Loop-and-allied QG bibliography

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marcus said:
more about the black hole radiation spectrum

http://arxiv.org/abs/hep-th/0506214
Selection Rules for Black-Hole Quantum Transitions
Shahar Hod, Uri Keshet
4 pages, 2 figures

"We suggest that quantum transitions of black holes comply with selection rules, analogous to those of atomic spectroscopy. In order to identify such rules, we apply Bohr's correspondence principle to the quasinormal ringing frequencies of black holes. In this context, classical ringing frequencies with an asymptotically vanishing real part
[tex]\omega_R[/tex]
correspond to virtual quanta, and may thus be interpreted as forbidden quantum transitions. With this motivation, we calculate the quasinormal spectrum of neutrino fields in spherically symmetric black-hole spacetimes. It is shown that
[tex]\omega_R \rightarrow 0[/tex]
for these resonances, suggesting that the corresponding fermionic transitions are quantum mechanically forbidden."


Shahar Hod was who started the uproar about quasinormal vibration modes of black holes in the first place. He cites his own 1998 paper

Marcus this paper may be of great interest:http://uk.arxiv.org/abs/quant-ph/0506228

he thanks Rovelli and Smolin to name but two!

and this paper may/will? be of interest to the Hod paper:http://uk.arxiv.org/abs/quant-ph/0506231
 
  • #353
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selfAdjoint said:
http://uk.arxiv.org/abs/quant-ph/0506228 is certainly a greatly interesting paper! I am going to bring it to the attention of the quantum physics subforum.

thanks for fielding that one!
 
  • #354
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another shoe drops re Pioneer anomaly

http://arxiv.org/abs/gr-qc/0506139

"...The existence of the Pioneer anomaly is no longer in doubt. Further, after much understandable hesitancy, a steadily growing part of the community has concluded that the anomaly should be subject to interpretation. Our program presents an ordered approach to doing this..."


"...This mission is designed to determine the origin of the discovered anomaly and to characterize its properties to an accuracy of at least three orders of magnitude below its measured value ..."

A MISSION TO EXPLORE THE PIONEER ANOMALY

the list of authors has some 39 names, they call themselves the Pioneer Collaboration.

it is an 8 page paper.

the noises keep getting louder that something about gravity needs adjustment
 
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  • #355
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http://arxiv.org/hep-th/0507012 [Broken]
Taming the cosmological constant in 2D causal quantum gravity with topology change

R. Loll (U. Utrecht), W. Westra (U. Utrecht), S. Zohren (U. Utrecht, RWTH Aachen)
19 pages, 4 figures

"As shown in previous work, there is a well-defined nonperturbative gravitational path integral including an explicit sum over topologies in the setting of Causal Dynamical Triangulations in two dimensions. In this paper we derive a complete analytical solution of the quantum continuum dynamics of this model, obtained uniquely by means of a double-scaling limit. We show that the presence of infinitesimal wormholes leads to a decrease in the effective cosmological constant, reminiscent of the suppression mechanism considered by Coleman and others in the four-dimensional Euclidean path integral. Remarkably, in the continuum limit we obtain a finite spacetime density of microscopic wormholes without assuming fundamental discreteness. This shows that one can in principle make sense of a gravitational path integral which includes a sum over topologies, provided suitable causality restrictions are imposed on the path integral histories."

this is the paper they will present this month in Paris at the Einstein2005 conference

Willem Westra did his Masters at Utrecht working for Loll, on this problem, and they published a paper in 2003 about it, and now he is doing his PhD.
Including topology-change in the path integral is very interesting.
one takes a weighted average not only over all possible spacetime geometries, but also adds up all possible spacetime topologies and all possible geometries of each topology. it could get amusing
 
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  • #356
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Abhay Ashtekar and Martin Bojowald have posted an updated version of this
http://arxiv.org/abs/gr-qc/0504029
Black hole evaporation: A paradigm
Abhay Ashtekar, Martin Bojowald
21 pages, 4 figures, v2: new references and discussion of relation to other ideas

"A paradigm describing black hole evaporation in non-perturbative quantum gravity is developed by combining two sets of detailed results: i) resolution of the Schwarzschild singularity using quantum geometry methods; and ii) time-evolution of black holes in the trapping and dynamical horizon frameworks. Quantum geometry effects introduce a major modification in the traditional space-time diagram of black hole evaporation, providing a possible mechanism for recovery of information that is classically lost in the process of black hole formation. The paradigm is developed directly in the Lorentzian regime and necessary conditions for its viability are discussed. If these conditions are met, much of the tension between expectations based on space-time geometry and structure of quantum theory would be resolved."

