Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/1008.4639
Any spacetime has a Bianchi type I spacetime as a limit
Authors: Bethan Cropp (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 27 Aug 2010)
Abstract: Pick an arbitrary timelike geodesic in an arbitrary spacetime. We demonstrate that there is a particular limiting process, an "ultra-local limit", in which the immediate neighborhood of the timelike geodesic can be "blown up" to yield a general (typically non-diagonal) Bianchi type I spacetime. This process shares some (but definitely not all) of the features of the Penrose limit, whereby the immediate neighborhood of an arbitrary null geodesic is "blown up" to yield a pp-wave as a limit.

http://arxiv.org/abs/1008.5006
New Horizons in Gravity: The Trace Anomaly, Dark Energy and Condensate Stars
Authors: Emil Mottola
(Submitted on 30 Aug 2010)
Abstract: General Relativity receives quantum corrections relevant at macroscopic distance scales and near event horizons. These arise from the conformal scalar degrees of freedom in the extended effective field theory of gravity generated by the trace anomaly of massless quantum fields in curved space. The origin of these conformal scalar degrees of freedom as massless poles in two-particle intermediate states of anomalous amplitudes in flat space is exposed. At event horizons the conformal anomaly scalar degrees of freedom can have macroscopically large effects on the geometry, potentially removing the classical event horizon of black hole and cosmological spacetimes, replacing them with a quantum boundary layer where the effective value of the gravitational vacuum energy density can change. In the effective theory, the cosmological term becomes a dynamical condensate, whose value depends upon boundary conditions near the horizon. In the conformal phase where the anomaly induced fluctutations dominate, and the condensate dissolves, the effective cosmological "constant" is a running coupling which has an infrared stable fixed point at zero. By taking a positive value in the interior of a fully collapsed star, the effective cosmological term removes any singularity, replacing it with a smooth dark energy interior. The resulting gravitational condensate star configuration resolves all black hole paradoxes, and provides a testable alternative to black holes as the final state of complete gravitational collapse. The observed dark energy of our universe likewise may be a macroscopic finite size effect whose value depends not on microphysics but on the cosmological horizon scale.

 

[marcus]

http://arxiv.org/abs/1008.5348
Noncommutative Geometry Spectral Action as a framework for unification: Introduction and phenomenological/cosmological consequences
Mairi Sakellariadou
11 pages; Invited contribution for Mario Castagnino's Festschrift, to be published in IJMPD
(Submitted on 31 Aug 2010)
"I will summarize Noncommutative Geometry Spectral Action, an elegant geometrical model valid at unification scale, which offers a purely gravitational explanation of the Standard Model, the most successful phenomenological model of particle physics. Noncommutative geometry states that close to the Planck energy scale, space-time has a fine structure and proposes that it is given as the product of a four-dimensional continuum compact Riemaniann manifold by a tiny discrete finite noncommutative space. The spectral action principle, a universal action functional on spectral triples which depends only on the spectrum of the Dirac operator, applied to this almost commutative product geometry, leads to the full Standard Model, including neutrino mixing which has Majorana mass terms and a see-saw mechanism, minimally coupled to gravity. It also makes various predictions at unification scale. I will review some of the phenomenological and cosmological consequences of this beautiful and purely geometrical approach to unification."

 

[MTd2]

http://arxiv.org/abs/1008.5154

Structure of Lanczos-Lovelock Lagrangians in Critical Dimensions

Alexandre Yale, ***T. Padmanabhan***
(Submitted on 30 Aug 2010)
The Lanczos-Lovelock models of gravity constitute the most general theories of gravity in D dimensions which satisfy (a) the principle of of equivalence, (b) the principle of general co-variance, and (c) have field equations involving derivatives of the metric tensor only up to second order. The mth order Lanczos-Lovelock Lagrangian is a polynomial of degree m in the curvature tensor. The field equations resulting from it become trivial in the critical dimension $D = 2m$ and the action itself can be written as the integral of an exterior derivative of an expression involving the vierbeins, in the differential form language. While these results are well known, there is some controversy in the literature as to whether the Lanczos-Lovelock Lagrangian itself can be expressed as a total divergence of quantities built only from the metric and its derivatives (without using the vierbeins) in $D = 2m$. We settle this issue by showing that this is indeed possible and provide an algorithm for its construction. In particular, we demonstrate that, in two dimensions, $R \sqrt{-g} = \partial_j R^j$ for a doublet of functions $R^j = (R^0,R^1)$ which depends only on the metric and its first derivatives. We explicitly construct families of such R^j -s in two dimensions. We also address related questions regarding the Gauss-Bonnet Lagrangian in $D = 4$. Finally, we demonstrate the relation between the Chern-Simons form and the mth order Lanczos-Lovelock Lagrangian.

¨¬Somewhere in the beginning of the text he says that this generalizations have a natural generalization of the thermo-holograhic principle to any dimension.
http://arxiv.org/abs/1008.5288

Relative entropy as a measure of inhomogeneity in general relativity

Nikolas Akerblom, Gunther Cornelissen
(Submitted on 31 Aug 2010)
We introduce the notion of relative volume entropy for two spacetimes with preferred compact spacelike foliations. This is accomplished by applying the notion of Kullback-Leibler divergence to the volume elements induced on spacelike slices. The resulting quantity gives a lower bound on the number of bits which are necessary to describe one metric given the other. For illustration, we study some examples, in particular gravitational waves, and conclude that the relative volume entropy is a suitable device for quantitative comparison of the inhomogeneity of two spacetimes.

http://arxiv.org/abs/1008.5192

Does Quantum Mechanics Clash with the Equivalence Principle - and Does it Matter?

