Questions tagged [loop-quantum-gravity]

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What is “one-loop cosmological constant”?

I'm following Witten's essay and I'm trying to understand the UV divergence. When he writes: In quantum field theory, this Feynman diagram with a single proper-time parameter τ, underlies the one-...
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What are the physical state invariants of loop quantum gravity? [closed]

What are the "physical state" invariants for Loop Quantum Gravity? The Wikipedia page talks about "physical states" being invariant, diffeomorphism invariance, "quantities ...
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Is background independence necessary for loop quantum gravity in itself, or only necessary to make LQG compatible with special relativity?

At first look the idea of particulate space in loop quantum gravity would seem to indicate a preferred frame or ether, but theorists have gone to great lengths to prove the background independence of ...
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Diffeomorphism constraint in loop quantum gravity

I am reading Canonical Quantization of Spherically Symmetric Gravity in Ashtekar’s Self-Dual Representation by Thiemann and Kastrup [a] and also the book Modern canonical quantum general relativity by ...
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I would like a simple description of what a sum of geometries means in loop quantum gravity

I have been studying quantum gravity theories. Many theories would have gravity (that is space and time) defined by the exchange of a gravitational quanta. That is simple in sense it is readily ...
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LQG: Why does gravity attract (not repel)?

I read that LQG can (with the spinfoam formalism) give the Einstein equation when $h$ tends to zero. See the metrics created by an object in the sky. The equations of the geodesics contain a parameter ...
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How does LQG explain attractivity? [duplicate]

Newton said that attraction was due to forces at a distance. RG said that matter fields change the metrics of space time Rovelli says the fields ARE space time (not enbedded in it). How does ...
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Loop quantum gravity

Is Loop Quantum Gravity trying to bring quantum mechanics from it's principles? Or is it taking ideas from quantum mechanics and merging them with GR to find a theory of quantum gravity?
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What is a particle in loop-quantum-gravity?

What exactly is the underlying mathematical structure of e.g. an electron in Loop-quantum-gravity? And is there a simple way to get the quantum mechanical limit so that one can see how the wave ...
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58 views

What is the interaction between quantum fields and Rovelli's particulate space of loop quantum gravity?

I've read Rovelli's book and papers, but he makes little mention of quantum fields. What is the interaction between fields and his particulate space of loop quantum gravity? I know that he says loop ...
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180 views

Loop Quantum Gravity falsified?

In the Wikipedia article "Loop Quantum Gravity" one can read ESA's INTEGRAL satellite measured polarization of photons of different wavelengths and was able to place a limit in the ...
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36 views

Compactness condition on boundary formalism for loop quantum gravity

I'm going over Rovelli's Covariant Loop Quantum Gravity notes http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf and in section 2.4.2 "Boundary formalism" he briefly develops how to ...
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A Loop Quantum Gravity question

Can someone answer this question, I have chosen this from Bodendorfer's article on 'An Elementary Introduction to Loop Quantum Gravity' from section 3 General relativity in the connection formulation ...
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Link between Horava-Lifshitz and Loop Quantum Gravity?

Most of the people say about that Combining String Theory with Loop Quantum Gravity, a beautiful example is given in a note by Nobert Bodendorfer, namely A note on quantum supergravity and AdS/CFT, ...
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84 views

Is Loop quantum cosmology replacing the Big Bang singularity with a big bounce?

Am I correct in understanding that in the cosmological version of Loop quantum gravity, namely Loop quantum cosmology, the universe in all models starts with a big bounce? Are there other models, for ...
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Loops in Loop Quantum Gravity

Am I correct in understanding that in Loop Quantum Gravity, loops are somehow interconnected with each other or interact, or is there some form of quantum entanglement between them? And if so, can we ...
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Clebsch-Gordan coefficients, spin networks and intertwiners

After spending some time with LQG books and articles i have still some problems regarding concepts of this theory. Spin network is built from lines labeled by spin label $j$ and since angular momentum ...
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Why does combining background independence and diffeomorphism invariance force you to conclude that spacetime must be quantized?

