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

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1answer
276 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
votes
2answers
607 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 ...
5
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0answers
130 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
votes
1answer
158 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 ...
1
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0answers
265 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
499 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 ...
0
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0answers
99 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
68 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
423 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
185 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
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2answers
239 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
212 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 ...
2
votes
2answers
676 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
179 views

Sun-Earth Virtual Gravitons?

How many virtual gravitons do the sun and earth exchange in one year? What are their wavelengths?
6
votes
2answers
318 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
434 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
268 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
359 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
416 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} ...
-3
votes
3answers
333 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
489 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 ...
-5
votes
2answers
186 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?
-3
votes
3answers
324 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 ...
3
votes
1answer
168 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. ...
18
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2answers
2k 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
534 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
241 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
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1answer
242 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
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2answers
581 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
votes
4answers
219 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
votes
1answer
744 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
649 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
716 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
212 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
399 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 ...
10
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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) ...
7
votes
1answer
550 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 ...
1
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0answers
246 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
162 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
589 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
284 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
votes
3answers
564 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 ...
11
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4answers
737 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
votes
3answers
874 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 ...
6
votes
3answers
585 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.
4
votes
2answers
549 views

Global symmetries in quantum gravity

In several papers (including a recent one by Banks and Seiberg) people mention a "folk-theorem" about the impossibility to have global symmetries in a consistent theory of quantum gravity. I remember ...
5
votes
1answer
265 views

Why can't fermions be 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 ...
8
votes
5answers
501 views

How important is the cosmic censorship conjecture?

I would like to know how important the cosmic censorship conjecture is? Should a quantum theory of gravity must obey this? It was never rigorously proved in classical GR too. What would be the ...
6
votes
2answers
500 views

Does perturbation theory break down for quantum gravity?

Perturbation theory presumes we have a valid family of models over some continuous (infinitely differentiable, in fact) range for some parameters, i.e. coupling constants. We have some special values ...
8
votes
2answers
796 views

Do traversable wormholes exist as solutions to string theory?

There has been some heated debate as to whether the laws of physics allow for traversable wormholes. Some physicists claim we require exotic matter to construct wormholes, but then others counter the ...