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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Can AdS/CFT still give bulk locality on small scales if the CFT has a nontrivial phase diagram?

The AdS/CFT correspondence is a well-supported conjecture about the equivalence between an ordinary conformal field theory (CFT) and a theory of quantum gravity in asymptotically anti-de Sitter (AdS) ...
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What happens if the Compton wavelength of a particle exceeds the inverse square root of the Ricci tensor (call it curvature)?

What happens if the Compton wavelength of a particle exceeds the inverse square root of the Ricci tensor (call it curvature) precisely? Can somebody please use some formal QFT in curved spacetime to ...
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If the exchange of virtual Gravitons isn't real why do bodies attract?

I have been told Earth is not really sending particles to me and keeping me on the ground. That it is just a convenient thing to do in calculations. However if the exchange of gravitons isn't real, ...
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Planck-scale curvature in covariant LQG and quantization of length: does LQG apply also to the Planck-regime?

In the covariant approach of loop quantum gravity (see http://www.cpt.univ-mrs.fr/~rovelli/IntroductionLQG.pdf ), the theory is defined on a "lattice", similar to lattice QCD. In this case ...
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How does string theory explain gravity? [duplicate]

I read once that string theory explains gravitation via invoking quanta of the electromagnetic field to transmit the force of gravity over large distances. Is this the truth? I read somewhere else ...
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How do Gravitons work?

Gravitons are supposed to mediate the force of gravity, but wouldn't that require Earth, Sun and basically everything else is constantly sending out gravitons to everything else?
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Does quantum gravity contradict general relativity? [duplicate]

As far as I know, quantum gravity basically says that the graviton mediates the force of gravity. And general relativity says that gravitation is the result of masses warping spacetime, and thus ...
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Black hole solutions to theories with no matter fields

The Schwarzschild black hole is described by a mass M but comes from the Einstein's field equations with the stress-energy tensor set to zero. If our theory does not include matter fields - say in the ...
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Do photons and gravitons emitted in quadrupole atomic transitions have the same momentum?

A transition of an electron from orbital $3\text d$ to $4\text s$ produces photons and in small chances gravitons ($\Delta l= \pm2$) through quadrupole transitions. In 2 atoms, if one does this ...
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What information is retained on the boundary of a black hole?

If an object were to be pulled into a black hole, I’ve been told its information would be retained on the boundary. But will that be just the information of the actual state it was in at the time? Or ...
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Quantum gravity in an accelerated frame of reference

It is said that we can't study quantum gravity because gravity is a weak force. But gravity and acceleration are the same. Why can't we study quantum gravity in a strongly accelerated frame of ...
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What is the true nature of gravity? [closed]

In 17th century, Sir Isaac Newton gave us the universal law of gravitation which stated that gravity is an inverse square force. In 1915, Albert Einstein recognised gravity as a curvature of space-...
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The linearized solution for the Vasiliev scalar field in stereographic coordinates [arXiv:1712.02401]

I am reading [arXiv:1712.02401] and struggling to reproduce equation (5.3), using iso(3) invariance. My goal is to write a similar so(1,3) invariant solution in using (3.12a). The equation I am trying ...
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Relic graviton background: how to observe it?

Question dedicated to the memory of Steven Weinberg, whose books I read as a child and undergraduated student! Beyond using gravitational waves, are out there any other ideas or proposals to detect ...
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Is it possible to describe gravitons in curved backgrounds?

I've been studying Quantum Field Theory in Curved Spacetimes through Wald's Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics. On Section 4.7, he discusses how to generalize the ...
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How to correctly understand the horizon complementarity in quantum cosmology?

In quantum cosmology, it is assumed that the degrees of freedom beyond the cosmological horizon of a given observer do not have an independent existence. How can this be understood? The analogy with ...
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Moore's function

In the dynamical Casimir effect, the Casimir force is given in terms of Moore's function R which satisfies $$R(t+L(t))-R(t-L(t))=2$$ where $L(t)$ is the trajectory of a moving mirror (while another ...
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Why do string theorists seem to ignore cosmology?

Related and sort of a follow-up question to: If string theory is inconsistent with observations, why hasn't it been rejected yet? From the answer to that question, string theorists are aware the ...
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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? [closed]

My question is philosophical, I'm just curious to hear people's opinions. I'm also interested in any resources that are related to this question. Have you read any papers that attempt to investigate ...
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Are "Selective Extra Dimensions" possible?

The large extra dimensions model also known as the ADD model proposes that the six hidden dimensions implied by string theory are not rolled up to the Planck length and some (or all) of them are ...
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Partition function in SYK Model

In SYK model, we have the partition function for $N$-interacting fermions as \begin{equation} z=\int d^{N} \psi \exp \left(\imath^{q / 2} \sum J_{a_{1} a_{2} \ldots a_{q}} \psi_{a_{1} a_{2} \ldots a_{...
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Physical principles and their rigidity

In many of his talks and interviews, Nima Arkani-Hamed mentions how incredibly constrained are the laws of physics. For example, in a recent interview he says: “[I]f we just took these general ...
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Heisenberg uncertainty principle: Have unusual methods of measuring position (other than photons) been proven equally imprecise?

I'm reading Hawking's A Briefer History of Time and it's explained that Heisenberg demonstrated his uncertainty principle based on fundamental limitations of positional meaurememt by basically ...
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Why can't we define mathematical observables in asymptotic $dS$ or flat space for gravitational theories?