Elias Okon, Craig Callender
(Submitted on 31 Aug 2010)
With an eye on developing a quantum theory of gravity, many physicists have recently searched for quantum challenges to the equivalence principle of general relativity. However, as historians and philosophers of science are well aware, the principle of equivalence is not so clear. When clarified, we think quantum tests of the equivalence principle won't yield much. The problem is that the clash/not-clash is either already evident or guaranteed not to exist. Nonetheless, this work does help teach us what it means for a theory to be geometric.

 

[qsa]

http://arxiv.org/PS_cache/arxiv/pdf/1008/1008.5169v1.pdf

Discrete mechanics: a kinematics for a particular case of causal sets
Authors: Alexey L. Krugly

Subjects: General Relativity and Quantum Cosmology (gr-qc)

The model is a particular case of causal set. This is a discrete model of spacetime in a microscopic level. In paper the most general properties of the model are investigated without any reference to a dynamics. The dynamics of the model is introduced in [arXiv: 1004.5077]. These two papers introduce a consistent description of the model.

 

[qsa]

http://arxiv.org/abs/1004.5077


Discrete mechanics: a sequential growth dynamics for causal sets, and a self-organization of particles


Authors: Alexey L. Krugly

(Submitted on 28 Apr 2010)
Abstract: A model of a discrete pregeometry on a microscopic scale is introduced. This model is a finite network of finite elementary processes. The mathematical description is a d-graph that is a generalization of a graph. This is the particular case of a causal set. The aim of this study is to construct the particles as emergent structures. The particles in this model must be cyclic processes. The general dynamics and several examples are given. A simple dynamics generates a hierarchy of cyclic processes. An algebraic representation of this dynamics is given. It is based on the algebra of creation and destruction operators. Loops are described by bosonic operators and causal connections are described by fermionic operators.

 

[atyy]

http://arxiv.org/abs/1009.0268
Stability of spin-0 graviton and strong coupling in Horava-Lifgarbagez theory of gravity
Anzhong Wang, Qiang Wu
(Submitted on 1 Sep 2010)
In this paper, we consider two different issues, stability and strong coupling, raised lately in the newly-proposed Horava-Lifgarbagez (HL) theory of quantum gravity with projectability condition. We find that all the scalar modes are stable in the de Sitter background, due to two different kinds of effects, one from high-order derivatives of the spacetime curvature, and the other from the exponential expansion of the de Sitter space. Combining these effects properly, one can make the instability found in the Minkowski background never raise even for small-scale modes, provided that the IR limit is sufficiently closed to the relativistic fixed point. At the fixed point, all the modes become stabilized, which is expected, as it is well-known that the de Sitter spacetime is stable in general relativity. We also show that the instability of Minkowski spacetime can be cured by introducing mass to the spin-0 graviton. The strong coupling problem is investigated following the effective field theory approach, and found that it cannot be cured by the Blas-Pujolas-Sibiryakov mechanism, initially designed for the case without projectability condition, but might be solved by the Vainshtein mechanism. In fact, we construct a class of non-perturbative solutions, and show explicitly that it reduces smoothly to the de Sitter spacetime in the relativistic limit.

 

[marcus]

http://arxiv.org/abs/1009.0669
Probing the small distance structure of canonical quantum gravity using the conformal group
Gerard 't Hooft
20 pages, 1 figure
(Submitted on 3 Sep 2010)
"In canonical quantum gravity, the formal functional integral includes an integration over the local conformal factor, and we propose to perform the functional integral over this factor before doing any of the other functional integrals. By construction, the resulting effective theory would be expected to be conformally invariant and therefore finite. However, also the conformal integral itself diverges, and therefore the actual situation is more delicate. The effects of a renormalization counter term are considered, including the associated problem of unitarity violation, such as a Landau-like ghost. Adding (massive or massless) matter fields does not change the picture; to confirm this, detailed calculations were necessary, and they are presented. Some alternative ideas are offered, including a more daring speculation, which is that no counter term should be allowed for at all. This has far-reaching and important consequences, which we discuss. A surprising picture emerges of quantized elementary particles interacting with a gravitational field that is "partly classical". This approach was inspired by a search towards the reconciliation of Hawking radiation with unitarity and locality, and it offers basic new insights there."

 

[atyy]

http://arxiv.org/abs/1008.4774
Tensor network states and algorithms in the presence of a global U(1) symmetrySukhwinder Singh, Robert N. C. Pfeifer, Guifre Vidal
(Submitted on 27 Aug 2010)
Tensor network decompositions offer an efficient description of certain many-body states of a lattice system and are the basis of a wealth of numerical simulation algorithms. In a recent paper [arXiv:0907.2994v1] we discussed how to incorporate a global internal symmetry, given by a compact, completely reducible group G, into tensor network decompositions and algorithms. Here we specialize to the case of Abelian groups and, for concreteness, to a U(1) symmetry, often associated with particle number conservation. We consider tensor networks made of tensors that are invariant (or covariant) under the symmetry, and explain how to decompose and manipulate such tensors in order to exploit their symmetry. In numerical calculations, the use of U(1) symmetric tensors allows selection of a specific number of particles, ensures the exact preservation of particle number, and significantly reduces computational costs. We illustrate all these points in the context of the multi-scale entanglement renormalization ansatz.

