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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 happens to the time evolution equations in canonical quantum gravity?

Many expositions on canonical quantum gravity start from a 3+1 type formalism, where spacetime is foliated along the time dimension. The Einstein equations then decompose into constraint equations on ...
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How do Graviton-based theories of gravity explain the expansion of the universe?

In General Relativity, the expansion of the universe is modeled using the Friedmann–Lemaître–Robertson–Walker metric, and the expansion itself is a metric expansion by which the scale of space itself ...
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Energy conservation and Quantum Gravity

According to QFT Energy is conserved because of time translation invariance but according to some speculative theories of Quantum Gravity time is an illusion and unreal and an emergent concept so if ...
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Is this a good clock for quantum cosmology?

I was thinking about Quantum Cosmology and the wave function of the universe $\Psi[g,\phi]$ in the no boundary proposal. I was thinking about what would make a good clock. It would be some function ...
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How does the direction of time work with timeless wave functions?

In thermodynamical theory, if we have a set of states for example: A) gas all in top left corner of box. B) gas spread out near the left. C) gas spread out to fill entire box. We can give the ...
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When GR is not valid passing the event horizon?

A test body crosses the event horizon of a black hole. When can we say that GR breaks down? Only at the singularity? At any point CLOSE enough to the singularity? How much close? In summary, do we ...
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Graviton and spacetime

General Relativity and the concept of curved spacetime replacing the "force" of gravitation is really beautiful, and I thought one could probably find similar descriptions of other forces like ...
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Are the factors in the de Broglie identities operators, scalar real numbers, or both? Or something else?

I understand that $\hbar$ is an honest element of $\mathbb{R}$. But I am trying to understand how the de Broglie wavelength is related to the wave function mathematically. In the identity $p = \hbar k$...
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If gravity turns out to be mediated by exchange particles, would that imply a problem with gravitational fields around a black hole?

In general relativity, gravity is a distortion of spacetime due to mass. Its effects travel (if that's the right word) at the speed of light. In the SM all 3 other known interactions are mediated by ...
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As anyone attempted to show that QM and GR cannot be unified? [duplicate]

Sometimes it possible to make very general statement about functional form of things and show, for example, that QM cannot be described by the use of hidden variable (I'm not a physicist in any sense, ...
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Can gravity be quantized in a way that doesn't require a particle? [duplicate]

I understand what gravitons are and why they are theorized to exist, but is there any way to quantize gravity without a boson? In other words, possibly geometrically with no particle at all? For ...
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Gauge fixing in canonical quantum gravity

In analogy with QFT, the partition function in canonical quantum gravity is defined as a functional integral over the metric tensor (which is now the quantum field), $$ \int \mathcal{D} g \mathcal{D}\...
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How to show no boundary wave function satisifies Wheeler-de-Witt equation?

I've seen various places it says the no boundary wave function (Hartle Hawking state) satisfies the Wheeler-de-Witt equation. But I don't think I've seen a proof. Is there a proof of this somewhere? ...
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What does it mean for gravity in $(2+1)$ dimensions to be topological?

I have been studying gravity in $(2+1)$-dimensions and I have come across the idea that gravity in this lower-dimensional spacetime is topological but I haven’t been able to find a simple explanation ...
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What is going on in “nonlinear gravity from entanglement in conformal field theories”?

EDIT: I am now convinced that the sign of the logarithmic terms in the equations after 3.29 and 3.30 are wrong (unless I have missed something else). These identites come from looking at ...
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Does the dictionary always map the bulk operator to the CFT operator?

Using the (extrapolate) dictionary, one can map a bulk field to a boundary CFT operator. The mapped operator is always a CFT operator? How is it guaranteed?
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Is the universe a closed system?

Most places say it is. But according to the multiverse hypothesis, gravitons and other quantum particles could travel from one universe to another. Wouldn't matter-energy then be transferred and thus ...
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What is the source of the difficulty (within the variational approach framework) in the attempt to unify quantum mechanics with general relativity? [duplicate]

It seems to me that quantum mechanics can be formulated within the general mathematical framework of variational  principles. Derivation of the equations of nonrelativistic quantum mechanics based on ...
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Where does the inconsistency between QM and general relativity stem from? [duplicate]

I understand special relativity works with QM (as seen by QFT) and I know that there is some discrepancy between GR and QM. Could someone elaborate on that? Where does the inconsistency stem from? ...
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What is the escape velocity of a neutron particle (not neutron star)

I'm not sure if this question makes sense (if not maybe you can explain why) But if the neutron has mass and have a size, then it should have a escape velocity in the "surface" right? I know the ...
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Does the center of a black hole have a physical body?

From science class, I'm led to belive that all matter breaks down once it's sucked into a black hole. I get that part, but doesn't all that matter still exist in the center of the black hole? yes it's ...
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Biggest string theory competitors

i am a layman in physics and i just want to know what are, as of today, the most importants framework for quantum gravity that contrast string theory and why is so?
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Is there is a concensus among physicists if spacetime actually curves and if so what is it?

Going off from what others have told me on here, and based on the Wikipedia page for Quantum Gravity, General Relativity can be described mathematically in a way different than the geometrical curved ...
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Are GWs made of gravitons (are gravitons the quanta of GWs) or not?

I have read this question: What is the difference between gravitons and gravitational waves? I have read this on wikipedia: However, if gravitons are the quanta of gravitational waves, then ...
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Polarization states of a massive graviton

How could I reconcile the fact that there are 6 polarization states for a gravitational wave (3 transverse and 3 with longitudinal components) with the fact that the spin-2 graviton should allow a ...
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Force Particles?

Since Einstein has shown that gravitational force is created by the warping of Space-Time, why are physicists looking for the "graviton" particle? Since gravitational force is created by a warping of ...
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What goes wrong with GR as a lattice gauge theory?

If one tried to formulate General Realativity in a similar manner to say lattice QCD, what goes wrong that makes it not work? My first thoughts are that for any particular grid, we might have a ...
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Spacetime as an independent yet interacting frame of reference for QM. Looking.for papers [duplicate]

We know that GR and QM are both valid and verified and yet for some reason they just don't want to go together. We say QM describes the microscopic while GR describes the macroscopic. I was thinking ...
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What qualifies as a quantum theory and why are we seeking a quantum theory of gravity? [duplicate]

When can a theory be called a quantum theory? Does it have to do with the existence of certain quantities which take discrete values (they increase in quanta)? Or does it have to do with the existence ...
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Is there any way gravitons can be found in gravitational waves? [duplicate]

I've been thinking for the past week about this thing, maybe due to my poor understanding of GR... So, if we try to measure the speed of a gravitational wave (I don't know if it's possible yet), and ...
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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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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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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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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 ...