Loop-and-allied QG bibliography

[marcus]

Thanks marcus, I have been waiting for this for some time. On another forum, and at PF under a different name, questions have I asked about the ILP (Information Loss Paradox) now you have yourself read the paper and it frustrates to an extent?
...

I've been waiting too, for some clarification of his position on "ILP". But I guess I am still waiting. the present paper doesn't resolve my doubts. Although it does have some additional content beyond his talk at GR17, it's generally quite similar.
To avoid overloading this thread, which is mostly a kind of library of links, I will put anything that occurs to me about this in the "new Hawking paper" thread.

 

[marcus]

http://arxiv.org/abs/gr-qc/0507106

Partial and Complete Observables for Canonical General Relativity

Bianca Dittrich
33 pages
Report-no: AEI-2005-128
"In this work we will consider the concepts of partial and complete observables for canonical general relativity. These concepts provide a method to calculate Dirac observables. The central result of this work is that one can compute Dirac observables for general relativity by dealing with just one constraint. For this we have to introduce spatial diffeomorphism invariant Hamiltonian constraints. It will turn out that these can be made to be Abelian. Furthermore the methods outlined here provide a connection between observables in the space--time picture, i.e. quantities invariant under space--time diffeomorphisms, and Dirac observables in the canonical picture."

 

[marcus]

http://arxiv.org/abs/hep-th/0507235

The case for background independence

Lee Smolin
46 pages, no figures

"The aim of this paper is to explain carefully the arguments behind the assertion that the correct quantum theory of gravity must be background independent. We begin by recounting how the debate over whether quantum gravity must be background independent is a continuation of a long-standing argument in the history of physics and philosophy over whether space and time are relational or absolute. This leads to a careful statement of what physicists mean when we speak of background independence. Given this we can characterize the precise sense in which general relativity is a background independent theory. The leading background independent approaches to quantum gravity are then discussed, including causal set models, loop quantum gravity and dynamical triangulations and their main achievements are summarized along with the problems that remain open. Some first attempts to cast string/M theory into a background independent formulation are also mentioned.
The relational/absolute debate has implications also for other issues such as unification and how the parameters of the standard models of physics and cosmology are to be explained. The recent issues concerning the string theory landscape are reviewed and it is argued that they can only be resolved within the context of a background independent formulation. Finally, we review some recent proposals to make quantum theory more relational."

 

[Kea]

http://arxiv.org/abs/hep-th/0507235

The case for background independence
Lee Smolin
46 pages, no figures

This should be fun to read, but I gave up after a couple of pages. Although it is true that Descartes deliberated relationalism he ended up espousing absolutism, whereas Newton only reluctantly followed, realising that his mathematics was not sufficient to deal with this issue.

Anyway, must be off ... NCG school in progress

Kea

 

[marcus]

http://arxiv.org/abs/hep-th/0507253

Quantum Gravity, Torsion, Parity Violation and all that
Laurent Freidel, Djordje Minic, Tatsu Takeuchi
11 pages
VPI-IPPAP-05-02

"We discuss the issue of parity violation in quantum gravity. In particular, we study the coupling of fermionic degrees of freedom in the presence of torsion and the physical meaning of the Immirzi parameter from the viewpoint of effective field theory. We derive the low-energy effective lagrangian which turns out to involve two parameters, one measuring the non-minimal coupling of fermions in the presence of torsion, the other being the Immirzi parameter. In the case of non-minimal coupling the effective lagrangian contains an axial-vector interaction leading to parity violation. Alternatively, in the case of minimal coupling there is no parity violation and the effective lagrangian contains only the usual axial-axial interaction. In this situation the real values of the Immirzi parameter are not at all constrained. On the other hand, purely imaginary values of the Immirzi parameter lead to violations of unitarity for the case of non-minimal coupling. Finally, the effective lagrangian blows up for the positive and negative unit imaginary values of the Immirzi parameter."

 

[marcus]

http://arxiv.org/abs/quant-ph/0507262
Fundamental gravitational limitations to quantum computing
Rodolfo Gambini, Rafael A. Porto, Jorge Pullin
3 pages no figures
LSU-REL-072105

"Lloyd has considered the ultimate limitations physics places on quantum computers. He concludes in particular that for an "ultimate laptop'' (a computer of one liter of volume and one kilogram of mass) the maximum number of operations per second is bounded by 10^{51}. The limit is derived considering ordinary quantum mechanics. Here we consider additional limits that are placed by quantum gravity ideas, namely the use of a relational notion of time and fundamental gravitational limits that exist on time measurements. We then particularize for the case of an ultimate laptop and show that the maximum number of operations is further constrained to 10^{47} per second."