 

[marcus]

Steve Carlip and George Ellis are both major figures in GR and cosmology. So this is news. A book edited by Ellis with a QG chapter by Carlip.
http://arxiv.org/abs/1009.1136
The Small Scale Structure of Spacetime
Steven Carlip
14 pages, 2 figures; to appear in Foundations of Space and Time, edited by George Ellis, Jeff Murugan, Amanda Weltman (Cambridge University Press)
(Submitted on 6 Sep 2010)
"Several lines of evidence hint that quantum gravity at very small distances may be effectively two-dimensional. I summarize the evidence for such 'spontaneous dimensional reduction', and suggest an additional argument coming from the strong-coupling limit of the Wheeler-DeWitt equation. If this description proves to be correct, it suggests a fascinating relationship between small-scale quantum spacetime and the behavior of cosmologies near an asymptotically silent singularity."

It sounds like the same talk he gave at the XXV Max Born conference on Planck Scale, in 2009.

Brief mention:
http://arxiv.org/abs/1009.0850
Natural Neutrino Dark Energy
Ilya Gurwich
14 pages, 6 figures
"A new class of neutrino dark energy models is presented. ... These predictions, can be verified (or disproved) with future experiments. At this point, the strongest constraints on these models are obtained from big bang nucleosynthesis, and lead to new bounds on the mass of the lightest neutrino."

 

[atyy]

http://arxiv.org/abs/1009.0879
Band-aid for information loss from black holes
Werner Israel, Zinkoo Yun
(Submitted on 4 Sep 2010)
We summarize, simplify and extend recent work showing that small deviations from exact thermality in Hawking radiation, first uncovered by Kraus and Wilczek, have the capacity to carry off the maximum information content of a black hole. This goes a considerable way toward resolving a long-standing "information-loss paradox".

http://arxiv.org/abs/1009.1135
The Ultraviolet Finiteness of N=8 Supergravity
Renata Kallosh
(Submitted on 6 Sep 2010)
We study counterterms (CT's), candidates for UV divergences in the four-dimensional N=8 supergravity. They have been constructed long ago in a Lorentz covariant on shell superspace and recently in the chiral light-cone (LC) superspace. We prove that all of these CT's are ruled out since they are not available in the real LC superspace. This implies the perturbative UV finiteness of d=4 N=8 supergravity under the assumption that supersymmetry and continuos E7 symmetry are anomaly-free. The proof, based on the chiral nature of CT's in the LC superspace, is a generalization of the perturbative F-term non-renormalization theorem for N=8 supergravity.

 

[marcus]

A PhD student of Fay Dowker (London Imperial) posts her dissertation:

http://arxiv.org/abs/1009.1593
Causal Set Phenomenology
Lydia Philpott
PhD Thesis, 145 pages
(Submitted on 8 Sep 2010)
"Central to the development of any new theory is the investigation of the observable consequences of the theory. In the search for quantum gravity, research in phenomenology has been dominated by models violating Lorentz invariance (LI) -- despite there being, at present, no evidence that LI is violated. Causal set theory is a LI candidate theory of QG that seeks not to quantise gravity as such, but rather to develop a new understanding of the universe from which both GR and QM could arise separately. The key hypothesis is that spacetime is a discrete partial order: a set of events where the partial ordering is the physical causal ordering between the events. This thesis investigates Lorentz invariant QG phenomenology motivated by the causal set approach. Massive particles propagating in a discrete spacetime will experience diffusion in both position and momentum in proper time. This thesis considers this idea in more depth, providing a rigorous derivation of the diffusion equation in terms of observable cosmic time. The diffusion behaviour does not depend on any particular underlying particle model. Simulations of three different models are conducted, revealing behaviour that matches the diffusion equation despite limitations on the size of causal set simulated. The effect of spacetime discreteness on the behaviour of massless particles is also investigated. Diffusion equations in both affine time and cosmic time are derived, and it is found that massless particles undergo diffusion and drift in energy. Constraints are placed on the magnitudes of the drift and diffusion parameters by considering the blackbody nature of the CMB. Spacetime discreteness also has a potentially observable effect on photon polarisation. For linearly polarised photons, underlying discreteness is found to cause a rotation in polarisation angle and a suppression in overall polarisation."

Apparently there are supercomputer/cluster simulation tools for Causal Sets QG, set up to run in the "Cactus" distributed computing environment. She thanks Joe Henson (currently at Perimeter) for making "Cactus CausalSets" simulation available.

http://arxiv.org/abs/1009.1735
Asymptotic Safety in Einstein Gravity and Scalar-Fermion Matter
G. P. Vacca, O. Zanusso
4 pages
(Submitted on 9 Sep 2010)
"Within the functional renormalization group approach we study the effective QFT of Einstein gravity and one self-interacting scalar coupled to N_f Dirac fermions. We include in our analysis the matter anomalous dimensions induced by all the interactions and analyze the highly non linear beta functions determining the renormalization flow. We find the existence of a non trivial fixed point structure both for the gravity and the matter sector, besides the usual gaussian matter one. This suggests that asymptotic safety could be realized in the gravitational sector and in the standard model. Non triviality in the Higgs sector might involve gravitational interactions."

