pp.
Published in Phys.Rev.D83:104006,2011.
e-Print: arXiv:1011.4249 [gr-qc] (7)

 

[marcus]

Besides cosmology (eg Higgs inflation in Loop cosmo) the other 6 topics to watch, that I listed in post #87 and don't want to completely forget about, are as follows:

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Unclamping the Immirzi parameter was a consequence of Bianchi's black hole entropy result S=A/4. It appears to me to have exciting and unpredictable implications for the theory.
http://arxiv.org/abs/1204.5122

The Tetrad's sign could be included in the classical theory upon which Loop gravity is based. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

========================

Actually something just came out today that relates to the "Tetrad sign" idea:

http://arxiv.org/abs/1207.5156
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli
(Submitted on 21 Jul 2012)
We suggest that large radiative corrections appearing in the spinfoam framework might be tied to the implicit sum over orientations. Specifically, we show that in a suitably simplified context the characteristic "spike" divergence of the Ponzano-Regge model disappears when restricting the theory to just one of the two orientations appearing in the asymptotic limit of the vertex amplitude.
10 pages, 5 figures

For example reference [13] is to the original tetrad sign paper by Rovelli&Wilson-Ewing which enters discussion on page 1 here:

==quote page 1 of Christodoulou et al==
We suggest here that the answer lies in the fact that the asymptotic limit of the Ponzano-Regge amplitude is not the exponential of the Regge action, but rather the sum of two exponentials of the Regge action, taken with certain flipped signs. With flipped signs, the invariant contribution comes when P is outside τ. In other words, the divergence is strictly dependent on the existence of the second term in the expansion of the vertex amplitude.

The geometrical origin of this second term can be traced to the fact that the asymptotic limit of the Ponzano-Regge model is not truly 3d general relativity in metric variables, but rather 3d general relativity in triad variables, with an action that flips sign under reversal of the orientation of the triad [13]. In three dimensions, it is this action (and not metric general relativity) which is equivalent to BF theory. In turn, BF theory has an additional gauge symmetry with respect to general relativity: the shift B → B+d_AΦ (where A is the connection variable: F = dA+A∧A), which can be shown to be related to the displacement of P all over the hyperplane [4].

In this paper, we present two arguments that provide some ground for these intuitions...
==endquote==

 

[marcus]

So as not to forget the active lines of research we're following:
PhenoCosmo (bounce early universe cosmology is where pheno enters most strongly)
Stacking
Histories
Unclamping
Tetrad-handedness
Thermo
Dust

I see that someone named David Craig has some potentially Loop-related consistent/decoherent "Histories" papers.
http://arxiv.org/find/gr-qc/1/au:+Craig_D/0/1/0/all/0/1
Somehow I was not aware of his research until now.
He has co-authored two papers with Loop researcher Param Singh, and also co-authored with Jim Hartle, Fay Dowker, Rafael Sorkin. Recently brought out his first explicitly Loop cosmology paper.

To keep track of a few of the authors involved in each of these research lines, for reference purposes:
PhenoCosmo (Barrau, Grain, Pawlowski, Cailleteau, Agullo, Nelson, Vidotto,...)
Stacking (Lew., ... )
Histories (Hartle, Schroeren?, Craig?)
Unclamping (Bianchi, ...)
Tetrad-handedness (Rov., ...)
Thermo (Jac., Smo., Pad., ...)
Dust (Lew., Thiem., Wise, ...)

 

[marcus]

