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

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How do black holes form without violating the uncertainty principle? [duplicate]

Many videos on YouTube while discussing black holes mention that it's born out of a heavy star when it collapses into a single point and that infinitely curves spacetime around it. When all the mass ...
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2answers
62 views

What does “spacetime becomes dominated by quantum effects” mean exactly?

I have read about Planck length on wikipedia. In some forms of quantum gravity, the Planck length is the length scale at which the structure of spacetime becomes dominated by quantum effects, ...
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78 views

Is gravity quantized? [closed]

Two uncharged particles are placed in an empty universe one light year apart. In one year, they will feel each other's presence and begin attraction. We are ignoring the uncertainty principle in this ...
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58 views

What sorts of problems does asymptotically safe gravity face as a potential quantum gravity solution?

I've recently read an article by Ethan Siegel over at Forbes (he also blogs at Starts with a Bang) on asymptotically safe gravity as a possible alternative to string theory. I read the author's blog ...
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1answer
210 views

Do the position-momentum uncertainty and time-energy uncertainty really exist in QFT?

It is well known from the Quantum Mechanics(QM) that for a particle, there is a position-momentum uncertainty relation: $$\Delta x\cdot \Delta p\geq \frac{1}{2}\hbar,$$ which bascically can be derived ...
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1answer
58 views

Definition of leading log terms in one loop corrections for gravity

One loop corrections for gravity usually includes non-local terms in the action such as $R\log(\frac{-\Box}{\mu^2})R$, where $\Box=g^{\mu\nu}\nabla_\mu\nabla_\nu$ is the D'Alembert operator, $R$ is ...
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2answers
108 views

Should dark matter play an important role in designing a quantum theory of gravity?

Since dark matter "accounts" for most of the matter of the universe, and its effect are inferred from the gravitational effects, should it play an important role in designing an unified theory between ...
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2answers
83 views

What subatomic particle is a blackhole composed of?

In a black hole, what is the "state" in which the mass that is present is in? Are they there as tightly packed neutron or soup of quark or are particles broken down into 10D strings and coiled as ...
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10 views

DGP model, minus one dimension

Is there a name for a theory like the DGP model, but with one fewer dimension? That is, a theory whose action consists of the 4D Einstein Hilbert action at long distances, and the 3D one at small ...
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3answers
90 views

Can a neutron decay to the gravitons?

Is it possible that a bunch of neutrons totally decay to the graviton? In other words, does the baryon number conserve in the quantum gravity interactions?
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74 views

Why is it that the conformal anomaly has to be scale invariant?

When reading about conformal anomalies, such as in this paper it is often stated that the anomaly (ie. $ \delta W[g]/ \delta \sigma$ where $ W[g]$ is the quantum effective action for gravity) must be ...
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3answers
290 views

Is there proof gravity bends space or is it just the most convenient explanation?

I have read this sentence in an article: The theory [of general relativity] holds that gravity is geometry: particles are deflected when they pass near a massive object not because they feel a ...
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112 views

What do quantum theory and general relativity have in common? [closed]

What areas of commonality are there between quantum theory and general relativity? Is it even possible to use the the two when calculating the same physical behaviour? Is there a correlation between ...
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1answer
51 views

If photons have a wave particle duality would gravitons have a wave particle duality? [duplicate]

If electromagnetic waves are the same as photons would gravitational waves be the same as gravitons?
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0answers
86 views

Topology and Quantum Field Theory

I am interested in finding any one particle state $\left| \Psi \right>$, mostly possibly topological in nature like a kink, such that $$ \left< VAC | R \widetilde{R} | \Psi \right> \neq ...
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0answers
15 views

Can measuring the refraction index in an object measure the strength of the quantum gravity between atoms of the object? [duplicate]

Is measuring the refraction index of an object a way of measuring the strength of the gravitational force between the atoms of the object (measuring quantum gravity)? From reading Snell's law of ...
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1answer
3k views

What is the difference between gravitons and gravitational waves?

