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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Why must gravity have a quantum mechanical description? [duplicate]

I know that to understand the Bigbang singularity or the blackholes, it is necessary to have a quantum description of gravity or quantum gravity. However, barring those kinds of explanations, is there ...
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The elusive graviton particle. Why does Hadron colider fail to find this elusive particle. It does exist but i think method for finding it is wrong [closed]

Why does Hadron colider fail to find this elusive particle. It does exist but i think method for finding it is floored.i think that trying to find it by collision is a floored method. My theory is ...
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Why quantum gravity can't be described by spin-1 bosons? [duplicate]

As the title suggests, why quantum gravity can't be described by spin-1 gravitons?
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Are Photons Entangled with Gravity? [closed]

Part 1: A photon's wavelength is red-shifted by expanding space. Since $E=hf$ the photon imparts some of it's energy to the curvature of space, slowing the expansion of space ever so slightly. Is that ...
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Understanding graviton propagators in Minkowski space

In Minkowski space, the graviton propagator is given by: $$\Pi_{\mu \nu\rho \sigma} = \frac{\mathcal{P}^2_{\mu\nu\rho\sigma}}{k^2} - \frac{\mathcal{P}^0_{s,\mu\nu\rho\sigma}}{2k^2} \tag{1}$$ where (...
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Mode expansion for $p$-branes

In the quantization of $p$-branes, for $p>1$ what is the mode expansion? What I am after, more specifically, is: what is the configuration around which the expansion is made? For example, for the ...
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Is String Theory actually the supreme theory of Physics that can unite the 'Theory of the Big' and the 'Theory of the Small'? [duplicate]

As Dr. Michio Kaku says, that in the current state of Physics, we have the 'Theory of the Big' that is black holes, the Big Bang, cosmology, astrophysics on one hand and the 'Theory of the Small' that ...
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Scale dependent dimensionality

Many theories of quantum gravity features the so called "dimensional reduction" (e.g. Asymptotic Safety, Causal Dynamical Triangulations, Euclidean Dynamical Triangulations, Loop Quantum ...
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Is there some mathematical or physical model that postulates that gravitons exist? [duplicate]

Is there some model, mathematical or physical, that postulates that gravitons exist? For example is there mass missing from some particle decay that is thought to form gravitons? Or something in the ...
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Is there any theory of fundamental physics that proposes that there are no symmetries at a fundamental level and that they are all emergent?

Quantum gravity theories usually propose that global symmetries are not fundamental. Are there any theories that propose that all symmetries (Lorentz symmetry, gauge symmetry...) and therefore the ...
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Has the fine structure constant been derived from theory? [closed]

The following paper from this year claims to have derived the fine structure constant: Singh, T.P. Quantum gravity effects in the infrared: a theoretical derivation of the low-energy fine structure ...
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Will a tiny particle size ensure that QFT's, and quantum gravity in particular, don't need renormalization?

If particles have an extended structure, as is for example the case in string theory, it seems that singularities will stay at bay. They can't gather at one point and it seems a natural cut-off is ...
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Operator insertions vs boundary conditions in AdS/CFT

This question is motivated by AdS/CFT, but really it's just about AdS quantum gravity. Consider quantum gravity in asymptotically AdS spacetime. For simplicity, assume there are no matter fields: the ...
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Why does gravity couple with a negative dimension?

I was reading this post where it states that "gravity does have a coupling with negative dimension and it's the Newton constant" However, I don't understand at all why Newton's constant ...
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What's the equivalent of the polarization vector of photons for gravitons?

Spin-0, spin-1/2, and spin-1 particles have an associated quality expressing the spin structure of the field. Spin-0 fields obviously have no extra quality. Spin-1/2 fields are expressed by spinors ...
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Why do we need Quantum Gravity [duplicate]

I know this question has been asked many times but I have not found a satisfying answer to it yet. I understand Gravity as an emergent phenomenon of mass/energy. I understand quantum mechanics as the ...
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Details on the string theory being a quantum gravity theory [duplicate]

So since string theory is a quantum gravity theory, how can we derive from string theory to Einstein gravity equation when $\hbar \rightarrow 0$, as what we did from quantum theory to classical theory?...
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When connections are fundamental objects, is LQG the only possible theory?