I forgot TWISTORS when listing active lines of Loop research which could feature in whatever reformulation takes shape at the July 2013 Perimeter conference. This just came out:
http://arxiv.org/abs/1207.6348
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland
(Submitted on 26 Jul 2012)
The spin foam formalism provides transition amplitudes for loop quantum gravity. Important aspects of the dynamics are understood, but many open questions are pressing on. In this paper we address some of them using a twistorial description, which brings new light on both classical and quantum aspects of the theory. At the classical level, we clarify the covariant properties of the discrete geometries involved, and the role of the simplicity constraints in leading to SU(2) Ashtekar-Barbero variables. We identify areas and Lorentzian dihedral angles in twistor space, and show that they form a canonical pair. The primary simplicity constraints are solved by simple twistors, parametrized by SU(2) spinors and the dihedral angles. We construct an SU(2) holonomy and prove it to correspond to the Ashtekar-Barbero connection. We argue that the role of secondary constraints is to provide a non trivial embedding of the cotangent bundle of SU(2) in the space of simple twistors. At the quantum level, a Schroedinger representation leads to a spinorial version of simple projected spin networks, where the argument of the wave functions is a spinor instead of a group element. We rewrite the Liouville measure on the cotangent bundle of SL(2,C) as an integral in twistor space. Using these tools, we show that the Engle-Pereira-Rovelli-Livine transition amplitudes can be derived from a path integral in twistor space. We construct a curvature tensor, show that it carries torsion off-shell, and that its Riemann part is of Petrov type D. Finally, we make contact between the semiclassical asymptotic behaviour of the model and our construction, clarifying the relation of the Regge geometries with the original phase space.
39 pages

So a revised list:
PhenoCosmo (bounce early universe cosmology is where pheno enters most directly)
TwistorLQG
FreeImmirzi
Tetrad-handedness
Stacking
Histories
Thermo
Dust

For reference purposes, helping to look up papers by author, I'll tag these lines of research with (very incomplete) lists of names:
PhenoCosmo (Barrau, Grain, Pawlowski, Cailleteau, Agullo, Nelson, Vidotto,...)
TwistorLQG (Levine, Dupuis, Speziale, Wieland,...)
FreeImmirzi (Bianchi, ...)
Tetrad-handedness (Rov., ...)
Stacking (Lew., ... )
Histories (Hartle, Schroeren?, Craig?)
Thermo (Jac., Smo., Pad., ...)
Dust (Lew., Thiem., Wise, ...)[/QUOTE]

 

[marcus]

Twistors are having a significant impact on Loop. We need to learn a bit about them.
Here is a nice tutorial with 15 transparencies sketched by Penrose. He is able to think and communicate in a highly graphic way, a bit like a cartoonist. The text is only 4 pages, if you print it out, but you might want to print out a few or all of the transparencies as well: just click on an individual slide and you can print it.
http://users.ox.ac.uk/~tweb/00006/index.shtml

The tutorial (based on a talk by Penrose) was prepared and put on line by Fedja Hadrovich, who also has this more mathy less visual introduction called Twistor Primer, that might be helpful as a supplement:
http://users.ox.ac.uk/~tweb/00004/index.shtml

My impression is that the entry of twistors into Loop geometry/gravity was by way of
work by Freidel, Livine, Dupuis, Tambornino, Speziale and Wieland.

One thing that served to whet my interest in this version ("twistorial LQG") was a Perimeter video talk by Wieland. Wieland is at Marseille but in February this year he was visiting at PI (doing some work with Bianchi I think) and gave a cogent and (to me unexpectedly understandable) seminar on a spinor/twistor way of treating Ashtekar variables and doing canonical Loop gravity. (!) I will get the PIRSA link to that video talk.

http://pirsa.org/12020129/
Spinor Quantisation for Complex Ashtekar Variables
Speaker(s): Wolfgang Wieland
Abstract: During the last couple of years Dupuis, Freidel, Livine, Speziale and Tambornino developed a twistorial formulation for loop quantum gravity.
Constructed from Ashtekar--Barbero variables, the formalism is restricted to SU(2) gauge transformations.
In this talk, I perform the generalisation to the full Lorentzian case, that is the group SL(2,C).
The phase space of SL(2,C) (i.e. complex or selfdual) Ashtekar variables on a spinnetwork graph is decomposed in terms of twistorial variables. To every link there are two twistors---one to each boundary point---attached. The formalism provides a clean derivation of the solution space of the reality conditions of loop quantum gravity.
Key features of the EPRL spinfoam model are perfectly recovered.
If there is still time, I'll sketch my current project concerning a twistorial path integral for spinfoam gravity as well.
Date: 29/02/2012 - 4:00 pm

In the sense used here, two spinors make a twistor. A twistor can be called a "bi-spinnor".
Basically just saying ℂ² x ℂ² = ℂ⁴
And Wieland is using pairs of spinnors on the links of his spinnetworks.

 

[marcus]

So the updated list of active Loop areas I want to watch are:

PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation. Recent papers by Ashtekar, Agullo, Nelson and by Artymowski, Dapor, Pawlowski.