What is the difference between gravitons and gravitational waves? So as I understand, gravitational waves are predicted by and part of the theory of general relativity, whereas gravitons are predicted ...
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1answer
61 views

Fundamentals for predicting the standard model? [closed]

. Now, the question(s). What must a theory include to be consistent with the standard model? What concepts/ fields must be predicted?(such as predicting a Higgs field, gauge groups, particles being ...
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1answer
73 views

What is the interpretation of a wave function of the Universe in Hawking's no boundary proposal?

In the path integral formalism we have an in state $\Psi_{in}[\phi]$ and and out state and we find the amplitude for going from one to the other: $$\Delta[\Psi_{in},\Psi_{out}] = \int ...
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3answers
214 views

What is the connection between gravitons and geometry?

I know there are two ways to do quantum gravity. One can pick a background space-time (usually Minkowski flat space-time) and then at any time slice one can define the state of the universe as the ...
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1answer
253 views

Is electron/positron annihilation to two gravitons forbidden?

This follows from the question Can stress energy tensor vanish in general relativity?. What I'm really asking is whether electron/positron annihilation to two gravitons is allowed, but the obvious ...
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1answer
68 views

Critique of various approaches to quantum gravity

Does anyone know of good essays/articles that critique the various approaches to canonical quantum gravity (as opposed to string theory)? I just want to get a sense of what all has been tried, and ...
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0answers
79 views

Significance of imaginary mass

Can real mass be thought of as producing a deformation in spacetime leading to a stable equilibrium (valley curve in gravitational potential energy) of the massive body, and imaginary mass as similar, ...
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2answers
113 views

I have been thinking about the unification of general relativity and quantum mechanics [closed]

I have been thinking about the unification of general relativity and quantum mechanics. after giving it much thought I'm wondering if the problem is not in the formulas used but the numbering system. ...
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1answer
34 views

Why integrate of metrics and not inverse metrics?

In quantum gravity it is generally said that the partition function is written as: $$ Z[J] = \int e^{ i S[g] +J.g } D[g] $$ My question is why do we functionally integrate over all possible metrics ...
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1answer
117 views

Can someone explain about M-theory? [closed]

Can someone explain about M-theory? I am very new to M-theory and string theory. I am very through with classical physics and little bits of quantum mechanics. Will you be able to explain why ...
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88 views

Is the Wheeler deWitt equation consistent with the holographic principle?

In this paper by Sean Carroll (What if Time Really Exists), there's a section "Lessons from Duality" where he says that the holographic principle (and in particular, that a lower dimensional ...
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30 views

Books on Spin-2 Gravity / Fierz-Pauli theory

I am trying to find a good pedagogical book/set of notes on Gravity as a spin-2 particle. I have been reading through Gravity & Strings by Ortin but it's very brief and offers little ...
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1answer
74 views

Is Hawking radiation valid for a microscopic black hole?

A black hole evaporates by Hawking radiation. The computation of the evaporation time uses some approximations. Question: Is the evaporation time valid for a microscopic black hole? In particular, ...
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1answer
230 views

What causes gravity in M-Theory?

New and updated, because people were misunderstanding what I meant! General relativity describes gravity as the result of....(very roughly) spacetime curvature Newtonian gravity describes gravity as ...
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4answers
264 views

How far apart do two objects have to be for there to be negligible force between them?

Inspired from the commentary on this question. How far apart do two objects have to be for the gravitational force between them to be negligible? By negligible I mean, that it could never be ...
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2answers
101 views

Do quantum wave functions curve spacetime before they are measured

Do wave functions cause spacetime curvature before they are measured, or would curvature only happen upon measurement? I guess the question becomes, do quantum wavefunctions carry energy while they ...
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3answers
429 views

Do gravitational waves exist at the quantum level?

Do gravitational waves exist at quantum level produced by electrons or by atoms or by molecules? If yes, which particle produces them and how are they produced? Spacetime exists at a quantum level ...
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2answers
143 views

What is it exactly that conflicts when trying to unify all 4 fundamental forces?

