Questions tagged [loop-quantum-gravity]
The loop-quantum-gravity tag has no usage guidance.
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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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43 views
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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1answer
49 views
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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52 views
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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48 views
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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1answer
60 views
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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49 views
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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2answers
121 views
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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50 views
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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1answer
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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2answers
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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1answer
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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96 views
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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38 views
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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1answer
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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1answer
91 views
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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1answer
72 views
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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63 views
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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1answer
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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28 views
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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1answer
87 views
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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60 views
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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1answer
75 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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1answer
24 views
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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3answers
228 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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2answers
235 views
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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1answer
81 views
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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1answer
465 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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1answer
77 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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0answers
26 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 ...
3
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1answer
50 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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1answer
96 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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87 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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1answer
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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281 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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1answer
105 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:
Given ...
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1answer
179 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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0answers
62 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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43 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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31 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 ...
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100 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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47 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 ...
2
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2answers
161 views
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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49 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 ...
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1answer
198 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
299 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 ...
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1answer
82 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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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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1answer
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 ...
2
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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 ...