Black hole evaporation seems to be a hot topic in quantum gravity now---thinking of the recent paper by Joshi, Goswami, and P.Singh. Also a couple recent papers by Bojowald solo.
BTW in this one Ashtekar and Bojo cite this highly readable and provocative paper by Sean Hayward
http://arxiv.org/abs/gr-qc/0504038
The disinformation problem for black holes (pop version)
Sean A. Hayward
6 pages
The supposed information paradox for black holes is based on the fundamental misunderstanding that black holes are usefully defined by event horizons. Understood in terms of locally defined trapping horizons, the paradox disappears: information will escape from an evaporating black hole. According to classical properties of trapping horizons, a general scenario is outlined whereby a black hole evaporates completely without singularity, event horizon or loss of energy or information.

as another BTW here are Ruth Williams' papers
http://arxiv.org/find/grp_physics/1/au:+Williams_Ruth/0/1/0/all/0/1
(she co-authored with Tullio Regge around 2000) and here is a new one
http://arxiv.org/abs/gr-qc/0506137
Nonlocal Effective Field Equations for Quantum Cosmology
Herbert W. Hamber, Ruth M. Williams
9 pages

"The possibility that the strength of gravitational interactions might slowly increase with distance, is explored by formulating a set of effective field equations, which incorporate the gravitational, vacuum-polarization induced, running of Newton's constant G. The resulting long distance (or large time) behaviour depends on only one adjustable parameter [itex] \xi [/itex], and the implications for the Robertson-Walker universe are calculated, predicting an accelerated power-law expansion at later times [itex]t \sim \xi \sim 1/H[/itex].
 
  • #357
marcus
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This week, Loll and Westra have posted an updated version of their 2003 paper
http://arxiv.org/hep-th/0306183 [Broken]
Sum over topologies and double-scaling limit in 2D Lorentzian quantum gravity
9 pages, 3 Postscript figures; added comments on strip versus bulk partition function

"We construct a combined non-perturbative path integral over geometries and topologies for two-dimensional Lorentzian quantum gravity. The Lorentzian structure is used in an essential way to exclude geometries with unacceptably large causality violations. The remaining sum can be performed analytically and possesses a unique and well-defined double-scaling limit, a property which has eluded similar models of Euclidean quantum gravity in the past."

this was a first. before, the moment you allowed wormholes the sum would blow up and you would get infinities. too many baby universes, too many possibilities. so you had to make a rule against topology-change at the outset.
I am oversimplifying. Anyway in earlier CDT the topology of spacetime had to be restricted to be simple, and then within that you could have all different shape geometries. But this little paper of Loll and Westra is a kind of landmark because at least in 2D they are allowing topology-change and it is not a complete disaster.

now there is a little more progress
http://arxiv.org/hep-th/0507012 [Broken]
Taming the cosmological constant in 2D causal quantum gravity with topology change
They are getting their stuff together for the Paris conference this month.
there is this curious result of a finite density of wormholes.
they are going back and polishing the 2003 paper a little, because it will be a footnote in the 2005 paper they give in Paris. what busy people
 
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  • #358
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Just a week or so ago this one came out, Parmapreet Singh being one of the co-authors.

http://arxiv.org/abs/gr-qc/0506129
Quantum evaporation of a naked singularity
Rituparno Goswami, Pankaj S. Joshi, Parampreet Singh
4 pages, 2 figures

"We investigate here gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that non-perturbative semi-classical modifications near the singularity, based on loop quantum gravity, give rise to a strong outward flux of energy. This leads to the dissolution of the collapsing cloud before a naked singularity can form. Quantum gravitational effects can thus censor naked singularities by avoiding their formation. Further, quantum gravity induced mass flux has a distinct feature which can lead to a novel observable signature in astrophysical bursts."

Today, another P. Singh paper:

http://arxiv.org/abs/gr-qc/0507029
Semi-classical States, Effective Dynamics and Classical Emergence in Loop Quantum Cosmology
Parampreet Singh, Kevin Vandersloot
8 pages, 4 figures
IGPG-05/7-1, AEI-2005-122

"We construct physical semi-classical states annihilated by the Hamiltonian constraint operator in the framework of loop quantum cosmology as a method of systematically determining the regime and validity of the semi-classical limit of the quantum theory. Our results indicate that the evolution can be effectively described using continuous classical equations of motion with non-perturbative corrections down to near the Planck scale below which the universe can only be described by the discrete quantum constraint. These results, for the first time, provide concrete evidence of the emergence of classicality in loop quantum cosmology and also clearly demarcate the domain of validity of different effective theories. We prove that discrete quantum geometry effects may become very significant and lead to various new phenomenological applications. Furthermore the understanding of semi-classical states allows for a framework for interpreting the quantum wavefunctions and understanding questions of a semi-classical nature within the quantum theory of loop quantum cosmology."

mounting evidence that in the cosmology sector LQG is consistent with classical cosmology----that it has the right largescale limit in other words.
 