Gambini and Pullin should be familiar to anyone watching the QG scene. their approach to QG is called "Consistent Discretizations". Last year they also published a resolution of the BH information paradox using relational time. An occasional poster here at PF, Edgar1813, has collaborated with Gambini and Pullin on QG research in much the same way as, for instance, the grad student Rafael Porto has. Here they are getting over into quantum computing (refering to Seth Lloyd) and applying gravitational limits to the ideal laptop.

In case anyone is interested here are other papers by Gambini Pullin et al.
http://arxiv.org/find/grp_physics/1/au:+Gambini/0/1/0/all/0/1
and a sampling of recent titles:

1. gr-qc/0505052
Classical and quantum general relativity: a new paradigm

2. gr-qc/0505043
Consistent discretizations: the Gowdy spacetimes

3. gr-qc/0505023
Discrete space-time

4. gr-qc/0501027
Fundamental decoherence in quantum gravity

5. gr-qc/0409057
Consistent discretization and loop quantum geometry

6. gr-qc/0409045
Unified model of loop quantum gravity and matter

7. gr-qc/0408050
Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance

9. hep-th/0406260
Realistic clocks, universal decoherence and the black hole information paradox

10. hep-th/0405183
No black hole information puzzle in a relational universe

15. gr-qc/0306095
Discrete quantum gravity: a mechanism for selecting the value of fundamental constants

 

[marcus]

John Stachel (philosophy of science) reviews main QG approaches

John Stachel gave the opening talk at the Paris Einstein Century conference
http://einstein2005.obspm.fr/programmer.php

(Gerard 't Hooft gave the final talk, other speakers included Ashtekar, Rovelli, Brian Greene...)

maybe we should pay attention to what Stachel says.

he is a philosopher and historian of science
a specialist in the Philosophy of Science as relates to the current tectonic merging of the Gen Rel and Quantum Theory plates. Stachel seems to be really interested in quantum gravity and thinking about it at a foundations level

he works with ideas, does not pursue this or that bunch of equations, though may be competent and savy as regards equations.

Rovelli said in his book that there are periods in physics when the hard philosophical questions are not superfluous and when, just in order to make progress in physics, people have to ask really fundamental questions about What is Space and What is Time.

for most of the latter half of the 20th Century physicists mostly thought they didnt have to consider Foundations or Philosophy questions, they just had to charge ahead with "doing physics". Foundations issues were considered a digression, a waste of time, those things will take care of themselves just keep your eye on getting the next formula.

but from time to time, according to Rovelli, it pays to consider these frustrating (to me a bit dumb-sounding) basic philosophical questions. Maybe you never get it right, but at least you could get some fresh ideas.

So I am going to include this one article of John Stachel
http://arxiv.org/abs/gr-qc/0507078
Structure, Individuality and Quantum Gravity
John Stachel
39 pages, to appear in "Structural Foundations of Quantum Gravity," edited by D.P. Rickles, S.R.D. French and J. Saatsi Oxford University Press

part of the abstract: "After reviewing various interpretations of structural realism, I adopt here a definition that allows both relations between things that are already individuated (which I call "relations between things'') and relations that individuate previously un-individuated entities ("things between relations"). Since both space-time points in general relativity and elementary particles in quantum theory fall into the latter category, I propose a principle of maximal permutability as a criterion for the fundamental entities of any future theory of "quantum gravity''; i.e., a theory yielding both general relativity and quantum field theory in appropriate limits..."

table of contents:
"Contents
1 What is Structural Realism? 3
2 Structure and Individuality 5
3 Effective field theory approach and asymptotic quantization 11
4 String Theory 15
5 Quantum general relativity - some preliminary problems 17
- 5.1 States or Processes: Which is primary ? . . . . . . . . . . . . 17
- 5.2 Formalism and measurability . . . . . . . . . . . . . . . . . . . 20
6 Canonical quantization (loop quantum gravity). 25
7 The causal set (causet) approach 29
8 What Structures to Quantize? 31
9 Acknowledgements 33"

 

[marcus]

http://arxiv.org/astro-ph/0507683
Inflation: A graceful entrance from Loop Quantum Cosmology
N. J. Nunes (U. Minnesota)
9 pages, 8 figures

Abstract: "Dynamical scalar fields in the framework of loop quantum cosmology have recently risen a considerable amount of attention. This interest relates mainly to the natural way the initial conditions for slow roll inflation are set and the avoidance of a big crunch in closed models. In this work, the evolution of a scalar field is explored..."

Start of Introduction: "Currently, the leading background independent and non-perturbative candidate for a quantum theory of gravity is loop quantum gravity [1, 2, 3] which is a canonical quantization of general relativity based in Ashtekar’s variables. ... Loop quantum Cosmology(LQC) is the application of loop quantum gravity to homogeneous and isotropic mini-superspaces [4]. An important featureof LQC is that eigenvalues of the inverse scale factor operator..."