I was recently watching a fermilab video on loop quantum gravity and at about 4:20 it is stated that, when combined, background independence and diffeomorphism invariance force you to conclude that ...
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39 views

Sinusoidal shape and continuity of the electromagnetic wave

I hope my question will be understandable. I wonder about the sinusoidal shape of an electromagnetic wave. I have the impression that something fundamental is behind this shape. To simplify things, ...
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Question about quantum gravity and super-fluid vacuum?

I've been reading a lot about super-fluid vacuum/quantum foam/quantum vacuum etc and how quantum gravity (as hypothesized) will emerge out of interaction of these virtual particles that make up said ...
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Relationship between classical $q$-deformed General Relativity and the cosmological constant

The frame-connection formulation of pure General Relativity in 4 dimensions is given by the action $$ S_{4d}[e, \omega] = \frac{1}{2 \kappa} \int \varepsilon_{IJKL} e^I \wedge e^J \wedge F^{KL}, $$ ...
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Would naked singularities have the same gravitational properties as black holes of the same mass? [duplicate]

This is just a thought experiment before someone mentions the cosmic censorship conjecture. If physicists are discussing it in papers and scholarship, I wanna see some of those theoretical angles too ...
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How does detection of Gravitational Waves affect Quantum Gravity Research?

After detection of gravitational waves in 2015, what are it's implications on quantum gravity theories? Are there any quantum gravity predictions validated through GW detection?
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How to get orientation entanglement in network?

Looking at this intuitive description of spinors. Is there a way to add something like this to a network graph. (e.g. the kinds of networks you get in LQG) or any kind of graph? It looks like ...
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Difference between Loop Quantum Gravity (LQG) and Causal Dynamical Triangulation (CDT)

Recently, I have read about Causal Dynamical Triangulation from here and Wikipedia. In Wikipedia it states, CDT is a modification of quantum Regge calculus where spacetime is discretized by ...
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Does Loop Quantum Gravity predict general relativity in semi-classical Limit?

Recently i read about Loop Quantum Gravity in Wikipedia and found this below statement here. Presently, no semiclassical limit recovering general relativity has been shown to exist. But i also ...
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What goes in what spaces in LQG?

I read again the first chapter of Rovelli 2005 quantum gravity book, and I am still puzzled by the different spaces for each concept. It seems spin foams inhabit Hilbert spaces. But i read elsewhere ...
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Discrete spacetime: what does it mean for spacetime fields and vacuum?

Suppose we imagine that the space-time continuum is really a discrete stuff of something. This something could be some kind of particles or "substance"/matter/energy field. Then, I see two conceptual ...
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Gauge invariance of 3-dimensional LQG action

I'm going through C. Rovelli's textbook. On pages 91-93 he defines: $$ U_e = {\cal{P}} \left\{ \exp \int_e \omega\right\} \in SU(2), \tag{4.30} $$ $$ L_s^i = \int_s e^i \in {\mathbb{R}}^3. \tag{4.31} $...
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Immirzi parameter and it’s Critical values

Where does the critical values of the Immirzi parameter, i.e. log(2)/pi(3^1/2), etc. come from? Is there an equation that relates these values to the general immirzi parameter? What is the ...
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Spin Foam Introductory Texts

What are some good introductory texts on spin foams / LQG for someone specialized in SUSY / string models? I’m somewhat familiar with some of the literature (Topological Amodel and crystal melting / ...
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Is Loop Quantum Gravity related with loops?

I read this article on wikipedia on loops. And I wondered if the loops of loop quantum gravity have the algebraic structure of loops or it's just a coincidence.
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What is the relationship between Loop Quantum Gravity and Conformal Cyclic Cosmology?

I have heard/read of both of these "theories" or concepts by Roger Penrose (et al.?) - pardon my possible misuse of the word "theory". Is CCC just one possible model of the universe based on the ...
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172 views

Is entropy quantized in Loop Quantum Gravity?