  • #359
marcus
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Thiemann decides which volume operator is right

http://arxiv.org/abs/gr-qc/0507036
Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity I
Kristina Giesel, Thomas Thiemann
20 pages, 5 figures

"The volume operator plays a pivotal role for the quantum dynamics of Loop Quantum Gravity (LQG). It is essential in order to construct Triad operators that enter the Hamiltonian constraint and which become densely defined operators on the full Hilbert space even though in the classical theory the triad becomes singular when classical GR breaks down. The expression for the volume and triad operators derives from the quantisation of the fundamental electric flux operator of LQG by a complicated regularisation procedure. In fact, there are two inequivalent volume operators available in the literature and, moreover, both operators are unique only up to a finite, multiplicative constant which should be viewed as a regularisation ambiguity. Now on the one hand, classical volumes and triads can be expressed directly in terms of fluxes and this fact was used to construct the corresponding volume and triad operators. On the other hand, fluxes can be expressed in terms of triads and therefore one can also view the volume operator as fundamental and consider the flux operator as a derived operator. In this paper we examine whether the volume, triad and flux quantisations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show: 1. The regularisation constant can be uniquely fixed. 2. One of the volume operators can be ruled out as inconsistent. 3. Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized. In this paper we present ideas and results of the consistency check. In a companion paper we supply detailed proofs."

http://arxiv.org/abs/gr-qc/0507037
Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity II

Kristina Giesel, Thomas Thiemann
67 pages, 6 figures, 36 pages paper, 31 pages appendix

"In this paper we provide the techniques and proofs for the resuls presented in our companion paper concerning the consistency check on volume and triad operator quantisation in Loop Quantum Gravity."
 
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  • #360
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A new LQG primer!

http://arxiv.org/abs/gr-qc/0507038
Loop Quantum Geometry: A primer
Alejandro Corichi
Comments: 25 pages. Contribution for the Proceedings of the VI Mexican School of Gravitation and Mathematical Physics

"This is the written version of a lecture given at the ``VI Mexican School of Gravitation and Mathematical Physics" (Nov 21-27, 2004, Playa del Carmen, Mexico), introducing the basics of Loop Quantum Geometry. The purpose of the written contribution is to provide a Primer version, that is, a first entry into Loop Quantum Gravity and to present at the same time a friendly guide to the existing pedagogical literature on the subject. This account is geared towards graduate students and non-experts interested in learning the basics of the subject."

This is by someone who used to visit here at PF fairly often and made some very useful posts (but under an internet "handle" name, not his own) according to my considered opinion.

I am very glad that there is a new Primer, introduction to the subject for grad students getting into it. for many years the most convenient LQG Primer was the 1998 one of Rovelli Upadhya and it is good to have another, so the beginner can have more choice.
 
  • #361
Chronos
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Thanks marcus. I'm a pedagogologist, as you probably know.
 
  • #362
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Chronos said:
Thanks marcus. I'm a pedagogologist, as you probably know.

a connoisseur of the art of teaching?
no I didn't know.
but actually Alejandro Corichi is aiming at a narrowly defined level of grad student who has completed a graduate course in General Relativity.
And one or two other substantial prerequisites. He is talking to a definite audience and not going out of his way to reach others.

If he was here i would tell him that on page 2 paragraphs 3 and 4 he misspells heart "hearth" and thought "though".
This is the pitfall of the spell-checker, which doesn't know what word you are trying to spell so just gives you SOME correctly spelled word.
 
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Bing that you are HERE marcus, I convey that "hearth" was meant to be 'Hearted', as in :not for the faint hearted! ;)

I myself have 'boo**ied' in spelling many times, actually in this very POST!..so I will refrain from altering it :(
 
  • #364
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New papers by Kristina Giesel and Thomas Thiemann

http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0507/0507036.pdf has the proofs.