N.J.Nunes is a comparative newcomer to LQC, having earlier done research in string/brane cosmology.
Thru 2004 he was at Queen Mary
and is now at U. Minnesota.
He has 14 papers going back to 1998, several in cosmology-related topics.
The string/brane stuff was back around 2003, and possibly earlier.

EXCELLENT TALK BY SMOLIN ON VIDEO

http://cfa-www.harvard.edu/colloquia/spring05/smolin.html

this was a colloquium at Harvard for astronomy/astrophysics audience given in May 2005
covers relation of QG to predictions and observations
general survey of problem of QG, basics of Loop approach, contrast with string, gives a lot of intuition about LQG. explains where the various predictions come from, which they are still working on to get in final shape for AUGER and GLAST. about 50 minutes talk followed by questions from
astronomers and Smolin answering.

we just got DSL, what a difference! less than a minute to download the video. probably never would have seen this if we still had our older slow connection.

title was Astrophysical Tests of Quantum Gravity
but talk was, as I indicated, much broader and more generally useful, could serve as a good introduction to both the full LQG theory and to Loop Quantum Cosmology.

NEW ROVELLI PAPER
http://www.arxiv.org/abs/gr-qc/0508007
From 3-geometry transition amplitudes to graviton states/B]
Federico Mattei (CPT), Carlo Rovelli (CPT), Simone Speziale (CPT), Massimo Testa (CPT)
18 pages
"In various background independent approaches, quantum gravity is defined in terms of a field propagation kernel: a sum over paths interpreted as a transition amplitude between 3-geometries, expected to project quantum states of the geometry on the solutions of the Wheeler-DeWitt equation. We study the relation between this formalism and conventional quantum field theory methods. We consider the propagation kernel of 4d Lorentzian general relativity in the temporal gauge, defined by a conventional formal Feynman path integral, gauge fixed a' la Fadeev--Popov. If space is compact, this turns out to depend only on the initial and final 3--geometries, while in the asymptotically flat case it depends also on the asymptotic proper time. We compute the explicit form of this kernel at first order around flat space, and show that it projects on the solutions of all quantum constraints, including the Wheeler-DeWitt equation, and yields the correct vacuum and n-graviton states. We also illustrate how the Newtonian interaction is coded into the propagation kernel, a key open issue in the spinfoam approach."

 

[Spin_Network]

Marcus this needs slotting in:http://arxiv.org/abs/gr-qc/0508007

 

[marcus]

Marcus this needs slotting in:http://arxiv.org/abs/gr-qc/0508007

thanks I actually got to it yesterday afternoon and slotted it into the preceding post
I am glad you are watching out Spin Network, so will be less worried about missing things.

 

[marcus]

relating triangle approach to loop and spin foam gravity

this paper was written a while back---an updated version would be nice to have---but it serves a useful purpose:

it is a kind of "Rosetta Stone" connecting LQG and Spin Foams to Dynamical Triangulations

http://arxiv.org/abs/gr-qc/0110026
Relating Covariant and Canonical Approaches to Triangulated Models of Quantum Gravity
Matthias Arnsdorf
28 pages, 10 figures
Class.Quant.Grav. 19 (2002) 1065-1092

"In this paper explore the relation between covariant and canonical approaches to quantum gravity and $BF$ theory. We will focus on the dynamical triangulation and spin-foam models, which have in common that they can be defined in terms of sums over space-time triangulations. Our aim is to show how we can recover these covariant models from a canonical framework by providing two regularisations of the projector onto the kernel of the Hamiltonian constraint. This link is important for the understanding of the dynamics of quantum gravity. In particular, we will see how in the simplest dynamical triangulations model we can recover the Hamiltonian constraint via our definition of the projector. Our discussion of spin-foam models will show how the elementary spin-network moves in loop quantum gravity, which were originally assumed to describe the Hamiltonian constraint action, are in fact related to the time-evolution generated by the constraint. We also show that the Immirzi parameter is important for the understanding of a continuum limit of the theory."

the first two references cited are papers by Ambjorn Jurkiewicz and Loll.

[1] J. Ambjorn, J. Jurkiewicz, and R. Loll. Lorentzian and euclidean quantum gravity: Analytical and numerical results. 1999, hep-th/0001124.

[2] J. Ambjorn, J. Jurkiewicz, and R. Loll. Dynamically triangulating lorentzian quantum gravity. Nucl. Phys., B610:347–382, 2001, hep-th/0105267.

It is clear that Arnsdorf is trying to put together a picture unifying AJL work with the quantum gravity of Rovelli, Baez, Smolin, Thiemann ...