From the Beckenstein-Hawking formula, we know that entropy is proportional to the area of the event horizon of a black-hole: $S\propto A$. From Loop Quantum Gravity, we know that length, area and ...
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What's wrong with lattice quantum gravity?

Assume one can write the metric field on a lattice, so on each lattice point one has a value of $g^{\mu\nu}$. Similar to the way lattice QCD is formulated. Then later taking the distance between ...
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What are exactly “norms” in spin networks? Are there any non-quantum spin networks?

Roger Penrose proposed a series of networks from which, fundamentally, space-time would emerge, called spin networks (https://en.wikipedia.org/wiki/Spin_network) In this article, it is said: Given ...
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By What Mechanism Does the Universe Expand in Loop Quantum Gravity Theory?

So, the universe is expanding, and, from what I've heard, that expansion is accelerating. However, I'm unclear as to the specific mechanism involved, particularly as would be described in a Loop ...
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Can mass still be observed in our universe after moving from a black-hole to a white-hole?

This question is based on the recent papers on LQG black-holes by Abhay Ashtekar. Quantum extension of the Kruskal spacetime and Quantum Transfiguration by Kruskal Black Holes When mass moves from ...
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In loop quantum gravity what is considered the initial conditions?

In LQG, the Universe starts off not from a singularity but of finite size. And there may be the idea of a pre-big bang crunch. Assuming the Universe is symmetric in time about the 'big bang'. What ...
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In LQG can two bosons occupy the same space?

Apart from the fact the LQG doesn't really have any well accepted theory of how to put matter and particles into it. In theory in LQG can two bosons occupy the same space? Because of course if you put ...
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Hierarchy problem and loop quantum gravity

According to https://en.wikipedia.org/wiki/Hierarchy_problem#Finite_Groups. Hierarchy problem can be theoretically solved using symmetry of certain bosonic string theory on the Leech lattice (though ...
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Could every particle encode the distances to all other particles?

I was thinking about quantum gravity and pre-geometry and wondering this question: "If space does not exist. How does a particle know how far away it is from another particle?" i.e. there are no ...
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Time and “fundamental” equations

I have been reading articles by Carlo Rovelli. About quantum mechanics, he says: “The Heisenberg and Schrödinger pictures are equivalent if there is a normal time evolution in history. In the ...
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Ad-hoc algebraic spin network quantization

In Loop Quantum Gravity, one usually embraces the functorial point of view towards (T)QFT. Canonical and spinfoam formalisms only differ in how the projection operator is defined. Canonical formalism ...
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Why the volume of a region is not a diffeomorphism invariant? (LQG)

In loop quantum gravity, the volume operator for a given region is not a diffeomorphism invariant. But classically we know that volume is a scalar quantity under a diffeomorphism even if we take the ...
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How to understand Carlo Rovelli's notion that time “does not exist”?

Media coverage of Carlo Rovelli's book The Order of Time has had headlines like "There is no such thing as past or future", or "Carlo Rovelli: 'Time does not exist'." Is there a way to explain what he ...
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Was $\kappa$-Minkowski model falsified by gamma ray burst measurements?

I'm considering the $\kappa$-Minkowski space – a certain model from non-commutative geometry which reduces to the usual Minkowski space in the limit $\kappa \rightarrow \infty$. The parameter $\kappa$ ...
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How to describe the strength of gravity in LQG?

In general relativity we have free falling observers who feel no gravity. and equations for geodesics. There are tools that describe the strength of gravitation field for a given observer. In Are ...
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510 views

Very basic questions about loop quantum gravity

I would like a very simple answer to this question with as little math as possible. In quantum loop gravity, we want to quantize the curvature of space. I understand that: First, we discretize our ...
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1answer
149 views

Why is a spin network a spinfoam's boundary?

i read in Wikipedia that spinfoams boudaries are spin networks, but nothing is said about what is a boundary in this case. i know that a spin foam is a spin network where other colours and ...