Loop-and-allied QG bibliography

[maddy]

The Bianchi IX model in Loop Quantum Cosmology
Authors: Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
Comments: 41 pages, 3 figures, revtex4
Report-no: IMSc/2004/04/16, AEI-2004-028

The Bianchi IX model has been used often to investigate the structure close to singularities of general relativity. Its classical chaos is expected to have, via the BKL scenario, implications even for the approach to general inhomogeneous singularities. Thus, it is a popular model to test consequences of modifications to general relativity suggested by quantum theories of gravity. This paper presents a detailed proof that modifications coming from loop quantum gravity lead to a non-chaotic effective behavior. The way this is realized, independently of quantization ambiguities, suggests a new look at initial and final singularities
http://arxiv.org/abs/gr-qc/0404039

How does LQG get rid of cosmological singularities in homogeneous models for e.g. Bianchi V? Any good references to papers?

Is the Belinskii-Khalatnikov-Lifschitz scenario mostly used in analyzing quantum gravity solutions for cosmological singularities?

Thanks for any help (am just a beginner ) .

 

[marcus]

How does LQG get rid of cosmological singularities in homogeneous models for e.g. Bianchi V? Any good references to papers?
.

Maddy, almost all the papers I know of by Bojowald etc have dealt with the isotropic case.

I will find some URL for ones dealing with anisotropic, but it is a comparatively recent effort AFAIK.
I haven't seen anything about Bianchi V. I only recall stuff about Bianchi IX.

People sometimes come here (stingray, nonunitary) who might be able to respond more usefully.

As for papers about the homogenous (but not isotropic) case here are some references:
the most recent review of progress in the field is
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053

this contains only two refs that are explicitly to the homogeneous case and they are from last year
"Homogeneous loop quantum cosmology" gr-qc/0303073
"Homogeneous loop quantum cosmology: the role of the spin connection" gr-qc/0311004

maybe you can find more if you look in the references of these

there was also this, which I have not looked at so cannot say if it mentions Bianchi V,
"Quantum suppression of the generic chaotic behavior close to cosmological singularities" gr-qc/0311003

it looks to me as if the homog. case is just barely being scratched at
and that the papers mostly go back only to November of last year
but I am not knowledgeable about homog. case and you might find out things
are different if you take a closer look

most LQC papers involve homogenenous and isotropic simplification and
in effect use a quantized form of the Friedmann equation (which is what most cosmology depends on anyway)---again this is just my limited view and we could hopefully get some expert comment responding to your question

 

[meteor]

http://arxiv.org/abs/gr-qc/0404055


Ln(3) and Black Hole Entropy
Authors: Olaf Dreyer
Comments: Contribution to the Proceedings of the 3rd International Symposium on Quantum Theory and Symmetries, Cincinnati, September 2003

"We review an idea that uses details of the quasinormal mode spectrum of a black hole to obtain the Bekenstein-Hawking entropy of A/4 in Loop Quantum Gravity. We further comment on a recent proposal concerning the quasinormal mode spectrum of rotating black holes. We conclude by remarking on a recent proposal to include supersymmetry. "






This paper try to fix the Immirzi parameter
Give a look to page 2. It contains the formula for the entropy of a Black hole according to Loop Quantum Gravity

 

[marcus]

this thread is serving as a surrogate sticky "reference library". Thanks to all who have contributed so far!

-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf

Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094

-----a recent review article----
http://arxiv.org./abs/gr-qc/0404018

Ashtekar and Lewandowski
"Background Independent Quantum Gravity: a Status Report"
125 pages
many references

---------a newsletter: "Matters of Gravity"----
Jorge Pullin's newsletter "Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue

-------Quantum Gravity Phenomenology and DSR---------

some recent phenomenology and DSR papers:

Ted Jacobson, Stefano Liberati, David Mattingly
"Quantum Gravity Phenomenology and Lorentz Violation"
http://arxiv.org./abs/gr-qc/0404067
15 April 2004

Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004

Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003

Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003

Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004

other less recent ones:

Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003

M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003


---------Loop Quantum Cosmology-------

Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)

The Bianchi IX model in Loop Quantum Cosmology
Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
41 pages
http://arxiv.org/abs/gr-qc/0404039

"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
http://arxiv.org./abs/gr-qc/0403106

Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003

Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.

Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003

Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."

Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.

as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003

-------recent conferences------

Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html

Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm

International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/

Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html

--------upcoming conferences--------

Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/

http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Feb.2
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10

General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference

------observational means for testing quantum gravity------

Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003

Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.


------links to an unselective assortment of current work------

Carlo Rovelli and Winston Fairbairn
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047

John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040

Livine's thesis
http://arxiv.org/gr-qc/0309028

Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032

Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"

Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.

Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060

Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110

Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111

Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112

Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113

Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076

Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062

Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)

----------fundamental constants, Planck units, time-keeping-------

Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm

In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic Planck units

http://physics.nist.gov/cuu/Constants/

choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature

A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf

-------science journalism----
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112

A translation of Rudy Vaas' article in the German
science magazine "Bild der Wissenschaft" roughly
comparable to the "Scientific American"

========
simply to have this link on LaTex handy:
https://www.physicsforums.com/misc/howtolatex.pdf

 

[marcus]

Woit's blog:
http://www.math.columbia.edu/~woit/blog/

the responses are getting interesting too
------------------
Recent paper by Olaf Dreyer
http://arxiv.org./gr-qc/0404055
-------------------
this is spillover from the main page of links
which is full
-------------------
a new QG Phenomenology paper
http://arxiv.org./gr-qc/0404113

"On alternative approaches to Lorentz violation invariance in loop quantum gravity inspired models
Jorge Alfaro, Marat Reyes, Hugo A. Morales-Tecotl and L.F. Urrutia
------------

there is also a new Quantum Gravity Phenomenology
paper by Ted Jacobson et al
dealing with QG predictions of Lorentz violation and
their testability

http://arxiv.org/gr-qc/0404067
"Quantum Gravity Phenomenology and Lorentz Violation"
Ted Jacobson, Stefano Liberati, David Mattingly

 

[marcus]

online resources for category theory

looks like some category theory may be needed to do
quantum gravity (e.g. Velhinho, also several Baez papers)

http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes

http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"

 

[marcus]

Hendryk Pfeiffer has a new preprint on arxiv
called
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088

"...d = 3 + 1, if it can indeed be constructed, will offer the same potential. This relationship is the main theme of the present article.
The special role of space-time dimension d = 3 + 1 in differential topology is summarized by the following result.