 

[marcus]

Chronos has supplied a paper which appears to refute the Smolin CNS-multiverse conjecture

http://www.arxiv.org/abs/astro-ph/0508050
A 2.1 Solar Mass Pulsar Measured by Relativistic Orbital Decay

David J. Nice, Eric M. Splaver (Princeton), Ingrid H. Stairs (UBC), Oliver Loehmer, Axel Jessner (MPIfR), Michael Kramer (Jodrell Bank), James M. Cordes (Cornell)
9 pages, Submitted to ApJ

Here are some supporting articles on the physics of neutron stars which the present one cites

http://www.arxiv.org/abs/astro-ph/0405262

http://www.arxiv.org/abs/astro-ph/0002232

Here is a recent discussion of the CNS conjecture

http://www.arxiv.org/abs/hep-th/0407213

just speaking for myself, this discovery further tips the balance of interest
from Loop towards the Loll Triangulation approach to QG.

 

[Chronos]

I totally agree, marcus. Not to claim any great insights, but, I believe the universe will ultimately submit to being described as a quantum computer.

 

[marcus]

I flagged this of Smolin in post #372 of this thread almost a month ago---on 25 July---and I didn't realize what a lot of fuss it would cause. There has been quite a racket in "the string community" about Smolin urging them to work out a non-perturbative formulation. No other paper, that has appeared in the month since, has had such repercussions. So I will bring this one forward to have the link handy.

http://arxiv.org/abs/hep-th/0507235

The case for background independence

Lee Smolin
46 pages, no figures

"The aim of this paper is to explain carefully the arguments behind the assertion that the correct quantum theory of gravity must be background independent. We begin by recounting how the debate over whether quantum gravity must be background independent is a continuation of a long-standing argument in the history of physics and philosophy over whether space and time are relational or absolute. This leads to a careful statement of what physicists mean when we speak of background independence. Given this we can characterize the precise sense in which general relativity is a background independent theory. The leading background independent approaches to quantum gravity are then discussed, including causal set models, loop quantum gravity and dynamical triangulations and their main achievements are summarized along with the problems that remain open. Some first attempts to cast string/M theory into a background independent formulation are also mentioned... "

Smolin has been discussing background independence in the open blog-place of ideas. for a sample, look at post #8 of this PF thread
https://www.physicsforums.com/showthread.php?t=85306
or scroll down the 100 or so posts in the "Landscape" thread at cosmicvariance.

Post #8 of the "smolin letter" thread is here
https://www.physicsforums.com/showthread.php?p=720463#post720463
and has a good sample of what smolin has been writing on that subject, besides what is already in the paper. It is interesting to see the reactions from the stringy flock, say from Jacques Distler (also on post #8) and also at the "String Coffee Table" blog.
http://golem.ph.utexas.edu/string/archives/000621.html
In that thread, Robert Helling displays a significant reaction to Smolin's paper. He says: "... Smolin’s arguments would appeal especially to outsiders of the field. We should prevent too many people from getting the idea that 'string theory is obviously wrong as it ignores the basic notion of background independence'. So I collected some thoughts that might help you arguing the next time you are envolved in this debate..."

Robert Helling's collected thoughts, to help in arguing against Smolin's suggestion that they put some effort into a nonperturbative string formulation, are online at:
http://www.damtp.cam.ac.uk/user/rch47/background.pdf

 

[marcus]

new Etera Livine paper, LQG black holes this time

http://www.arxiv.org/abs/gr-qc/0508085
Quantum Black Holes: Entropy and Entanglement on the Horizon
Etera R. Livine, Daniel R. Terno
25 pages, 4 figures

"We are interested in black holes in Loop Quantum Gravity (LQG). We study the simple model of static black holes: the horizon is made of a given number of identical elementary surfaces and these small surfaces all behaves as a spin-s system accordingly to LQG. The chosen spin-s defines the area unit or area resolution, which the observer uses to probe the space(time) geometry. For s=1/2, we are actually dealing with the qubit model, where the horizon is made of a certain number of qubits. In this context, we compute the black hole entropy and show that the factor in front of the logarithmic correction to the entropy formula is independent of the unit s. We also compute the entanglement between parts of the horizon. We show that these correlations between parts of the horizon are directly responsible for the asymptotic logarithmic corrections. This leads us to speculate on a relation between the evaporation process and the entanglement between a pair of qubits and the rest of the horizon. Finally, we introduce a concept of renormalisation of areas in LQG."

All I can say right now is that I've watched Livine's research for a couple of years and I'm impressed. If he is doing something with LQG and black holes it is probably worth doing. Livine's thesis came out in 2003, if I remember, and we flagged it at PF and had a look. The name Terno is a new one to me.

