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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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Does Graviton bend light?

In the weak gravitational regime where the low-energy effective action holds,(linearized) gravity can be quantized, and we can treat graviton as quantum fields on a Minkowski background. One can also ...
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Kinematic aspects of quantum gravity

Quantum gravity proposals at the leading perturbative order, such as the quantisation of linearized gravity, or the low-energy effective action like in string theory, provide the dynamics for the ...
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Reference on quantization of linearized gravity

I often see claims saying that quantization of linearized gravity can be done. Is there some standard references for it? such as original papers/ review papers / textbooks etc.
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Implication of the Jacobian map for the structure of the Euclidean space-time

I'm listening to Alain Connes "On the Fine-Structure of Space-Time" around minute 23 saying that it was disappoing that the solution Y to the equation $$ <Y[D,Y]^{2m} >= \gamma $$ with $D$ a ...
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QFT Hilbert space and analysis of quantum black holes

QFT Hilbert space is infinite dimensional and it is known that given a region $A$ and its complement $A^c$ of the spacetime, the QFT Hilbert space $\textbf{does not}$ decompose into $\mathcal{H}_A \...
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Quantum gravity: How do we know gravity operates at quantum scales? [duplicate]

Edit: An alternate angle on the original question I asked below: Would general relativity still create singularities in special cases (center of black holes) if we assumed that gravity had a lower ...
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Black hole fundamental laws [closed]

As the Laws of physics breakdown during inside a black hole, would we have to create new fundamental Laws?
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Does theoretical physics suggest that gravity is the exchange of gravitons or deformation/bending of spacetime?

Throughout my life, I have always been taught that gravity is a simple force, however now I struggle to see that being strictly true. Hence I wanted to ask what modern theoretical physics suggests ...
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Would a massive graviton emit more massive gravitons?

In the theory of massive gravity, suppose that two bodies start interacting by exchanging gravitons. Wouldn't these gravitons, having mass, have to exchange gravitons between themselves, with these ...
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Is quantum gravity even detectable? [duplicate]

Since gravity is several orders of magnitude weaker than the rest of the fundamental forces, the force of gravity acting on a single particle is almost *nonexistant. Since there is still a lot left ...
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How is it possible that quantum mechanics and general relativity will be united? [duplicate]

The newspaper articles state that physicists are making attempts to unite quantum mechanics and general relativity. However, other articles stress that they are not compatible with each other.
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What is the relationship between spin network spacetime and tensor network (entanglement) spacetime?

In 1971, Sir Roger Penrose, suggested a combinatorial construction of spacetime using the angular momentum of particles. This work led to and introduced the idea of spin networks which are ...
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Boundary conditions of temporal progress [duplicate]

If one wanted to assume time as a sequence of smallest units, what would be their physically possible sizes ?
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What equations of general relativity and quantum mechanics are we trying to unify? [duplicate]

In other words, what equations hold the most promise in this endeavour? Are we all looking at the same equations or is this pursuit subjective?
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Is there a connection between these two results on soft hair on black holes?

In 2016 Strominger, Hawking and Perry published the paper "Soft Hair on Black Holes" proposing new results that could have importance to the study of the black hole information problem. One ...
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Hamiltonian ordering ambiguity in quantum cosmology/gravity

I am trying to study several quantum cosmology models. The standard procedure for quantization consists typically in several steps: People write the theory as an action, or Hamiltonian $H(p^i,q^j)$ ...
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Does a Black-hole Represent a Limit To The Value of Energy-density? [closed]

I've recently read an article that stated the singularity of a black hole could be a string, which is stipulated to be pure energy. If that's the case, does this mean that the singularity of a black ...
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Skobelev graviton-photon cross section diverges

Skobelev calculated in 1975 the cross-section of graviton+photon to graviton +photon and the graviton+graviton to photon+photon. For the latter, he gave the integrated cross-section, but for the first,...
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Are there other types or versions of holographic principle?

Holographic principle (https://en.wikipedia.org/wiki/Holographic_principle) establishes a correlation between a bulk and its boundary. It says, in layman terms, that the amount of information in a ...
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What if quantum mechanics and general relativity are not connected? [duplicate]

The main objective in modern physics research is to find a way to unify quantum mechanics and general relativity thorugh a series of theoretical approaches called "quantum gravity theories" But what ...
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1answer
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Problems with Twistor theory

Penrose developed a theory called "twistor theory" that tries to describe the universe using twistors. (https://en.wikipedia.org/wiki/Twistor_theory) From what I've read, there are a lot of papers ...
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What is the motivation for developing a “theory of everything”?

What is the reason physicists are working so hard looking for a "theory of everything" and trying to unify gravity with quantum mechanics? Historically, new theories were always developed to explain ...
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Calculating vacuum expectation value of graviton field

I've been reading a section in this thesis (pp 25-27) which reviews Duff's paper (pp 6-7 in particular) in which he calculates the tree-level vacuum expectation value (vev) of the graviton field (upon ...
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Understanding Moonshine via Heterotic E8xE8 (resubmitted, originally asked on mathematics stack exchange)

Recently I have become familiar with the conjectured relationship of monstrous moonshine and pure (2+1)-dimensional quantum gravity in AdS with maximally negative cosmological constant and, it’s being ...
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Can quantum black hole be in superposition state?

Quantum black hole can have electric charge and angular momentum or spin, I am wondering if a quantum black hole can be described as a probability wavefunction or not? If so can it quantum tunnels too?...
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What's wrong with lattice quantum gravity?