 

[atyy]

http://arxiv.org/abs/1009.1643
E7(7) constraints on counterterms in N=8 supergravity
Niklas Beisert, Henriette Elvang, Daniel Z. Freedman, Michael Kiermaier, Alejandro Morales, Stephan Stieberger
(Submitted on 8 Sep 2010)
We prove by explicit computation that the operators D^4R^4 and D^6R^4 in N=8 supergravity have non-vanishing single-soft scalar limits at the 6-point level, and therefore they violate the continuous E7(7) symmetry. The soft limits precisely match automorphism constraints. Together with previous results for R^4, this provides a direct proof that no E7(7)-invariant candidate counterterm exists below 7-loop order. At 7-loops, we characterize the infinite tower of independent supersymmetric operators D^4R^6, R^8, phi^2 R^8,... with n>4 fields and prove that they all violate E7(7) symmetry. This means that the 4-graviton amplitude determines finiteness at 7-loop order. We show that the corresponding candidate counterterm D^8R^4 has a non-linear supersymmetrization such that its single- and double-soft scalar limits are compatible with E7(7) up to and including 6-points. At loop orders 7, 8, 9 we provide an exhaustive account of all independent candidate counterterms with up to 16, 14, 12 fields, respectively, together with their potential single-soft scalar limits.

 

[marcus]

http://arxiv.org/abs/1009.2445
Gravity quantized
Marcin Domagala, Kristina Giesel, Wojciech Kaminski, Jerzy Lewandowski
(Submitted on 13 Sep 2010)
"...'but we do not have quantum gravity.' This phrase is often used when analysis of a physical problem enters the regime in which quantum gravity effects should be taken into account. In fact, there are several models of the gravitational field coupled to (scalar) fields for which the quantization procedure can be completed using loop quantum gravity techniques. The model we present in this paper consist of the gravitational field coupled to a scalar field. The result has similar structure to the loop quantum cosmology models, except for that it involves all the local degrees of freedom because no symmetry reduction has been performed at the classical level."

http://arxiv.org/abs/1009.2157
The Problem of Time in Quantum Gravity
Edward Anderson
25 pages, including 1 figure
(Submitted on 11 Sep 2010)
"The problem of time in quantum gravity occurs because 'time' is taken to have a different meaning in each of general relativity and ordinary quantum theory. This incompatibility creates serious problems with trying to replace these two branches of physics with a single framework in regimes in which neither quantum theory nor general relativity can be neglected, such as in black holes or in the very early universe. Strategies for resolving the Problem of Time have evolved somewhat since Kuchar and Isham's well-known reviews from the early 90's. These come in the following divisions:

I) Time before quantization, such as hidden time or matter time.

II) Time after quantization, such as emergent semiclassical time.

III) Timeless strategies of
Type 1: naive Schrodinger interpretation, conditional probabilities interpretation and various forms of records theories, and
Type 2 'Rovelli': in terms of evolving constants of the motion, complete observables and partial observables.

IV) I argue for histories theories to be a separate class of strategy. Additionally, various combinations of these strategies have begun to appear in the literature; I discuss a number of such. Finally, I comment on loop quantum gravity, supergravity and string/M-theory from the problem of time perspective."

Briefly noted longshot. Kostecki has been involved in QG research and meetings--his approach to QM foundations may be helpful:
http://arxiv.org/abs/1009.2423
Quantum theory as inductive inference
Ryszard Paweł Kostecki
7 pages. Slightly extended version of a paper submitted to Proceedings of the 30th International Workshop on Bayesian Inference and Maximum Entropy Methods, Chamonix July 4-9, 2010. (Earlier title: Algebraic bayesian quantum theory.)
(Submitted on 10 Sep 2010)
"We present the elements of a new approach to the foundations of quantum theory and information theory ... It enables us to deal with conceptual and mathematical problems of quantum theory without any appeal to Hilbert space framework and without frequentist or subjective interpretation of probability."

 

[Jonnyb42]

Hi everyone, I most likely do not belong here at all, but I wanted to ask a few questions. This topic seems very heated and I am liking physics a lot right now. I am a freshman in college and I am studying math my guts out because String theory, and other proposed GUT theories require the craziest math I know of. Is the "math path" on the superstringhtheory website accurate? I am basing my studies off of that, although I don't know any other math than that anyways haha. Is it possible to learn any technical information about string theory with the mathematics level of multivariable calculus? I assume no, but if so please let me know.

I am currently in multivariable calculus class, while studying differential equations, linear algebra, and I am walking into a Classical Mechanics and graduate differential equations (that class I understand much less than the others but I try to pick up on a few things. For example, they went over lp norms and I researched that, I am sad to realize that I only know of one space - Euclidean space. I cannot wait to try to imagine the others.)

Thanks.

 

[marcus]

Jonny, this thread simply lists new non-string quantum gravity articles as they come out. We try to be selective and only list those that might be of interest to people following non-string QG research or involved in it. It's what you call a bibliography thread. Not for discussion.

Since your post doesn't really belong here, I will copy it to start a "Jonnyb42 introduces himself" thread. It may be moved (at mod discretion) to the special forum for discussing academic experience, what courses to take, different major/minor choices, grad school etc.
There's a separate forum (up at the top of the menu) for that. But for now I'll just start a "Hello" thread for you here in this forum.

 

[marcus]

http://arxiv.org/abs/1009.2525
Eternal inflation without metaphysics
James Hartle, S.W. Hawking, Thomas Hertog
4 pages
(Submitted on 13 Sep 2010)
"In the usual account of eternal inflation the universe is supposed to be a de Sitter background in which pocket universes nucleate at a steady rate. However this is metaphysics because there is no way this mosaic structure can be observed. We don't see the whole universe but only a nearly homogeneous region within our past light cone. We show that we can use the no-boundary wave function to calculate small departures from homogeneity within our past light cone despite the possibility of much larger fluctuations on super horizon scales. We find that the dominant contribution comes from the history that exits eternal inflation at the lowest value of the potential and predict, in a certain class of landscape models, a tensor to scalar ratio of about 10%. In this way the no-boundary wave function defines a measure for the prediction of local cosmological observations."