TwistorLQG Papers by Freidel, Livine, Dupuis, Speziale, Wieland... For example http://arxiv.org/abs/1207.6348
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland

FreeImmirzi was a consequence of Bianchi and others' black hole entropy result S=A/4. It appears to have exciting and unpredictable implications for the theory.
http://arxiv.org/abs/1204.5122

Tetrad-handedness The Tetrad's sign could start to be included both in the classical theory upon which Loop gravity is based and in the quantum theory. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733
http://arxiv.org/abs/1207.5156

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

========================

 

[marcus]

Connes is back in the game!
That means that Grimstrup's effort to implement the Spectral Standard Model of particle theory in the Loop QG is likely to get some attention at next July's Loops conference.
http://pirsa.org/index.php?p=speaker&name=Jesper_Grimstrup
http://pirsa.org/09100143/
On Semi-classical States of Quantum Gravity and Noncommutative Geometry
Speaker(s): Jesper Grimstrup
Abstract: The idea behind an intersection between loop quantum gravity and noncommutative geometry is to combine elements of unification with a setup of canonical quantum gravity. In my talk I will first review the construction of a semi-finite spectral triple build over an algebra of holonomy loops. Here, the loop algebra is a noncommutative algebra of functions over a configurations space of connections, and the interaction between the Dirac type operator and the loop algebra captures information of the kinematical part of canonical quantum gravity. Next, I will show how certain normalizable, semi-classical states are build which connects the spectral triple construction to the Dirac Hamiltonian in 3+1 dimensions. Thus, these states can be interpreted as one-particle fermion states in an ambient gravitational field. This analysis indicates that the spectral triple construction involves matter degrees of freedom.
Date: 14/10/2009 - 4:00 pm

Here is Connes' recent paper. MTd2 spotted it and added it to our bibliography.
http://arxiv.org/abs/1208.1030
Resilience of the Spectral Standard Model
Ali H. Chamseddine, Alain Connes
(Submitted on 5 Aug 2012)
We show that the inconsistency between the spectral Standard Model and the experimental value of the Higgs mass is resolved by the presence of a real scalar field strongly coupled to the Higgs field. This scalar field was already present in the spectral model and we wrongly neglected it in our previous computations. It was shown recently by several authors, independently of the spectral approach, that such a strongly coupled scalar field stabilizes the Standard Model up to unification scale in spite of the low value of the Higgs mass. In this letter we show that the noncommutative neutral singlet modifies substantially the RG analysis, invalidates our previous prediction of Higgs mass in the range 160--180 Gev, and restores the consistency of the noncommutative geometric model with the low Higgs mass.
13 pages

This August paper consists largely of a re-examination of their April 2010 paper (which is reference [2] and is cited over and over again). The 2010 paper treats the Spectral Standard Model and a sketch of the unification of forces roughly along "Big Desert" lines. As I understand it, in the analysis for the earlier paper a "Higgs singlet" appeared, as well as a Higgs doublet. The assumption was made that this scalar field would not affect the Higgs mass. Unless I'm mistaken it is this part of the 2010 picture which they are now revising. I should include the abstract.

http://arxiv.org/abs/1004.0464/
Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I
Ali H. Chamseddine, Alain Connes
(Submitted on 3 Apr 2010)
We examine the hypothesis that space-time is a product of a continuous four-dimensional manifold times a finite space. A new tensorial notation is developed to present the various constructs of noncommutative geometry. In particular, this notation is used to determine the spectral data of the standard model. The particle spectrum with all of its symmetries is derived, almost uniquely, under the assumption of irreducibility and of dimension 6 modulo 8 for the finite space. The reduction from the natural symmetry group SU(2)xSU(2)xSU(4) to U(1)xSU(2)xSU(3) is a consequence of the hypothesis that the two layers of space-time are finite distance apart but is non-dynamical. The square of the Dirac operator, and all geometrical invariants that appear in the calculation of the heat kernel expansion are evaluated. We re-derive the leading order terms in the spectral action. The geometrical action yields unification of all fundamental interactions including gravity at very high energies. We make the following predictions:
(i) The number of fermions per family is 16.
(ii) The symmetry group is U(1)xSU(2)xSU(3).
(iii) There are quarks and leptons in the correct representations.
(iv) There is a doublet Higgs that breaks the electroweak symmetry to U(1).
(v) Top quark mass of 170-175 Gev.
(v) There is a right-handed neutrino with a see-saw mechanism.
Moreover, the zeroth order spectral action obtained with a cut-off function is consistent with experimental data up to few percent. We discuss a number of open issues. We prepare the ground for computing higher order corrections since the predicted mass of the Higgs field is quite sensitive to the higher order corrections. We speculate on the nature of the noncommutative space at Planckian energies and the possible role of the fundamental group for the problem of generations.
56 pages

