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

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206 views

How is black hole complementarity derived from path integrals or string theory?

How is the black hole complementarity version of the holographic principle derived from path integrals and/or string theory? That has never been obvious to me. Can someone show me how to do it step by ...
13
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1answer
183 views

6d Massive Gravity

Massive gravity (with a Fierz-Pauli mass) in 4 dimensions is very well-studied, involving exotic phenomena like the vDVZ discontinuity and the Vainshtein effect that all have an elegant and physically ...
4
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1answer
720 views

What do “tachionic” neutrinos mean for QG?

Reading about the spectacular Opera claim, I`m (again ;-P) wondering if a confirmation of superliminous neutrinos could help settle some still open quantum gravity issues ...? In this post, Lumo ...
15
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1answer
153 views

Instantons and Non Perturbative Amplitudes in Gravity

In perturbative QFT in flat spacetime the perturbation expansion typically does not converge, and estimates of the large order behaviour of perturbative amplitudes reveals ambiguity of the ...
2
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2answers
379 views

Quantum mechanics + General relativity =?

I have learned (the basics) of how one can incorporate the principles of special relativity to quantum mechanics to obtain quantum field theory. Can the same be done with GR and QM to obtain a new ...
2
votes
1answer
326 views

Why does gravity forbid local observables?

I heard in a conference that gravity forbids to construct local gauge invariants like $\mathrm{Tr}\left\{−\frac{1}{4} F_{μν}^{a}F_{a}^{μν}\right\}$ and only allows non-local gauge invariant quantities ...
3
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2answers
646 views

What are the implications for quantum gravity if the LHC sees no higgs?

Following the news in serious "non-hype" physics blogs I`ve learnd that as things are now one needs a lot of patience and more data to learn what happens in the higgs sector. There are already a ...
6
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0answers
144 views

Can we have consistent histories inside a black hole?

A consistent history is a POVM set of observables corresponding to a time-ordered product of projection operators. For gauge theories, not any old operator will do, only gauge-invariant observables. ...
4
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1answer
164 views

AdS to dS uplifting and its opposite

So as I understand it, localized structures in AdS can wick rotated to dS, the boundary has to be complixified as can be seen here. Also, uplifting is another technique that can be used to move from ...
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0answers
274 views

Gravitation and the QFT vacuum

I'm asking this to get yet another lessson in the inability of QFT and GR to cohabit. Many people believe GR must yield to quantization. The question here is as to why the activity of the vacuum ...
7
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4answers
711 views

Why should one expect closed timelike curves to be impossible in quantum gravity?

From the Wikipedia article, it seems that physicists tend to view closed timelike curves as an undesirable attribute of a solution to the Einstein Field Equations. Hawking formulated the Chronology ...
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0answers
101 views

Might quantum gravity be a barely inconsistent theory running as a quantum computer simulation? [duplicate]

Possible Duplicate: Does Quantum Physics really suggests this universe as a computer simulation? Might our universe described by "quantum gravity" really be a finite but superastronomical ...
2
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0answers
71 views

Complementarity between the laws of physics? [closed]

Is this following proposal plausible, worth considering, or dismissable as lunatic fringe science? What if the universe isn't really what we think it is but some universal quantum computer where we ...
2
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2answers
481 views

Does spacetime really exist in quantum gravity?

If there are no localized observables in quantum gravity, does spacetime really exist, or might spacetime really be an illusion?
3
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1answer
206 views

Do spacelike singularities really exist in quantum gravity? [duplicate]

Do spacelike singularities really exist in quantum gravity? If the memory of anything which falls into a black hole can't get out, is there any sense in which the interior of the black hole is real? ...
6
votes
2answers
244 views

Possibility of “graviballs”?

Looking at the relevant wikipedia page, one can read that the graviton should be massless. Is it 100 % certain that it is massless or is there room in any "nonstandard" models for a tiny non-zero mass ...
5
votes
2answers
233 views

Hayden-Preskill informational mirrors and decryption

I do have a question about an assumption made in the very interesting Hayden-Preskill paper of black holes as informational mirrors. Alice throws her top secret quantum diary which is $k$ qubits long ...
3
votes
2answers
718 views

Is string theory a quantum theory of gravity?