Theorem 1.1. Let M be a topological d-manifold, d = 1, 2, 3, 4,... If M admits an infinite number of pairwise inequivalent differentiable structures, then d = 4.

This is a corollary of several theorems by various authors. We explain in this article why this result is related to the search for a quantum theory of general relativity..."

here is something unique about the dimension 4
It is the only possible dimension for spacetime to be if you want to have
plenty of possible smooth-manifold structures.

In this sense, dim = 5 is not OK, and dim = 6 is not OK.

This is a surprising theorem. I didnt know that d = 3+1 was mathematically so special as that. It is so surprising that I think I must be failing to understand. but Pfeiffer is I think very good and there it is written in black and white as theorem 1.1, so will go back and try to understand.

-------had to do something else and just got back-----
Pfeiffer says, on page 16:

"In d ≥ 3 + 1, no analogous result is available. There exist countably infinite families of (compact) smooth 4-manifolds [13] which are pairwise non-diffeomorphic, but which have homeomorphic underlying topological manifolds. There is therefore a considerable discrepancy between C∞- and C₀-QFTs in d = 3 + 1 space-time dimensions.

The most striking result even concerns the standard space R4 [14, 15].
Theorem 4.1. Consider the topological manifold R^d, d ε N.

• If d < 4, then there exists a differentiable structure for R^d which is unique up to diffeomorphism.
• If d = 4, then there exists an uncountable family of pairwise non diffeomorphic differentiable structure for R^d.

Non-uniqueness of differentiable structures persists in higher dimensions, for example, there are 28 inequivalent differentiable structures on the sphere S⁷, or 992 inequivalent differentiable structures on S¹¹, [16], but in dimension d ≥ 4 + 1 (d ≥ 5 + 1 if the manifold has a non-empty boundary), there never exists more than a finite number of non-diffeomorphic differentiable structures on the same underlying topological manifold.

The space-time dimension d = 3 + 1 is distinguished by the feature that there can exist an infinite number of homeomorphic, but non-diffeomorphic smooth manifolds."

------then on page 19 Pfeiffer says------
"Scenario for quantum gravity.

We have reached a first goal: the diffeomorphism gauge symmetry of general relativity on a closed space-time manifold has been translated into a purely combinatorial problem involving triangulations that consist of only a finite number of simplices, and their manipulation by finite sequences of Pachner moves.

If not only the partition function, but also the full path integral of general relativity in d ≤ 5+1 is given by a PL-QFT, we know that all observables are invariant under Pachner moves.

The partition function of quantum general relativity is an invariant of PL-manifolds, too, and can be computed by purely combinatorial methods for any given combinatorial manifold.

A generic expression of such a partition function is the state sum,
Z = <br /> <br /> \sum_{ colourings } \prod_{ simplices }<br /> (amplitudes),

where the sum is over all labelings of the simplices with elements of some set of colours, and the integrand is a number that can be computed for each such labeling. In Section 5 below, we give examples and illustrate that the partition function of quantum general relativity in d = 2 + 1 is precisely of this form.

If quantum general relativity in d = 3+1 is indeed a PL-QFT, the following two statements which sound philosophically completely contrary,

• Nature is fundamentally smooth.
• Nature is fundamentally discrete.

are just two different points of view on the same underlying mathematical structure: equivalence classes of smooth manifolds up to diffeomorphism."

also on page 20, right after this, there is a picture which illustrates what are Pachner moves in 2 dimensions and 3 dimensions.

this business on pages 19 and 20 of Pfeiffer paper seems interesting. I never heard talk like this. I have bolded some exerpts for emphasis.

 

[meteor]

http://arxiv.org/abs/gr-qc/0404083

Spectrum of quantized black hole, correspondence principle, and holographic bound
Authors: I.B. Khriplovich
Comments: 9 pages

An equidistant spectrum of the horizon area of a quantized black hole does not follow from the correspondence principle or from general statistical arguments. On the other hand, such a spectrum obtained in loop quantum gravity (LQG) either does not comply with the holographic bound, or demands a special choice of the Barbero-Immirzi parameter for the horizon surface, distinct from its value for other quantized surfaces. The problem of distinguishability of edges in LQG is discussed, with the following conclusion. Only under the assumption of partial distinguishability of the edges, the microcanonical entropy of a black hole can be made both proportional to the horizon area and satisfying the holographic bound

 

[selfAdjoint]

That's a new thought. All edges are equal but some edges are more equal.

 

[Sauron]

I knew long time ago about these peculiarity of 4-dimensional manifolds. I thought it was a mainstream knowledge.

By some stupid reason i have no acces to arwiv for around a week so i couldn´t read the article. So while waitng i´ll ask about other, slighly related, things.

I have a few generic questions/refelxions aobut some of the themes LQG is addressing.


Let´s beging by the question of entropy. My deal is whether the concept of entropy makes sense in GR at all. A lest in the same sense that in ordinay statistichal mehcanics.

I know about two main results. The one, of which i have a reasonable understanding , about the black hole area behaving like entropy. I also have notice about (but no understanding at all) results of Penrose relating the Weyl tensor to entropy, at least in cosmologicla scenarys

The question is that in the microcanonical device the entropy is reltaed to the number of micro-states compatible with an energy. But in GR there is no a good (and less local) definition of the energy of the gravitatory field.

I think these is commonly known. Anyway i would like to know how it has been addressed.

In order to get my own understanding i revised the whole idea (i never have had a deep basics in statistichal mecanics and it was a good exercice for me) of micro-states.

It works fine because it is used mainly for quantum mechanical systems with a discrete spectrum. But i wanted to understand how it could work incontinuous systems, other than the gravitatory field, an dalso in continuous spectrum of quantum systems.

I begined revising the black-body radiation. There the thermal equiibrium is suposed to be achieved by influence of the walls of a cavity.