I spent some time searching through the April 2010 paper and could not find the relevant passage. There was mention of something possibly relevant on page 26, right before equation (6.17), and also section 9.4 on page 33. But I couldn't be certain.Steven Weinberg gave some useful perspective in this 2009 wide-audience talk, link to which I should keep handy:

 

[marcus]

Back in post #93, when listing significant areas of Loop gravity development to watch I gave links to sample research in all but the first on the list: what for want of a handier term I am calling "PhenoCosmo" for phenomenological quantum cosmology. I think of this as perhaps the most critical research front, because cosmology is the main arena in which QG (quantum relativity, quantum geometry and matter) theories will necessarily be tested.

In the Loop case the Pheno studies involve calculating the observable effects of the Bounce and subsequent Bounce-triggered inflation. This search, while not perfect and containing a few of what may be considered "false positives", currently finds 62 papers of which most are of the desired sort. The papers all appeared in 2009 or later, and are ranked by number of times cited.

http://www-library.desy.de/cgi-bin/spifaacce/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+inflation+or+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29

Here's a revised listing with some PhenoCosmo sample links:
PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation.
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1204.1288 (Perturbations in loop quantum cosmology)
Artymowski, Dapor, Pawlowski http://arxiv.org/abs/1207.4353 (Inflation from non-minimally coupled scalar field in loop quantum cosmology)
By various of the following: Barrau, Grain, Cailleteau, Vidotto, Mielczarek
http://arxiv.org/abs/1206.6736 (Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology)
http://arxiv.org/abs/1206.1511 (Loop quantum cosmology in the cosmic microwave background)
http://arxiv.org/abs/1111.3535 (Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology)
http://arxiv.org/abs/1011.1811 (Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters)
http://arxiv.org/abs/1003.4660 (Inflation in loop quantum cosmology: Dynamics and spectrum of gravitational waves)

TwistorLQG Papers by Freidel, Livine, Dupuis, Speziale, Wieland... For example Speziale and Wieland http://arxiv.org/abs/1207.6348(The twistorial structure of loop-gravity transition amplitudes)

FreeImmirzi was a consequence of Bianchi and others' black hole entropy result S=A/4. It appears to have exciting and unpredictable implications for the theory.
http://arxiv.org/abs/1204.5122

Tetrad-handedness The Tetrad's sign could start to be included both in the classical theory upon which Loop gravity is based and in the quantum theory. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733
http://arxiv.org/abs/1207.5156

Stacking refers to Lewandowski group's way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Thermodynamics of geometry is the theme of some recent papers by Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

 

[marcus]

One of the categories in the preceding post needs enlargement.

FreeImmirzi and Operator Spectra
http://arxiv.org/abs/1204.5122 This, and several others along the same lines establish the Loop black hole entropy relation S = A/4 independent of the the Immirzi parameter γ. At the same time, there is another approach to studying the range of possible values of this parameter, since the geometric operator spectra depend on γ. It turns out that it is possible to define semiclassical (Bohr-Sommerfeld) volume OUTSIDE the LQG context and thus have semiclassical eigenvalues to compare with those of LQG. I have the sense that this work is just getting started. Here is a recent paper along those lines.

Note however the footnote on page 4:
"Here l_P is the Planck length and γ is the Barbero-Immirzi parameter. They should both be understood as coupling constants of the theory. Throughout the remainder of the paper we will take l_P = γ = [STRIKE]h[/STRIKE] = 1."