Assume one can write the metric field on a lattice, so on each lattice point one has a value of $g^{\mu\nu}$. Similar to the way lattice QCD is formulated. Then later taking the distance between ...
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What are the pros and cons of Causal Dynamical Triangulation (CDT)?

I understand that Causal Dynamical Triangulation (CDT) is a theory of quantum gravity, but i don’t hear it talked about nearly as often as say String Theory or Loop Quantum Gravity. What are it’s pros ...
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Time is the potential?

Perhaps time can be expressed as $$ t = \frac {Gh} {c ^ 4} \int \frac {dS} {r} $$ Where S is the entropy of entanglement of an arbitrary closed surface. r is the radius to the surface point. ...
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Can a no boundary proposal work without the big bang?

In Hawking's no boundary proposal he assumed that the the amplitude for a state A (such as a given 3-manifold) is the sum of all amplitudes for histories starting at a special big-bang-state (like the ...
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Does quantum cosmology work for open spacetime manifolds?

Hawkings no boundary proposal assumes a closed spacetime. But why would it not work with a infinite spacetime? With instead if periodic boundary conditions you might have asymptotic boundary ...
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Towards a quantum maximal supergravity?

Is there any evidence of ultraviolet quantumness and well defined behaviour in eleven dimensional supergravity? Of course, since it is one limit of M-theory, I see no good review of this topic in the ...
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Could the gravitational field be a cloud of particles not curved spacetime? [closed]

The more I've been looking into quantum gravity the more I've been coming to the conclusion that the graviton must be some sort of particle within a cloud of identical particles surrounding massive ...
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If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

As far as I’m aware, gravity in general relativity arises from the curvature of spacetime and is equivalent to an accelerated reference frame. Objects accelerating in a gravitational field are in fact ...
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Does the Dirac sea have any mass or gravitational effect?

Dirac sea is a model for vacuum which considers the empty space as a sea full of negative-energy particles. Anti-particles are holes in this sea. Dirac sea is used to model Quantum field theory in the ...
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Is quantizing acceleration equivalent to quantizing gravity?

It's been said that gravity is locally equivalent to acceleration. It is also been said, that we need to quantize gravity. But doesn't that imply that this would have to be equivalent to quantizing ...
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Gravity-Induced Quantum State Reduction

Roger Penrose (along with two other co-authors) recently released a paper on the arXiv which proposes a test of quantum gravity with a Bose-Einstein condensate acting as a very massive 'particle'. The ...
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How to fuse quantum mechanics and general relativity?

I am very new to this topic but I have started reading Kevin Wray's lecture notes about string theory (PDF) and in the introduction he says: "Sometimes it is said that we don’t understand how to ...
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Question regarding the degeneracy of vacuum state in various spacetimes

In a talk, it was mentioned that if one attempts to do quantum gravity in the following spacetimes (of any dimension), then the vacuum state has the following degeneracy. In $AdS$ - There is a ...
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Emergent physics in the gap between the Planck scale and the “size” of quarks [closed]

The smallest probed length scale to date is around $\Delta x \approx 10^{-18} m$ (see here). If we compare this to the Planck scale of $\Delta x \approx 10^{-35} m$ we are left with almost 20 orders ...
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Why can't the Schwarzschild radius be larger than the Compton wavelength?

In the second paragraph of the third page of this paper, the author makes a statement The dynamical process must now involve, not the length scale $R_{s}$, but rather the universal length $L_{CS}$....
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Boundary bulk intuitive comparison between Planckian black holes and QCD jets

One of the key intuitions that we derive from trying to extrapolate QFT to the Planck scale is that when one tries to resolve features at those scales, you only get a larger uncertainty as your ...
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Why high energy physicists study complexity? [closed]

Recently, studying complexity is a trend in high energy physics. I have read there are two motivations behind these studies. First, Einstein-Rosen bridge behind the horizon keeps evolving even after ...
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In Causal Set Theory what constitutes a final state?

Causal Set Theory consists of a set of events and a partial time ordering. But what would constitute a final state? If you took as a final state simply a set of events which have no time-ordering, ...
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$\rho-3P$ for ultra-relativistic regime in early universe

I am trying to read this paper, in which they try to get dark energy by modifying the EFEs to its unimodular form. As interesting as it may be for someone here, I'm struggling to understand the ...
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Quantum state of a black hole

On page 8 of the paper "Soft Hair on Black Holes" the authors say: Let $|M\rangle$ denote the incoming quantum state of a black hole defined on $\mathcal{H}$. We take it to be formed with neutral ...
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Can LIGO be explained in terms of gravitons?

If electromagnetic waves from a star are so faint, all that can be detected are single photons on a photographic plate. For the LIGO experiment, the gravitational waves were so weak, I would have ...
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Do these particles actually reach future null infinity, and if not where they end up?

In this paper by analogy with a process $e^+e^-\to e^+e^-$ Andrew Strominger proposes that in black hole evaporation there is creation of soft photons and gravitons. This can be argued by infrared ...
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Why any theory of spacetime atoms would be wrong from start? What about causal sets?

After watching this public talk by Nina Arkani-Hamed titled "The doom of spacetime", I can't fully understand what he said in the answer to an audience question about gravity. He said we can't simply ...
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Could every particle encode the distances to all other particles?

I was thinking about quantum gravity and pre-geometry and wondering this question: "If space does not exist. How does a particle know how far away it is from another particle?" i.e. there are no ...
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Spacetime geometry of black hole formed by the scattering of two gravitons

In the paper "Breakdown of predictability in gravitational collapse" Stephen Hawking mentions the following on section 5: Consider, for example, the scattering of two gravitons. In this case the ...