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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How does black hole thermodynamics contains hints to quantum gravity?

This is kind of vague question: I have seen a lot of work been done in calculating out entropy of BH (ex BTZ, extremal BH using string theory). I think as entropy is given by log of microstates and ...
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Settling Copenhagen Interpretation / Measurement in QM & Existence of Quantum Gravity

It is my understanding that there are many interpretations of observation/measurement in Quantum Mechanics (I am only familiar with the Copenhagen one). The Schrodinger's Cat experiment forms a ...
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Who can give metaphors to differentiate fermion string from bosonic string?

From my rough elementary understanding the diff between fermion and bosonic particle/strings are fermion is more materialistic, and bosonic somehow relates to the energy/function part of the common ...
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Why is a static potential between supergravitons problematic?

When deriving the one and two-loop result for the effective potential between two scattering supergravitons, for example from here, we see that it is always a velocity dependent potential. ...
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Validity of DLCQ Matrix Theory near the Big Bang

In this paper,section 3.2, Craps, Sethi and Verlinde claim that DLCQ matrix theory is valid near the big bang if The open string oscillators decouple and Gravity decouples from the matrix description ...
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Obtaining Inverse configuration space metric $M^{AB}$ in canonical gravity

I am considering phase space of configuration variables in general scalar tensor theory $(q^A=(h_{ab},\phi))$, characterised by the metric: $ M_{AB}=\frac{\sqrt{h}}{N}\begin{pmatrix} \frac{U(\phi)}{2}...
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Quantum Fluctuations of Light-Cones

We typically draw light-cones to study causal relation between two space-time points. Source : Wikipedia Space-Time is manifest as the metric in various calculations. If one considers that something ...
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Debunking thought experiment purporting to show that gravity is not classical

As far as I know, there are no known observations that require a quantum theory of gravity. Now, that's not to say we don't need a quantum theory of gravity. In fact, I get the sense that we do, at ...
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Do bodies radiate energy by emitting gravitons?

As gravitons are formulated in current theories, are bodies supposed to radiate energy as they emit gravitons? I thought that if bodies radiate energy, for big bodies this would be difficult to ...
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New Quantum Relativity [closed]

Professor Marco Fedi, has published a paper in which he considers vacuum as a dilating superfluid (oobleck type), using the Lorentz factor as the viscosity term in the Stokes equation. This allowed ...
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Why does a phase shift in a light pulse imply a non-commutative structure of space (which implies gravity has a quantum structure)?

In the news report Physicists propose test of quantum gravity using current technology (Lisa Zyga, Phys.org, 27 October 2017), a test is proposed to determine if gravity has a quantum structure. From ...
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Why does combining background independence and diffeomorphism invariance force you to conclude that spacetime must be quantized?

I was recently watching a fermilab video on loop quantum gravity and at about 4:20 it is stated that, when combined, background independence and diffeomorphism invariance force you to conclude that ...
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Does the fact that light (photons) are bent by a sufficiently big mass prove that a quantum theory of gravity doesn't exist?

As is proven, light can be bent by a heavy mass in the context of classical General Relativity. In a quantum theory of gravity, the gravity field around a heavy mass consists of a condensate of (...
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Why graviton theory fails? [duplicate]

What are the different reasons why theory of gravity mediated through gravitons fails?
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Why do we think the graviton is not the photon?

Why couldn't the photon be the graviton? How sure are we that the photon could not fill the role of gravitons?
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Does the notion of blurriness of lightcone appear in any theory of quantum gravity?

In the above diagram, black lines are $x=\pm t$ and the highlighted space is what I meant by the blurriness of the lightcone. Personally I have 2 arguments and both seems to suggest there shouldn't be ...
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What does “continuous spacetime” mean?

I often encounter discussions, such as seen here, about whether spacetime is discrete or continuous. However, I am only familiar with continuity as being a property of functions. I saw this question ...
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Is it mathematically as well physically alright (consistent) to not quantize metric when doing QFT in curved spacetime?