From the abstract of gr-qc/0507036:

In this paper we examine whether the volume, triad and flux quantisations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show: 1. The regularisation constant can be uniquely fixed. 2. One of the volume operators can be ruled out as inconsistent. 3. Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized. In this paper we present ideas and results of the consistency check. In a companion paper we supply detailed proofs.

From the introduction:
First of all, there are in fact two unitarily
inequivalent volume operators [5, 6] which come from two, a priori equally justified background independent regularisation techniques. We will denote them by Rovelli – Smolin (RS) and Ashtekar – Lewandowski (AL) volume respectively for the rest of this paper. Secondly, both volume operators are anyway only determined up to a multiplicative regularisation constant C_reg [12] which remains undetermined when taking the limit, quite similar to finite regularisation constants that appear in counterterms of standard renormaisation of ordinary QFT. The ambiguity is further enhanced by factor ordering ambiguities once we consider triad operators. These ambiguities are parameterized by a spin quantum number ℓ = 1/2, 1, 3/2, ...
In this paper we will be able to remove all those ambiguities by the following consistency check: As we mentioned above, the volume and triad can be considered as functions of the fluxes. But the converse is also true: The fluxes can be written in terms of triads and thus the volume. Is it then true that there exists a regularisation constant for the volume operator and a factor ordering of the flux operator considered as a function of the triad operator or volume operator such that the corresponding alternative flux operator agrees (at least in the correspondence limit of large eigenvalues of the volume operator) with the fundamental flux operator, independent of the choice of ℓ? This better be possible as otherwise the inescapable conclusion would be that the volume operator is inconsistently quantised

Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.
 
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  • #365
marcus
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selfAdjoint said:
...

Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.

Hi selfAdj, concise and nicely crafted. It's a definite plus to have more than one person scouting and flagging papers. So it looks like Ashtekar's volume wins over Smolin and Rovelli volume.
Here's a snapshot of Kristina Giesel
http://math.ucr.edu/home/baez/marseille/giesel.jpg
 
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  • #366
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selfAdjoint said:
http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0507/0507036.pdf has the proofs.


From the abstract of gr-qc/0507036:



From the introduction:


Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.

This is actually going to be very..very interesting, I have not gone through both papers fully, I actually was reading the proof one first, which set alarm bells ringing, and I am indulged in a number of Rovelli's papers, I beleive Rovelli has allready highlighted a specific relevant aspect, but then again I might just go straight to the Volume operator Einstein detailed in an much overlooked correspondance, I believe Rovelli's veiwpoint not be 'over-ruled' just yet!

Measure..measure..measure! :smile:
 
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  • #367
marcus
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Hawking finally comes out with it!

http://arxiv.org/abs/hep-th/0507171


he gave the talk almost exactly one year ago, and everybody said
they'd have to wait to read the paper

this 5-pager may not settle all the questions.

In the paper Hawking says information is not lost down a black hole, however reconstructing the information that fell in from the hawking radiation that comes from the hole as it evaporates is (in some sense) like reconstructing an encyclopedia from the smoke and ashes it leaves when consumed by fire.

At the end Hawking reflects that when he paid off the bet to John Preskill and gave him the Encyclopedia
perhaps he should have burned the Encyclopedia first and given Preskill the ashes.

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

WARNING: this paper may leave you quite frustrated. that's how it left me anyway.

In the paper he says:
"I adopt the Euclidean [path integral] approach, the only sane way to do quantum gravity nonperturbatively."

Hawking Euclidean QG path integral is a 1980s and 1990s precursor to Loll Lorentzian QG path integral. Loll papers refer back to Hawking Euclidean sum over histories and related Hawking QG matters. I'm persuaded that path integral sum over geometries is a promising way to do QG and Hawking deserves credit for establishing this research direction. But this paper has not convinced me that Hawking's specific (Euclidean) approach to it is necessarily the right one.
 
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  • #368
marcus
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new paper by Martin Reuter

http://arxiv.org/abs/hep-th/0507167

From Big Bang to Asymptotic de Sitter: Complete Cosmologies in a Quantum Gravity Framework

M. Reuter, F. Saueressig
47 pages, 17 figures
"Using the Einstein-Hilbert approximation of asymptotically safe quantum gravity we present a consistent renormalization group based framework for the inclusion of quantum gravitational effects into the cosmological field equations. Relating the renormalization group scale to cosmological time via a dynamical cutoff identification this framework applies to all stages of the cosmological evolution. The very early universe is found to contain a period of "oscillatory inflation'' with an infinite sequence of time intervals during which the expansion alternates between acceleration and deceleration. For asymptotically late times we identify a mechanism which prevents the universe from leaving the domain of validity of the Einstein-Hilbert approximation and obtain a classical de Sitter era."