As yet I do not see it constraining the variation of γ, but this line of investigation could lead to that. So far what it does is tend to confirm that the LQG geometric operators are correct, have the right spectra, because of the agreement with an alternative quantization of space.

http://arxiv.org/abs/1208.2228
Bohr-Sommerfeld Quantization of Space
Eugenio Bianchi, Hal M. Haggard
(Submitted on 10 Aug 2012)
We introduce semiclassical methods into the study of the volume spectrum in loop gravity. The classical system behind a 4-valent spinnetwork node is a Euclidean tetrahedron. We investigate the tetrahedral volume dynamics on phase space and apply Bohr-Sommerfeld quantization to find the volume spectrum. The analysis shows a remarkable quantitative agreement with the volume spectrum computed in loop gravity. Moreover, it provides new geometrical insights into the degeneracy of this spectrum and the maximum and minimum eigenvalues of the volume on intertwiner space.
32 pages, 10 figures

 

[marcus]

The main idea of this thread is a hunch that a reformulation of Loop is in progress and I'm trying to identify the areas to watch, in order to spot the main direction. In earlier posts I identified 7 areas and () may have missed the most, or one of the most significant ones: holonomy spin foams, where e.g. edges can be labeled with group elements instead of representations.
Here are a couple of papers that were recently cited "in preparation".

[28] B. Bahr, B. Dittrich, F. Hellmann, and W. Kaminski, “Holonomy Spin Foam Models:
Boundary Hilbert spaces and canonical dynamics,” (2012) .

[29] F. Hellmann and W. Kaminski, “Holonomy Spin Foam Models: Asymptotic dynamics of EPRL type models,” (2012) .

These have not come out yet but should appear this year. I'll try to explain why I think this line of investigation is important. Here is the paper which cites them.

http://arxiv.org/abs/1208.3388
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski
(Submitted on 16 Aug 2012)
We propose a new holonomy formulation for spin foams, which naturally extends the theory space of lattice gauge theories. This allows current spin foam models to be defined on arbitrary two-complexes as well as to generalize current spin foam models to arbitrary, in particular finite groups. The similarity with standard lattice gauge theories allows to apply standard coarse graining methods, which for finite groups can now be easily considered numerically. We will summarize other holonomy and spin network formulations of spin foams and group field theories and explain how the different representations arise through variable transformations in the partition function. A companion paper will provide a description of boundary Hilbert spaces as well as a canonical dynamic encoded in transfer operators.
36 pages, 12 figures

As an interested non-expert observer I now think this is probably the most significant Loop QG paper that has appeared so far this quarter (or perhaps a longer period of time).

The transfer operator concept, in spinfoam context, is introduced here:
and also here:
http://arxiv.org/abs/1103.6264
Spin foam models with finite groups
Benjamin Bahr, Bianca Dittrich, James P. Ryan
(Submitted on 31 Mar 2011)
Spin foam models, loop quantum gravity and group field theory are discussed as quantum gravity candidate theories and usually involve a continuous Lie group. We advocate here to consider quantum gravity inspired models with finite groups, firstly as a test bed for the full theory and secondly as a class of new lattice theories possibly featuring an analogue diffeomorphism symmetry. To make these notes accessible to readers outside the quantum gravity community we provide an introduction to some essential concepts in the loop quantum gravity, spin foam and group field theory approach and point out the many connections to lattice field theory and condensed matter systems.
47 pages, 6 figures

See equations (6.1) (6.8) (6.15) (6.20) starting on page 19
Further reference on page 37.
For possibility of slicing spinfoams see Dittrich Höhn 0912.1817
There is a type of transfer operator which is based on "tent moves".
For tent move concept see http://arxiv.org/abs/0912.1817 Fig.1 on page 6 and Fig.2 on page 7.

 

[marcus]

We should also have these links handy, to help understand the connection of this paper with the topic I earlier called "Stacking". Maybe I should have called it "Stacking, group labels, coarse graining, and the transfer operator." :-) All these ideas seem to be interrelated, where they involve spinfoam QG. The presence of finite groups is interesting.
http://arxiv.org/abs/1112.3567
Operator Spin Foams: holonomy formulation and coarse graining
Benjamin Bahr
(Submitted on 15 Dec 2011)
A dual holonomy version of operator spin foam models is presented, which is particularly adapted to the notion of coarse graining. We discuss how this leads to a natural way of comparing models on different discretization scales, and a notion of renormalization group flow on the partially ordered set of 2-complexes.
5 pages, 3 figures, to appear in Journal of Physics: Conference Series. (JPCS)