In higher spin currents, the boundary CFT is dual to an asymptotic $AdS$. I have heard that $dS$ is not quantizable. But I don't understand why we want it to be in the first place. Isn't Chern-Simons ...
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Terminology: "Sum over Geometries" vs. "Sum over Topologies" in Quantum Gravity

I am currently studying loop quantum gravity and related approaches like simplicial quantum gravity, spin foam models, tensor models and group field theories. In texts about this topics, one often ...
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Nonrenormalizable but quantizable theory: gravity?

In p.8 of Michio Kaku book Introduction to Superstrings and M-Theory-Springer (1998), he said The gravitational force. Gravity research was totally uncoupled from research in the other interactions. ...
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What are the advantages of string theory? [closed]

I'm following Witten's essay and it seams that he only presented one advantage of string theory (the fact that there are no UV divergence). Are there more advantages to string theory? It seems to me ...
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Motivation for 3D Quantum Gravity

I was briefly going through the idea of 3d quantum gravity on nLab, where it is stated that: The case of dimension 3 is noteworthy, because in this case the quantum theory can be and has been fairly ...
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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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What are the physical state invariants of loop quantum gravity?

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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Are gravitons produced in an atomic transition?

Atomic transitions produce photons. You can create beams of photons with a laser. In lasers rays of light are produced. Will rays of gravitons (or small directed gravitational waves) be produced also?
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Does gravity bend gravity?

Let's say that there is a large mass $M$ a light-year or so away from a black hole merger, which causes a very large gravitational wave to be produced. When the gravitational wave reaches $M$, does it ...
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Regarding a possible duality between (2+1)D gravity and Chern-Simons Theory

Is there a duality between (2+1)D gravity and Chern-Simons Theory? Or they merely have related features? If so, of which type and why?
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Graviton-graviton scattering in tree level. Based on a talk by Bryce DeWitt

In this talk given by Bryce DeWitt, at page 6 he mentions a problem he gave to one of his PhD students. The problem is to compute the graviton-graviton scattering amplitude in tree level approximation....
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Difference between resolvent and degeneracy

I am studying https://arxiv.org/abs/1903.11115. In equation (62), the resolvent is defined as the integral transform of partition function as $$ R(E) = -\int_0^\infty d\beta\ e^{\beta E} Z(\beta) $$ ...
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Gravitational non-scattering [closed]

When gravitation is modelled after real life, there is no such thing as a gravitational shadow. Does this mean that gravitons travel through matter without being scattered?
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Quantum correction of event horizon in QFT in curved spacetime or in any QG theory

As in the loop correction in QFT we get some correction to various physical observables (magnetic moment, charge, mass), do we get some sort of correction of EH of black hole when quantum phenomenon ...
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Cosmic censorship conjecture: Why naked singularities are expected not to exist?

According to wikipedia, the weak form of the Cosmic Censorship Hypothesis says the following: Singularities [...] are typically hidden within event horizons, and therefore cannot be observed from the ...
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Differences between observations of light versus gravity for a satellite traveling by the sun

A satellite travels in a geodesic by the sun with sufficient velocity to escape the sun's orbit. The distance of closest approach is 100 light seconds when the satellite's velocity is perpendicular ...
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Research Proposal in Statement of purpose for integrated Phd Theoretical Physics [closed]

I wrote this following paragraph in my statement of purpose for my Integrated PhD Physics program in Theoretical physics at best research institute in India. My research proposal in this field is to ...
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How are the hypothetical intermediate "dark photons" bosons theorized to couple dark matter with normal matter?

I believe that dark matter is said to couple with normal matter only gravitationally? Do these relatively new, hypothetical so far, "dark photons" suggest that there can be also an unknown ...
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How are gravitons compatible with general relativity?

I have been reading about how gravity has 2 equivalents descriptions: General Relativity. Explained by the graviton. How are these two things compatible? How can it be that gravity is explained ...
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Lowermost limit on energy

We know the lowest possible temperature that can exist -even if it is impossible to reach - being 0K. We also know the highest temperature with our current model of physics, that being the Planck ...
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Integration with respect to Haar measure and reduced density matrix [closed]

Consider a bipartite system $\mathcal{H}_A \otimes \mathcal{H}_B$, with $|A|,|B|>>1$ and not necceserly $|A|=|B|$. Following Jerusalem Lectures on Black Holes and Quantum Information (eq. 5.8) ...
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Gravitons and general relativity

First I want to say that I am a layperson, so I want intuitive answers. So all the 3 fundamental forces in nature has a carrier particle except gravity. So we have hypothesized the existence of ...
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Can you have quantum gravity without gravitons? [duplicate]

I was thinking about whether quantum gravity needs gravitons. One can do a sum over histories of curved spaces perhaps without having to have gravitons. But the term "quantization" implies ...
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How to incorporate both the ideas of field and particle while describing the working of a force?

So basically, the mechanism which explains the working of a fundamental force is just exchange of particles e.g. photons interaction in electromagnetic force. But then there is also the concept of ...
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Why do we integrate up to the invariances

Following Witten's essay What every physicist should know about string theory I understood that in the Hilbert-Einstein action is invariant under diffeomorphism in 1D and under conformal mapping in 2D....
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Why is the proper time $\tau$ the only invariant under diffomorphizem?

I'm following Witten's essay: What Every Physicist Should Know About String Theory . When applying path integral in 1D Witten mention: Part of the process of evaluating the path integral in our ...
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How can the graviton spin-2 string have emerged from the strong interaction, mediated by spin-1 gluons?

The origin of string theory can be traced back to 1969: String theory was first studied in the late 1960s as a theory of the strong nuclear force, before being abandoned in favor of quantum ...

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