11,576 research outputs found
Modelling Planck-scale Lorentz violation via analogue models
Astrophysical tests of Planck-suppressed Lorentz violations had been
extensively studied in recent years and very stringent constraints have been
obtained within the framework of effective field theory. There are however
still some unresolved theoretical issues, in particular regarding the so called
"naturalness problem" - which arises when postulating that Planck-suppressed
Lorentz violations arise only from operators with mass dimension greater than
four in the Lagrangian. In the work presented here we shall try to address this
problem by looking at a condensed-matter analogue of the Lorentz violations
considered in quantum gravity phenomenology. Specifically, we investigate the
class of two-component BECs subject to laser-induced transitions between the
two components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. We shall show that such a
model can be considered to be an explicit example high-energy Lorentz
violations where the ``naturalness problem'' does not arise.Comment: Talk given at the Fourth Meeting on Constrained Dynamics and Quantum
Gravity (QG05), Cala Gonone (Sardinia, Italy) September 12-16, 200
Entropy bounds in terms of the w parameter
In a pair of recent articles [PRL 105 (2010) 041302 - arXiv:1005.1132; JHEP
1103 (2011) 056 - arXiv:1012.2867] two of the current authors have developed an
entropy bound for equilibrium uncollapsed matter using only classical general
relativity, basic thermodynamics, and the Unruh effect. An odd feature of that
bound, S <= A/2, was that the proportionality constant, 1/2, was weaker than
that expected from black hole thermodynamics, 1/4. In the current article we
strengthen the previous results by obtaining a bound involving the (suitably
averaged) w parameter. Simple causality arguments restrict this averaged
parameter to be <= 1. When equality holds, the entropy bound saturates at the
value expected based on black hole thermodynamics. We also add some clarifying
comments regarding the (net) positivity of the chemical potential. Overall, we
find that even in the absence of any black hole region, we can nevertheless get
arbitrarily close to the Bekenstein entropy.Comment: V1: 14 pages. V2: One reference added. V3: This version accepted for
publication in JHE
An Internet Portal based on 'Twenty Questions'
An efficient Internet portal should contain a search engine or maybe even a decision support system to supply the user with the information (s)he may be looking for. In this report an intelligent agent is suggested that relates different sites to each other, based on the answers supplied by the users looking for certain information. For this purpose a self-learning system has been made, based on the neural network of the game Twenty Questions, but with a strategy that relates different objects or sites by correlating the list of answers to the questions
Riemannian geometry of irrotational vortex acoustics
We consider acoustic propagation in an irrotational vortex, using the
technical machinery of differential geometry to investigate the ``acoustic
geometry'' that is probed by the sound waves. The acoustic space-time curvature
of a constant circulation hydrodynamical vortex leads to deflection of phonons
at appreciable distances from the vortex core. The scattering angle for phonon
rays is shown to be quadratic in the small quantity , where
is the vortex circulation, the speed of sound, and the impact
parameter.Comment: 4 pages, 2 figures, RevTex4. Discussion of focal length added; to
appear in Physical Review Letter
Stability of thin-shell wormholes supported by ordinary matter in Einstein-Maxwell-Gauss-Bonnet gravity
Recently in (Phys. Rev. D 76, 087502 (2007) and Phys. Rev. D 77, 089903(E)
(2008)) a thin-shell wormhole has been introduced in 5-dimensional
Einstein-Maxwell-Gauss-Bonnet (EMGB) gravity which was supported by normal
matter. We wish to consider this solution and investigate its stability. Our
analysis shows that for the Gauss-Bonnet (GB) parameter stability
regions form for a narrow band of finely-tuned mass and charge. For the case
, we iterate once more that no stable, normal matter thin-shell
wormhole exists.Comment: 11 pages, 4 figure
Warped space-time for phonons moving in a perfect nonrelativistic fluid
We construct a kinematical analogue of superluminal travel in the ``warped''
space-times curved by gravitation, in the form of ``super-phononic'' travel in
the effective space-times of perfect nonrelativistic fluids. These warp-field
space-times are most easily generated by considering a solid object that is
placed as an obstruction in an otherwise uniform flow. No violation of any
condition on the positivity of energy is necessary, because the effective
curved space-times for the phonons are ruled by the Euler and continuity
