30
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Why is Standard Model + Loop Quantum Gravity usually not listed as a theory of everything
One can pinpoint the technical error in LQG explicitly:
To recall, the starting point of LQG is to encode the Riemannian metric in terms of the parallel transport of the affine connection that it ...
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26
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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?
There are a lot of projects going on, and I'll try to sum them up with pithy one-liners that are as accurate as my own (admittedly limited) understanding of them. The solutions include:
Classical ...
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22
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Why there is no unique "recipe" for quantization of a classical theory?
1. Reverse the burden: Why should there be a unique quantization method? The classical theory is a limit of the quantum theory, why should this limit be reversible? It's like asking thermodynamics to ...
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12
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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?
As Heterotic said in the comments, the "fundamental" change, regardless of how fundamental you think it really is, is most likely the change from the old view of renormalization as an arbitary choice ...
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11
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Discrete spacetime: what does it mean for spacetime fields and vacuum?
You should really look into Loop Quantum Gravity for a quantitative example. While unconfirmed and highly speculative, it does offer a toy example for a background independent quantum field theory, ...
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9
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Very basic questions about loop quantum gravity
Am I right by saying that the curvature will be given by a group element?
As any observable in any quantum-mechanical theory, the curvature of space in LQG is an operator acting on the kinematical ...
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7
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Does Loop Quantum Gravity predict general relativity in semi-classical Limit?
The confusion arises because there are essentially two different approaches to the LQG dynamics in the literature.
The canonical approach pioneered by Thiemann is to define the matrix elements of the ...
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6
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Loop Quantum Gravity falsified?
This looks like a typical case where a scientist publishes a paper that makes certain specific and limited claims, but public relations people at their organization then put out a press release making ...
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5
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Difference between Loop Quantum Gravity (LQG) and Causal Dynamical Triangulation (CDT)
CDT is based on the idea to calculate the path integral over spacetime geometries by summing over the Regge triangulations. The discreteness of these triangulations is put in by hand, moreover, it isn'...
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4
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Is loop quantum gravity the only theory concerning the structure of spacetime?
I think the general viewpoint is that any theory of quantum gravity must have the geometry of spacetime emerge at low energy from more fundamental degrees of freedom. One argument for this emergence ...
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4
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Clebsch-Gordan coefficients, spin networks and intertwiners
Clebsh-Gordan coefficients don't play a part in the definition of LQG. The notion of intertwining tensors is enough to build spin network states.
They are, however, a very convenient tool for ...
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4
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LQG: Why does gravity attract (not repel)?
Asking this is like asking how the Standard Model explains why friction is exothermic. There’s a complex chain of approximations involved in both cases.
LQG (at least it’s spinfoam formulation) ...
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4
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Is Loop quantum gravity an unadulterated quantisation of general relativity, or does it have additional assumptions?
The issue is a bit more subtle:
Canonical quantization generally gives different results when you use different sets of classical canonical coordinates. For example, classical-mechanical systems will ...
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3
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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?
I think that Dirac was unsatisfied with the mathematical meaninglessness of the way renormalization was done.
This has changed with causal perturbation theory. The latter is a covariant and ...
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3
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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?
Regarding the question of whether such fundamental change has come along, I would say yes and no. Yes because Wilson's point of view has provided a much clearer picture of renormalization which by the ...
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3
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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?
Why do you think something has changed? Although I totally agree with ACuriousMind's viewpoint that viewing QFT as an effective theory relieves some of the pressure about the nature of renormalization,...
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3
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Why there is no unique "recipe" for quantization of a classical theory?
The answers above are great, but they don't address the last question of yours, so here goes.
$-$ Can spin networks be used to quantize QCD?
$-$ Only if it is coupled to gravity.
The spin network ...
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3
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Why there is no unique "recipe" for quantization of a classical theory?
Firstly, it should be stressed that different quantisation approaches to a classical theory will lend different insights. Secondly, one quantisation method for a system may be particularly ...
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3
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What is the connection between Braided Matter, Loop Quantum Gravity and the Rishon Model?
Yes, the rishon model is very similar to the Bilson-Thompson model. In the original paper, Bilson-Thompson cited both Shupe and Harari [1]. According to Bilson-Thompson, the earlier works introduced ...
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Discretizing spacetime
I've never heard of such claim, especially taken into account that LQG is based on a particular discretization of space on graphs (the spin network basis), while the evolution of the spin network ...
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3
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What experiment would disprove loop quantum gravity?
First of all, Loop Quantum Gravity doesn't postulate the granularity of space: it's a logical consequence of the canonical quantization of Einstein's Lagrangian and it's associated whit the spectra of ...
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Is Loop Quantum Gravity related with loops?
It's just a coincidence, the word loop in loop quantum gravity holds for the use of closed curves to rewrite the degrees of freedom of gravity theory in terms of gauge invariant traces of path ...
3
votes
How does detection of Gravitational Waves affect Quantum Gravity Research?
The observation of gravitational waves allows us to probe physical regimes that thus far had been inaccessible to us. This allows us to search for deviations from general relativity caused by quantum ...
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3
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Since spacetime can bend and form waves, does this mean it must be made of sub-units of matter? How can something bend if it does not have sub-units?
When we say "spacetime is curved" in General Relativity, we mean that the Riemann curvature tensor field - which loosely tells you how a vector will change when parallel-transported around a ...
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3
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GHY term in loop quantum gravity
I'll take a stab at answering this.
Short answer: it does not play a major role in neither the canonical or covariant LQG.
A bit about boundary terms
Terms like that aren't at all mysterious, in fact, ...
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3
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Triangulation of the Hamiltonian constraint in Loop quantum gravity
I'm working on this regularization too.
The triangulated Hamiltonian $H_{E}^{\Delta}$ you wrote it's such that in the limit it tends to (here I take $k = 1$)
\begin{equation}
2 \int_{\Delta} \mathrm{d}...
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3
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From a quant-info perspective why are the reals indexing irreps of the Lorentz group less suspect than continuousness in space-time and general QM?
The UV infinities in perturbative QFT is a mathematical artifact coming from the indiscriminate use of distributions. In causal perturbation theory, where more careful use of distributions is made, ...
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3
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Is there a quantum gravity theory where ''space is discrete''?
Empty space or vacuum cannot be a simple lattice with Planck length spacing, because such a lattice contradicts special relativity.
The idea of a lattice is that it has a smallest length.
But due to ...
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2
votes
Accepted
Black hole singularity in loop quantum gravity
There has been some funny progress recently, as described in a paper by Ashtekar, Olmedo and Singh, about which Rovelli wrote this article:
https://physics.aps.org/articles/v11/127
Apparently, loop ...
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