Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.

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Could gravitational waves have a large entropy in quantum gravity?

The question is broad, and we still don't know what is quantum gravity. But let me be more specific. In the question (on this site, see below) on whether gravitational waves have entropy the answers ...
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164 views

From Quantum Mechanics to Quantum field theory to String theory?

Today during a very "unique" study session, I might have internalized why Quantum mechanics was not enough, and Quantum field theory makes sense. It seems the reasons are that When a potential is ...
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21 views

LIGO Gravitational Wave Discovery and Gravitons [duplicate]

Now that gravitational waves have been detected, how hard will it be to detect a graviton? Will the equipment have to be more sensitive or are the difficulties more complicated than that?
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99 views

What kind of fundamental interaction is gravitation?

Einstein showed that gravitation is not a force but a effect caused by curvature of space time. Then why does theories of quantum gravity treats gravitation as one of fundamental force?
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34 views

What is quantum foam?

Can someone please explain me what quantum foam is? Is it the space-time fabric or just any other field? Also please explain this image
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39 views

Has anyone explored a possible duality between quantum gravity and statistical mechanics?

There is a well known duality between quantum mechanics in D dimensions and classical statistical mechanics in D+1 dimensions. Specifically, the Euclidean version of the path integral in D dimensions ...
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37 views

Scale invariance

Physically why should scale invariance break down at the Planck limit? If not what are the other possibilities to save the quantum granularity of spacetime? I would really appreciate professional ...
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97 views

A doubt regarding Black Hole Complementarity

A friend was explaining Black Hole Complementarity to me, and at one point he said that to get a (horrendously) mixed quantum state, i.e. a thermal density matrix without a heat bath, one takes a ...
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108 views

A spin zero graviton?

Why can the graviton not be a spin 0 particle? On a similar note why can it not be a spin 4, spin 6 particle?
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17 views

Why is graviton not found yet? [duplicate]

Why is graviton not found yet? Newton said gravity pulls us, Einstein said space is bent and it pushes us, hawking predicts the existence of graviton and 'hawking radiation'. How far are we from ...
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39 views

What´s the physical foundation of the assumption that the curvature of spacetime can be quantised? [duplicate]

At the moment different paths (by percentual very few people in the world) are taken to arrive (that is, if an arrival exists) at a theory that can quantise the curvature of spacetime. Considering the ...
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9 views

Is viscosity excluded by Lorentz symmetry in spatio-temporal discreteness?

"In the case of spatio-temporal discreteness, viscosity is excluded by Lorentz symmetry". This statement is from here. Can anyone explain why this is the case? I'm looking for an intuitive way to ...
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124 views

Why are string theorist so indifferent to the gauge structure of gravity? [closed]

Gravity shares many of the characteristics of Yang-Mills gauge theory. For example, the affine connection plays the similar role as the gauge potential in gauge theory, the Riemann tensor plays the ...
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52 views

Uncertainty relation and quantum gravity

Suppose a quantum version of gravity exists (which I don´t think). What will be the quantities in the uncertainty relation connected with this quantum gravity? Some time ago I sent a letter to Lee ...
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167 views

What is the problem with quantizing GR in the Effective Field Theory approach?

In the modern view due to Wilson, the cut-off $\Lambda$ is an intrinsic property of a theory and renormalization just means that the theory is invariant under scale transformations below $\Lambda$. ...
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75 views

If gravitons are 'real' and analogous to photons are they also being 'stretched' by the universe's expansion?

Since photon wavelengths are stretched by our expanding universe, appearing to us as a redshift, would graviton wavelengths similarly be stretched? For that matter, do gravitons even have a ...
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135 views

The integration of Einstein's equations [closed]

Einstein's equation is $$G_{\mu\nu} + \Lambda g_{\mu\nu} = {8 \pi G \over c^4} T_{\mu\nu}$$ where $G_{\mu\nu} = R_{\mu\nu} - (1/2)g_{\mu\nu}\,R$ is the Einstein tensor, which combines the Ricci ...
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67 views

Why is quantising gravity so difficult? [duplicate]

Since gravity is so similar with the Yang-Mills theory, the Christoffel connection is the gauge potential, the Riemann curvature is the field strength, then why is quantising gravity so difficult when ...
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104 views

Heisenberg's uncertainty principle at the Planck scale

A particle of mass has a reduced Compton wavelength $$\overline{\lambda}_{C} = \frac {\lambda_{C}}{2 \pi} = \frac {\hbar}{m c}$$ Schwarzschild radius of the particle is $$r_s = ...
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83 views

Can gravity prevent quantum superposition of positions for a massive object?

Theoretically, nothing prevents a really massive object to be in a superposition of two spatial locations, even far away one from the other. Then I guess spacetime would also show the superposition of ...
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96 views

What is the justification for continuing to model space-time as a manifold in string theory?

Background The question I have arises from taking seriously the idea that, in a physical theory, one should first define all physical quantities via (thought) experiment. Justification and ...
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1answer
81 views

Quantum Gravity vs. Quantum Field Theory in Curved Space-time [closed]

I'm a freshman on Physics course, espite of this fact I have a quite interest on Gravitation. My question is: What is the difference between Quantum Gravity and QFT in curved space-time? The great ...
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90 views

Could there be a “massive gravity” theory?

If we talk about a "quantum theory" of General Relativity, we know that the particle that mediates the gravitational force would be the so called Graviton, a massless particle with spin $2$. I ...
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57 views

Wave function of the universe - universes similar to ours [closed]

According to this image, the first peak in the graph corresponds to our universe while the other peaks correspond to other, less probable universes. But what do points slightly right (or left, ...
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77 views
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Singularity - exactly what is it?