I have read that string theory predicts (or requires ?) the existence of gravitons. So, would that make it a quantum theory of gravity ? If so, I have also read that quantum gravity would allow us to ...
1
vote
1answer
196 views

Sun-Earth Virtual Gravitons?

How many virtual gravitons do the sun and earth exchange in one year? What are their wavelengths?
8
votes
2answers
489 views

The energy of a Graviton

Maybe another stupid question, but what's the energy of a graviton? Is it $\hbar \omega$? Does it emit gravitons when an apple falls onto the ground, like photons be emitted when an electron transits ...
6
votes
1answer
445 views

Do we need a quantum deformation of the diffeomorphism group in string theory?

Let me justify my question before I go on. In string theory, gravitons are strings extended over space. Longitudinal gravitons are pure gauge modes of the diffeomorphism group. However, in string ...
6
votes
2answers
279 views

How can one reconcile the temperature of a black hole with asymptotic flatness?

A stationary observer very close to the horizon of a black hole is immersed in a thermal bath of temperature that diverges as the horizon is approached. $$T^{-1} = 4\pi \sqrt{2M(r-2M)}$$ The ...
9
votes
3answers
386 views

Can the entropy density of a spacelike singularity arbitrarily exceed the inverse Planck volume?

For the purpose of this question, let's restrict ourselves to BKL singularities. BKL cosmologies are homogeneous Bianchi type XIII and IV cosmologies which exhibit oscillatory chaotic behavior, ...
3
votes
2answers
441 views

Brans-Dicke gravity and Black-Hole “vacuum”

The field equations of the Brans-Dicke gravity are $$\Box\phi = \frac{8\pi}{3+2\omega}T$$ $$G_{ab} = \frac{8\pi}{\phi}T_{ab}+\frac{\omega}{\phi^2} ...
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votes
3answers
363 views

Incommensurability between different observers describing the same universe?

According to black hole complementarity, if there is a black hole and Alice falls into it carrying a qubit, but Bob stays out, then Alice can measure the qubit inside the black hole, and confirm it ...
2
votes
2answers
537 views

Three-Dimensional Gravity

Does anyone have any references that discuss gravity in three-dimensions? I'm trying to make my way through some papers by Witten relating $SL(2,\mathbb{C})$ Chern-Simons theory and gravity in three ...
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votes
2answers
198 views

Do events exist after our death if we can't measure them? [closed]

The great physicist Raphael Bousso predicted time will end in this article. We can't measure anything after our death in principle. So, does time end when we die?
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3answers
345 views

Logical positivism and black hole interiors

A black hole exists. Eventually, it will completely evaporate away. Alice falls into the hole. Imagine you are Alice. According to logical positivism, the interior of the black hole exists. But you ...
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1answer
176 views

Please justify invoking logical positivism to causal patches and black hole interiors in quantum gravity!

Logical positivism is often invoked to explain why external observers can't talk about black hole interiors, or why we can't talk about what happens outside our causal patch in inflationary models. ...
31
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2answers
3k views

Does the Planck scale imply that spacetime is discrete?

On a quantum scale the smallest unit is the Planck scale, which is a discrete measure. There several question that come to mind: Does that mean that particles can only live in a discrete grid-like ...
1
vote
1answer
552 views

What is the smoking gun signature of string theory?

What is the smoking gun signature of string theory? Suppose we have a complete and consistent model of quantum gravity with a zero or negative cosmological constant, but all we are given is its ...
7
votes
1answer
251 views

Does the AdS/CFT correspondence for thermal states really imply time evolution for evaporating black holes is unitary?

You always hear theoreticians proudly proclaim the AdS/CFT correspondence implies time evolution for evaporating black holes is unitary. But if you examine the argument carefully, you find AdS black ...
7
votes
1answer
251 views

Scale set by cosmological constant

Following on Jim Graber's answer to: Can "big rip" rip apart an atomic nucleus? If the cosmological constant is large enough, even the ground state of a hydrogen atom can be affected. So ...
10
votes
2answers
672 views

The Reeh-Schlieder theorem and quantum geometry

There have been some very nice discussions recently centered around the question of whether gravity and the geometry and topology of the classical world we see about us, could be phenomena which ...
2
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4answers
223 views

What are the conditions to be satisfied by a theory in order to be a quantum theory?

