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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Gravitational Waves and Quantum Gravity

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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Would a quantum theory of gravity necessarily violate relativity? (quantisation of speed of light?) [closed]

The way I understand Quantum Mechanics, observables are "promoted" to operators. Relativity (General and Special) are defined around the constancy of speed of light $c$. This is the speed of ...
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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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Why doesn't the Weinberg-Witten theorem forbid collinear photons?

The Weinberg-Witten theorem tells us that any theory that has an effective graviton, i.e. a massless helicity-2 particle as a state in the free-particle Fock space, cannot have a gauge-invariant and ...
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What is boundary cosmological constant in boundary Liouville field theory?

In this paper, Liouville field theory with conformally invariant boundary is studied. The action is: $$ \int_{\Gamma}\left(\frac{1}{4 \pi}\left(\partial_{a} \phi\right)^{2}+\mu e^{2 b \phi}\right) d^{...
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How is gravitational time dilation different from the time dilation from Special Reativity? [duplicate]

In Special Relativity, acceleration i.e. a changing velocity 4-vector results in time dilation, that is asymmetric aging of observers. In General Relativity, the 4-vector does not change along a ...
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What does it mean that gravitation separated from the four forces at $10^{-36}$ seconds?

Physicists have used math to calculate that gravitation separated from the 4 forces at 10^-36 seconds after the big bang, which is even before the inflationary period. The universe was still smaller ...
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Is there a wave function in loop quantum gravity?

In the postulates of quantum mechanics, http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/qm.html a wavefunction is fundamental. Is there a wavefunction - or an analogon - in loop quantum gravity? As ...
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Simple explanation on 'How vacuum is treated in the different quantum gravity theories?' And is there a theory where there is NO space without matter?

Could you please make an easy-to-understand synopsis on how the different quantum gravity theories (loop quantum gravity, causal dynamic triangulation, and the many other) treat empty space? That ...
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If quantum gravity is a TQFT, why isn't the Wheeler-De Witt equation satisfied automatically?

It is often said that QG is a topological QFT: given a bordism between $D$-manifolds $\Sigma_1$ and $\Sigma_2$, QG assigns a unitary between the Hilbert spaces associated with $\Sigma_1$ and $\Sigma_2$...
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Are there quantum gravity theories in which spacetime itself is regarded as quantum in nature?

In quantum gravity, it's tried to quantize the gravitation. However, if I got it correctly, most quantum gravity approaches try only to quantize gravity as a force, the curvature of spacetime, not the ...
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What's the current theory on final stages of black hole decay? [duplicate]

Okay, firstly, I know this question was asked here 11 years ago. And I know the correct answer is that we don't know because we haven't run the experiments and don't have a solid theory of quantum ...
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What physics laws justify Planck's units? [duplicate]

It is usually said that Planck units have no scientific ground, yet they are useful indeed because many laws collapse, make no more sense at, say, Plancks length or time. Can you mention a couple of ...
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Are black hole singularities fermions? [closed]

I was just wondering. I apologize if this is a dumb question. Is it possible that the mass of a black hole is converted into quantum energy that gets distributed across the universe uniformly so that ...
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Virtual Graviton Polarisations

Is it true that in canonically quantised gravity "non-traceless" and "non-transverse" gravitons are allowed to exist as virtual particles, whereas any graviton that corresponds to ...
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How is TQFT connected to 1+1D and 2+1D quantum gravity?

Quantum gravity is believed to be background-independent (in some suitable sense), and TQFTs provide examples of background independent quantum field theories. This has prompted ongoing theoretical ...
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What speaks against perturbative quantum gravity?

The past decades of research in perturbative quantum gravity has shown that calculations are possible, that results are unique, and that corrections to classical results are extremely small. (Donoghue'...
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Question on "proof" of the holographic feature of euclidean quantum gravity

In a preceding question, where more details about what I'm doing here are given, I pointed out the possibility for euclidean quantum gravity to exhibit a holographic property. Technically there is no ...
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Why can't we use expected values for a "quantum general relativity"? [duplicate]

So, the equations for general relativity are as follows: $$G (mu, nu) = ĸ T (mu,nu)$$ I was told, that since energy, momentum and other observables are quantum then the stress-energy tensor $T$ is ...
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What is the shortest form way to fully describe the way our universse functions? [closed]

Let's say we wanted to explain to some alien living in another universe with different laws of physics to how our universe worked, what is the shortest way of doing this? A different way of looking at ...
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What is meant by GR being nonrenormalisable, given that "scale" is the very thing being quantised?

TL;DR Usually, RG flow involves scaling the metric $g_{\mu\nu}\to\lambda^2 g_{\mu\nu}$ and seeing how couplings change. But if we're quantising $g_{\mu\nu}$ itself, I don't see how this process can be ...
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What is self-action in quantum theory?

