Questions tagged [loop-quantum-gravity]

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33
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4answers
5k views

Give a description of Loop Quantum Gravity your grandmother could understand

Of course, assuming your grandmother is not a theoretical physicist. I'd like to hear the basics concepts that make LQG tick and the way it relates to the GR. I heard about spin-networks where one ...
27
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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 ...
23
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2answers
3k views

Why is Standard Model + Loop Quantum Gravity usually not listed as a theory of everything

I have often seeen statements on physics.SE such as, The only consistent theory of everything which we know of to date (2013) is string theory. Why exactly is this so? Adding the Loop Quantum ...
20
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3answers
2k views

Space-time in String Theory

I would like to understand how Physicists think of space-time in the context of String Theory. I understand that there are $3$ large space dimensions, a time dimension, and $6$ or $7$ (or $22$) extra ...
19
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1answer
1k views

How is matter modeled in loop quantum gravity?

I have a basic understanding of how gravitational degrees of freedom are modeled in loop quantum gravity, but as far as I know, the main machinery, spin network states and observables, does not encode ...
15
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5answers
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What is the role spin-networks play in Loop Quantum Gravity?

This is a second question (in what will probably become a series) in my attempt to understand LQG a little. In the previous question I asked about the general concepts behind LQG to which space_cadet ...
13
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5answers
6k views

Can Loop Quantum Gravity connect in any way with string theory?

The one difficulty I see with LQG is that it requires an enormous number of degrees of freedom, e.g. these spin variables in the net. This is in contrast to stringy holographic theory where the ...
13
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3answers
1k views

What experiment would disprove loop quantum gravity?

Loop Quantum Gravity (LQG) is a theory of quantum space-time that attempts to describe the interconnection between general relativity and quantum mechanics. It's main postulate is the granularity of ...
12
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7answers
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Does the discreteness of spacetime in canonical approaches imply good bye to STR?

In all the canonical approaches to the problem of quantum gravity, (eg. loop variable) spacetime is thought to have a discrete structure. One question immediately comes naively to an outsider of this ...
12
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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 = ...
10
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4answers
2k views

Discussion of the Rovelli's paper on the black hole entropy in Loop Quantum Gravity

In a recent discussion about black holes, space_cadet provided me with the following paper of Rovelli: Black Hole Entropy from Loop Quantum Gravity which claims to derive the Bekenstein-Hawking ...
10
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4answers
2k views

Question regarding the validity of the big bounce

I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows. 1) How one reconciles it with the fact that it is now ...
10
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1answer
525 views

Area law for Entropy in Loop Quantum Gravity

In connection with the long saga of the (claimed) microscopic calculations of the Hawking-Bekenstein entropy in (3+1) Loop Quantum Gravity (LQG) and related approaches I have the following question: ...
9
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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 ...
9
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1answer
1k views

Are there any versions of LQG that claim to not violate Lorentz symmetry?

LQG formulations have a minimum length/area. Since say, a Planck area can always be boosted, any minimum area in space can be shrunk. Do LQG proponents worry about local Lorentz invariance violation, ...
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 ...
9
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0answers
311 views

Significance for LQG of Sen's result on entropy of black holes?

Sen 2013 says, ...we apply Euclidean gravity to compute logarithmic corrections to the entropy of various non-extremal black holes in different dimensions [...] For Schwarzschild black holes in ...
8
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2answers
1k views

Preparatory knowledge to study Quantum Gravity

I am an old (41-Years-old) physics graduate. I pretty much quit to "frequent physics" 10 years ago and my ability to manage physics is a bit rusty. I was fascinated with Quantum Gravity (QG) theory ...
8
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2answers
451 views

Coherent $U(N)$ intertwiners in Loop Quantum Gravity (LQG) and a measure on the Grassmannian

This is a detailed question about $U(N)$ intertwiners in LQG, and it comes from the the paper by Freidel and Livine (2011 - archive). It is very specific but related to finding a measure on a quotient ...
8
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2answers
515 views

Is taking the “square root” of the densitized inverse triad irregular in loop quantum gravity?