In an interview Abhay Ashtekar claims: if you have a background independence and if you want to have Einstein connections this (Loop Quantum Gravity) is the only thing you can write down. Where the ...
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Path integral of Quantum Gravity while keeping Einstein's relation satisfied

Suppose we have a field $\phi(x)$ and the metric field is $g_{\mu \nu}(x)$. The action is the functional $S[\phi (x) , g_{\mu \nu } (x)] $. We want to do the path integral: $$\int d[\phi (x)] d[g_{\mu ...
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Standard resources outlining the Planck Epoch in detail?

The Wikipedia page on the Planck Epoch links to the universeadventure.org website, from UC Berkeley, which I so happened to have worked on (for a semester or so) when I was an undergraduate there. But ...
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Is there an uncertainty in spacetime at Planck length due to uncertainty principle? And if so, what causes it?

I understand that trying to measure particles precisely for different variables simultaneously at subatomic scales is impossible due to uncertainty principle. Does it also hold true for spacetime ...
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Conceptually, why is the entropy of black hole related to Planck length?

I was watching a lecture on the holographic principle, and it presented the equation for the entropy of a black hole as $$S_{BH} = \frac{A}{4l_p^2}$$ My question is why, conceptually, the entropy of a ...
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Imaginary part of IR divergence in gravitational S-matrix

I was recently studying Weinberg's computation of the IR divergence in QED and gravity from virtual soft exchanges (https://doi.org/10.1103/PhysRev.140.B516). He does the computation where the matter ...
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How (if) is the gauge group for gravity incorporated in the Calabi-Yau manifold of string theory?

The Calabi-Yau manifold is a stable complex 3D (or real 6D) manifold on the geometry of which information about strings can be stored as fibre bundles of tensors, more or less like the electromagnetic ...
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Is there a quantum gravity theory where ''space is discrete''?

I have been reading up on some approaches to quantum gravity apart from string theory. The popular conception of loop quantum gravity is that it says that space is actually physically discrete at ...
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Breaking down of 2nd law of thermodynamics [closed]

Do you know a scenario where the second law of thermodynamics breaks down?
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Proof probability distribution of number of particles seen in different basis connected by Bogoljubov transformation is Poissonian

Let we have two bases connected by Bogoljubov transformations. In the first basis, creation and annihilation operators are ($a,a^\dagger$) and in the second basis ($a',a'^\dagger$). They are connected ...
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Is a path integral over a region with a bottleneck still unitary?

Consider the following hourglass-shaped region of spacetime: The red circular cross sections are the initial and final conditions, which can vary, while we fix a particular boundary condition on the ...
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ER = EPR and Heisenberg uncertainty principle

Following the Maldacena-Susskind "ER = EPR" correspondence, what gravitational fact would the Heisenberg uncertainty principle correspond to?
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0+1D dimensional quantum gravity (QG) action?

I'm trying to understand how we might sum over the metric (or vielbein) in a QG theory. So I'm looking at a simple (perhaps too simple) 0+1D action. Imagine a scalar field in 0+1D. The vielbein in 0+...
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Is randomness in the collapse of the wavefunction in quantum mechanics simply a manifestation of chaos, and not inherently random at all?

Chaos is sensitivity to initial conditions. Could the randomness in quantum mechanics simply be a manifestation of chaos?? The initial conditions would be both the initial state of a particle, and ...
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Is the any experimental evidence that gravity exists in the quantum realm?

I saw someone speaking about the gravity of an electron when discussing quantum gravity. What is the smallest mass for which we have experimentally observed a gravitational field? Maybe gravity simply ...
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Vasiliev equations Cartan gauge invariance - arXiv:1208.3880

I am going through https://arxiv.org/abs/1208.3880 and I am somehow struggling to show that the Vasiliev field equation (2.24) \begin{equation} d_{x}W + W *W = 0 \end{equation} is invariant under the ...
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Type III von Neumann Algebras and Quantum Field theory

In https://arxiv.org/abs/1803.04993 , https://arxiv.org/abs/2112.12828, it is written that local algebras in quantum field theory are of Type III and these do not admit pure states, density matrices ...
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Is the Quantum Gravity mystery already solved by Susskind? [closed]

Very recently, Leonard Susskind presented a lecture presenting his proposal of Quantum Mechanics = Gravity (equality means duality). Quantum Origins of Gravity Is this really the end of Quantum ...
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Why is gravitational Chern-Simons action invariant under conformal transformation?

