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Questions tagged [quantum-gravity]

Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.

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What are the differences and similarities between inflaton, dilaton, scaleron, and other similar hypothetical particles?

Lately I've been reading about modified gravity and I'm a bit lost on the names of hypothetical particles. Let me give an example. I was reading about Starobinsky inflation, which prescribes a ...
Níckolas Alves's user avatar
1 vote
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Is there a good overview or introduction to "de Sitter-invariant Special Relativity"?

Is there a good overview or introduction to de Sitter-invariant Special Relativity? I searched for various materials, but it was difficult to find materials that were easy to read.
2 votes
1 answer
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Bekenstein experiment to reveal the quantum foam

Bekenstein proposed an experiment to experimentally verify the Planck-scale space-time quantization. However, this has not been verified by subsequent actual experiments. What is it that cannot be ...
정윤우's user avatar
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Probabilistic curvature of spacetime [duplicate]

I was wondering since matter tells space-time how to curve, and since matter is probabilistic in position (say hydrogen atom) is the curvature also probabilistic? black holes slowly shrink by ...
Mantu Das's user avatar
2 votes
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BTZ black hole as a quotient of AdS space

I am trying to understand this paper 1 and trying to reproduce some calculations and had some questions about that. In section 3.2, page 12, eq. 3.9, the authors are writing normal geodesics of an ...
Goodfellow's user avatar
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1 answer
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Can you calculate the radius of a hypothetical singular surface inside a black hole from observing changes to its linear momentum?

Say there is a ball of unknown radius surrounded by a bubble. The ball represents a hypothetical singular surface inside a black hole and the bubble represents the event horizon. If you threw marbles ...
user414142's user avatar
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3 answers
194 views

Is the size of a black hole singularity smaller than a fundamental particle?

I am wondering about the size of a black hole singularity. We know that a classical black hole is infinitely dense. I am not asking about size of event horizon. I am asking about actual size of the ...
Arpan Purkait's user avatar
6 votes
0 answers
88 views

What does the word "Observable" mean in Quantum Gravity?

I have seen the statement in several places that the only "observables" in general relativity or Quantum Gravity are measured at temporal or spatial infinity. This is often used as a ...
Josh Newey's user avatar
9 votes
1 answer
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Does the cosmological constant entail a mass for the graviton?

If I consider the Einstein equations into the form $$ R_{\mu\nu}=\kappa \left(T_{\mu\nu}-\frac{1}{2}g_{\mu\nu}T\right)+\Lambda g_{\mu\nu} $$ and then linearize them, we should get by moving to ...
Jon's user avatar
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What Does Feynman Mean When He Says Amplitude and Probabilities?

In Feynman lectures on gravitation section 1.4, he tries to debate over whether one should quantize the gravitation or not. He provides a two-slit diffraction experiment with a gravity detector, which ...
Ting-Kai Hsu's user avatar
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Inflation in background free models of the universe

There are many authors who are attempting to construct a model of physics that doesn't rely on the objective existence of spacetime. This is part of the work in quantum gravity. This leads to things ...
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What is the difference between Hawking radiation and a black hole laser?

While reading this paper (https://arxiv.org/abs/1409.6550), I got a little bit puzzled: what is the difference between Hawking radiation and a black hole laser? Is it the same thing? From my ...
Andris Erglis's user avatar
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Quantum experiments at different heights from Earth

If one were to perform the same Quantum mechanical experiment say the double slit experiment or any other quantum mechanical experiments with identical conditions, set ups and elements. while ...
Precious Adegbite's user avatar
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0 answers
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Graviton metric interaction

In most discussions on quantum-gravity, graviton is considered as a perturbation that is being added linearly to a flat metric (the $h_{\mu\nu}$ term in $g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu}$). ...
physics_2015's user avatar
1 vote
2 answers
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Why is Perturbative expansion of gravity in terms of $GE^2$?

From General Relativity by Weinberg p.797 edited by Hawking & Israel: This is to be used to generate a perturbation series in powers of $GE^2$ or $G/r^2$ (where $E$ and $r$ are an energy and a ...
Arevilov 3's user avatar
32 votes
8 answers
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Explain to a non-physicist what goes wrong when trying to quantize gravity

I am not a physicist, but I'm trying to get a little bit of an understanding of why it is hard to extend the standard model with quantum gravity (i.e. why it's hard to combine QM and GR), cf. e.g. A ...
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Planck Length Calculation

On this question, from long ago, the Planck length is calculated as the length at which the Reduced Compton Wavelength is equal to the Schwarzschild Radius. However, in the calculation, the scalar &...
Earl Whitney's user avatar
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1 answer
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Quantised Newtonian potential as an operator in non-relativistic QM [closed]

Suppose we have two slowly moving (effectively static) masses $m_1,m_2$, interacting through gravity, that are not occupying a definite state of position i.e. that matter is being treated quantum ...
Theoreticalhelp's user avatar
2 votes
0 answers
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3-Particle Kinematics and Parity of Operators

Recall that if the momentum of scattering amplitudes is taken to be complex and from little group scaling that the 3-particle interaction for massless particles of any spin is given as \begin{equation}...
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5 votes
1 answer
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Where does Planck's constant come from in non-renormalizability of quantum gravity?