I now see that Livine and Terno have already published a paper this year in Physical Review A
and also I see that they have a paper with Girelli in preparation, called
F. Girelli, E. R. Livine, D. R. Terno, Reconstructing Quantum Geometry from Quantum Information: Entanglement as a Measure of Distance
(as Kea once said, "where is SetAI when we need him?")

http://www.arxiv.org/abs/gr-qc/0508088
Finiteness and Dual Variables for Lorentzian Spin Foam Models
Wade Cherrington
"We describe here some new results concerning the Lorentzian Barrett-Crane model, a well-known spin foam formulation of quantum gravity. Generalizing an existing finiteness result, we provide a concise proof of finiteness of the partition function associated to all non-degenerate triangulations of 4-manifolds and for a class of degenerate triangulations not previously shown. This is accomplished by a suitable re-factoring and re-ordering of integration, through which a large set of variables can be eliminated. The resulting formulation can be interpreted as a 'dual variables' model that uses hyperboloid variables associated to spin foam edges in place of representation variables associated to faces. We outline how this method may also be useful for numerical computations, which have so far proven to be very challenging for Lorentzian spin foam models."

The name Cherrington is also a new one. I can only take note of this paper, to evaluate later. Cherrington is at UWO, where Dan Christensen is. He might be a grad student or postdoc working with Dan. Looks like they may collaborate on a paper. At UWO they do computer calculation with spin foams, John Baez worked with them on this at UWO. It is one of the places where advanced computer facilities and techniques is paired with QG.

http://www.arxiv.org/abs/gr-qc/0508091
Background independent quantizations: the scalar field I
Wojciech Kaminski, Jerzy Lewandowski, Marcin Bobienski
13 pages
"We are concerned with the issue of quantization of a scalar field in a diffeomorphism invariant manner. We apply the method used in Loop Quantum Gravity. It relies on the specific choice of scalar field variables referred to as the polymer variables. The quantization, in our formulation, amounts to introducing the 'quantum' polymer *-star algebra and looking for positive linear functionals, called states. The assumed in our paper homeomorphism invariance allows to determine a complete class of the states. Except one, all of them are new. In this letter we outline the main steps and conclusions, and present the results: the GNS representations, characterization of those states which lead to essentially self adjoint momentum operators (unbounded), identification of the equivalence classes of the representations as well as of the irreducible ones. The algebra and topology of the problem, the derivation, all the technical details and more are contained in the paper-part II."

Lewandowski is by now a familiar face. he is the L in the LOST (Lewandowski, Okolow, Sahlmann, Thiemann) uniqueness theorem.
Also a frequent-coauthor with Ashtekar.
Lewandowski cites the recent Smolin paper. Here is the first paragraph

The phrase "background independent theory" means in Physics a theory defined on a bare manifold endowed with no extra structure like geometry or fixed coordinates. A prominent example is the theory of matter fields coupled to Einstein’s gravity. In the case of a background independent classical theory it is natural to assume the background independence in a corresponding quantum theory.
A profound polemic devoted to that issue can be found in recent paper of Smolin [CITES "THE CASE FOR BACKGROUND INDEPENDENCE"]. The canonical formulation of the field theory relies on the 3 + 1 decomposition of space-time into the "space" M and "time" R. Then, the background independence implies invariance with respect to the diffeomorphisms of M. The invariance concerns in particular any matter fields in question: they have to be quantized in an often new, background independent way. In this letter and the accompanying paper [2] we are concerned with the issue of a diffeomorphism invariant quantization of a scalar field.[/color]

 

[marcus]

http://www.arxiv.org/abs/gr-qc/0508100
On the quantum origin of the seeds of cosmic structure
Alejandro Perez, Hanno Sahlmann, Daniel Sudarsky

"The current understanding of the quantum origin of cosmic structure is discussed critically. We point out that in the existing treatments a transition from a symmetric quantum state to an (essentially classical) non-symmetric state is implicitly assumed, but not specified or analyzed in any detail. In facing the issue we are led to conclude that new physics is required to explain the apparent predictive power of the usual schemes. Furthermore we show that the new way of looking at the relevant issues opens new windows from where relevant information might be extracted regarding cosmological issues and perhaps even clues about aspects of quantum gravity."

We know Sahlmann from the LQG uniqueness theorem. here he is taking quite a different direction

------from conclusions section pages 44 through 48----
We have discussed the problematic part of the standard analysis that is supposed to predict the primordial spectrum of fluctuations responsible for the deviation of our universe from perfect homogeneity and isotropy and in particular for the eventual evolution of galaxies stars and our own. We have argued that there is an essential element that is missing in existing proposals. We have argued that the missing element must contain some new physics. We have considered this issue following the line of thought exposed by Penrose, that such new physics might be tied to some quantum aspect of gravitation, and we have employed this idea in what we called the collapse hypothesis, which is reflected concretely in our model in the fact that we take the Newtonian potential to couple to expectation values of the quantum matter degrees of freedom, and have allowed such expectation values to “jump” in association with the so called collapse process in a particular set of states. It should thus be emphasized that this can be justified only if we declare that gravitation is, at the quantum level profoundly different from other degrees of freedom as only such posture would justify the different treatment awarded to both the gravitational and the scalar sectors in the present work. We have shown that a relatively simple proposal concerning a collapse of the wave function induced by some unknown mechanism, possibly tied to Quantum Gravity can in fact account in a transparent way, for the scale invariant spectrum that seems to fit very well with the observations. ...
...
...
We end by noting a paradoxical aspect of the situation in our field of study: On the one hand there is an almost frenetic search for any form of experimental manifestations of any conceivable aspect of quantum gravity, while on the other hand, when faced with one such clear arena for these type studies, as the one we have treated in this work, the prevailing attitude seems to be to hide the mysteries under the rug and declare that everything is fine. It is our hope that this paper contributes to changing this situation.
----end quote----