By general (non rigurous) arguments (kirchov laws and it´s subleties/consecuences) it is assumed that an arbitrary (as far as i know expermintally tested for simply connected) shaped body can be simulated by a box shaped resonant cavity.

Them the number of microstates of the EM field can be charazterized by thre numbers (k1.k2,k3). And by standard manipulations they can be related to the frecuency.

Ok, i will not reexplain all the remaining basic facts. My point is here. We can give a meaning to the number of microstates because we are dealing with a configuration in which the EM field is in a compact region. ALso it is important to note that it seems that we are faced with a topological nature of the black body radiation. It is invariant under topological changes of the shap of the cavity.

So i try to think on the vibration numbers like something with a topologicla meaning. ¿woul the black body reaction could have another behaviour for non topologically trivial shapes of the caity?

Well, i know these hass addresed me far form the original question of the entropy of the gravitatory field. But my gues is that we are triying to aplly blindly a concept which works fine in non relativistic discret spectred hamiltonians. Maybe in some more general situations it needs some refinements.

For example, ¿whay about yang-mills fields?

Of course you will be saying ¿What about at finite temperature field theory?

I have not studied it in deep. But the whole idea seems to me so merely formal that doesn´t convince me as a good way to gain deep understanding of the problems. But of course probably it is my problem.

Well, i have more conceptual issues. But i´ll expose them in another moment.

 

[marcus]

this is interesting
QG has already challenged the Big Bang singularity
quantizing seems to remove it (Bojowald and others say)

maybe there is no black hole singularity, just a very deep well
maybe quantizing GR will remove this singularity also

also even without quantizing GR some alternative models avoid a singularity


http://arxiv.org/astro-ph/9908113
"Alternate Explosions: Collapse and Accretion Events with Red Holes instead of Black Holes"

html version:
http://arxiv.org./html/astro-ph/9908113

http://arxiv.org./astro-ph/9912322
"Red Hole Gamma-Ray Bursts: A New Gravitational Collapse Paradigm
Explains the Peak Energy Distribution And Solves the GRB Energy Crisis"

gammaray bursts are especially interesting and may even now not be
satisfactorily explained---awful lot of energy in them, more than supernovas

what has happened to these 1999 conjectures?

EDIT: by the way the author, Jim Graber, sometimes posts here at PF.
why doesn't someone ask him about this alternative picture of gravitational collapse

 

[marcus]

categories, as they impinge on Quantum Gravity

looks like some category theory may be needed to do
quantum gravity (e.g. Velhinho, also several Baez papers)

http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes

http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"

not being a whiz with categories or else they're just not very familiar, I'm
a bit bothered by their infiltrating into QG

there was just this paper by John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040

and this other recent paper by Hendryk Pfeiffer
is categorical in its approach
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088

and back in February there was this paper by Velhinho
"On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060

and now there is a woman mathematician who has published with Louis Crane and lives in New Zealand---her name is Marni Dee Sheppeard.
Her paper is so categorical that it seems the same to me whether I read it front to back or back to front. That is, I see a lot of diagrams with arrows and don't understand anything. but it purports to be about quantum gravity and I like the name Marni Dee so here's the link:

http://arxiv.org/gr-qc/0404121


On state sums, internalisation and unification
M. D. Sheppeard
35 pages

Abstract: "In this mostly expository article, elements of higher category theory essential to the construction of a class of four dimensional quantum geometric models are reviewed. These models improve current state sum models for Quantum Gravity, such as the Barrett-Crane model, in that they appear, for instance to remove degeneracies which swamp the partition function. Much work remains to be done before a complete construction is reached, but the crucial categorical notion of internalisation already illuminates the idea that a full unified model may result from few, albeit as yet poorly understood, additional principles. In particular, a spacetime and matter duality principle is employed through an understanding of the role of pseudomonoidal objects in categorified cohomology."

the good news: these models improve on the Barrett-Crane model which is pretty much the main spin-foam studied and which has some numerical crankiness (Baez published a paper in 2002 about this)

the good news: somehow putting spacetime and matter on the same footing?

the bad news: what the devil is pseudomonoidal objects in categorified cohomology?! it sounds like a new disease and I just hope it's not painful.
well fraid there's something else to learn about in QG

 

[selfAdjoint]

The Sheppeard paper looks good! I am going to read it, and see if I can keep up with it.


I just love the sniffy prose style too, which I associate with Oxbridge and its colonies like The Economist :

"The first and simplest way of regarding a manifold M as a category is to give it, albeit rather trivially, a groupoid structure. That is, the points of the manifold are the objects of the category and each point has attached an identity arrow, which is of course invertible."

Of course!

 

[marcus]

The Sheppeard paper looks good! I am going to read it, and see if I can keep up with it.

I am much cheered by your positive reaction.
I think I might be able to get something out of pages 6 thru 10
as they are a basic expostion of category theory.
although I'm less than confident of getting much of the rest of the paper.

I didnt notice at first that she even had a prose style! when I first looked i was just very impressed: she seemed awesomely intelligent and mainly bewildering. Will try to attribute some of this to her English.

This is a nice efficient way to start talking about categories (maybe it is the standard definition):

"Whereas a set has elements, and a map between sets takes elements to elements, a category has both elements, called objects, and relationships between elements, called arrows. Every object A is equipped with at least an identity arrow 1_A from A to A. Maps between categories, called functors, take objects to objects and arrows to arrows. Arrows may be composed f ◦g..."

 

[marcus]

she makes this point first off that I don't believe I've heard put so clearly by anyone:
background independence is basic to QG, she says, and
if you want background independence
then you have to go categories.

rovelli never told me that, baez only hinted at it, she flatly asserts it.
there is a bold magesterial quality turn to her thinking. Doesnt mince words. Says right out front where she's going. E.g. here is the first of the introduction:


"1 Introduction
The philosophy behind the construction outlined here is that any reasonable
attempt to describe quantum gravity within a unified framework ought to
respect some quantum principle of general covariance. Recall that in coming
to accept general covariance in the first place [37], Einstein needed to rid
himself of the idea that spacetime points had physical meaning outside of
their use in the metric tensor field. Put another way: no gravitational field,
no spacetime.
It is argued here that categorical internalisation is an essential element
required of a successful mathematical description of such a principle. There is mounting support for this point of view from studies of, for instance,..."