I am trying to understand the idea that gravity breaks down at the Planck scale, but I am confused by the use of natural units ($c = \hbar = 1$). The Einstein-Hilbert action in natural units is: \...
Caspar201's user avatar
1 vote
1 answer
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How do hyperbolic tessellations like {7,3} in Anti de-Sitter space relate to our intuition of 3D space (or 4D structure if you include time)?

I just ran into the AdS/CFT correspondence, as I am looking at various use-cases of hyperbolic tessellations, specifically related to the pentagrid and heptagrid as defined by Maurice Margenstern in ...
Lance's user avatar
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Replica wormholes in $AdS_5 \times S^5$ holography

I have a question about replica wormholes and the CFT ensembles in AdS/CFT. To make sure that my question isn't coming from a simple misunderstanding, I'll first sketch out my current understanding on ...
11zaq's user avatar
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2 answers
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Does information always gravitate?

I'm trying to wrap my head around Bekenstein's loose argument that a bit of information added to the black hole corresponds to an added Planck surface area to its horizon. In it, he argues that one ...
Lourenco Entrudo's user avatar
11 votes
11 answers
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What would one use a theory of quantum gravity for?

I am generally wondering how useful new more ambitious theories would be considering that even with standard non-relativistic electrostatic QM one usually has to employ unsatisfyingly crude ...
Zaph's user avatar
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Is the background independence of dynamics a necessary condition for physical theories?

I read in the answer of Lubos Motl to this question that the dynamics of string theory is demonstrably background-independent while the (manifest) background independence is an aesthetic ...
leonardo ricca's user avatar
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1 answer
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Planck-scale measurements of sub-light-speed motion

Firstly, I want to make clear that I am not under the impression that the Planck scale represents a 'resolution' of physical reality, though I can understand why this question might make it seem that ...
P...'s user avatar
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(Sufficiently) Rigorous argument for Planck scale and formation of black holes

I used to hear the arguments about how if we go down to Planck scale (see, e.g., this lecture notes that I am reading but by no means the only place I saw this) then "you would need energies of ...
Evangeline A. K. McDowell's user avatar
19 votes
5 answers
14k views

Is quantum gravity research implying that gravity is actually a force and not spacetime curvature according to GR?

I am all the time reading that gravity is actually the curvature of spacetime according to general relativity (GR) established theory and not a force, like the known three fundamental forces of nature,...
Markoul11's user avatar
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Is the Hilbert space of a Black Hole factorizable?

I am doing research on black hole information and have a question: Is the black hole's Hilbert space factorizable? In other words, can we write the Hilbert space of the black hole as a multiplication ...
TheFyziker's user avatar
2 votes
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Relation between the Wheeler–DeWitt equation and string theory

Can we derive the Wheeler–DeWitt equations from string theory? Since they are both quantum gravity theory. A simple way seems to be the following logic: The Wheeler–DeWitt equation is the canonically ...
feng lin's user avatar
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2 answers
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Renormalizability of Quantum Gravity

At the end of chapter 6 on p. 210 in David's Griffiths' book Introduction to Elementary Particle Physics he says that 't Hooft proved that all gauge theories are renormalizable. I have also read ...
KaraboMadisa's user avatar
-2 votes
1 answer
86 views

Le Sage's Quantum Gravity Theory and the 5th force of Nature? [closed]

Le Sage's theory of gravitation when you first learn about it is surprisingly a 16th century theory of quantum gravity, a precursor to the modern term referring to this field of research which by many ...
Markoul11's user avatar
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2 votes
0 answers
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BRST quantization of Hilbert-Palatini gravity, question regarding a choice of gauge fixing fermion

I am interested in the BRST quantization of the Hilbert-Palatini gravity theory. In the paper https://arxiv.org/abs/gr-qc/9806001, Alexandrov and Vassilevich write down the BRST procedure for defining ...
Jeanbaptiste Roux's user avatar
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Why are the zero modes of the below operator Killing vectors? (2+1 dimensional gravity)

I'm trying to understand the eigenmodes of the following operator: $$(\Delta_{(1)}^{L L}-\frac{2}{3} R)V_\nu \equiv -\nabla^\mu \nabla_\mu V_\nu+R_{\nu \mu} V^\mu -\frac{2}{3} RV_\nu $$ Where $R_{\mu\...
faker 23's user avatar
6 votes
1 answer
251 views

Are Weinberg's soft theorems relevant when making predictions about collider physics?