 

[atyy]

http://arxiv.org/abs/1009.3176
A Shifted View of Fundamental Physics
Michael Atiyah, Gregory W. Moore

But, if we need new ideas, where will they come from? Youth is the traditional source of radical thoughts, but only a genius or a fool would risk their whole future career on the gamble of some revolutionary new point of view. The weight of orthodoxy is too heavy to be challenged by a PhD student.

So it is left to the older generation like me to speculate. The same friend who likened string theory to poetry encouraged me to have wild ideas, saying ”you have nothing to lose!” That is true, I have my PhD. I do not need employment and all I can lose is a bit of my reputation. But then allowances are made for old-age, as in the case of Einstein when he persistently refused to concede defeat in his battle with Niels Bohr.

So my birthday present to Is is to tell him that we senior citizens can indulge in wild speculations!

Congrats MTd2!

 

[MTd2]

Well, I will make it my marriage gift, because I married today ;).

 

[marcus]

Congratulations MTd2!

My respects and warm best wishes to you and your wife!

 

[MTd2]

Thank you people! :) Here it goes 2 papers you guys did not notice about LQG/Holography and Quantum Graphity:

http://arxiv.org/abs/1009.3093
Generalized second law of thermodynamics in modified FRW cosmology with corrected entropy-area relation

K. Karami, A. Sheykhi, N. Sahraei, S. Ghaffari
(Submitted on 16 Sep 2010)
Using the corrected entropy-area relation motivated by the loop quantum gravity, we investigate the validity of the generalized second law of thermodynamics in the framework of modified FRW cosmology. We consider a non-flat universe filled with an interacting viscous dark energy with dark matter and radiation. The boundary of the universe is assumed to be the dynamical apparent horizon. We find out that the generalized second law is always satisfied throughout the history of the universe for any spatial curvature regardless of the dark energy model.

http://arxiv.org/abs/1009.3195

Space as a low-temperature regime of graphs


Florian Conrady (Perimeter Inst. Theor. Phys.)
(Submitted on 16 Sep 2010)
I define a statistical model of graphs in which 2-dimensional spaces arise at low temperature. The configurations are given by graphs with a fixed number of edges and the Hamiltonian is a simple, local function of the graphs. Simulations show that there is a transition between a low-temperature regime in which the graphs form triangulations of 2-dimensional surfaces and a high-temperature regime, where the surfaces disappear. I use data for the specific heat and other observables to discuss whether this is a phase transition. The surface states are analyzed with regard to topology and defects.

 

[marcus]

http://arxiv.org/abs/1009.3195
Space as a low-temperature regime of graphs
...
Florian Conrady (Perimeter Inst. Theor. Phys.)
...
... Simulations show that there is a transition between a low-temperature regime in which the graphs form triangulations of 2-dimensional surfaces and a high-temperature regime, where the surfaces disappear...

To go with this paper (good find, MTd2!) the author has provided two animations:
http://www.florianconrady.com/simulations.html
The second one of these two simulations ("Model with 2D interactions") is the one to watch if you want to get an idea of something recognizably like 2D space crystalizing out of non-space chaos as it cools.

 

[MTd2]

http://arxiv.org/abs/1009.3267

Hausdorff dimension of a particle path in a quantum manifold

Piero Nicolini, Benjamin Niedner
(Submitted on 16 Sep 2010)
After recalling the concept of Hausdorff dimension, we study the fractal properties of a quantum particle path. As a novelty we consider the possibility for the space where the particle propagates, to be endowed with a quantum gravity induced minimal length. We show that the Hausdorff dimension accounts for both the quantum mechanics uncertainty and manifold fluctuations. In addition the presence of a minimal length breaks the self similarity property of the erratic path of the quantum particle. Finally we establish a universal property of the Hausdorff dimension as well as the spectral dimension: they both depend on the amount of resolution loss which affects both the path and the manifold when quantum gravity fluctuations occur.

http://arxiv.org/abs/1009.3402

Polyhedra in loop quantum gravity

Eugenio Bianchi, Pietro Dona', Simone Speziale
(Submitted on 17 Sep 2010)
Interwiners are the building blocks of spin-network states. The space of intertwiners is the quantization of a classical symplectic manifold introduced by Kapovich and Millson. Here we show that a theorem by Minkowski allows us to interpret generic configurations in this space as bounded convex polyhedra in Euclidean space: a polyhedron is uniquely described by the areas and normals to its faces. We provide a reconstruction of the geometry of the polyhedron: we give formulas for the edge lengths, the volume and the adjacency of its faces. At the quantum level, this correspondence allows us to identify an intertwiner with the state of a quantum polyhedron, thus generalizing the notion of quantum tetrahedron familiar in the loop quantum gravity literature. Moreover, coherent intertwiners result to be peaked on the classical geometry of a polyhedron. We discuss the relevance of this result for loop quantum gravity. In particular, coherent spin-network states with nodes of arbitrary valence represent a collection of semiclassical polyhedra. Furthermore, we introduce an operator that measures the volume of a quantum polyhedron and examine its relation with the standard volume operator of loop quantum gravity. We also comment on the semiclassical limit of spinfoams with non-simplicial graphs.