 

[selfAdjoint]

Yes. The only problem with doing the first pages is that she front loads the abstract stuff and expects her readers to have some problems with that. Of course with my weird background, that's the part I like!

 

[meteor]

http://arxiv.org/abs/hep-th/0405036
Remarks on the black hole entropy and Hawking spectrum in Loop Quantum Gravity
Authors: A. Alekseev, A.P. Polychronakos, M. Smedback
Report-no: CCNY-HEP-04/3, UUITP-13/04

In this note we reply to the criticism by Corichi concerning our proposal for an equidistant area spectrum in loop quantum gravity. We further comment on the emission properties of black holes and on the statistics of links.

 

[Mike2]

Doesn't LQG propose disconnected quanta of space-time? Or are these quanta of space-time connected by infinitesimal strings between them? Does LQG really propose communication through something of no dimensional thickness?

 

[Sauron]

Sorry for the off-topic.

As I told in the previous post I couldn´t download the last pdf´s because my acces to arxiv was temporarilly locked.

The nnouncement said "one week", but it was taking more time to be unlocked so I mailed arxiv and the lock was deletted.

But i have been warned about using prefetching software. ¿Wha´s that?, could it be the acrobat reader active-w which executes from inside the explorer.

I ask because i wouldn´t want to be locked agian. I can´t follow properly these interesting threads without acces to the pdf´s, you know.

thanks in advance and sorry again for the off-topic.

 

[meteor]

Sauron I was also banned one week from arxiv, but it was because my acrobat reader was of an old version. They said that if i updated to the newer version of acrobat I wouldn't be banned again, so I did and so I haven't been banned anymore

 

[marcus]

Sauron and Meteor, I haven't yet experienced this difficulty with arxiv
but if it happens, knowing about the
possibility will make it less of a shock. thanks for the warning!
----------------
a September 2004 conference in quantum gravity has been announced
by Jonathan Halliwell of Imperial College London

"Current Themes in Quantum Gravity: A Two-day Conference
in Honour of the 60th Birthday of Chris Isham

Imperial College, September 6-7, 2004.

Inspired by the 60th birthday of Prof. Chris Isham, we are pleased to announce a conference whose aim is to take stock of the current status of quantum gravity, its mathematical foundations, conceptual problems and physical predictions for cosmology and black holes. Aspects of quantum theory related to gravity will also be considered. The main themes of the meeting will be as follows:

The canonical Hamiltonian approach to gravity and the loop variable approach to quantum gravity.

Alternatives to the canonical approach, such as causal sets and other discrete models.

The phenomenology of quantum gravity, in particular, quantum cosmology and quantum black holes.

Aspects of the foundations of quantum theory related to quantum gravity, in particular, the decoherent histories approach and topos-theoretic ideas.

Conceptual problems in quantum gravity, in particular, the problem of time.


The list of speakers will include,

Abhay Ashtekar (Penn State)
Stanley Deser (Brandeis)
Gary Gibbons (Cambridge)
Jim Hartle (Santa Barbara)
Stephen Hawking (Cambridge)
Karel Kuchar (Utah)
Renate Loll (Utrecht)
Roger Penrose (Oxford)
Carlo Rovelli (Marseille)
Rafael Sorkin (Syracuse)
"

the announcement says more detail will eventually be available on the
website
http://theory.ic.ac.uk .

http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Apr.11

 

[marcus]

why the world is 4D---a possible explanation from quant grav

a recent paper co-written by Renate Loll

http://arxiv.org./hep-th/0404156

"Emergence of a 4D World from Causal Quantum Gravity"

my unqualified&inexpert opinion: Renate Loll kicks butt, anybody studying quantum gravity would be lucky to get a chance to study with her at Utrecht (netherlands)

they (Ambjorn, Jurkiewicz, Loll) did a monte carlo simulation (massive randomized computer experiment) and 4 dimensions emerged probabilistically from their quantum gravity setup---they probably helped and it isn't conclusive but it is a suggestive preliminary finding at the least

Renate Loll is one of the featured speakers (with Stephen Hawking, Carlo Rovelli, Roger Penrose,...) at that Quantum Gravity conference happening in London this september. what odds she talks about this monte carlo QG research

 

[marcus]

A few hours after I wrote this (about the AJL paper) John Baez posted the "Marseille Workshop" thread at PF calling attention to his "Week 206" which is mostly about the Ambjorn Jurkiewicz Loll paper. Here is a link to Baez Week 206:

http://math.ucr.edu/home/baez/week206.html

Judging from what Baez says, and from today's reaction by Thomas Larsson on SPR, it could be a landmark paper.

Larsson's comment on AJL in reply to Baez week 206 was:
"If the numerical evidence in this paper is true, and it seems quite strong, then we see a new field open up here...I would not be surprised if this is the next bandwagon and a lot of smart people will jump onto it."

 

[Olias]

Hi Marcus (ranyart here) you not be aware of this 'amazing' paper:http://uk.arxiv.org/abs/gr-qc/0405060

One question before the storm?..the Dynamical Volume Operator here proposed is 3-D, Volumes do not exist in 2-Dimensional Spacetimes.

Straight Lengths for 2-D spaces, Curved Lengths for 3-D!
we are on our way ;)

 

[marcus]

...not be aware of...:http://uk.arxiv.org/abs/gr-qc/0405060

another by Thomas Thiemann!
he is busy this year
hope all is well with you
thanks for the link

 

[marcus]

getting back what we were talking about earlier
here is a photograph of Renate Loll

http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm

for some reason it does not form an active link and must be copy-pasted into the browser

the snapshot appears to have been taken at Utrecht institute for theoretical physics

with ears discretely concealed (in case of being a Vulcan)

a recent paper co-written by Renate Loll

http://arxiv.org./hep-th/0404156

"Emergence of a 4D World from Causal Quantum Gravity"

my unqualified&inexpert opinion: Renate Loll kicks butt, anybody studying quantum gravity would be lucky to get a chance to study with her at Utrecht (netherlands)

they (Ambjorn, Jurkiewicz, Loll) did a monte carlo simulation (massive randomized computer experiment) and 4 dimensions emerged probabilistically from their quantum gravity setup---they probably helped and it isn't conclusive but it is a suggestive preliminary finding at the least

