752 research outputs found
Black hole lasers in Bose-Einstein condensates
We consider elongated condensates that cross twice the speed of sound. In the
absence of periodic boundary conditions, the phonon spectrum possesses a
discrete and finite set of complex frequency modes that induce a laser effect.
This effect constitutes a dynamical instability and is due to the fact that the
supersonic region acts as a resonant cavity. We numerically compute the complex
frequencies and density-density correlation function. We obtain patterns with
very specific signatures. In terms of the gravitational analogy, the flows we
consider correspond to a pair of black hole and white hole horizons, and the
laser effect can be conceived as a self-amplified Hawking radiation. This is
verified by comparing the outgoing flux at early time with the standard black
hole radiation.Comment: iopams, 37 pages, 14 figures, 1 table; for associated gif animations,
see http://people.sissa.it/~finazzi/bec_bhlasers/movies/ or
http://iopscience.iop.org/1367-2630/12/9/095015/media. Published on New. J.
Phys. (http://iopscience.iop.org/1367-2630/12/9/095015/). V2: few new
comments, modified figure
Impossibility of superluminal travel in Lorentz violating theories
Warp drives are space-times allowing for superluminal travel. However, they
are quantum mechanically unstable because they produce a Hawking-like radiation
which is blue shifted at their front wall without any bound. We reexamine this
instability when local Lorentz invariance is violated at ultrahigh energy by
dispersion, as in some theories of quantum gravity. Interestingly, even though
the ultraviolet divergence is now regulated, warp drives are still unstable.
Moreover the type of instability is different whether one uses a subluminal or
a superluminal dispersion relation. In the first case, a black-hole laser
yields an exponential amplification of the emitted flux whereas, in the second,
infrared effects produce a linear growth of that flux. These results suggest
that chronology could still be protected when violating Lorentz invariance.Comment: 5 pages, 3 figure
Black holes and Hawking radiation in spacetime and its analogues
These notes introduce the fundamentals of black hole geometry, the thermality
of the vacuum, and the Hawking effect, in spacetime and its analogues.
Stimulated emission of Hawking radiation, the trans-Planckian question, short
wavelength dispersion, and white hole radiation in the setting of analogue
models are also discussed. No prior knowledge of differential geometry, general
relativity, or quantum field theory in curved spacetime is assumed.Comment: 31 pages, 9 figures; to appear in the proceedings of the IX SIGRAV
School on 'Analogue Gravity', Como (Italy), May 2011, eds. D. Faccio et. al.
(Springer
Thermite-for-Demise (T4D): Preliminary Assessment on the Effects of a Thermite Charge in Arc-Heated Wind Tunnel Experiments
The treatment of polycythaemia vera: an update in the JAK2 era
The clinical course of polycythaemia vera is marked by a high incidence of thrombotic complications, which represent the main cause of morbidity and mortality. Major predictors of vascular events are increasing age and previous thrombosis. Myelosuppressive drugs can reduce the rate of thrombosis, but there is concern that their use raises the risk of transformation into acute leukaemia. To tackle this dilemma, a risk-oriented management strategy is recommended. Low-risk patients should be treated with phlebotomy and low-dose aspirin. Cytotoxic therapy is indicated in high-risk patients, with the drug of choice being hydroxyurea because its leukaemogenicity is low. The recent discovery of JAK2 V617F mutation in the vast majority of polycythaemia vera patients opens new avenues for the treatment of this disease. Novel therapeutic options theoretically devoid of leukaemic risk, such as alpha-interferon and imatinib, affect JAK2 expression in some patients. Nevertheless, these drugs require further clinical experience and, for the time being, should be reserved for selected cases
The circadian clock in the diatom <i>Phaeodactylum tricornutum</i>
International audienceno abstrac
Critical velocity for a toroidal Bose-Einstein condensate flowing through a barrier
We consider the setup employed in a recent experiment (Ramanathan et al 2011
Phys. Rev. Lett. 106 130401) devoted to the study of the instability of the
superfluid flow of a toroidal Bose-Einstein condensate in presence of a
repulsive optical barrier. Using the Gross-Pitaevskii mean-field equation, we
observe, consistently with what we found in Piazza et al (2009 Phys. Rev. A 80
021601), that the superflow with one unit of angular momentum becomes unstable
at a critical strength of the barrier, and decays through the mechanism of
phase slippage performed by pairs of vortex-antivortex lines annihilating.
While this picture qualitatively agrees with the experimental findings, the
measured critical barrier height is not very well reproduced by the
Gross-Pitaevskii equation, indicating that thermal fluctuations can play an
important role (Mathey et al 2012 arXiv:1207.0501). As an alternative
explanation of the discrepancy, we consider the effect of the finite resolution
of the imaging system. At the critical point, the superfluid velocity in the
vicinity of the obstacle is always of the order of the sound speed in that
region, . In particular, in the hydrodynamic regime
(not reached in the above experiment), the critical point is determined by
applying the Landau criterion inside the barrier region. On the other hand, the
Feynman critical velocity is much lower than the observed critical
velocity. We argue that this is a general feature of the Gross-Pitaevskii
equation, where we have with being a
small parameter of the model. Given these observations, the question still
remains open about the nature of the superfluid instability.Comment: Extended versio
Understanding Hawking radiation from simple models of atomic Bose-Einstein condensates
This chapter is an introduction to the Bogoliubov theory of dilute Bose
condensates as applied to the study of the spontaneous emission of phonons in a
stationary condensate flowing at supersonic speeds. This emission process is a
condensed-matter analog of Hawking radiation from astrophysical black holes but
is derived here from a microscopic quantum theory of the condensate without any
use of the analogy with gravitational systems. To facilitate physical
understanding of the basic concepts, a simple one-dimensional geometry with a
stepwise homogenous flow is considered which allows for a fully analytical
treatment.Comment: 41 pages. to appear in the proceedings of the IX SIGRAV School on
'Analogue Gravity', Como (Italy), May 201
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