Martin Reuter is one of the invited speakers at the Loops 05 conference in October. He has his own approach to QG, which is different from LQG but when it is applied to cosmology it gets some similar results. Here he gets some results similar to Martin Bojowald's Loop Quantum Cosmology. So there is an interesting convergence. Bojowald derived this "oscillatory inflation" business earlier. Now reuter is getting it by a different method.
 
  • #369
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marcus said:
http://arxiv.org/abs/hep-th/0507171


he gave the talk almost exactly one year ago, and everybody said
they'd have to wait to read the paper

this 5-pager may not settle all the questions.

In the paper Hawking says information is not lost down a black hole, however reconstructing the information that fell in from the hawking radiation that comes from the hole as it evaporates is (in some sense) like reconstructing an encyclopedia from the smoke and ashes it leaves when consumed by fire.

At the end Hawking reflects that when he paid off the bet to John Preskill and gave him the Encyclopedia
perhaps he should have burned the Encyclopedia first and given Preskill the ashes.

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

WARNING: this paper may leave you quite frustrated. that's how it left me anyway.

In the paper he says:
"I adopt the Euclidean [path integral] approach, the only sane way to do quantum gravity nonperturbatively."

Hawking Euclidean QG path integral is a 1980s and 1990s precursor to Loll Lorentzian QG path integral. Loll papers refer back to Hawking Euclidean sum over histories and related Hawking QG matters. I'm persuaded that path integral sum over geometries is a promising way to do QG and Hawking deserves credit for establishing this research direction. But this paper has not convinced me that Hawking's specific (Euclidean) approach to it is necessarily the right one.

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?

Lets read between the path-integral lines? straight away I see Hawking has confirmed my 'past' question of Blackhole 'Time-dependant' reasoning. Take the last sentence in the paper:I gave John an encyclopedia of baseball, but maybe I should just have given him the ashes.

Is this really cricket? :blushing: ..or is this fact that one can derive an integral of information entering a Blackhole, but cannot derieve the same integral of the information that 'rebounds' , scatters back out?

What this really means is that if you throw an English Encyclopedia into a Blackhole, the only thing you can guarantee, is that an English Encyclopedia will never emerge, the particles that went into the construction of the pre-blackhole Encyclopedia, can never re-construct it , the particles that scatter from a certain blackhole horizon, are 'Time-Stamped', and are thus 'younger', 'older' but never the same 'age' as those that entered the Blackhole.

Into the fire, Out of the ashes ?..you will never get the particles of the Encyclopedia back scattered, but you could theoretically Get the Particles of the Trees that went into 'before' the Paper was created, and thus only re-create an Encyclopedia that has 'no-written-words', a sort of Encyclopedia Template!..not an 'Historical' Documentation of Factual Writings!

"The information loss corresponds to the classical relaxation of black holes according to the no hair theorem. One can not ask when the information gets out of a black hole because that would require the use of a semi-classical metric which has already lost the information"

The Time-Dependant paths of Galactic Blackholes have no Branching off to 'other-universes' . "If you jump into a black hole, your mass energy will
be returned to our universe but in a mangled form which contains the information about what you were like but in a state where it can not be easily recognized. It is like burning an encyclopedia. Information is not lost, if one keeps the smoke and the ashes. But it is difficult to read."

This paper contravines one of the most rigourous time-evolution paramiters of Big-Bang theory, if one rewinds our Galaxy, with the theorized Blackhole at its Core, then our Galaxy has a Time-Stamp 'information' that is Unique to our Galaxy. It is evident that all Galaxies that have theorized Blackhole's at their core's, are thus themselves 'Unique'.

The only way in GR to travel to another 'Time-PAST/FUTURE', is to remain inside you Galaxy, wait for another Galaxy to head your way as a merging process, then to 'jump-ship' when the intertwined Galaxies exchange information during the collision process.

You cannot leave our Galaxy and travel to Andromeda, which observationally is within our 'information-time' locally, but yet according to Hawking, if one waits for the 'future' collision of Andromeda and Milkyway, then this collision harbours a good chance of 'Time-Travel'..infact the ONLY chance.
 
  • #370
marcus
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Spin_Network said:
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 doesnt 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.
 
  • #371
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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."
 
  • #372
marcus
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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."
 
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  • #373
Kea
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marcus said:
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 :smile:
 
  • #374
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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."
 
  • #375
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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
 
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