The Future of LQG - Oldey's Perspective

[julian]

Can I mention that researchers in LQG have themselves stated that they don't think LQG is the final answer...it's all part of the fun.

 

[marcus]

Tom, your post #28 provides a nice summary of topics to consider in discussing "the future of LQG". It's something worth thinking about because LQG has moved much more into the limelight recently.

I recently looked back at the program of the 2003 MG 10 conference and compared the relative attention paid to String and Loop then versus now. When you compare that program to the one for 2012 MG 13 you see something like a 4-to-one ratio in both cases only it has flipped around. So there is a lot more research interest focused on Loop now, a lot of people want the program to succeed and want to see what it offers for cosmology (and for extreme gravity as well, assume).

There is quite a difference in our perspectives on "the future of LQG" and I realized just now that much of the difference simply has to do with timescale. By my standards you are looking FAR OUT into the longterm future---all sorts of things could develop say on a 10year timespan.

I am focusing primarily on near term. I would like to be able to envisage the research emphasis at the Loops 2013 conference at Perimeter Institute next year. And I would like to envisage what some of the parallel sessions will be like at the Warsaw GR 20 conference July 8-12, 2013, just one year from now.

Naturally I'd like to be able to anticipate developments on beyond that, but first I want to be able to check my perceptions of what the trends and directions are---soon---within a twelve-month. So I can see if I'm wrong and need to correct my perceptions.

So my picture of "the Loop future" differs from yours (largely I think because of the timescale and the desire to be able to check by watching how the research community behaves). I want to try to sketch what I see happening in the next post or two.

A lot of it has to do with the fact that almost nothing has happened with "Master Constraint" or with any other kind of Dirac (constraint) quantization for such a long time. And the fact that I see the Erlangen people getting into Spinfoam and Dust-Hamiltonian, or what Thiemann calls "physical" Hamiltonian. Sometimes a paper will deal with both, it doesn't seem especially hard to bridge across there.

Comparing 2003 with 2012:
MG10 http://www.cbpf.br/mg10/WelcomeNew.html
MG13 http://www.icra.it/mg/mg13/parallel_sessions.htm

 

[julian]

w.r.t Dirac observables...there are certain quanities that can be measured (partial observables) but which are not Dirac observables. What GR predicts is the relationship (complete observables) between these measurable quantities - http://arxiv.org/pdf/gr-qc/0110035.pdf.

 

[julian]

Hello atty

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

I'm also interested in the holographic principle but how does it arise in string theory? I just read the other day in a review by Ashtekar that in the AdS/CFT conjecture the curled up extra dimensions are n-spheres with a radius the same order of the cosmological length!

 

[julian]

"
The last problem is that in the construction of LQG we use global spacelike foliations which restricts the manifold not only topologically but even w.r.t. its smoothness structures. That means that in (canonical) LQG we may lose physics and that this is the reason why (canonical) LQG may essentially fail!

In the book "Approaches to quantum gravity" edited by Oriti on page 332 Crane asks Thiemann the question about foliations and Thiemann replies:

"...LQG starts from this classical framework and so one may think it cannot deal with topology change. However, very beautifully this is not the case: vectors in the LQG Hilbert space are superpositions of spin network states. These describe polymerlike excitations of the gravitational field on finite graphs. Consider the volume operator of LQG associated with some spatial region. If that region has empty intersection with the given graph then the volume vanishes. Physically this means that the given state assigns no volume to that region, i.e. that there is a hole in that hypersurface. Hence we see that topology change is all over the place in LQG..."

I think what Thiemann is saying is quite subtle.

 

[julian]

Hello tom

I have a limited knowledge of the issues of differentiable structure of spatial diffs in LQG. I know that if the valence of the nodes is great enougth that using the smooth diff structure makes the Hilbert space non-seperable (http://arxiv.org/pdf/gr-qc/0403047.pdf):"Indeed, as we show below, the nodes of sufficiently high valence have a surprising “rigidity” under smooth transformation, and this rigidity turns out to be the one responsible for the moduli. Therefore the non-separability of H_{diff} is a bizarre remnant of the initial choice of the smooth category. It is therefore natural to explore the possibility of using a slightly different functional class of fields to start with."I know in the LOST theorem that they consider piecwise analytic structures. This is to avoid the union of two graphs having an infinite number of edges (if piecewise analytic curves intersect at least a countable number of times they will coincide everywhere) - it is crucial that they be piecwise becuase otherwise everything would be determined by the data in an arbitrarily small region (analyticity) and there would be no local degrees of freedom.

I'd be interested to hear more about what you think about the whole issue. Maybe you are right about topology change and diff structure in LQG. Was this not part of the motivation for Thiemann's Algebriac quantum gravity where there is no fumdamental topology or differential structure?