 

[MTd2]

Mentions Benedetti, Carlip, etc... :

http://arxiv.org/abs/1009.3267

Hausdorff dimension of a particle path in a quantum manifold

Piero Nicolini, Benjamin Niedner
(Submitted on 16 Sep 2010)
After recalling the concept of Hausdorff dimension, we study the fractal properties of a quantum particle path. As a novelty we consider the possibility for the space where the particle propagates, to be endowed with a quantum gravity induced minimal length. We show that the Hausdorff dimension accounts for both the quantum mechanics uncertainty and manifold fluctuations. In addition the presence of a minimal length breaks the self similarity property of the erratic path of the quantum particle. Finally we establish a universal property of the Hausdorff dimension as well as the spectral dimension: they both depend on the amount of resolution loss which affects both the path and the manifold when quantum gravity fluctuations occur.

 

[marcus]

http://arxiv.org/abs/1009.3559
Conformal Superspace: the configuration space of general relativity
Julian Barbour, Niall Ó Murchadha
(Submitted on 18 Sep 2010)
"It has long been considered that conformal superspace is the natural configuration space for canonical general relativity. However, this was never definitively demonstrated. We have found that the standard conformal method of solving the Einstein constraints has an unexpected extra symmetry. This allows us to complete the project. We show that given a point and a velocity in conformal superspace, the Einstein equations generate a unique curve in conformal superspace."

http://arxiv.org/abs/1009.3528
Quantum Gravity Effects in the Kerr Spacetime
M. Reuter, E. Tuiran
57 pages, 30 figures
(Submitted on 18 Sep 2010)
"We analyze the impact of the leading quantum gravity effects on the properties of black holes with nonzero angular momentum by performing a suitable renormalization group improvement of the classical Kerr metric within Quantum Einstein Gravity (QEG). In particular we explore the structure of the horizons, the ergosphere, and the static limit surfaces as well as the phase space avilable for the Penrose process. The positivity properties of the effective vacuum energy momentum tensor are also discussed and the 'dressing' of the black hole's mass and angular momentum are investigated by computing the corresponding Komar integrals. The pertinent Smarr formula turns out to retain its classical form. As for their thermodynamical properties, a modified first law of black hole thermodynamics is found to be satisfied by the improved black holes (to second order in the angular momentum); the corresponding Bekenstein-Hawking temperature is not proportional to the surface gravity."

 

[MTd2]

http://arxiv.org/abs/1009.3786

A universe of processes and some of its guises

Bob Coecke
(Submitted on 20 Sep 2010)
Our starting point is a particular `canvas' aimed to `draw' theories of physics, which has symmetric monoidal categories as its mathematical backbone. In this paper we consider the conceptual foundations for this canvas, and how these can then be converted into mathematical structure. With very little structural effort (i.e. in very abstract terms) and in a very short time span the categorical quantum mechanics (CQM) research program has reproduced a surprisingly large fragment of quantum theory. It also provides new insights both in quantum foundations and in quantum information, and has even resulted in automated reasoning software called `quantomatic' which exploits the deductive power of CQM. In this paper we complement the available material by not requiring prior knowledge of category theory, and by pointing at connections to previous and current developments in the foundations of physics. This research program is also in close synergy with developments elsewhere, for example in representation theory, quantum algebra, knot theory, topological quantum field theory and several other areas.

 

[John86]

http://arxiv.org/abs/1009.3817
Undecidability as solution to the problem of measurement: fundamental criterion for the production of events
Authors: Rodolfo Gambini, Luis Pedro Garcia-Pintos, Jorge Pullin
(Submitted on 20 Sep 2010)
Abstract: In recent papers we put forth a new interpretation of quantum mechanics, colloquially known as ``the Montevideo interpretation''. This interpretation is based on taking into account fundamental limits that gravity imposes on the measurement process. As a consequence one has that situations develop where a reduction process is undecidable from an evolution operator. When such a situation is achieved, an event has taken place. In this paper we sharpen the definition of when and how events occur, more precisely we give sufficient conditions for the occurrence of events. We probe the new definition in an example. In particular we show that the concept of undecidability used is not ``FAPP'' (for all practical purposes), but fundamental.

 

[MTd2]

http://arxiv.org/abs/1009.3677

Extended Hořava Gravity with Physical Ground-State Wavefunction

Fu-Wen Shu
(Submitted on 20 Sep 2010)
We propose a new extended theory of Ho\v{r}ava gravity based on the following three conditions: (i) UV completion, (ii) healthy IR behavior and (iii) a stable vacuum state in quantized version of the theory. Compared with other extended theories, we stress that any realistic theory of gravity must have physical ground states when quantization is performed. To fulfill the three conditions, we softly break the detailed balance but keep its basic structure unchanged. It turns out that the new model constructed in this way can avoid the strong coupling problem and remains power-counting renormalizable, moreover, it has a stable vacuum state by an appropriate choice of parameters.

************

This is another healthy version of Horava gravity. Worth mentioning here.

 

[John86]

http://arxiv.org/abs/1005.5496
Enhancement of Compton Scattering by an Effective Coupling Constant
Authors: Bernardo Barbiellini, Piero Nicolini
(Submitted on 30 May 2010 (v1), last revised 21 Sep 2010 (this version, v2))
Abstract: A robust thermodynamic argument shows that a small reduction of the effective coupling constant $\alpha$ of QED greatly enhances the Compton scattering cross section and that the Thomson scattering length is connected to a fundamental scale $\lambda$. A discussion provides a possible quantum interpretation of this enormous sensitivity to changes in the effective coupling constant $\alpha$.

