Questions tagged [loop-quantum-gravity]

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Conformal dimension of fields over groups

Group Field Theory (GFT) deals with field theory over group space for instance $$S_2 = \int dg_1dg_2 \mathcal{K}(g_1, g_2)\phi(g_1)\phi(g_2) +\int dg_1dg_2dg_3\mathcal{V}(g_1, g_2, g_3)\phi(g_1)\phi(...
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Resource recommendation: Relation between q-deformed groups and non-commutative field theory

I would like to know if non-commutative field theory is related to $q$-deformed groups in any way. There is a relation between non-commutative field theory and group field theory which I understand is ...
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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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116 views

Biggest string theory competitors [closed]

i am a layman in physics and i just want to know what are, as of today, the most importants framework for quantum gravity that contrast string theory and why is so?
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Relation of spin networks, graphs, polyhedra and emergence of 3-space in loop quantum gravity

I have some questions regarding the current status of spin networks and their relation to geometry in loop quantum gravity. The picture I got many years ago is that spin networks in LQG are coloured ...
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111 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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102 views

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: ...
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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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How do Feynman diagrams and loop quantum gravity fit together?

Feynman diagrams are a good way of calculating effects in quantum electrodynamics on a constant background space-time. Spin foams (LQG) are conjectured to be a good way of calculating quantum effects ...
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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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115 views

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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39 views

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 upon a time, 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 ...
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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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418 views

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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180 views

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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2answers
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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 ...
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Why there is no unique “recipe” for quantization of a classical theory?

I have seen in Wikipedia that different quantization methods exist (see Wiki article with name "Quantization"). Moreover, Wikipedia stated that there is more than one way to quantize a classical ...
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1answer
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What is the meaning of $O(r^{-n})$ odd/even?

I am studying the book "Modern canonical quantum general relativity" written by Thomas Thiemann. In this book, it has been written since $\vec{N}$ is an asymptotic Killing field of $δ_{ab}$ we ...
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543 views

Is loop quantum gravity the only theory concerning the structure of spacetime?

As said in wikipedia's page More precisely, space can be viewed as an extremely fine fabric or network "woven" of finite loops. These networks of loops are called spin networks. The evolution of a ...
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Spin networks - resources

I am very interested in studying spin networks. Where can I begin? I want to understand them at their basic level. Which reference is good to get more technical details?
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Experimental bounds on Lorentz-violating dispersion relation

It has been predicted by several background-independent approaches to Quantum Gravity (like LQG or spinfoams) that the physical dispersion relations in vacuum could take the following form: $$ p^2 = ...
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Does a good path integral exist in Loop Quantum Gravity?

The Hamiltonian operator of Loop quantum gravity is a totally constraint system $$H = \int_\Sigma d^3x\ (N\mathcal{H}+N^a V_a+G)$$ Here, $\Sigma$ is a 3-dimensional hypersurface; a slice of ...
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161 views

Good mathematical introduction to loop quantum gravity [duplicate]

All websites or papers I could find did only have a mathless introduction for laymen. Does anybody know where to find a precise and mathematical introduction to LQG, for example containing the "...
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153 views

A brief description of a plausible and possible candidate for Quantum Gravity [closed]

I wanted to know the specifics(essence) of a particular theory that is present for quantum gravity (QG),and which seems to have the best potential. I am no expert on field theories and QG, cause ...
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489 views

Number of free parameters in String Theory and LQG

In the Standard Model, we have these free parameters: 6 quark masses 6 lepton masses 3 coupling constants Higgs mass Higgs VEV 4 CKM parameters 4 PMNS parameters QCD vacuum angle That's 26 in total. ...
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Definition of evolution equation in Rovelli's quantum gravity book or how a function can determine a curve

I'm reading the book Quantum Gravity by Carlo Rovelli. In page 107, he says that the evolution equation of the system can be cast into $f=0$, where $f$ is a map such that $f: \mathcal{C}\times \Gamma \...
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1answer
182 views

Discretizing spacetime

It is often claimed in quantum gravity circles (I'm thinking of LQG in particular) that it is "not possible" to discretize spacetime. The issue, as I understand it, is that should one attempt to ...
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350 views

Zeno effect in quantum gravity?

Given that "the quantum zeno effect can "freeze" the evolution of the system by measuring it frequently enough in its known initial state." Is there any equivalent/similar Zeno effect in quantum ...
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1answer
881 views

What is the basic idea of loop quantum gravity? [duplicate]

I have researched a little on loop quantum gravity (LQG), but all I got were very ambiguous explanations about loops and nodes that all end with mathematical equations. If someone could give me a non-...