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

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229 views

Why are geons unstable? Are there other problems with geons?

I read in various places geons are "generally considered unstable." Why? How solid is this reasoning? Is the reason geons are not studied much anymore because we can't make more progress without ...
1
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2answers
268 views

Could gravity hold electron charge together?

Could the gravitational force be what holds the charge of the electron together? It seems to be the only obvious possibility; what other ideas have been proposed besides side-stepping the issue and ...
3
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1answer
211 views

Could strings be geons?

Is it possible that string theory strings are geons? This may be an overly speculative or naive question, but is there an obvious reason why not? Both strings and geons seem to have roughly the same ...
3
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0answers
86 views

Pohlmeyer reduction of string theory for flat- and AdS- spaces

The definition of Pohlmeyer invariants in flat-space (as per eq-2.16 in Urs Schreiber's DDF and Pohlmeyer invariants of (super)string) is the following: $ Z^{\mu_1...\mu_N} (\mathcal{P}) = ...
8
votes
1answer
317 views

Derivation of the basic equation for Witten diagrams

I could understand the derivation of the "bulk-to-boundary" propagators ($K$) for scalar fields in $AdS$ but the iterative definition of the "bulk-to-bulk" propagators is not clear to me. On is ...
43
votes
2answers
722 views

Analog Hawking radiation

I am confused by most discussions of analog Hawking radiation in fluids (see, for example, the recent experimental result of Weinfurtner et al. Phys. Rev. Lett. 106, 021302 (2011), ...
11
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2answers
300 views

How is causal patch complementarity compatible with behavior during inflation?

Causal patch complementarity is the conjecture that in de Sitter space with a positive cosmological constant, the states within the causal patch are sufficient to fully describe the universe with the ...
2
votes
1answer
420 views

How is the expansion of space itself explained in LQG?

To explain the expansion of space I have often heard people saying that space is continuously created. This picture is usually applied to cosmological scales but I`m nevertheless curious if some ...
16
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3answers
1k views

Is decoherence even possible in anti de Sitter space?

Is decoherence even possible in anti de Sitter space? The spatial conformal boundary acts as a repulsive wall, thus turning anti de Sitter space into an eternally closed quantum system. Superpositions ...
9
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1answer
128 views

Quantum gravity at D = 3

Quantization of gravity (general relativity) seems to be impossible for spacetime dimension D >= 4. Instead, quantum gravity is described by string theory which is something more than quantization ...
8
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1answer
403 views

Physical interpretation of Wheeler - Dewitt equation

What is the mainstream ( if there is one ) interpretation of the Wheeler - Dewitt equation $\hat{H}\Psi =0$ ?
0
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2answers
334 views

Momentum Energy and Higgs

So, as an object accelerates it gains energy. And energy is mass. So an object becomes more massive as it approaches the speed of light. But, if mass is ONLY due to an object's interaction with the ...
2
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2answers
185 views

Can the implications of dark energy be used to bridge the gap between Quantum Mechanics and General Relativity?

Can the findings of the Physics Nobel Laureates of 2011, namely the overpowering existence of dark energy (vacuum energy) have any implications in the quest the combine Quantum Mechanics and General ...
4
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2answers
1k views

Physical laws prior the big bang (quantum fluctuations)

A theory among scientists says that quantum fluctuations caused the big bang and created the universe. This seems plausible to me. What I can't grasp yet is how a quantum fluctuation can even start ...
7
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1answer
107 views

Simple question on the foundations of spin foam formalism

To make it simple, take the spin foam formalism of ($SU(2)$) 3D gravity. My question is about the choice of the data that will replace the (smoothly defined) fields $e$ (the triad) and $\omega$ (the ...
5
votes
1answer
89 views

Quantum causal structure

We take causal structure to be some relation defined over elements which are understood to be morphisms of some category. An example of such a relation is a domain, another is a directed acyclic ...
5
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0answers
142 views

Geometric entropy vs entanglement entropy (dependent on curvature coupling parameter)

I have a quick question. In hep-th/9506066, Larsen and Wilczek calculated the geometric entropy (which I believe is just another name for entanglement entropy) for a non-minimally coupled scalar field ...
6
votes
1answer
126 views

Quantum mechanical gravitational bound states

The quantum mechanics of Coloumb-force bound states of atomic nuclei and electrons lead to the extremely rich theory of molecules. In particular, I think the richness of the theory is related to the ...
19
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3answers
183 views

Twistors in Curved Spacetime

I am looking for good and recent references to constructing twistor space for curved spacetime. This could be a general spacetime, or specific ones (say maximally symmetric spaces different from ...
5
votes
2answers
791 views

Critics of Mannheim's Conformal Gravity Theory?

I'm looking for more articles/reactions/critiques/support for Philip Mannheim's recent conformal gravity theory. See here: http://arxiv.org/abs/1101.2186v1 Any ideas on where to start?
15
votes
6answers
508 views

Classic Literature in Quantum Gravity?

I've seen it said in various places that a major reason people like string theory as a theory of quantum gravity is that it does a good job of matching our prejudices about how a quantum gravity ...
3
votes
3answers
568 views

Does the Opera result hint to a discrete spacetime?

Could the Opera result be interpreted as some kind of hint to a discrete spacetime that is only seen for high enough energy neutrinos? I think I've read (some time ago) something like this in a ...
8
votes
5answers
2k views

Graduate School for Theoretical Physics

First off, let me just say that I am unsure if this question is appropriate for this site, and if the community deems it necessary, the question should be closed. So right now I am a fourth year ...
2
votes
0answers
39 views

What methods are there to deal with quantum spatiotemporal chaos?

By now, there has been enough grasp on quantum chaos for systems with a small number of degrees of freedom. The major tool used is periodic orbit theory to approximate the spectral distribution. Is ...
9
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3answers
400 views

Does the dimensionality of phase space go up as the universe expands?

Ever since Hubble, it is well known that the universe is expanding from a Big Bang. The size of the universe had gone up by many many orders of magnitude as space expanded. If the dimensionality of ...
4
votes
1answer
204 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
169 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
717 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
138 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
364 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
297 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
625 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
140 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
159 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
271 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
votes
4answers
610 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
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
votes
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
446 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
197 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
241 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
222 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
693 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
189 views

Sun-Earth Virtual Gravitons?

How many virtual gravitons do the sun and earth exchange in one year? What are their wavelengths?
7
votes
2answers
373 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
441 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
275 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
370 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
428 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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3answers
340 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 ...