 

[marcus]

http://arxiv.org/abs/1009.4475
Critical Overview of Loops and Foams
Sergei Alexandrov, Philippe Roche
(Submitted on 22 Sep 2010)
"This is a review of the present status of loop and spin foam approaches to quantization of four-dimensional general relativity. It aims at raising various issues which seem to challenge some of the methods and the results often taken as granted in these domains. A particular emphasis is given to the issue of diffeomorphism and local Lorentz symmetries at the quantum level and to the discussion of new spin foam models. We also describe modifications of these two approaches which may overcome their problems and speculate on other promising research directions."

 

[atyy]

http://arxiv.org/abs/1009.4603
Chiral fermions in noncommutative electrodynamics: renormalizability and dispersionM. Buric, D. Latas, V. Radovanovic, J. Trampetic
(Submitted on 23 Sep 2010)
We analyze quantization of noncommutative chiral electrodynamics in the enveloping algebra formalism in linear order in noncommutativity parameter $\theta$. Calculations show that divergences exist and cannot be removed by ordinary renormalization, however they can be removed by the Seiberg-Witten redefinition of fields. Performing the redefinitions explicitly, we obtain renormalizable lagrangian and discuss the influence of noncommutativity on field propagation. Noncommutativity affects the propagation of chiral fermions only: half of the fermionic modes become massive and birefringent.

 

[MTd2]

http://arxiv.org/abs/1009.5100

Géométrie quantique dans les mousses de spins : de la théorie topologique BF vers la relativité générale
Valentin Bonzom
Comments: PhD. Thesis 2010, in French, Centre de Physique Th\'eorique
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
Loop quantum gravity has provided us with a canonical framework especially devised for background independent and diffeomorphism invariant gauge field theories. In this quantization the fundamental excitations are called spin network states, and in the context of general relativity, they give a meaning to quantum geometry. Spin foams are a sort of path integral for spin network states, supposed to enable the computations of transition amplitudes between these states. The spin foam quantization has proved very efficient for topological field theories, like 2d Yang-Mills, 3d gravity or BF theories. Different models have also been proposed for 4-dimensional quantum gravity. In this PhD manuscript, I discuss several methods to study spin foam models. In particular, I present some recurrence relations on spin foam amplitudes, which generically encode classical symmetries at the quantum level, and are likely to help fill the gap with the Hamiltonian constraints. These relations can be naturally interpreted in terms of elementary deformations of discrete geometric structures, like simplicial geometries. Another interesting method consists in exploring the way spin foam models can be written as path integrals for systems of geometries on a lattice, taking inspiration from topological models and Regge calculus. This leads to a very geometric view on spin foams, and gives classical action principles which are studied in details.

 

[MTd2]

http://arxiv.org/abs/1009.5161

Information Physics: The New Frontier

Kevin H. Knuth
(Submitted on 27 Sep 2010)
At this point in time, two major areas of physics, statistical mechanics and quantum mechanics, rest on the foundations of probability and entropy. The last century saw several significant fundamental advances in our understanding of the process of inference, which make it clear that these are inferential theories. That is, rather than being a description of the behavior of the universe, these theories describe how observers can make optimal predictions about the universe. In such a picture, information plays a critical role. What is more is that little clues, such as the fact that black holes have entropy, continue to suggest that information is fundamental to physics in general.
In the last decade, our fundamental understanding of probability theory has led to a Bayesian revolution. In addition, we have come to recognize that the foundations go far deeper and that Cox's approach of generalizing a Boolean algebra to a probability calculus is the first specific example of the more fundamental idea of assigning valuations to partially-ordered sets. By considering this as a natural way to introduce quantification to the more fundamental notion of ordering, one obtains an entirely new way of deriving physical laws. I will introduce this new way of thinking by demonstrating how one can quantify partially-ordered sets and, in the process, derive physical laws. The implication is that physical law does not reflect the order in the universe, instead it is derived from the order imposed by our description of the universe. Information physics, which is based on understanding the ways in which we both quantify and process information about the world around us, is a fundamentally new approach to science.

 

[MTd2]

http://arxiv.org/abs/1009.4944

Nonhomogeneous Cooling, Entropic Gravity and MOND Theory

Jorge Ananias Neto
(Submitted on 24 Sep 2010)
In this paper, using the holographic principle, a modified equipartition theorem where we assume that below a critical temperature the energy is not equally divided on all bits, and the Unruh temperature, we derive MOND theory and a modified Friedmann equation compatible with MOND theory.

 

[MTd2]

http://arxiv.org/abs/1009.5385

An operational view on the holographic information bound

Karel Van Acoleyen
(Submitted on 27 Sep 2010)
We study the covariant holographic entropy bound from an operational standpoint. Therefore we consider the physical limit for observations on a light-sheet. A light-sheet is a particular null hypersurface, and the natural measuring apparatus is a screen. By considering the physical properties of the screen - as dictated by quantum mechanics - we derive an uncertainty relation. This connects the number of bits of decoded information on the light-sheet to two geometric uncertainties: the uncertainty on the place where the bits are located and the uncertainty on the local expansion of the light-sheet. From this relation we can argue a local operational version of the (generalized) covariant entropy bound: the maximum number of bits decoded on a light-sheet interval goes like the area difference (in Planck units) of the initial and final surface spanned by the light rays.