 

[Spin_Network]

Great paper marcus, thanks again, and a great excuse to hit a significant number! :) shuttle re-entry speed !

 

[marcus]

Glad you approve, Spin_Network! Just to condense this down to the essentials:

http://www.arxiv.org/abs/gr-qc/0508100
On the quantum origin of the seeds of cosmic structure
Alejandro Perez, Hanno Sahlmann, Daniel Sudarsky

---quote conclusions page 48---
We end by noting a paradoxical aspect of the situation in our field of study: On the one hand there is an almost frenetic search for any form of experimental manifestations of any conceivable aspect of quantum gravity, while on the other hand, when faced with one such clear arena for these type studies, as the one we have treated in this work, the prevailing attitude seems to be to hide the mysteries under the rug and declare that everything is fine. It is our hope that this paper contributes to changing this situation.
---end quote---

I normally refrain from complaining about getting my brain fried because I think it is more considerate of other people not to fuss about it. an inevitable side-effect we all have to cope with occasionally. but I will acknowledge that the seeds of structure in quantum fluctuations business is something of an enigma.

 

[marcus]

http://www.arxiv.org/abs/gr-qc/0508106

On the perturbative expansion of a quantum field theory around a topological sector

Carlo Rovelli, Simone Speziale
7 pages

"The idea of treating general relativistic theories in a perturbative expansion around a topological theory has been recently put forward in the quantum gravity literature. Here we investigate the viability of this idea, by applying it to conventional Yang--Mills theory on flat spacetime. We find that the expansion around the topological theory coincides with the usual expansion around the abelian theory, though the equivalence is non-trivial. In this context, the technique appears therefore to be viable, but not to bring particularly new insights. Some implications for gravity are discussed."


With this next one, Noldus is a postdoc of Loll at Utrecht. I don't know how this fits in. It does not look like CDT.

http://www.arxiv.org/abs/gr-qc/0508104
A fully consistent relativistic quantum mechanics and a change of perspective on quantum gravity

Johan Noldus
16 pages

"This paper can be seen as an exercise in how to adapt quantum mechanics from a strict relativistic perspective while being respectful and critical towards the experimental achievements of the contemporary theory. The result is a fully observer independent relativistic quantum mechanics for N particle systems without tachyonic solutions. A remaining worry for the moment is Bell's theorem."

 

[marcus]

Reuter confirms Loll---or "makes contact" anyway

Reuter and Lauscher are going to be at the October Loops 05
and they have a different approach to quantum gravity where they just flat out quantize classical Einstein equation and renormalize the sucker.
(no loops, no strings, no networks, no foams, no triangles)
this was supposed not to be possible, but Reuter apparently very determined and stubborn---insists on it being possible.

selfAdjoint was reading an earlier Reuter paper and had some comments, maybe we can get some clues from him about this paper too. ohwilleke too.

Reuter is one of the invited speakers at Loops 05 conference..

He says he has "made contact" with Loll triangulation path integral gravity.

that would be news if it is what it sounds like

http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter
20 pages

"Four-dimensional Quantum Einstein Gravity (QEG) is likely to be an asymptotically safe theory which is applicable at arbitrarily small distance scales. On sub-Planckian distances it predicts that spacetime is a fractal with an effective dimensionality of 2. The original argument leading to this result was based upon the anomalous dimension of Newton's constant. In the present paper we demonstrate that also the spectral dimension equals 2 microscopically, while it is equal to 4 on macroscopic scales. This result is an exact consequence of asymptotic safety and does not rely on any truncation. Contact is made with recent Monte Carlo simulations."

Interestingly, Loll got something fractally and 2D looking at very small scale in a macro-4D world----measuring the spectral dimension with a diffusion process. looks like Reuter got something like that (very different approach)

Because this looks like a CONVERGENCE of Loll and Reuter approaches to a quantum spacetime dynamics, I have put some discussion of this Reuter paper in the Loll thread "Quantum Graffiti":

https://www.physicsforums.com/showthread.php?p=727388#post727388

 

[selfAdjoint]

The business about quantizing GR turns on the existing concept of asymptotic safety, which itself turns on the behavior of the renormalization group fixed point. I don't have my notes and I'm too tired tonight to post on it, but I'll try to put something up tomorrow. Asymptotic safety, if it is accepted, could revolutionize a lot of old abandoned ideas.

