Loop-and-allied QG bibliography

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  • #351
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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
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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.
 
  • #363
Spin_Network
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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 believe 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
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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
Spin_Network
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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 black hole 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 doesn't resolve my doubts. Although it does have some additional content beyond his talk at GR17, it's generally quite similar.
To avoid overloading this thread, which is mostly a kind of library of links, I will put anything that occurs to me about this in the "new Hawking paper" thread.
 
  • #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
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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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  • #376
marcus
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John Stachel (philosophy of science) reviews main QG approaches

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

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

maybe we should pay attention to what Stachel says.

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

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

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

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

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

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

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

table of contents:
"Contents
1 What is Structural Realism? 3
2 Structure and Individuality 5
3 Effective field theory approach and asymptotic quantization 11
4 String Theory 15
5 Quantum general relativity - some preliminary problems 17
- 5.1 States or Processes: Which is primary ? . . . . . . . . . . . . 17
- 5.2 Formalism and measurability . . . . . . . . . . . . . . . . . . . 20
6 Canonical quantization (loop quantum gravity). 25
7 The causal set (causet) approach 29
8 What Structures to Quantize? 31
9 Acknowledgements 33"
 
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  • #377
marcus
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http://arxiv.org/astro-ph/0507683 [Broken]
Inflation: A graceful entrance from Loop Quantum Cosmology
N. J. Nunes (U. Minnesota)
9 pages, 8 figures

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

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


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

EXCELLENT TALK BY SMOLIN ON VIDEO

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

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

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

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

NEW ROVELLI PAPER
http://www.arxiv.org/abs/gr-qc/0508007
From 3-geometry transition amplitudes to graviton states/B]
Federico Mattei (CPT), Carlo Rovelli (CPT), Simone Speziale (CPT), Massimo Testa (CPT)
18 pages
"In various background independent approaches, quantum gravity is defined in terms of a field propagation kernel: a sum over paths interpreted as a transition amplitude between 3-geometries, expected to project quantum states of the geometry on the solutions of the Wheeler-DeWitt equation. We study the relation between this formalism and conventional quantum field theory methods. We consider the propagation kernel of 4d Lorentzian general relativity in the temporal gauge, defined by a conventional formal Feynman path integral, gauge fixed a' la Fadeev--Popov. If space is compact, this turns out to depend only on the initial and final 3--geometries, while in the asymptotically flat case it depends also on the asymptotic proper time. We compute the explicit form of this kernel at first order around flat space, and show that it projects on the solutions of all quantum constraints, including the Wheeler-DeWitt equation, and yields the correct vacuum and n-graviton states. We also illustrate how the Newtonian interaction is coded into the propagation kernel, a key open issue in the spinfoam approach."
 
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  • #379
marcus
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Spin_Network said:
Marcus this needs slotting in:http://arxiv.org/abs/gr-qc/0508007

thanks I actually got to it yesterday afternoon and slotted it into the preceding post :smile:
I am glad you are watching out Spin Network, so will be less worried about missing things.
 
  • #380
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relating triangle approach to loop and spin foam gravity

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

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

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

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

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

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

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

It is clear that Arnsdorf is trying to put together a picture unifying AJL work with the quantum gravity of Rovelli, Baez, Smolin, Thiemann ...
 
  • #381
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Chronos has supplied a paper which appears to refute the Smolin CNS-multiverse conjecture

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

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

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

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

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

Here is a recent discussion of the CNS conjecture

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

just speaking for myself, this discovery further tips the balance of interest
from Loop towards the Loll Triangulation approach to QG.
 
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  • #382
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I totally agree, marcus. Not to claim any great insights, but, I believe the universe will ultimately submit to being described as a quantum computer.
 
  • #383
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I flagged this of Smolin in post #372 of this thread almost a month ago---on 25 July---and I didn't realize what a lot of fuss it would cause. There has been quite a racket in "the string community" about Smolin urging them to work out a non-perturbative formulation. No other paper, that has appeared in the month since, has had such repercussions. So I will bring this one forward to have the link handy.
http://arxiv.org/abs/hep-th/0507235

The case for background independence

Lee Smolin
46 pages, no figures

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

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

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

Robert Helling's collected thoughts, to help in arguing against Smolin's suggestion that they put some effort into a nonperturbative string formulation, are online at:
http://www.damtp.cam.ac.uk/user/rch47/background.pdf
 
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  • #384
marcus
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new Etera Livine paper, LQG black holes this time

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

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

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

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

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

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

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

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

The phrase "background independent theory" means in Physics a theory defined on a bare manifold endowed with no extra structure like geometry or fixed coordinates. A prominent example is the theory of matter fields coupled to Einstein’s gravity. In the case of a background independent classical theory it is natural to assume the background independence in a corresponding quantum theory.
A profound polemic devoted to that issue can be found in recent paper of Smolin [CITES "THE CASE FOR BACKGROUND INDEPENDENCE"]. The canonical formulation of the field theory relies on the 3 + 1 decomposition of space-time into the "space" M and "time" R. Then, the background independence implies invariance with respect to the diffeomorphisms of M. The invariance concerns in particular any matter fields in question: they have to be quantized in an often new, background independent way. In this letter and the accompanying paper [2] we are concerned with the issue of a diffeomorphism invariant quantization of a scalar field.
 
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  • #385
marcus
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http://www.arxiv.org/abs/gr-qc/0508100
On the quantum origin of the seeds of cosmic structure
Alejandro Perez, Hanno Sahlmann, Daniel Sudarsky

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

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

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