I am not well-versed with the theory of renormalization of QFT as of now (I just know that when you redefine your observables you get rid of the UV divergences and then get finite predictions from ...
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Are following statements valid for quantum gravity?

In the concluding section of this post user Chiral Anomaly states following: On the other hand, since any stable marriage of quantum theory and gravity (in the sense of general relativity, not just ...
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Swampland and Weak Gravity Conjecture (WGC)

I think I've understood the general idea of the WGC (proposed in https://arxiv.org/abs/hep-th/0601001). From a more recent review, I quote: In a consistent effective field theory (EFT) coupled to ...
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Experiments on a Black Hole [closed]

Recently it's been theorized that "planet nine" may be a black hole. If this is the case and we were to fly something out there, what unsolved problems in physics could be answered? ...
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Relationship between classical $q$-deformed General Relativity and the cosmological constant

The frame-connection formulation of pure General Relativity in 4 dimensions is given by the action $$ S_{4d}[e, \omega] = \frac{1}{2 \kappa} \int \varepsilon_{IJKL} e^I \wedge e^J \wedge F^{KL}, $$ ...
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$cGh$-Physics Cube: When Do (General) Relativistic / Quantum Effects Become Relevant?

The $cGh$-cube is a way to visualise todays theoretical physics framework(s). This representation gives the impression $c$, $G$, and $\hbar$ could be viewed as variables between $0$ and $1$. This is ...
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How does one (physically) interpret the relationship between the graviton and the vielbein?

One can naturally think of the vielbein $e_\mu^a$ as a gauge field corresponding to local translation invariance. Moreover, the metric may be written $$g_{\mu\nu}=e_\mu^a e_\nu^b \eta_{ab}.$$ I have ...
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Does the GKPW rule imply there's only one unique theory of quantum gravity?

The GKPW (Gubser-Klebanov-Polyakov-Witten) prescription relates the partition function of a CFT to that of a bulk theory of quantum gravity. Since the CFT partition function is fixed, does that mean ...
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Why are gravitational forces much weaker than electrostatic while both the graviton and photon are massless?

I know that in quantum mechanics the stronger the force, the shorter the range it mediates between (maybe I have problem in this postulation, if so please correct me). So how can gravitational forces ...
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Reasons to leave gravity classical in QFT on curved spacetime

From what I have learnt after studying QFT on curved spacetime is that we replace the Minkowski metric, used in flat spacetime QFT, with the general metric and partial derivatives goes to covariant ...
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What is a 'quantum break'?

In the July issue of Scientific American, in a brief article by Charlie Wood on detecting gravitons by looking for long-wavelength radio waves that some of the graviton have turned into, it says, ......
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How could an Effective Field Theory (EFT) have no UV completion?

In the context of quantum gravity, but also in other places, I have heard the question "does this effective field theory admit an UV completion?". However, I really don't understand this ...
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Nordström's Second Theory of Gravity, and Quantum Mechanics?

I read about [Nordström's Theory of Gravitation][1] [1]: https://en.wikipedia.org/wiki/Nordstr%C3%B6m%27s_theory_of_gravitation and how it was proposed before General Relativity. There were two ...
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How does the non-degeneracy of the metric imply that gravity is not fundamental?

In his General Relativity notes, on page 149, David Tong remarks that when we look for solutions to Einstein's equations, we can't just take any metric, such as $g_{\mu \nu} = 0$; we must pick one ...
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Does the Ryu-Takayanagi conjecture only apply to vacua?