 

[Kea]

http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter

I hadn't come across these guys before. Very nice program they have going, called Quantum Einstein Gravity. See the series of papers

http://www.arxiv.org/find/hep-th/1/au:+Lauscher_O/0/1/0/all/0/1

They appear to have put CDT into a slightly more fundamental context (if the results do indeed match up). selfAdjoint, I'm looking forward to you explaining your notes on this.

Cheers
Kea

 

[Kea]

...this was supposed not to be possible, but Reuter apparently very determined and stubborn...insists on it being possible.

The family of Lagrangians idea that they work with is very nice. The so-called impossibility stems from not thinking this way, and insisting that something like a standard path integral over geometries should do the trick. They also stress that they study effective theories and make no claims that the full theory should be described in these terms, unlike some of the CDT papers.

 

[marcus]

if anyone is feeling extra interested in Reuter's work we have this thread from around a year ago:

https://www.physicsforums.com/showthread.php?t=47209

and also I think some more recent (e.g. July 18) in this "reference link library" thread

https://www.physicsforums.com/showthread.php?p=682965#post682965

(this is the "Big Bang to asympt. de Sitter" paper, the earlier one from last October might be of more interest, obviating dark energy by running Newton G)

 

[marcus]

new Bojowald, new Kiefer, new Dowker

http://arxiv.org/gr-qc/0508118
Degenerate Configurations, Singularities and the Non-Abelian Nature of Loop Quantum Gravity
Martin Bojowald
24 pages, 1 figure
Report-no: AEI-2005-133

"Degenerate geometrical configurations in quantum gravity are important to understand if the fate of classical singularities is to be revealed. However, not all degenerate configurations arise on an equal footing, and one must take into account dynamical aspects when interpreting results: While there are many degenerate spatial metrics, not all of them are approached along the dynamical evolution of general relativity or a candidate theory for quantum gravity. For loop quantum gravity, relevant properties and steps in an analysis are summarized and evaluated critically with the currently available information, also elucidating the role of degrees of freedom captured in the sector provided by loop quantum cosmology. This allows an outlook on how singularity removal might be analyzed in a general setting and also in the full theory. The general mechanism of loop quantum cosmology will be shown to be insensitive to recently observed unbounded behavior of inverse volume in the full theory. Moreover, significant features of this unboundedness are not a consequence of inhomogeneities but of non-Abelian effects which can also be included in homogeneous models."

http://arxiv.org/gr-qc/0508120
Quantum Gravity: General Introduction and Recent Developments
Claus Kiefer
Comments: 21 pages, 6 figures, invited contribution for "Annalen der Physik"

"I briefly review the current status of quantum gravity. After giving some general motivations for the need of such a theory, I discuss the main approaches in quantizing general relativity: Covariant approaches (perturbation theory, effective theory, and path integrals) and canonical approaches (quantum geometrodynamics, loop quantum gravity). I then address quantum gravitational aspects of string theory. This is followed by a discussion of black holes and quantum cosmology. I end with some remarks on the observational status of quantum gravity."

http://arxiv.org/gr-qc/0508109
Causal sets and the deep structure of spacetime
Fay Dowker
17 pages. Article contributed to "100 Years of Relativity - Space-time Structure: Einstein and Beyond" ed Abhay Ashtekar (World Scientific)

"The causal set approach to quantum gravity embodies the concepts of causality and discreteness. This article explores some foundational and conceptual issues within causal set theory."

 

[marcus]

getting back to this, probably important, paper. I will get some Wiki links that could help people interested in reading it

First, here is the paper

http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter
20 pages

"Four-dimensional Quantum Einstein Gravity (QEG) is likely to be an asymptotically safe theory which is applicable at arbitrarily small distance scales. On sub-Planckian distances it predicts that spacetime is a fractal with an effective dimensionality of 2. The original argument leading to this result was based upon the anomalous dimension of Newton's constant. In the present paper we demonstrate that also the spectral dimension equals 2 microscopically, while it is equal to 4 on macroscopic scales. This result is an exact consequence of asymptotic safety and does not rely on any truncation. Contact is made with recent Monte Carlo simulations."

... a CONVERGENCE of Loll and Reuter approaches to a quantum spacetime dynamics, I have put some discussion of this Reuter paper in the Loll thread "Quantum Graffiti":

https://www.physicsforums.com/showthread.php?p=727388#post727388

Now here are some relevant Wiki links

http://en.wikipedia.org/wiki/Path_integral_formulation

http://en.wikipedia.org/wiki/Effective_field_theory

http://en.wikipedia.org/wiki/Renormalizable

I should warn that Lubos Motl seems to have inserted string buzz into a number of Wiki entries, which one can sometimes discover by clicking on the entry's "history" tab. In connection with some biased language one may find in the history that it was supplied by "Lumidek" or other signature. As for example in "Renormalizable" entry where one sees Lumidek in connection with this:

"On the other hand, Fermi's interaction or general relativity are non-renormalizable. This problem is a hint that these theories should be replaced by a more complete theory at very short distance. These more complete theories are the electroweak theory and string theory, respectively."

