Any of the various explanations of gravity as a quantum theory, including string theory and loop quantum gravity.
3
votes
0answers
71 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
173 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 ...
38
votes
2answers
575 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), ...
9
votes
2answers
211 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 ...
1
vote
1answer
336 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 ...
15
votes
3answers
917 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 ...
8
votes
1answer
80 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 ...
7
votes
1answer
303 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
votes
2answers
271 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
votes
2answers
164 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 ...
7
votes
1answer
77 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
61 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
votes
0answers
88 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
81 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 ...
17
votes
3answers
105 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 ...
4
votes
2answers
452 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?
14
votes
6answers
307 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 ...
2
votes
3answers
504 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
765 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
26 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 ...
8
votes
3answers
339 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
187 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 ...
12
votes
1answer
92 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
votes
1answer
706 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 ...
14
votes
1answer
62 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
votes
2answers
293 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
194 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
552 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 ...
4
votes
0answers
114 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
145 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 ...
0
votes
0answers
254 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
333 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
votes
0answers
97 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
63 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 ...
1
vote
1answer
318 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?
0
votes
0answers
147 views
Do spacelike singularities really exist in quantum gravity?
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? ...
5
votes
2answers
220 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
188 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 ...
1
vote
3answers
562 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 ...
0
votes
1answer
153 views
Sun-Earth Virtual Gravitons?
How many virtual gravitons do the sun and earth exchange in one year?
What are their wavelengths?
6
votes
1answer
413 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
246 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
319 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
380 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
304 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
436 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 ...
-4
votes
2answers
169 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
294 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
157 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. ...
7
votes
2answers
1k views
Is reality discrete at the quantum level? (…and what does it imply not only mathematically?)
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 ...
