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

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600 views

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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1answer
140 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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38 views

Is mass in the sub-atomic particles caused by their angular momentum? [closed]

When they found the God Particle, they said it's responsible for creating mass in all other particles except photons and neutrinos. But how it creates mass is a mystery to me. My suspicion is that ...
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2answers
57 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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1answer
97 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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33 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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1answer
80 views

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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1answer
46 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 ...
3
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1answer
86 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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2answers
122 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 ...
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0answers
61 views

Do any quantum gravity theories deal with closed timelike curves?

As far as I'm aware, there are no quantum gravity theories that deal directly with closed timelike curves. Some of them (like canonical QG, CDT and LQG) forbid them outright, others merely seem to not ...
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127 views

What are the remaining obstacles to low-energy quantum gravity?

In a 2003 review Burgess outlined how the QFT perturbative methodology is being extended to gravity, and described some effective quantum gravity expansions that reproduce general relativity in the ...
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1answer
47 views

Can many universes exist inside black holes? [closed]

As temperature and pressure grows, matter changes. The energy imposed on matter forces particles to disintegrate and recombine in different ways and subatomic orbitals fold over each other into new, ...
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0answers
21 views

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
60 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, ...
2
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0answers
75 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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0answers
52 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 ...
3
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1answer
166 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
44 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
105 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
69 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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0answers
7 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
82 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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0answers
59 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
239 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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0answers
101 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
36 views

If photons have a wave particle duality would gravitons have a wave particle duality?

If electromagnetic waves are the same as photons would gravitational waves be the same as gravitons?
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82 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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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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2answers
845 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
60 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 ...
2
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1answer
69 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 ...
3
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3answers
199 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
228 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
66 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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75 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
110 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
30 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
109 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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0answers
71 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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28 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
73 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
201 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
235 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 ...
3
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2answers
85 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 ...
3
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3answers
342 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
135 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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58 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
79 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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15 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 ...