In a seminal paper, Weinberg has shown that one can relate a $n \to m$ scattering amplitude to the $n \to m + k$ scattering amplitude that involves the same particle content plus $k$ additional ...
Níckolas Alves's user avatar
0 votes
0 answers
29 views

Does the Existence of Planck Units Suggest Discontinuity in Time and Space? [duplicate]

I’ve posed the following inquiry on Philosophy Stack Exchange: Can the idea of continuity make sense in the real world? A summary of it is presented here: Continuity in mathematics means no jumps or ...
user avatar
2 votes
7 answers
2k views

Does quantum mechanics respect the principle of relativity?

The principle of relativity says that all observers see the same laws of physics. It is, to my knowledge, the underlying principle behind General Relativity; put alternatively, General Relativity is ...
Allure's user avatar
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Lanczos-Lovelock gravity Lagrangian

I am studying the Lanczos-Lovelock theory of gravity where the Lagrangian density is $$ \mathcal{L} = \sqrt{-g} \sum_{i=0}^{t} \alpha_{i} \mathcal{R}^i $$ With $ \alpha $ being the coupling constants ...
Tomás's user avatar
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-2 votes
2 answers
115 views

What is the current most widely-accepted explanation of gravity? [closed]

What do physicists typically say gives gravity the ability to act on a pair of objects? I am not asking for a description of gravity as a scalar field, but rather what the current accepted theory is ...
EngineeringMind's user avatar
1 vote
2 answers
115 views

What do we learn from quantizing the relativistic point particle?

In many textbooks on string theory, some time is spend on quantizing the relativistic point particle as a warming-up for quantizing the Nambu-Goto action for relativistic strings. However, I have not ...
Fraxinian's user avatar
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0 answers
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How does the asymptotic metric fluctuation in $n \to m$ scattering relates to the soft factor in Weinberg's soft graviton theorem?

I'm reading arXiv: 1411.5745 [hep-th]. In Sec. 5, the authors show how the memory effect and Weinberg's soft graviton theorem are two faces of the same coin. I'm interested in understanding a specific ...
Níckolas Alves's user avatar
5 votes
1 answer
108 views

Low-energy string effective action valid for large dilaton field?

The low-energy effective action of the bosonic string in the critical dimension $D=26$ is given by: $$S=\frac{1}{2\kappa_0^2}\int d^{26}x\sqrt{-G} \left[ \phi^2\left( R-\frac{1}{12}H_{\mu\nu\lambda}H^{...
John Eastmond's user avatar
1 vote
0 answers
35 views

Invariant Tensor in spin representations

Exercise 1.9 of the book Covariant LQG by Rovelli and Vidotto reads: If we raise the index of the Pauli matrices with $\epsilon$ we obtain the 2-index spinors $(\sigma_i)^{AB} = (\sigma_i)^A_C\epsilon^...
K. Sadri's user avatar
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5 votes
1 answer
230 views

Why and how we study different limits in quantum gravity?

While I'm reading an article, I get confused by why and how we study different limits in quantum limit. In this paper, the author introduced four limits in D0-brane quantum mechanics: the DKPS (...
Errorbar's user avatar
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0 votes
0 answers
66 views

How can Planck-length elements exists in spacetime? [duplicate]

My question is simple, how can the theory of finite-sized elements (Planck-sized elements) in spacetime be correct, when you find the number $\pi$ in the Schwartzchild representation of the black hole,...
Superunknown's user avatar
1 vote
1 answer
110 views

On Gravity and the Path Integral

The path integral, in the simplest case, usually attributes a classical action to every conceivable trajectory a particle can take between to points in spacetime. This assumes a flat, Minkowski ...
Gabriel Turner's user avatar
3 votes
1 answer
207 views

Problems with fuzzball black hole creation and low energy physics

The fuzzball proposal intrigues me, but I doubt its ability to match up with semi-classical low energy physics. A variation of my question was asked by the awkwardly titled post:"At the instant ...
Michael C.'s user avatar
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0 answers
73 views

Question about Witten 0+1 quantum gravity

I am trying to follow Witten’s article as much as I can. What every physicist should know about string theory. Physics Today 68 (11), 38–43 (2015); my knowledge is very basic GR and QFT. I am really ...
Pato Galmarini's user avatar
3 votes
0 answers
221 views

How Do Gravitons Follow Geodesics?

It is known that all particles follow a geodesic in spacetime. Presumably gravitons follow geodesics as well. However, how does one describe that mathematically? For the case of other particles it is ...
physics_2015's user avatar
2 votes
0 answers
115 views

Gravity dual of the string world-sheet CFT?

The AdS/CFT correspondence conjectures a duality between a $(D+1)$ dimensional gravity theory in asymptotic AdS spacetime with a $D$ dimensional conformal field theory. Is there any sense in asking ...
Michael C.'s user avatar

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