In quantum mechanics and gravity theories one always meets the word "singularity" in connection with talks about black holes and the big bang, etc. Now in mathematics a singularity is well defined – ...
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121 views

How do people go about looking for asymptotic safety in quantum gravity?

Do we have (proposed?) methods to look for fixed points in the renormlaization group flow of the Einstein-Hilbert action? My understanding of the RG is still somewhat sketchy at this point and I am ...
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122 views

How do we choose how to discretise spacetime in loop quantum gravity?

In LQG we choose a two-complex (or other) on which to discretise GR before quantising. I'm a little uncertain about the nature of this choice. Is the two-complex an a priori property of the universe ...
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92 views

Is there a path to go from GR, to Gravitational Waves, to Gravitational Particles (Gravitons), to Quantum Gravity?

The articles on the subject often say : a part of the mass of the 2 celestial bodies became energy for gravitational waves after merging : E(gravity waves) = (M1 + M2 - M_merged).c^2 Intuitively I ...
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80 views

Why do black holes need a quantum mechanical description?

I read about black holes, about the Schwarzschild metric, Einstein field equations and their solution in the vacuum for a spherical body. I understood black holes are object whose gravity is ...
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100 views

Can light escape a black hole? [duplicate]

I heard that a black hole is not black because it's escape velocity is greater than or equals to the speed of light. But instead it is black because the light that gets emitted from a black hole gets ...
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194 views

Why doesn't gravity fit into quantum theory?

Before you read, I want to point out that I probably don't know nearly as much as you guys about quantum theory, even though I love learning about it, so I would prefer explanations in relatively ...
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65 views

Where would you go to get feedback, advice and assistance for a new theory of physics? [closed]

I know that crackpots are constantly running down physicists to pitch them on a "new theory" that solves everything. This has understandably led to extreme skepticism for anyone not affiliated with a ...
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137 views

What does the finding of gravitational waves mean for M Theory? [duplicate]

I was wondering if the finding of gravitational waves proves M theory. I know at Fermi Lab scientists were looking for gravitons in distant places in our universe so that they can prove string theory. ...
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190 views

Using open system dynamics to define a quantum state

Background The density matrix of a closed quantum system with Hilbert space $\mathscr H$ evolves according to the von Neumann equation \begin{align*} i\hbar\dot\rho=[H,\rho]. \end{align*} Given a ...
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Can Newton's law of gravitation and Einstein's GR be considered true and complete laws of gravitation? [closed]

Newton's law of gravitation and Einstein's general relativity (GR) are empirical laws of gravitation that are strong on description and prediction but lack deterministic and/or causal rigor, which ...
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1answer
104 views

Are gravitons affected by the force of gravity? [closed]

1) Every particle, including photons, is affected by gravity. Would this be true for gravitons as well? 2) Gravitons should always move at the speed of light, so, if photons are red-shifted while ...
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105 views

Prospects for detection of gravitons?

With the announcement of the detection of gravitational waves, questions about the implications proliferate. Some relate to the possible existence of gravitons. The analogous relationship between ...
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272 views

Does wave-particle duality exist for gravitational waves?

For electromagnetic waves there exists a wave/particle duality: light sometimes behaves as a wave, and other times as a particle (photons). Does such a duality exist for gravitational waves? In other ...
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445 views

Does the existence (now proved) of gravitational waves imply the existence of Gravitons?

I studied the theoretical part about the Gravitational waves in General Relativity (linearization of gravity and small perturbations of the metric and so on). But I was wondering about: since ...
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Evidence for quantum gravity from gravitational waves

The rumor spreads that physicists will make their big gravitational wave announcement this thursday. I am far from being an experimentalist, but I want to know if there is any chance that the ...
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174 views

Entropy of the cosmological constant and the laws of thermodynamics?

Convention The convention being used is: $ A_{C} = $ The classical variable Premise Consider the following toy-model universe: A universe with a positive cosmological constant. Basic ...
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73 views

Space-time and gravitational fields

I am reading a book by Carlo Rovelli, Seven Brief Lessons On Physics, and would like to check if I have understood something. Apologies if my question is badly phrased, feel free to edit where ...
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106 views

Can Quantum Field Theory be right even though it doesn't include gravity? [closed]

Quantum Field Theory doesn't include gravity, so does that mean it can't be right?
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36 views

Does the universe need Quantum Gravity? [duplicate]

A lot of supporters of string theory say that it has to be right because the quantum gravity is necessary for the universe, is this true? Does the universe need quantum gravity?
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Doesn't string theory have to exist? [closed]

String theory is a theory of quantum gravity, and the standard model doesn't include gravity. But since there is gravity down at the sub atomic level, then doesn't string theory have to be right since ...
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66 views

Graviton detector thought experiment

I was recently thinking of a thought experiment: Assumptions Graviton detectors can exist The equivalence principle will hold in the final theory of quantum gravity We can accelerate the graviton ...
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92 views

Could there exist (and if not: why?) non-elementary particles composed of gravitons?

I read about glueballs, which should be particles made purely out of gluons. This is a quite theoretical thing but its existence has at least been theorized. Now, I don't know so much about ...
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154 views

Hotter than the Absolute Hot?

Is the Planck temperature ($1.416×10^{32}$ kelvin), the hottest possible temperature that can ever be reached, with absolute zero as it's opposite analogue ? All I know is a particle with that ...