Renate Loll is one of the featured speakers (with Stephen Hawking, Carlo Rovelli, Roger Penrose,...) at that Quantum Gravity conference happening in London this september. what odds she talks about this monte carlo QG research

the best introductory presentation of Loll's style of quantum gravity that I have found so far is
http://arxiv.org/hep-th/0212340
"A Discrete History of the Lorentzian Path Integral"

this is dated 13 January 2003 and describes results in d = 2 and d = 3.
These foreshadow the results just announced for d = 4.
On page 16 she says:
"... In d = 4, the first numerical simulations are currently being set up."

so this is a good pedagogical exposition of just before the recent major result

 

[marcus]

Nesvizhevky's bouncing neutron experiment

http://arxiv.org./hep-ph/0306/0306198[/URL]

Measuring quantum states of neutrons in the Earth's gravitational field

this experiment was a first
(objects falling in the Earth's grav field seem to descend in jumps from level to level because their grav potential energy is quantized)
it was a beautiful experiment
first announced in a 2-page note in Nature vol 415 (2002)
http://www.nature.com/nsu/020114/020114-8.html
but this 9-page (Phys Rev D) article is online and beautifully illustrated
and more complete than the earlier one in Nature

[this thread is serving as a "surrogate sticky" quantum gravity link-basket,
or a reference library of links to QG stuff, the quantization of energy levels of a falling neutron seems at least periferally related to quantum gravity]

Nesvizhevsky's team is at Grenoble France but now
according to ZapperZ in another thread
there may be related work at Mainz

 

[selfAdjoint]

Umm, Marcus, that experiment has been interpreted as showing not that gravity is quantized but that the particle's ENERGY is quantized. So whatever the gravity potential is, they can only acquire energy from it in steps. But of course that's old news.

 

[marcus]

so does the link belong in General Physics?
and not in the Quantum Gravity link-basket?
I would like to keep tabs on it one place or the other
but don't care about the wording

if you suggest an editing change I will be glad to make it

[PS I think I see the objection so I went ahead and changed
the wording in the post to emphasize that the connection with QG may be
only periferal.]

 

[pelastration]

Marcus,

This Loll et All approach reminds me somehow to Buckminster Fuller's work , but he didn't started from 'dust-type' particles.

Buckminster was a very gifted guy. He was very consequent in his idea's, also on politics. This is the offical website: http://www.bfi.org

In his giant standard work http://www.rwgrayprojects.com/synergetics/synergetics.html you will some find some amazing concepts. I believe you have to enter that link first (for copyright).
Then I find it helpful to go first to the index about images : http://www.rwgrayprojects.com/synergetics/findex/findex.html[/URL].
By these image numbers you can look for the real texts (paragraph numbers) in the Table of content [URL]http://www.rwgrayprojects.com/synergetics/toc/toc.html[/URL].

Buckminster: "Please now think of all the tensional forces of Universe as one single membrane containing all the radiational, explosive forces we have enumerated. Now think of the original compression sphere exploding into many ... individual, exploded-apart, spherical mass components, each of which is tightly embraced by the membrane - leaving only intervening perpendicular linear tubes."

I think you will appreciate his logic.

Dirk

 

[marcus]

A new paper by Amelino-Camelia
http://arxiv.org/gr-qc/0405084

A new one by Livine and Oriti
http://arxiv.org/gr-qc/0405085

any comments are welcome. I have to go out for part of the evening, so
may not get around to discussing these for a while. I would guess the
Livine/Oriti paper is interesting.

L/O:
----quote---
Does a quantum gravity theory with an invariant length and a discrete spectrum for geometric observables necessarily break Lorentz symmetry or necessarily require some sort of modification/deformation of it? The answer, as we will see, is simply “no”.
---end quote---

 

[Olias]

I believe they are Two Gems!

 

[marcus]

this thread is serving as a surrogate sticky "reference library" for useful LQG links. Thanks to all who have contributed so far!
the last time I updated the main list of references was post #163 about thirty posts back, so it's about time to update the list again

I will break it down into some categories, with textbooks and introductory survey lectures and such coming first

------- texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf

Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094

-----a recent review article----
http://arxiv.org./abs/gr-qc/0404018

Ashtekar and Lewandowski
"Background Independent Quantum Gravity: a Status Report"
125 pages
many references

another recent survey:
Enrique Alvarez
http://arxiv.org/gr-qc/0405107
"Quantum Gravity"
( Lectures given at Karpacz. 40 pages)

this next is older and interesting partly for historical and broader perspective.
it is a Rovelli survey at a 1997 GR conference (plenary at GR15)
and you get not just LQG and string but some other approaches that
were tried in the 1990s:
http://arxiv.org/gr-qc/9803024
Carlo Rovelli
"Strings, loops and others: a critical survey of the present approaches to quantum gravity"
" I illustrate the main achievements and the main difficulties in: string theory, loop quantum gravity, discrete quantum gravity (Regge calculus, dynamical triangulations and simplicial models), Euclidean quantum gravity, perturbative quantum gravity, quantum field theory on curved spacetime, noncommutative geometry, null surfaces, topological quantum field theories and spin foam models..."

---------a newsletter: "Matters of Gravity"----
Jorge Pullin's newsletter "Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue

-------Quantum Gravity Phenomenology and DSR---------

some recent phenomenology and DSR papers:

A new paper by Amelino-Camelia
http://arxiv.org/gr-qc/0405084


http://arxiv.org./gr-qc/0404113
"On alternative approaches to Lorentz violation invariance in loop quantum gravity inspired models
Jorge Alfaro, Marat Reyes, Hugo A. Morales-Tecotl and L.F. Urrutia


Ted Jacobson, Stefano Liberati, David Mattingly
"Quantum Gravity Phenomenology and Lorentz Violation"
http://arxiv.org./abs/gr-qc/0404067
15 April 2004

Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004

Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003

Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003

Jerzy Lukierski
"Relation between quantum ?-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004

other less recent ones:

Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003

M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003


---------Loop Quantum Cosmology-------

Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)

The Bianchi IX model in Loop Quantum Cosmology
Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
41 pages
http://arxiv.org/abs/gr-qc/0404039

"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
http://arxiv.org./abs/gr-qc/0403106

Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003

Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.

Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003

Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."

Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.

as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003

-----in case of category theory----

http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes

http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"[/QUOTE]

 

[marcus]

here are some recent conferences and other stuff (I still need to edit and bringh some of it up to date)
------recent conferences------

Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html

Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm

International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/

Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html


Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
Baez report on it
http://math.ucr.edu/home/baez/week206.html


--------upcoming conferences------

General Relativity Conference (GR17) at Dublin 3 July 2004

http://www.dcu.ie/~nolanb/gr17.htm

more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference

Chris Isham's 60th Birthday conference
Imperial College London, around September 6-7th
for info go to the Imperial College site and click on
"Isham 60 Conference"
http://theory.ic.ac.uk/




------links to an unselective assortment of current work------

Ambjorn Jurkeiwicz Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"


Carlo Rovelli and Winston Fairbairn
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047

John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040

Hendryk Pfeiffer has a new preprint on arxiv
called
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088


Livine's thesis
http://arxiv.org/gr-qc/0309028

Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032

Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"

Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.

Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060

Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110

Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111

Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112

Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113

Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076

Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062

Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)

http://arxiv.org/abs/hep-th/0405036
Remarks on the black hole entropy and Hawking spectrum in Loop Quantum Gravity
Authors: A. Alekseev, A.P. Polychronakos, M. Smedback
Report-no: CCNY-HEP-04/3, UUITP-13/04

In this note we reply to the criticism by Corichi concerning our proposal for an equidistant area spectrum in loop quantum gravity. We further comment on the emission properties of black holes and on the statistics of links.

A new one by Livine and Oriti
http://arxiv.org/gr-qc/0405085


----------fundamental constants, Planck units, time-keeping-------

Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm

In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic Planck units

http://physics.nist.gov/cuu/Constants/

choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature

A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf

------prospects for testing quantum gravity observationally------

Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003

Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.

-------science journalism----
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112

A translation of Rudy Vaas' article in the German
science magazine "Bild der Wissenschaft" roughly
comparable to the "Scientific American"

========
simply to have this link on LaTex handy:
https://www.physicsforums.com/misc/howtolatex.pdf
quotes about physics:
http://www.angelo.edu/faculty/kboudrea/cheap/cheap2_physics.htm
Michael Flohr's great set of notes on group theory in physics:
http://www.itp.uni-hannover.de/~flohr/lectures
(scroll down to "Physical Applications of Group Theory")

 

[marcus]

A Secret Tunnel Through The Horizon

http://arxiv.org/hep-th/0405160
Maulik K. Parikh
A Secret Tunnel Through The Horizon
(First prize in the Gravity Research Foundation Essay Competition)
7 pages

 

[marcus]

A poster on SPR named Chris Weed has noted Parikh's paper and recommended it, together with another that has recently appeared
http://arxiv.org/gr-qc/0405111

some more new ones:

http://arxiv.org/hep-th/0405183
"No black hole information puzzle in a relational universe"
Rodolfo Gambini, Rafael Porto, Jorge Pullin
4 pages

http://arxiv.org/gr-qc/0405119
"Automorphism covariant representations of the holonomy-flux *-algebra"
Andrzej Okolow, Jerzy Lewandowski
32 pages


Lewandowski/Okolow abstract: "We continue an analysis of representations of cylindrical functions and fluxes which are commonly used as elementary variables of Loop Quantum Gravity. We consider an arbitrary principal bundle of a compact connected structure group and following Sahlmann's ideas define a holonomy-flux *-algebra whose elements correspond to the elementary variables. There exists a natural action of automorphisms of the bundle on the algebra; the action generalizes the action of analytic diffeomorphisms and gauge transformations on the algebra considered in earlier works. We define the automorphism covariance of a *-representation of the algebra on a Hilbert space and prove that the only Hilbert space admitting such a representation is a direct sum of spaces L^2 given by a unique measure on the space of generalized connections. This result is a generalization of our previous work (Class. Quantum. Grav. 20 (2003) 3543-3567, gr-qc/0302059) where we assumed that the principal bundle is trivial, and its base manifold is R^d."

 

[marcus]

the topcite 50+ feature at Spires

there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:

http://www.slac.stanford.edu/spires/hep/

to illustrate, suppose you want to find influential or highly cited papers by Jan Ambjorn.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D World from Causal Quantum Gravity")

In the main search field if you type
find a ambjorn and topcite 50+

then it will list those of Jan Ambjorn's papers which have received 50 or more citations to date

It turns out he has authored 34 papers which topped 50 citations.

Or you can say "topcite 100+" to restrict the search still further,
and find several of Ambjorn's papers which have topped 100.

The search engine takes several different formats and for one of them instead of saying "find a thiemann" you have to say
"find author thiemann". but the default seems to be use the letter A to stand for author.

 

[jeff]

there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:

http://www.slac.stanford.edu/spires/hep/

In the main search field if you type
find author ambjorn and topcite 50+

then it will list Jan Ambjorn's papers which have received 50 or more
citations to date

or you can say "topcite 100+" to restrict the search still further.

Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D world...")

It turns out Ambjorn has published several "100+" citations papers.

This is an lqg thread. Of course maybe your posting this because you now believe lqg is wrong and you've decided to change religions.

 

[marcus]

A new PF poster named setAI pointed out a good 6-page essay by Lee Smolin today, so I will add it to this collection of links:

Sample from page 5:
"The debate between proponents of background-dependent and background independent theories is in fact just the modern version of an ancient debate. Since the Greeks, the argument has raged between those who believed that space and time have an eternally fixed, absolute character and those who thought space and time are no more than relations between events that themselves evolve in time. Plato, Aristotle, and Newton were absolutists. Heraclites, Democritus, Leibniz, Mach, and Einstein were relationalists. When we demand that the quantum theory of gravity be background-independent, we are saying we believe that the triumph that general relativity represented for the relational point of view is final and will not be reversed.