Everyone knows that there are 4 fundamental forces: Gravity Electromagnetic Weak Strong At school and university you're taught that no one has been able to unify these 4 fources. However, not a ...
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61 views

Gravity dual of N free scalars in 2D

I have a very basic (and might be very naive) question. What should be the dual gravity description of $N$ (with $N>>1$) free scalars in two dimensions? I was wondering whether it would be ...
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2answers
91 views

Probing beyond the black hole event horizon

Black holes are interpreted to have a "break down" of general relativity at their point of singularity. The region near the singularity is expected to be described by some theory of quantum gravity. ...
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17 views

Boundary term from isolated horizon?

In Quantum Black Hole Physics one must add a boundary term to the Einstein-Hilbert action because of a geometric restriction on the horizon. Why? Stokes theorem doesn't apply since the horizon isn't ...
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43 views

Problems of General Relativity on small and large scales [duplicate]

As far as I know, the most important problem of GR on large scales is the cosmological constant problem which in some manner can be thought of as a dark energy problem (please correct me if I am ...
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48 views

How much do we know about Quantum Gravity at energies larger than Planck energy?

What kind of predictions do current approaches to quantum gravity (superstrings/M-theory, loop quantum gravity,...) make at energies LARGER than Planck scale? Or is that a meaningless question? Does ...
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3answers
663 views

What are the technical obstructions that prevent scale relativity from being a viable theory of quantum-gravity? [closed]

This post has been imported on physicsoverflow, see here. The astrophysicist Laurent Nottale develops since 1984 the scale relativity, which aims to unify quantum physics and relativity theory, ...
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4answers
3k views

Why do physicists trust black hole physics?

Based on popular accounts of modern physics and black holes (articles, video lectures), I have come to understand the following: Black holes are predicted by General Relativity, a classical theory ...
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0answers
195 views

Can we create graviton particle or gravitational wave? [duplicate]

Can we create graviton particle or gravitational wave? I want to know if we able to generate graviton particle out of some energy. Is it theoretically possible? And could we generate gravitational ...
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2answers
71 views

Do the stellar remnant in a black hole have spatial extent? [duplicate]

I am not talking about event horizons. I am talking about the actual remnant of stellar collapse. Is it just a point, hence the problem with the singularity? Or does it have a finite volume that we ...
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43 views

Palatini type gravities: Where could I find them?

I read the article Quantum gravity with torsion and non-metricity. Although I found interesting the analysis in the paper, I found quite interesting an statement in the abstract, The class of ...
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2answers
331 views

What experiment would disprove loop quantum gravity?

Loop Quantum Gravity (LQG) is a theory of quantum space-time that attempts to describe the interconnection between general relativity and quantum mechanics. It's main postulate is the granularity of ...
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1answer
264 views

Absolute time as a common basis of quantum gravity? [closed]

Quantum mechanics has an absolute time concept, and general relativity has the dynamic time concept of spacetime. The incompatibility of both concepts is called the problem of time in quantum gravity. ...
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1answer
96 views

Can we observe a star collapse into a black hole? [duplicate]

Let's say that I happen by a star that has just burned out and is collasping into a black hole. I seat at infinity to watch it as a Schwarzschild observer. Will I ever see the black hole collapse into ...
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29 views

How is the first law of black hole thermodynamics formulated for a non-vacuum solution with event horizon?

How is the first law of black hole thermodynamics formulated for a non-vacuum solution with event horizon?
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1answer
562 views

Graviton and principle of equivalence

Is it possible that in a theory of gravitons, i.e., a quantum field theory of gravitation, general relativity's principle of the equivalence of gravitational mass and inertial mass, no longer holds?
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330 views

Why should the modes of the linearized metric perturbation be “wavefunctions” of gravitons (in the Randall-Sundrum model)?

In "An Alternative to Compactification" by Randall and Sundrum, they discuss the localization of "graviton modes" around the Planck brane in the Randall-Sundrum model where we have a compact fifth ...