This is in continuation to my previous question. It is not a duplicate of the previous one. This question arises because of the answers and discussions in that question. Can we call a theory, quantum ...
3
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1answer
866 views

Mathematical justification of Hartle-Hawking “no boundary” proposal

In Hartle-Hawking "no boundary" proposal it is proposed that Riemannian spacetimes rather than Lorentzian dominated the path integral near the big bang. Moments after the big bang however spacetimes ...
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1answer
716 views

What do gravitons interact with?

The other three forces' mediating particles (photons etc.) are absorbed by their appropriate charge-carrying particles, but I can't seem to find a clear answer that applies to the gravitational force ...
4
votes
2answers
820 views

What is energy in string theory?

Facts agreed on by most Physicists - GR: One can't apply Noether's theorem to argue there is a conserved energy. QFT: One can apply Noether's theorem to argue there is a conserved energy. String ...
4
votes
1answer
225 views

Can T-duality resolve spacelike singularities?

Schwarzschild singularities are described by the Kantowski-Sachs metric with a contracting S2. Of course, T-duality doesn't apply to S2. But what about a Kasner-type singularity with two contracting ...
3
votes
1answer
421 views

Can PEPS explain the holographic principle in quantum gravity?

Condensed matter physicists have shown using quantum information that in many condensed matter systems, entanglement entropy only scales as the area of the boundary, and not the volume. This is the ...
11
votes
3answers
1k views

What happens when a black hole and an “anti-black-hole” collide?

Let's say we have one black hole that formed through the collapse of hydrogen gas and another that formed through the collapse of anti-hydrogen gas. What happens when they collide? Do they (1) ...
8
votes
1answer
635 views

Quantization of Gravitational Field: Quantization conditions

I'm begining to study Quantization of field with the second quantization formalism. I've studied phononic field, electromagnetic field in the vacuum and a generic relativistical scalar field. I ...
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0answers
249 views

Are virtual photons affected by effective gravity in non-linear quantum electrodynamics?

Quantum electrodynamics based upon Euler-Heisenberg or Born-Infeld Lagrangians predict photons to move according to an effective metric, which is dependent on the background electromagnetic field in a ...
8
votes
1answer
194 views

When periodic solutions are combined with timelessness, do we get closed timelike curves?

In quantum gravity, ADM wavefunctional solutions have to satisfy the Wheeler-DeWitt equation. This leads to timelessness. What happens if we have a time periodic solution? In classical general ...
4
votes
4answers
877 views

Question regarding the validity of the big bounce

I have several questions regarding the "big bounce" theory. It appears to be popular among LQG researchers. My questions are as as follows. 1) How one reconciles it with the fact that it is now ...
3
votes
2answers
314 views

Gravitational and gauge-gravitational anomalies in N=1 D=4 supergravity coupled to a SUSY gauge theory with chiral matter

When people talk about the first superstring revolution they often mention the miraculous cancellation of anomalies via the Green Schwarz mechanism. My question is whether such a string-theoretic ...
8
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3answers
621 views

Is anyone studying how the general topology of spacetime arises from more fundamental notions?

Stephen Wolfram in his book A New Kind of Science touches on a model of space itself based on automata theory. That it, he makes some suggestions about modelling not only the behaviour of matter ...
12
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4answers
871 views

Why is there a search for an exchange particle for gravity?

If I understand correctly, according to Einstein's General Theory of Relativity, mass results in a distortion in space-time. In turn, the motion of the mass is affected by the distortion. A result of ...
4
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3answers
993 views

Give a description of M-theory your grandmother can understand

Inspired by this question, let me ask a similar question. Is it possible to do the same (give a description of M-theory your grandmother could understand)for M theory? While I know even experts don't ...
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3answers
729 views

Does Wick rotation work for quantum gravity?

Does Wick rotation work for quantum gravity? The Euclidean Einstein-Hilbert action isn't bounded from below.