http://arxiv.org/abs/1009.5414

Gravity is not an entropic force

Archil Kobakhidze
(Submitted on 27 Sep 2010)
We argue that experiments with ultra-cold neutrons in the gravitational field of Earth disprove recent speculations on the entropic origin of gravitation.

http://arxiv.org/abs/1009.5561

Entropic Corrections to Coulomb's Law

A. Sheykhi, S. H. Hendi
(Submitted on 28 Sep 2010)
Starting from the holographic principle and an equipartition rule, Verlinde derived Newton's law of gravitation as well as Einstein equation. In this Letter, we apply Verlinde's approach on the entropic force to derive the quantum corrections to Coulomb's Law. We adopt the viewpoint that gravity can be emerged as an entropic force and assume the entropy-area relation can be modified from the inclusion of quantum effects, motivated from the loop quantum gravity.

http://arxiv.org/abs/1009.5133

Equations of motion in General Relativity and Quantum Mechanics

Paul O'Hara
(Submitted on 26 Sep 2010)
In a previous article a relationship was established between the linearized metrics of General Relativity associated with geodesics and the Dirac Equation of quantum mechanics. In this paper the extension of that result to arbitrary curves is investigated. The Dirac equation is derived and shown to be related to the Lie derivative of the momentum along the curve. In addition,the equations of motion are derived from the Hamilton-Jacobi equation associated with the metric and the wave equation associated with the Hamiltonian is then shown not to commute with the Dirac operator. Finally, the Maxwell-Boltzmann distribution is shown to be a consequence of geodesic motion.

 

[marcus]

http://arxiv.org/abs/1009.5436
Timeless path integral for relativistic quantum mechanics
Dah-Wei Chiou
30 pages
(Submitted on 28 Sep 2010)
"Starting from the canonical formalism of relativistic (timeless) quantum mechanics, the formulation of timeless path integral is rigorously derived. The transition amplitude is reformulated as the sum, or functional integral, over all possible paths in the constraint surface specified by the (relativistic) Hamiltonian constraint, and each path contributes with a phase identical to the classical action divided by $$\hbar$$. The timeless path integral manifests the timeless feature as it is completely independent of the parametrization for paths. For the special case that the Hamiltonian constraint is a quadratic polynomial in momenta, the transition amplitude admits the timeless Feynman's path integral over the (relativistic) configuration space."

http://arxiv.org/abs/1009.5632
Asymptotes in SU(2) Recoupling Theory: Wigner Matrices, 3j Symbols, and Character Localization
Joseph Ben Geloun, Razvan Gurau
(Submitted on 28 Sep 2010)
In this paper we employ a novel technique combining the Euler Maclaurin formula with the saddle point approximation method to obtain the asymptotic behavior (in the limit of large representation index J) of generic Wigner matrix elements $$D^{J}_{MM'}(g)$$. We use this result to derive asymptotic formulae for the character $$\chi^J(g)$$ of an SU(2) group element and for Wigner's 3j symbol. Surprisingly, given that we perform five successive layers of approximations, the asymptotic formula we obtain for $$\chi^J(g)$$ is in fact exact. This result provides a non trivial example of a Duistermaat-Heckman like localization property for discrete sums."

This could be useful in spinfoam calculations, see this quote:
"Our results are relevant for computing topological (Turaev Viro like [5]) invariants and
in connection to the volume conjecture [6]. From a theoretical physics perspective they are
of consequence for spin foam models [7], Group Field Theory [8, 9], discretized BF theory
and lattice gravity [10],[11], [12]. Continuous SPA has been extensively used in this context
to derive asymptotic behaviors of spin foam amplitudes [13], [14], [15], and [16], [17], [18]."
Over a dozen citations to LQG spinfoam papers, indicating possible applications of the math.

http://arxiv.org/abs/1009.5514
Varying constants, Gravitation and Cosmology
Jean-Philippe Uzan
145 pages, 10 figures, Review for Living Reviews in Relativity
(Submitted on 28 Sep 2010)
"Fundamental constants are a cornerstone of our physical laws. Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. It is thus of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy. We thus detail the relations between the constants, the tests of the local position invariance and of the universality of free fall. We then review the main experimental and observational constraints that have been obtained from atomic clocks, the Oklo phenomenon, Solar system observations, meteorites dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic microwave background and big bang nucleosynthesis. At each step we describe the basics of each system, its dependence with respect to the constants, the known systematic effects and the most recent constraints that have been obtained. We then describe the main theoretical frameworks in which the low-energy constants may actually be varying and we focus on the unification mechanisms and the relations between the variation of different constants. To finish, we discuss the more speculative possibility of understanding their numerical values and the apparent fine-tuning that they confront us with."

http://arxiv.org/abs/1009.5595
Consistent matter couplings for Plebanski gravity
Felix Tennie, Mattias N.R. Wohlfarth
20 pages
(Submitted on 28 Sep 2010)
"We develop a scheme for the minimal coupling of all standard types of tensor and spinor field matter to Plebanski gravity. This theory is a geometric reformulation of vacuum general relativity in terms of two-form frames and connection one-forms, and provides a covariant basis for various quantization approaches. Using the spinor formalism we prove the consistency of the newly proposed matter coupling by demonstrating the full equivalence of Plebanski gravity plus matter to Einstein--Cartan gravity. As a byproduct we also show the consistency of some previous suggestions for matter actions."

MTd2 already spotted this one! Looks interesting:
http://arxiv.org/abs/1009.5414
Gravity is not an entropic force
Archil Kobakhidze