Much of the argument between string and loop theorists is a continuation of this debate. Most string theorists were trained as elementary-particle physicists and worked their whole lives in a single fixed spacetime. Many of them have never even heard of the relational/absolute debate, which is the basic historical and philosophical context for Einstein's work. Most people who work in loop quantum gravity do so because at some point in their education they understood the relational, dynamical character of spacetime as described in general relativity, and they believe in it. They don't work on string theory because they cannot take seriously any candidate for a quantum theory of gravity that is background-dependent and hence loses (or at best hides) the relational, dynamical character of space and time."
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html

It is a good essay because it combines a clearsighted overview with a personal insider's take, and also tells the history of this approach to quantum gravity from a participant's perspective.


Sample from page 3:
"Loop quantum gravity started in the early 1980s with some discoveries about classical general relativity by Amitaba Sen, then a postdoc at the University of Maryland. These were made into a beautiful reformulation of Einstein's theory by Abhay Ashtekar, then at Syracuse University and now director of the Center for Gravitational Physics at Penn State—a reformulation that brought the mathematical and conceptual language we use to describe space and time closer to the language used in particle physics and quantum physics."
http://www.edge.org/3rd_culture/smolin03/smolin03_p3.html

Another quote from page 5:
"Another reason that string theory cannot be the final word is that in string theory one studies strings moving in a fixed classical spacetime. Thus, string theory is what we call a background-dependent approach. It means that one defines the strings as moving in a fixed space and time. This may be a useful approximation, but it cannot be the fundamental theory. One of the fundamental discoveries of Einstein is that there is no fixed background. The very geometry of space and time is a dynamical system that evolves in time. The experimental observations that energy leaks from binary pulsars in the form of gravitational waves—at the rate predicted by general relativity to the unprecedented accuracy of eleven decimal places—tells us that there is no more a fixed background of spacetime geometry than there are fixed crystal spheres holding the planets up. The fundamental theory must unify quantum theory with a completely dynamical description of space and time. It must be what we call a background-independent theory. Loop quantum gravity is such a one; string theory is not."
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html

From page 6:
"So while I disagree with the leading string theorists about methodology, this hasn't kept me from working on string theory. After all, they don't own it; its open problems are there for anyone to try to solve. So I decided a few years ago to ignore their advice and try to construct the background independent form of M theory. In the process of inventing loop quantum gravity, we gained a lot of knowledge about how to make quantum theories of space and time that are background-independent."
http://www.edge.org/3rd_culture/smolin03/smolin03_p6.html

the essay seems to have been written in latter half of 2003, so is fairly recent.
thanks to setAI for flagging this one

 

[selfAdjoint]

Thinking about all this LQG vs. string business, I wonder if the "beauitiful reformulation" of Ashtekar isn't like the "beautiful reformulation of string theory" of Schwartz and Witten that has motivated so much string research. I am wondering whether in the final analysis, beauty is all it's cracked up to be as a search strategy.

The two great historical exemplars of beauty first were Einstein and Dirac. In each case their approach achieved a great success early but then led them into unproductive wastelands. And it is at least arguable that both string physics and LQG research in the Ashtekar tradition are right now spinning their wheels. Maybe it's time for a younger generation, playing Feynman and Dyson to the Witten - Ashtekar version of Einstein-Dirac to have their say. Which is why I am very interseted in the AJL paper, a possibly rough hewed (remember Feyman's early rep?) but undoubtedly novel approach to the problem of background independent quantum mechanics (and THAT, not just quantum gravity is the big kahuna).

This post is possibly not in line with your intent to have this as a colllection of documents, but I just couldn't resist, seeing the same old same old out of Smolin being posted once again.

 

[marcus]

Thinking about all this LQG vs. string business,...

...Which is why I am very interseted in the AJL paper, a possibly rough hewed (remember Feyman's early rep?) but undoubtedly novel approach to the problem of background independent quantum mechanics (and THAT, not just quantum gravity is the big kahuna)...

Amen to that.
Background independent quantum mechanics is the big kahuna.

this turns up the lights on something that was creeping around the edges of my mind also

I'm very interested in the AJL dynamical triangulations approach too. We could continue in the Marseille thread that Baez started (it is largely about AJL but Marseille was billed as a Loop+Foam conference---a lot of family resemblances: loop to foam and foam to simplicial QG---making too sharp distinctions could be a mistake.
Anyway, if we leave this thread as a catchbasket for LQG-and-related links we could followup on AJL etc at the Marseille thread if that suits you, or start a new one on the Big Kahuna!

 

[marcus]

Doubly Special Relativity, new paper by Kowalski-Glikman

http://arxiv.org/abs/hep-th/0405273

the title is "Introduction to DSR"
there was a Quantum Gravity symposium in poland
this February and Jerzy Kowalski-Glikman (the organizer) lectured on
DSR and its relation to QG

this paper was developed from his lectures at the Winterschool, and
submitted to Springer for publication in its "lecture notes in physics" series.

anything calling itself an Introduction could potentially
be useful.

the idea of DSR is that the usual minkowski space and lorentz group symmetries
of special relativity are what results from forcing c to be the same
for all observers
what if you force TWO physical quantities, not just the speed of light but also the Planck length or the Planck mass, to be the same for all observers.

----------------
more DSR news, this time from Alejandro's city of Zaragoza

just out:
http://arxiv.org/hep-th/0405285

Quantum Uncertainty in Doubly Special Relativity
Authors: Jose Luis Cortes, J. Gamboa
4 pages, no figures

The modification of the quantum mechanical commutators in a relativistic theory with an invariant length scale (DSR) is identified...

---------QG phenomenology-----
a new paper:

http://arxiv.org/quant-ph/0406007

"Could Energy Decoherence due to Quantum Gravity be observed?"
Christoph Simon, Dieter Jaksch
7 pages, no figures

Sample from abstract:
"It has recently been proposed that quantum gravity might lead to the decoherence of superpositions in energy, corresponding to a discretization of time at the Planck scale...
... We also show how local energy decoherence, which acts separately on system and phase reference, could be detected with remarkable sensitivity and over a wide range of length scales by long-distance Ramsey interferometry with metastable atomic states. The sensitivity of the experiments can be further enhanced using multi-atom entanglement."

