Questions tagged [loop-quantum-gravity]
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106
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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 ...
2
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1answer
232 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 ...
2
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1answer
58 views
Gauge invariance of 3-dimensional LQG action
I'm going through C. Rovelli's textbook. On pages 91-93 he defines:
$$
U_e = {\cal{P}} \left\{ \exp \int_e \omega\right\} \in SU(2), \tag{4.30}
$$
$$
L_s^i = \int_s e^i \in {\mathbb{R}}^3. \tag{4.31}
...
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19 views
Immirzi parameter and it’s Critical values
Where does the critical values of the Immirzi parameter, i.e. log(2)/pi(3^1/2), etc. come from? Is there an equation that relates these values to the general immirzi parameter?
What is the ...
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18 views
Spin Foam Introductory Texts
What are some good introductory texts on spin foams / LQG for someone specialized in SUSY / string models? I’m somewhat familiar with some of the literature (Topological Amodel and crystal melting / ...
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29 views
Coordinate space of loops in loop quantum gravity
I have a basic and perhaps admittedly misguided question about the loops in LQG. Broadly speaking I think I understand the logic that lead to them, namely that working in the ADM formalism (using the ...
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58 views
Has Loop Quantum Gravity Been Axiomized?
I was just wondering if any work had been done to axiomize loop quantum gravity. If so what are the axioms of LQG?
Thanks
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1answer
79 views
Is Loop Quantum Gravity related with loops?
I read this article on wikipedia on loops. And I wondered if the loops of loop quantum gravity have the algebraic structure of loops or it's just a coincidence.
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24 views
Conformal dimension of fields over groups
Group Field Theory (GFT) deals with field theory over group space for instance
$$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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9 views
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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54 views
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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67 views
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 ...
2
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1answer
137 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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146 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 ...
0
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1answer
86 views
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:
...
0
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1answer
125 views
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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59 views
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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40 views
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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25 views
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 ...
2
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1answer
78 views
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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43 views
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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1answer
85 views
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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46 views
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 ...
0
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1answer
141 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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1answer
243 views
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 ...
2
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1answer
71 views
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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56 views
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 ...
6
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1answer
447 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 ...
2
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1answer
112 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 ...
2
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1answer
118 views
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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1answer
40 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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40 views
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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176 views
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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7answers
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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 ...
4
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1answer
135 views
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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71 views
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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67 views
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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1answer
475 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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181 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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151 views
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
57 views
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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87 views
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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94 views
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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3answers
805 views
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
78 views
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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4answers
614 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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1answer
148 views
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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1answer
268 views
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:
$$ p^2 = ...
9
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0answers
434 views
Does a good path integral exist in Loop Quantum Gravity?
The Hamiltonian operator of Loop quantum gravity is a totally constraint system
$$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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186 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 "...