We know the action of topologically massive gravity in 3-dimentional spacetime is \begin{equation} \label{eq:EH} S=S_{\mathrm{EH}}+S_{\mathrm{CS}}=\frac{1}{\kappa^2}\int d^3x\sqrt{-g}R+\frac{1}{\mu}\...
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How can a graviton curve space(time) if it only couples to particles in spacetime?

In the theory of quantum gravity (insofar it's incomplete version exists) gravitons couple to particles like photons or other gauge fields. Unlike the other gauge fields, which are vector-like or ...
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Lagrangian of a graviton

Recently in an interview for a phd program I was asked how would you write the lagrangian of a graviton. I answered that since graviton is a massless particle it's lagrangian should be similar to the ...
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How does string theory relate superposition and general relativity?

So I know that in general relativity, superposition cannot be true. However, since string theory supposedly connects quantum mechanics and gravity, how does it relate superposition and gravity?
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From where does the fact that there are $10^{500}$ possible compactifications in string theory? [duplicate]

I have heard for quite a few times that there are $10^{500}$ possible compactifications in string theory. And in one of his lectures, physicist Leaonard Susskind explains that, this comes from the ...
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Gravitational Waves and Quantum Gravity [duplicate]

Since we can now directly observe gravitational wave signals, can any type of future experiment be set up that might manifest the quantum nature of gravity? For example, perhaps a version of LIGO ...
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Boundary conditions for bulk partition function in AdS/CFT

In AdS/CFT, we are told that the bulk and boundary functions are equal: $$ \tag{1}Z_{bulk}[J]= Z_{CFT}[J], $$ where on the left hand side of the equality, $J$ is interpreted as a boundary condition at ...
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Can the stretched horizon be entangled with an external qubit even if the qubit is unentangled with the black hole?

Suppose at time $t_0$, we have a black hole unentangled with an external mass and an external qubit $\left( |0\rangle+|1\rangle \right)/\sqrt 2$. We then perform a controlled gate operation on the ...
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How many massless particles are there in closed bosonic string theory?

How many massless particles are there for closed strings in bosonic string theory? Page 53 of String Theory and $M$-Theory by Becker and Schwarz states that there are 576 massless states for the $N = ...
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What experiments could be done *in principle* to help down-select between different versions of string theory?

I understand that string theory (broadly defined) is a solution to quantum gravity. That is, it is a unified theory the explains both quantum phenomena (such as the particles of the standard model ...
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Is Planck length constant in String Theory? Does it have a renormalization flow?

Is Planck length constant? Planck length $l_p$ is dependent on Newton constant $G_N$ which is related to coupling constant of interaction of gravitons, but from field theory point of view, we know ...
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On the range of validity of General Relativity and Quantum Field Theory in terms of energy and impact parameter (from Rovelli & Vidotto's book)

In Fig. 1.1 on page 5 in Rovelli & Vidotto's 2015 book Covariant Loop Quantum Gravity: An Elementary Introduction to Quantum Gravity and Spinfoam Theory (PDF), there is this graph giving a general ...
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Do there already exists solutions to quantum gravity which are mathematically consistent and consistent with PRESENT observations? [closed]

This question is inspired by a recent video by Sabine Hossenfelder on the topic of black hole information loss. At about 8:23 the makes the claim that "...there are many possible solutions to the ...
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Is it OK to do this manipulation at the partition function level? (auxiliary fields in quadratic gravity)

Background I am working with the following action in the Euclidean signature ($C^2$ is the Weyl quadratic term): \begin{equation} S_B = -\frac{1}{2\kappa^2}\int d^4x\sqrt{g}\left(2\Lambda_C+\zeta R-\...
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Is it possible to distinguish the intensity and the frequency of the graviton?

The wave equation of the graviton was assumed to be similar to that of the EM waves, which a "frequency" parameter could be identified by comparison. However, in EM, there was intensity as ...

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