I read that the gravitational field in any quantum theory will be self-acting. What does it mean? How can a field interact with itself?
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AdS/CFT correspondence [closed]

Can the AdS/CFT correspondence, which states that the anti-de Sitter space in $n$ dimensions is equivalent to a Conformal Field Theory in $n-1$ dimensions, for quantum gravity be seen as an analogue ...
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Obligated Bibliography for Quantum Gravity

I am a graduate student who wants to begin to get into Quantum Gravity. I've looked on internet for bibliography and I've found a lot of great books, but I would like to start from the beginning (of ...
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What are the energies (in eV) that we need to reach to create 'strings', gravitons and a 'Theory of Everything' (quantum gravity)?

Somehow, I cannot find an answer on the Web.... Possibly because nobody knows, of course, but I would still like to find a 'ballpark' answer... Creating the particles or whatever (and whatever they ...
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D0, D1, D2... DN branes consist of D0 branes?

In BFSS Matrix theory, D0-branes connect the ends of open strings. Do other D-branes (D1, D2... DN) consist of D0-branes?
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Implications of M-Theory on the correctness of String Theory

So we know that there are 5 types of string theories (Type 1, Type IIA, Type IIB, $SO(32)$ heterotic, and $E_8 \times E_8$ heterotic). It was shown that these 5 types are just limits of something ...
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What are the basic building blocks in M-Theory?

In particle theory, the basic building blocks are particles (okay... perhaps I should say quantum fields). In string theory, the basic building blocks are strings. I heard that there's something ...
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What does M-theory predict at the singularity of a black hole?

how does M-theory avoid problems like infinite density and geodesic incompleteness. like, is there a fundamental limit to where matter can collapse or simply does matter decay and form a condensate of ...
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Momentum Conserving Delta Function and Soft Gravitons

I am reading a paper called "Testing subleading multiple soft graviton theorem for CHY prescription" written by Subhroneel Chakrabarti, Sitender Pratap Kashyap, Biswajit Sahoo, Ashoke Sena ...
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Does discovery of graviton disprove wormhole since latter is applicable in GR only?

I know graviton is only a hypothetical particle invented probably to serve as a placeholder in standard model, but suppose one day we discovered graviton, does this disprove the existence of wormhole ...
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Would Gravitons have Gravity in it of themselves?

If you held a handful of gravitons, would you be holding a handful of gravity, drawing things towards it, or do gravitons have to be exchanged or transmitted for gravity to take effect, in which case, ...
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How do Gravitons work? as compared to photons

Photons are to Electrons as Gravitons are to ... what? What is it that 'emits' a graviton? And what 'absorbs' it? I've been looking for a good layman's description of how gravitons interact with ... ...
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Sir Roger Penrose's claim on quantum gravity in relation to brain function

I'm hoping that this board may be able to offer an explanation. First, my apologies for quoting slightly lengthy fragment but I wanted to provide the full context for my question. I'm reading ...
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How does existence of graviton helps explain 2 different objects fall at the same rate?

Actually I want to see how gravitons help to explain why a feather and a bowling ball would fall at the same rate towards the ground assuming no air resistance, I would imagine bowling ball to emit ...
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Feynman diagram for two-dimensional nonlinear sigma model with antisymmetric coupling

I am studying the review by C. Calland and L. Thorlacius (link: https://www.damtp.cam.ac.uk/user/tong/string/sigma.pdf) on sigma models in string theory. In particular I am trying to evaluate the ...
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Component field expansion of $(1,1)$ supersymmetric Polyakov action

I am working on the renormalizibility of two-dimensional nonlinear sigma models in string theory and particularly the $(1,1)$ supersymmetric extension of it. The following is based on the review by C. ...
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Background field expansion of supersymmetric string action

For a reasearch project I am studying the paper by L. Alvarez-Gaumé, D. Freedman and S. Mukhi called "The Background Field Method and the UV Structure of the Supersymmetric Nonlinear $\sigma$ ...
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How do gravitons escape each other's gravity to spread out and form a field?

The hypothetical graviton I'm familiar with is a spin-2 particle. That implies it has angular momentum, which further implies it has energy. If energy attracts energy, what prevents gravitons from ...
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Good Introductory Text for Quantizing Geometry?

In this video she makes an interesting claim. In loop quantum gravity on can think of the gravity as: $$ G^{\mu \nu} = \kappa T^{\mu \nu} $$ Now in loop quantum gravity one can quantize geometry as ...
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What is common between gravitons and Higgs bozon? [duplicate]

The General Relativity started from the consideration of Einstein that the heavy mass (due to gravitational field and inertial mass (Higgs field) are locally equivalent. What is the connection between ...
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Wouldn't a simple scalar field fix the non-renormalizability of gravity?

It is well known that quadratic gravity is renormalizable. On the other hand it is possible to transform the partition function of Einstein-Hilbert + free minimally coupled complex scalar field into a ...
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