 

[julian]

What did ever happen to Markopoulou's approach to renormalisation via Kreimer Hopf algebras? (http://arxiv.org/pdf/hep-th/0006199.pdf). It was such a nice idea.

 

[tom.stoer]

I am not sure whether we talk about the same issue. Thiemann asks for topology change, but I am asking for same topology with non-diffeomorphic smoothness structures.

 

[tom.stoer]

@marcus, right, we are talking about different time scales regarding "future of LQG".

regarding our everlasting debate on spin networks vs. foams, constraints and anomalous quantization etc.: I think this is the central point simply b/c this the only area of research where I think LQG as of today can be provable wrong (mathematically).

 

[marcus]

@marcus, right, we are talking about different time scales regarding "future of LQG".

regarding our everlasting debate on spin networks vs. foams, ...

I'm glad you agree about the timescale difference. We could do a lot better debate-wise if we got clear about basic terms.
I gather from something you said in the "Reformulation" thread that you thought I was thinking "spin networks VERSUS foams".
For me there is no conflict. Each are a necessary part of the theory. Both are purely combinatorial objects. No manifold is needed to define either one. Manifold is extra baggage (in both cases) and out the window. :-)

Basically I try to stay up to date with the majority of the Loop community and adjust my terminology accordingly, so less liklihood for confusion, as I see it.

 

[audioloop]

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

but rovelli base a lot from them (and neglecting time)

Lorentz covariance of loop quantum gravity
http://arxiv.org/pdf/1012.1739v3.pdf

...The possibility of a Lorenz covariant formulations of spin networks has been extensively studied by Alexandrov in [12–14], where several of of the results presented here can be already found...
...See also [31]...[12–14]
The new vertices and canonical quantization
http://arxiv.org/pdf/1004.2260.pdf

[31]
Towards Loop Quantum Gravity without the time gauge.
http://arxiv.org/pdf/0811.1916.pdf

 

[marcus]

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

The future of LQG is an interesting topic. In his original post (later edited) Julian quoted Ashtekar's overview of the Loop program and then asked "what do you think is the most important direction?"
I replied by highlighting selected parts of the long Ashtekar passage in Julian's original post.

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

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

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

3. As you originally asked, but I would put in the plural: What do you think are the most important directions?

It's a question that we should ask periodically. One thing to note that has bearing on the LQG future is that next year's conference has begun to take shape. The normally biennial Loops conference in effect defines the field and gives a snapshot of the current status of the Loops research program. We should reflect on the people who have joined the Loops 2013 international Advisory Committee. They constitute an interesting assortment.

http://www.perimeterinstitute.ca/en/Events/Loops_13/Loops_13/

International Advisory Committee

Giovanni Ameliano-Camelia, University of Rome
Abhay Ashtekar, Pennsylvania State University
Fernando Barbero, Instituto de Estructura de la Materia
John Barrett, University of Nottingham
James Bjorken, SLAC
Martin Bojowald, Pennsylvania State University
Robert Brandenberger, McGill University
Alejandro Corichi, Pennsylvania State University
Fay Dowker, Imperial College, London
Rodolfo Gambini, Instituto de Fisica Facultad de Ciendias
Steve Giddings, University of California, Santa Barbara
Viqar Husain, University of New Brunswick
Ted Jacobson, University of Maryland
Kirill Krasnov, University of Nottingham
Jerzy Lewandowski, University of Warsaw
Stefano Liberati, SISSA
Etera Livine, Ens de Lyon
Renate Loll, Universiteit Utrecht
Joao Magueijo, Imperial College, London
Alex Maloney, McGill University
Matilde Marcolli, California Institute of Technology
Guillermo Mena, Instituto de Estructura de la Materia
Djordje Minic, Virginia Tech
Daniele Oriti, Albert Einstein Institute
Roberto Percacci, SISSA
Alejandro Perez, Centre de Physique Theorique
Jorge Pullin, Lousiana State University
Martin Reuter, Johannes Gutenberg Universitat
Vincent Rivasseau, Laboratoire de Physique Théorique d'Orsay
Carlo Rovelli, Centre de Physique Theorique
Thomas Thiemann, Institut für Theoretische Physik III
William Unruh, University of British Columbia

To make the mix visual, I colored different areas of expertise:
Loop, not colored
Competing QG theories orange (Spectral Geometry, AsymSafe, CDT, CausalSets...)
QG phenomenology (both concrete and speculative) green,
String magenta
with blue for uncategorized all-purpose great people.