http://arxiv.org/abs/1010.4787
Operator Spin Foam Models
Benjamin Bahr, Frank Hellmann, Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 22 Oct 2010)
The goal of this paper is to introduce a systematic approach to spin foams. We define operator spin foams, that is foams labelled by group representations and operators, as the main tool. An equivalence relation we impose in the set of the operator spin foams allows to split the faces and the edges of the foams. The consistency with that relation requires introduction of the (familiar for the BF theory) face amplitude. The operator spin foam models are defined quite generally. Imposing a maximal symmetry leads to a family we call natural operator spin foam models. This symmetry, combined with demanding consistency with splitting the edges, determines a complete characterization of a general natural model. It can be obtained by applying arbitrary (quantum) constraints on an arbitrary BF spin foam model. In particular, imposing suitable constraints on Spin(4) BF spin foam model is exactly the way we tend to view 4d quantum gravity, starting with the BC model and continuing with the EPRL or FK models. That makes our framework directly applicable to those models. Specifically, our operator spin foam framework can be translated into the language of spin foams and partition functions. We discuss the examples: BF spin foam model, the BC model, and the model obtained by application of our framework to the EPRL intertwiners.
19 pages, 11 figures. Published in Classical and Quantum Gravity (2011)

There was also a third, related, paper:
http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
36 pages, 23 figures. Published in Classical and Quantum Gravity (2012)

The idea of TRANSFER OPERATOR, highlighted in red in preceding post, is also introduced in Dittrich's 2011 Escorial talk:
http://www.ucm.es/info/giccucm/Escorial2011/Dittrich.pdf
See the slide immediately before the Summary, at the end. And also a couple of slides before that.
The index for the July 2011 Escorial QInfo+StatM school is here:
http://www.ucm.es/info/giccucm/Escorial2011/
There seem to have been several interesting talks given at that school.

 

[marcus]

http://arxiv.org/abs/1208.3388
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

the "Groups 29" conference has been running all this past week. These four people are all invited speakers. What do you imagine their talks have been about?

http://www.cim.nankai.edu.cn/activites/conferences/hy20120820/index.htm
There is an important biennial series of conferences held once every two years, called the
International Colloquium on Group-Theoretical Methods in Physics
Most recently one was held this past week at Tianjin China This is the 29th in the series so it's called "Groups 29". It concludes tomorrow, 26 August.

The invited Loop speakers are almost all young researchers---postdocs plus some first-time faculty. It's a remarkable list.
Session 8: Loop Quantum Gravity
Chair: Jerzy Lewandowski (University of Warsaw, Poland)

Invited Speakers (Titles and Abstracts)

Emanuele Alesci (University of Erlangen-Nurnberg, Germany)
Benjamin Bahr (University of Cambridge, UK)
Norbert Bodendorfer (University of Erlangen-Nuremberg, Germany)
You Ding (Beijing Jiaotong University, China)
Bianca Dittrich (Perimeter Institute for Theoretical Physics, Canada)
Jonathan Engle (Florida Atlantic University, USA)
Marc Geiller (APC-University Paris 7, France)
Hal Haggard (Centre de Physique Theorique de Luminy, France)
Frank Hellmann (Albert Einstein Institute, Germany)
Wojciech Kaminski (Albert Einstein Institute, Germany)
Marcin Kisielowski (University of Warsaw, Poland)
Yongge Ma (Beijing Normal University, China)
Wolfgang Wieland (Universite de la Mediterranee (Marseille), France)
Mingyi Zhang (Aix-Marseille Universite, France)

Though I have some guesses about the conference presentation topics, I can't say for sure because the "Titles and Abstracts" link does not work with either of my browsers.
Maybe someone else can get the talk titles and post them here.

A list of the Tianjin talks might have clues as to what direction the changing formulation of Loop QG is going.

 

[atyy]

http://www.ucm.es/info/giccucm/Escorial2011/Dittrich.pdf

No more spin networks - just spin nets

I think it's interesting that spin nets are dual to spin foams. I had wrongly thought they'd be like spin networks.

Her final point: "can apply tensor network renormalization schemes: stay tuned" !