Questions tagged [loop-quantum-gravity]

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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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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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Do theoretical naked singularities have the same gravitational properties as black holes of the same mass? [duplicate]

Just a thought experiment before someone mentions the cosmic censorship conjecture. If you take a black hole and turn it into a naked singularity, does the naked singularity have the same ...
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66 views

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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156 views

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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339 views

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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66 views

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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Coordinate space of loops in loop quantum gravity

I have a basic and perhaps admittedly misguided question about the loops in LQG. Broadly speaking I think I understand the logic that lead to them, namely that working in the ADM formalism (using the ...
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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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151 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 “fundamenta”l 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 the 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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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
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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 ...
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What happens to the masses in the center of two black holes in the moment they merger?

This an attempt to improve my recent question What happens to the singularities of two black holes in the moment they merger?. What is the background talking about „singularities“ or „masses in the ...
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Evolution of a Planck Star relative to its Schwarzschild radius

As particles fall into a black hole they accelerate and thus the mass of the Planck star increases. The more the star grows, the stronger its gravitational pull and thus the same particle will add ...
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Quantum gravity: should it stabilize inspirals?

Bohr model and Quantum Mechanics stabilize the atoms in order to have a minimum state (due to some effective or centrifugal force) avoiding electrons to hit the nuclei. Thus, electrons (accelerated ...
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Why is a theory of quantum gravity expected to solve the BigBang singularity problem?

This a follow-up question related to this. A successful theory of Quantum gravity is expected to solve the BigBang singularity problem. Why (and how) is the quantization of gravity supposed to solve ...
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Dirac once said that renormalization is just a stop gap procedure, and there had to occur a fundamental change in our ideas. Did something change?

Once, Dirac said the following about renormalization in Quantum Field Theory (look here, for example): Renormalization is just a stop-gap procedure. There must be some fundamental change in our ...
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Non-separable Hilbert space in LQG

Most of Hilbert spaces appearing in real life are separable ones: even such large for the first spaces as Fock space, spaces of functions in infinite number of variables and so on. However I heard ...
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Expansion Mechanism in Loop Quantum Gravity [duplicate]

Loop Quantum Gravity suggests that spacetime is quantized. Given that, what is the mechanism of expansion? Do more quanta appear out of nowhere or do the quanta grow larger?
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Why are strings chosen over loops? [duplicate]

The basis of my question is as follows: I am very aware that, within the realm of quantum gravity research, string theory is a far more popular formulation than loop quantum gravity (LQG). Why is ...
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Loop quantum gravity and time

Loop quantum gravity (LQG), if I understand correctly, describes time as emerging from the interactions of quanta of the gravitational field that can be described by spinfoam networks. (See photos) ...
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How can a Loop Quantum Gravity Theory relate to a Gravity theory? [closed]

The Loop Quantum Gravity as I heard states that there are indivisible chunks of space and time in the space-time fabric and therefore it is possible for an object to occupy least possible space and ...
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Is there a connection between Loop Quantum Gravity and the Rishon Model?

In the picture below you can see how the first generation of quarks and leptons is seen in Loop Quantum Gravity. In the Rishon Model of my beloved Haim Harari you can see that the first generation of ...
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Notation issue in a paper on loop quantum gravity/geometrodynamics

I was reading a paper and I came across some notation that confused me. This was concerning the extrinsic curvature tensor defined as $K_{ab} = \frac{\mu}{2}K^i_{(a}K^i_{b)}$. I am normally ...
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Can loop quantum gravity use known techniques to cure the fermion doubling problem?

It is well known that LQG has a fermion doubling problem and therefore cannot include chiral fermions. https://arxiv.org/abs/1507.01232 However, there are already many known techniques to dealing ...
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Physical interpretation of polymer quantization

I have read a bit about polymer quantization, and I want to know what this quantization means physically. I already know that it enables us to incorporate an additional length scale that solves the ...