equations, and not by the Einstein field equations.Comment: 7 pages, 1 figure. Version as published; references update
From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture
The recent interest in ``time machines'' has been largely fueled by the
apparent ease with which such systems may be formed in general relativity,
given relatively benign initial conditions such as the existence of traversable
wormholes or of infinite cosmic strings. This rather disturbing state of
affairs has led Hawking to formulate his Chronology Protection Conjecture,
whereby the formation of ``time machines'' is forbidden. This paper will use
several simple examples to argue that the universe appears to exhibit a
``defense in depth'' strategy in this regard. For appropriate parameter regimes
Casimir effects, wormhole disruption effects, and gravitational back reaction
effects all contribute to the fight against time travel. Particular attention
is paid to the role of the quantum gravity cutoff. For the class of model
problems considered it is shown that the gravitational back reaction becomes
large before the Planck scale quantum gravity cutoff is reached, thus
supporting Hawking's conjecture.Comment: 43 pages,ReV_TeX,major revision
Cylindrical thin-shell wormholes and energy conditions
We prove the impossibility of cylindrical thin-shell wormholes supported by
matter satisfying the energy conditions everywhere, under reasonable
assumptions about the asymptotic behaviour of the - in general different -
metrics at each side of the throat. In particular, we reproduce for singular
sources previous results corresponding to flat and conical asymptotics, and
extend them to a more general asymptotic behaviour. Besides, we establish
necessary conditions for the possibility of non exotic cylindrical thin-shell
wormholes.Comment: 9 pages; slightly improved version of the article accepted in Int. J.
Mod. Phys.
Conformational Transitions Accompanying Oligomerization of Yeast Alcohol Oxidase, a Peroxisomal Flavoenzyme
Alcohol oxidase (AO) is a homo-octameric flavoenzyme which catalyzes methanol oxidation in methylotrophic yeasts. AO protein is synthesized in the cytosol and subsequently sorted to peroxisomes where the active enzyme is formed. To gain further insight in the molecular mechanisms involved in AO activation, we studied spectroscopically native AO from Hansenula polymorpha and Pichia pastoris and three putative assembly intermediates. Fluorescence studies revealed that both Trp and FAD are suitable intramolecular markers of the conformation and oligomeric state of AO. A direct relationship between dissociation of AO octamers and increase in Trp fluorescence quantum yield and average fluorescence lifetime was found. The time-resolved fluorescence of the FAD cofactor showed a rapid decay component which reflects dynamic quenching due to the presence of aromatic amino acids in the FAD-binding pocket. The analysis of FAD fluorescence lifetime profiles showed a remarkable resemblance of pattern for purified AO and AO present in intact yeast cells. Native AO contains a high content of ordered secondary structure which was reduced upon FAD-removal. Dissociation of octamers into monomers resulted in a conversion of β-sheets into α-helices. Our results are explained in relation to a 3D model of AO, which was built based on the crystallographic data of the homologous enzyme glucose oxidase from Aspergillus niger. The implications of our results for the current model of the in vivo AO assembly pathway are discussed.
Theorems on gravitational time delay and related issues
Two theorems related to gravitational time delay are proven. Both theorems
apply to spacetimes satisfying the null energy condition and the null generic
condition. The first theorem states that if the spacetime is null geodesically
complete, then given any compact set , there exists another compact set
such that for any , if there exists a ``fastest null
geodesic'', , between and , then cannot enter . As
an application of this theorem, we show that if, in addition, the spacetime is
globally hyperbolic with a compact Cauchy surface, then any observer at
sufficiently late times cannot have a particle horizon. The second theorem
states that if a timelike conformal boundary can be attached to the spacetime
such that the spacetime with boundary satisfies strong causality as well as a
compactness condition, then any ``fastest null geodesic'' connecting two points
on the boundary must lie entirely within the boundary. It follows from this
theorem that generic perturbations of anti-de Sitter spacetime always produce a
time delay relative to anti-de Sitter spacetime itself.Comment: 15 pages, 1 figure. Example of gauge perturbation changed/corrected.
Two footnotes added and one footnote remove
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