Does the Ryu-Takayanagi conjecture only apply to vacua, or does it also apply to arbitrary excited states? For excited CFT states, the entanglement entropy can be proportional to the volume, not ...
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Unifying quantum mechanics and general relativity [duplicate]

I am very new to physics and I want to pursue Unified Field theory now when I read about how to unify quantum mechanics with relativity as they are essentially the same thing and a relation must be ...
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Bern-Carrasco-Johansson (BCJ) Double Copy and Color-Kinematic duality

According to the wikipedia page on Strong Gravity, the theory is considered "non-mainstream", but from what I can gather there have been some very interesting progress and results since it ...
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What formally means when physicists say “relativity theory is inconsistent with quantum theory”? [duplicate]

I hear this sentence sometimes: "quantum theory is inconsistent with relativity theory". Is it possible to interpret this really in a formal system logically? I tried to understand the term &...
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Equations of gravitation in Bronstein's article

I'm studying Matvei Bronstein's 'Quantum theory of weak gravitational fields' and in the general remarks whilst he gives the correct expression for the Riemann-Christoffel curvature tensor in first ...
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Fran De Aquino's theories on gravity and electromagnetism; reputable? [closed]

I've been transiently interested in the research of a Brazilian physicist named Fran De Aquino. The majority of his papers are related to gravity manipulation, which raises a lot of red flags. This ...
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Gauge symmetry implies global symmetry and quantum gravity

It is folklore that quantum gravity cannot have any exact global symmetry (see Global symmetries in quantum gravity). This follows for example from thought experiments involving black holes (no-hair). ...
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Absence of topological phases in quantum gravity and the BFSS matrix model of $M$-theory

McNamara and Vafa have recently conjectured in their paper Cobordism Classes and the Swampland that any vacua in string theory can be reached from any other one (possibly by a process that require an ...
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Are gravitons equivalent to extra localised lattice points?

So imagine space is a regular square mesh or lattice. In a theory like QCD, the photon lines are placed along the edges of this graph to form paths. The space is supposed to represent simple ...
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True Hamiltonian in Geometrodynamics

If I set $N=1$ and $N^a=0$ in the Einstein-Hilbert action $$S[N,N^a,q_{ab}]=\int\sqrt{\mathrm{det}(g)}\,R\,\mathrm d^4x \,\text{,}$$ expressed in terms of ADM variables, then $N$ and $N^a$ are no ...
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Can a theory of everything solve the oldest problem of turbulence? [closed]

Can a theory of everything solve the oldest problem of turbulence? Can we have unified theory of universe without solving the problem of turbulence?
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Assuming the big rip scenario is possible, what happens exactly at $t=t_{rip}$? Is spacetime really ripped?

I want to start by prefacing a scenario. Say equation of state is $w<-1$ and we live in a phantom dark energy dominated universe. This means in a finite time the cosmological scale factor reaches ...
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How does the philosophy of 'Emergent Gravity' differ from that of 'Quantum Gravity'?

How are the two approaches of understanding gravity, the so-called 'Emergent/entropic gravity' and 'Quantum gravity' (String theory, LQG etc) differ from each other? I am very curious and will ...
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Would naked singularities have the same gravitational properties as black holes of the same mass? [duplicate]

This is just a thought experiment before someone mentions the cosmic censorship conjecture. If physicists are discussing it in papers and scholarship, I wanna see some of those theoretical angles too ...
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Is the “Rotation System” of a graph linked to spin?

If one takes the limit of certain meshes, one can approximate a Riemannian manifold. Is there a way to generalise this to Riemannian manifolds with a spin structure or a super-Riemannian manifold? I ...
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Do theoretical naked singularities have the same gravitational properties as black holes of the same mass? [duplicate]

Just a thought experiment before someone mentions the cosmic censorship conjecture. If you take a black hole and turn it into a naked singularity, does the naked singularity have the same ...
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How does the “Space from Hilbert space” program incorporate Lorentz invariance?

There is a loose set of ideas going under various names ("It from Qubit"; "Space from Hilbert Space"; "Geometry from Entanglement") which propose that spacetime is not fundamental, but instead emerges ...
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The reality of gravitons

In quantum field theory, gravity arises from the exchange of gravitons. However, to an observer in a local inertial reference frame, there is no gravity. How can a particle that exists in one ...
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Why is the graviton spin supposed to be 2, and how can it interact with matter? [duplicate]

As far as I know, gravitons are predicted to be spin-2 particles because some extrapolation from GR about non-linearity and antisymmetry. I think I want some clarification on that. What are some ...

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