In loop quantum gravity, the canonical (Ashtekar) variables are chosen to be the densitized inverse triad $\mathbf{E}$ and some rotation connection field $\mathbf{A}$. To get the ordinary triad from $\...
8
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1answer
372 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 ...
6
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1answer
448 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 ...
6
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1answer
943 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-...
6
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1answer
1k views

LQG Demystified Book?

Does a similar easy to read introduction like David Mc Mohan's Demystified books exist for LQG? I'd like something easier to start with than a paper but deeper than a popular book.
6
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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 ...
6
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1answer
263 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 ...
5
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1answer
449 views

What are the arguments for gravity not being a force? (in quantum gravity)

In quantum gravity the standard assumtion is that gravity is a force, although there is a small but persistent group of theorethical physicists who think otherwise. What gives us the motivation to ...
4
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2answers
809 views

Entropy of a naked singularity

According to the wikipedia article http://en.wikipedia.org/wiki/Naked_singularity: "Some research has suggested that if loop quantum gravity is correct, then naked singularities could exist in nature, ...
4
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1answer
771 views

What do “tachionic” neutrinos mean for QG?

Reading about the spectacular Opera claim, I`m (again ;-P) wondering if a confirmation of superliminous neutrinos could help settle some still open quantum gravity issues ...? In this post, Lumo ...
4
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1answer
730 views

How is the expansion of space itself explained in LQG?

To explain the expansion of space I have often heard people saying that space is continuously created. This picture is usually applied to cosmological scales but I`m nevertheless curious if some ...
4
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2answers
424 views

Black hole singularity in loop quantum gravity

How is the singularity of a Black Hole treated in Loop Quantum gravity ? Does it go away ? And if it does, what's after the event horizon ?
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 ...
4
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1answer
570 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. ...
4
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1answer
289 views

How do we choose how to discretise spacetime in loop quantum gravity?

In LQG we choose a two-complex (or other) on which to discretise GR before quantising. I'm a little uncertain about the nature of this choice. Is the two-complex an a priori property of the universe (...
4
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0answers
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 ...
4
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0answers
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 ...
3
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3answers
764 views

Does the Opera result hint to a discrete spacetime?

Could the Opera result be interpreted as some kind of hint to a discrete spacetime that is only seen for high enough energy neutrinos? I think I've read (some time ago) something like this in a ...
3
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1answer
378 views

A loop quantum gravity toy inspired by an Aharonov-Bohm ring

Comparing my question to Give a description of Loop Quantum Gravity your grandmother could understand what I'm looking for here is a toy for a toddler ($\approx$ a pre-QFT graduate student). I seek ...
3
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1answer
634 views

Variable speed of light in loop quantum gravity

Loop quantum gravity predicts a variable speed of light. What is the range of such a variation in its speed?
3
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1answer
318 views

Are background independence and diffeomorphism invariance the same?

What is the difference between background independence, which loop quantum gravity formalism claims to have, and diffeomorphism invariance?
3
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0answers
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?
3
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0answers
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 ...
3
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0answers
103 views

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 \...
3
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0answers
276 views

What are elementary particles according to Loop Quantum Gravity? [closed]

Are elementary particles still excitations of quantum fields in LQG?
3
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0answers
136 views

Homeomorphism between the space of all Ashtekar connections and spacetime?

Excerpt from an essay of mine: Let $\Psi(\varsigma)$ be the wavefunction in the loop representation, where $\varsigma:[0,1]\to\mathcal{M}$, where $\mathcal{M}$ is spacetime. Then, let $\mathcal{A}$ ...
2
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1answer
233 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
673 views

The future of supersymmetry [duplicate]

Considering the fault of any experimental evidence from LHC for supporting the supersymmetry idea until now, can we say that it is dead? Generally the people who are working on this subject say that ...
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 $\kappa$ ...
2
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3answers
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How much energy does a photon need to form a black hole?

I was wondering if it is possible for a single photon to form a black hole if it has a small enough wavelength.If so, what would this wavelength be? I came across this question because I am reading ...
2
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1answer
178 views

If the graph isomorphism problem can't be solved in polynomial time, do spin-networks in loop quantum gravity violate the Church-Turing thesis?

If the graph isomorphism problem can't be solved in polynomial time, do spin-networks in loop quantum gravity violate the Church-Turing thesis? To determine whether or not two graphs are isomorphic (...