The nonrenormalizability of Gen Rel is discussed in the literature and several conclusions are drawn. The slanted and overly simple inference (that Motl gives here) is challenged, for example by Rovelli in his book. Rovelli gives an argument why the simple analogy with electroweak doesn't work and that a different conclusion can be drawn. But this does not matter, I think----soon one adjusts to the fact that string propaganda has been inserted in parts of the Wiki environment, and one can filter it out.

On the whole Wiki is extremely useful! Here is more Wiki stuff:

http://en.wikipedia.org/wiki/Renormalization

http://en.wikipedia.org/wiki/Renormalization_group

http://en.wikipedia.org/wiki/Asymptotic_freedom

 

[marcus]

http://www.arxiv.org/abs/gr-qc/0508124
Graviton propagator from background-independent quantum gravity
Carlo Rovelli
6 pages

"We study the graviton propagator in euclidean loop quantum gravity, using the spinfoam formalism. We use boundary-amplitude and group-field-theory techniques, and compute one component of the propagator to first order, under a number of approximations, obtaining the correct spacetime dependence. In the large distance limit, the only term of the vertex amplitude that contributes is the exponential of the Regge action: the other terms, that have raised doubts on the physical viability of the model, are suppressed by the phase of the vacuum state, which is determined by the extrinsic geometry of the boundary."

 

[marcus]

new Loll paper, CDT for wider physics audience

http://www.arxiv.org/abs/hep-th/0509010

The Universe from Scratch

R. Loll, J. Ambjorn, J. Jurkiewicz
30 pages, 5 figures; review paper commissioned by Contemporary Physics and aimed at a wider physics audience

I started a thread for this, for any comments:

https://www.physicsforums.com/showthread.php?p=732021#post732021

a paper like this has been needed.

the authors discuss the connection with Martin Reuter work ("quantum einstein gravity", assymptotic safety)

 

[marcus]

http://www.arxiv.org/abs/gr-qc/0509039
Bibliography of Publications related to Classical Self-dual variables and Loop Quantum Gravity

Alejandro Corichi, Alberto Hauser
45 pages
"This bibliography attempts to give a comprehensive overview of all the literature related to what is known as the Ashtekar-Sen connection and the Rovelli-Smolin loop variables, from which the program currently known as Loop Quantum Gravity emerged..."

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

Some Loops 05 links:
http://loops05.aei.mpg.de/
http://loops05.aei.mpg.de/index_files/Programme.html
http://loops05.aei.mpg.de/index_files/Participants.html
http://loops05.aei.mpg.de/index_files/abstract_rovelli.html
http://loops05.aei.mpg.de/index_files/abstract_reuter.html
http://loops05.aei.mpg.de/index_files/abstract_loll.html
http://loops05.aei.mpg.de/index_files/abstract_freidel.html
http://loops05.aei.mpg.de/index_files/abstract_starodubtsev.html
..and so on..

the full program was posted Friday 9 September, a month before the start of conference

 

[marcus]

http://www.arxiv.org/abs/gr-qc/0509049
Eigenvalues of the volume operator in loop quantum gravity
Krzysztof A. Meissner
12 pages
"We present a simple method to calculate certain sums of the eigenvalues of the volume operator in loop quantum gravity. We derive the asymptotic distribution of the eigenvalues in the classical limit of very large spins which turns out to be of a very simple form. The results can be useful for example in the statistical approach to quantum gravity."

Sample from the conclusions section at the end:

"...Besides being mathematically interesting, these results can be important for example in the statistical approach to loop quantum gravity. Several conceptual problems arise in this context by the results of the present paper. The first one is what observables (besides the volume) should be specified to actually describe a macroscopic state in a given region. Since the contribution to the volume from a given 4-valent vertex grows like j₁j₂j₃ it is important to know whether these observables provide suppression of the contribution from very large spins rendering final expectation value of the volume finite. The second problem is connected with the first one and concerns the bulk entropy: there are many sets of graphs with many different assignments of spins on legs that give (macroscopically) the same volume. If we specify both the volume and the other observables we are left with a certain number of graphs. The question is, whether the logarithm of this number is connected with the bulk entropy much as sequences of spins are connected with the black hole surface entropy [7]."

we know Meissner from his earlier work with Lewandowski, calculating BH entropy (http://arxiv.org/gr-qc/0407052 ) and estimating Immirzi parameter.