 

[marcus]

the Spires database is an amazing resource for keeping track of activity in various research lines and seeing what topics are attracting interest

several links within that site have proven useful (for me) recently

Here is the 2003 edition of the topcites for all categories, gr-qc as well as hep-th and the rest
http://www.slac.stanford.edu/library/topcites/2003/eprints/index.shtml

Here is the general index for topcites for all the years 1992-2003:
http://www.slac.stanford.edu/library/topcites/

I posted earlier another special Spires feature which let's you find the most-cited papers by a particular author. To use it you need to know that in their code the letter A stands for author, so you say "a ambjorn" to find papers authored by ambjorn.

there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:

http://www.slac.stanford.edu/spires/hep/

to illustrate, suppose you want to find influential or highly cited papers by Jan Ambjorn.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D World from Causal Quantum Gravity")

In the main search field if you type
find a ambjorn and topcite 50+

then it will list those of Jan Ambjorn's papers which have received 50 or more citations to date

It turns out he has authored 34 papers which topped 50 citations.

Or you can say "topcite 100+" to restrict the search still further,
and find several of Ambjorn's papers which have topped 100.

The search engine takes several different formats and for one of them instead of saying "find a thiemann" you have to say
"find author thiemann". but the default seems to be use the letter A to stand for author.

lot of good information to get out of Spires, bravo to Stanford and SLAC for hosting it, I feel I've just scratched the surface

 

[marcus]

PAM Dirac was (one of) the first to try
to construct a quantum version of
General Relativity------to quantize gravity.
So this thread being for LQG links we should have a Dirac link
and Pelastration just supplied one with two photos of Dirac and
a newspaper interview

http://faculty.rmwc.edu/tmichalik/dirac.htm

 

[marcus]

cant recommend but thought it interesting enough to keep tabs on


"Toward a Background Independent Quantum Theory of Gravity"
Authors: Vishnu Jejjala, Djordje Minic, Chia-Hsiung Tze
Comments: Awarded Honorable Mention, 2004 Gravity Research Foundation Essay Competition; 8 pages

http://arxiv.org/gr-qc/0406037

 

[marcus]

This just appeared on arXiv today.
It looks like a keeper:

http://arxiv.org/gr-qc/0406042

"Oscillatory Universes in Loop Quantum Cosmology and Initial Conditions for Inflation"
James E. Lidsey, David J. Mulryne, N. J. Nunes, Reza Tavakol
6 pages, 4 figures

Several of them wrote a paper with Martin Bojowald that posted a couple of months back. Otherwise I don't recall seeing their names before

 

[marcus]

Rovelli has redone his homepage.

The book is planned to hit the bookstores in October

there is a picture of the book and a link to the Cambridge U. P.
catalog entry for it
planned price is 45 pounds sterling
the draft is still available free at his site, by agreement with C.U.P.

He mentions another piece of writing---popular----
called "What is space? What is time?"
so far just in Italian. I would guess there will be an English version

If you want to look for it in Italian (which I understand some people can read) the title is
Che cos' e lo spazio? Che cos' e il tempo?

Rovelli homepage:
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html

 

[Olias]

Rovelli has redone his homepage.

The book is planned to hit the bookstores in October

there is a picture of the book and a link to the Cambridge U. P.
catalog entry for it
planned price is 45 pounds sterling
the draft is still available free at his site, by agreement with C.U.P.

He mentions another piece of writing---popular----
called "What is space? What is time?"
so far just in Italian. I would guess there will be an English version

If you want to look for it in Italian (which I understand some people can read) the title is
Che cos' e lo spazio? Che cos' e il tempo?

Rovelli homepage:
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html

Thanks Marcus, its a must buy!

I like the cover of the book, the use of Geometry is very evident.

The cover outline is framed with precise measurment at its edge, with the Backgound uniform colour.

As one moves inward the Author and Title are framed by a 'casimir-effect'..and a sea of points 'Quantum-Background' are pretty hazy?

Cool1

 

[sol2]

http://assets.cambridge.org/0521837332/cover/0521837332.jpg

Quantum gravity is perhaps the most important open problem in fundamental physics. It is the problem of merging quantum mechanics and general relativity, the two great conceptual revolutions in the physics of the twentieth century. The loop and spinfoam approach, presented in this book, is one of the leading research programs in the field. The first part of the book discusses the reformulation of the basis of classical and quantum Hamiltonian physics required by general relativity. The second part covers the basic technical research directions. Appendices include a detailed history of the subject of quantum gravity, hard-to-find mathematical material, and a discussion of some philosophical issues raised by the subject. This fascinating text is ideal for graduate students entering the field, as well as researchers already working in quantum gravity. It will also appeal to philosophers and other scholars interested in the nature of space and time.

http://titles.cambridge.org/catalogue.asp?isbn=0521837332

It will be a must buy for myself as well.

But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?

The Elegant Universe, by Brian Greene, pg 231 and Pg 232

"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of general relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as Planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."

I am seeing similarities arising not only from this perspective but from the current link Marcus supplied on the cosmological association (LQC ).

 

[marcus]

Olias and Sol2, I too am glad to see "Quantum Gravity" well on its
way to being available, and I will certainly buy a copy although
45 pounds sterling is a fair-size chunk of cash.

It was thoughtful of Carlo to work out an agreement with Cambridge where
the publisher will allow him to keep a draft version available free online at his website. that way if someone can't afford the book they can at least get the draft and print it out at home, or just keep it on the computer---which is already pretty useful.

speculation is always risky but I am speculating that because it's a fast-moving field there will be several editions of this book
what is coming out this year (planned for October) will be the first edition
but----with ongoing developments in quantum cosmology and the simplicial or "dynamical triangulations" approach, and whatever else (so hard to predict)----there may be a second edition, and possibly others as years go along.
and then the draft on rovelli's website will be a kind of "zero-th edition".

my favorite page in the draft version of the book is page 7---the part about the whale. I also like some things around page 52
I also really like the philosophical essays at the end
and the historical accounts

although it has a lot for the general reader, the book is primarily for graduate students looking for PhD thesis work to do and for established researchers wanting to move into the field of QG.
That is to say, it has generally accessible portions (which are admirable and enlightening, in my opinion) but also (in case other people besides Olias and Sol2 are reading this thread I want to stress) plenty that is not so accessible.