16 primarily loop research (with interrelated spinfoam, spinnorial versions, GFT, TQFT)
6 specializing in other QG programs (spectral, asymsafe, triangulations, causal sets)
3 primarily phenomenology---ideas (both solid and speculative) related to testing.
4 string
3 uncategorized blue
Totaling 32, so just about half are drawn from what is usually considered Loop community.

 

[marcus]

Another pointer to the future of LQG is the paper which Ashtekar et al just posted on arxiv.
http://arxiv.org/abs/1209.1609
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 7 Sep 2012)
Since the standard inflationary paradigm is based on quantum field theory on classical space-times, it excludes the Planck era. Using techniques from loop quantum gravity, the paradigm is extended to a self-consistent theory from the Planck scale to the onset of slow roll inflation, covering some 11 orders of magnitude in energy density and curvature. This pre-inflationary dynamics also opens a small window for novel effects, e.g. a source for non-Gaussianities, which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
4 pages, 2 figures

This is one of a number of papers that have appeared in the last 2 years all moving in a similar direction. Early universe phenomenology is one of the (perhaps the single strongest) determinants of the immediate future of LQG. A bunch of research effort uncovering features one could look for in the Cosmic Microwave Background. Often related to inflation--both usual inflation and Loops own type of faster-than-exponential inflation that occurs naturally (without inflaton field) as a result of the bounce. A substantial part of the Loops 2013 conference is almost certainly going to be about this sector of Loop research. So that is one window on the future of LQG (the thread topic) right there.

 

[audioloop]

The of LQG is an interesting . In his post (later edited) Julian quoted Ashtekar's overview of the Loop program and then asked "what do you think is the most important direction?"
I replied by highlighting selected parts of the long Ashtekar passage in Julian's post.



It's a question that we should ask periodically. One thing to note that has bearing on the LQG is that next year's conference has begun to take shape. The normally biennial Loops conference in effect defines the field and gives a snapshot of the current status of the Loops research program. We should reflect on the people who have joined the Loops 2013 international Advisory Committee. They constitute an interesting assortment.

http://www.perimeterinstitute.ca/en/Events/Loops_13/Loops_13/

International Advisory Committee

Giovanni Ameliano-Camelia, of
Abhay Ashtekar, Pennsylvania State University
Fernando Barbero, Instituto de Estructura de la Materia
John Barrett, University of
James Bjorken, SLAC
Martin Bojowald, Pennsylvania State University
Brandenberger, McGill University
Alejandro Corichi, Pennsylvania State University
Fay Dowker, Imperial College,
Rodolfo Gambini, Instituto de Fisica Facultad de Ciendias
Steve Giddings, University of California,
Viqar Husain, University of New Brunswick
Ted Jacobson, University of Maryland
Kirill Krasnov, University of
Jerzy Lewandowski, University of Warsaw
Stefano Liberati, SISSA
Etera Livine, Ens de Lyon
Renate Loll, Universiteit

Joao Magueijo, Imperial College,

Maloney, McGill University
Matilde Marcolli, California Institute of Technology
Guillermo Mena, Instituto de Estructura de la Materia
Djordje Minic, Virginia Tech
Daniele Oriti, Albert Einstein Institute
Roberto Percacci, SISSA
Alejandro Perez, Centre de Physique Theorique
Jorge Pullin, Lousiana State University
Martin Reuter, Johannes Gutenberg Universitat
Rivasseau, Laboratoire de Physique Théorique d'Orsay
Carlo Rovelli, Centre de Physique Theorique
Thiemann, Institut für Theoretische Physik III
William Unruh,

To make the mix visual, I colored different areas of expertise:
Loop, not colored
Competing QG theories orange (Spectral Geometry, AsymSafe, CDT, CausalSets...)
QG phenomenology (both concrete and speculative) green,
magenta
with blue for uncategorized all-purpose great people.

16 primarily loop research (with interrelated spinfoam, spinnorial versions, GFT, TQFT)
6 specializing in other QG programs (spectral, asymsafe, triangulations, causal sets)
3 primarily phenomenology---ideas (both solid and speculative) related to testing.
4 string
3 uncategorized blue
Totaling 32, so just about half are from what is usually considered Loop community.

After seeing some articles of the cited researchers, I saw an alternative to the propositions of the inflationary models, written by Magueijo, is nice to see alternatives to the inflationary models

http://arxiv.org/pdf/gr-qc/0007036v1.pdf
...The varying speed of light (VSL) theory provides an elegant solution to the cosmological problems - the horizon,flatness, and Lambda problems of Big-Bang cosmology...


http://arxiv.org/pdf/astro-ph/0305457v3.pdf
...brought a varying speed of light (VSL) into the arenas of cosmology, quantum gravity...



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