941 research outputs found
Mode-coupling and nonlinear Landau damping effects in auroral Farley-Buneman turbulence
The fundamental problem of Farley-Buneman turbulence in the auroral
-region has been discussed and debated extensively in the past two decades.
In the present paper we intend to clarify the different steps that the auroral
-region plasma has to undergo before reaching a steady state. The
mode-coupling calculation, for Farley-Buneman turbulence, is developed in order
to place it in perspective and to estimate its magnitude relative to the
anomalous effects which arise through the nonlinear wave-particle interaction.
This nonlinear effect, known as nonlinear ``Landau damping'' is due to the
coupling of waves which produces other waves which in turn lose energy to the
bulk of the particles by Landau damping. This leads to a decay of the wave
energy and consequently a heating of the plasma. An equation governing the
evolution of the field spectrum is derived and a physical interpration for each
of its terms is provided
Photon Statistics; Nonlinear Spectroscopy of Single Quantum Systems
A unified description of multitime correlation functions, nonlinear response
functions, and quantum measurements is developed using a common generating
function which allows a direct comparison of their information content. A
general formal expression for photon counting statistics from single quantum
objects is derived in terms of Liouville space correlation functions of the
material system by making a single assumption that spontaneous emission is
described by a master equation
Non-deterministic Boolean Proof Nets
16 pagesInternational audienceWe introduce Non-deterministic Boolean proof nets to study the correspondence with Boolean circuits, a parallel model of computation. We extend the cut elimination of Non-deterministic Multiplicative Linear logic to a parallel procedure in proof nets. With the restriction of proof nets to Boolean types, we prove that the cut-elimination procedure corresponds to Non-deterministic Boolean circuit evaluation and reciprocally. We obtain implicit characterization of the complexity classes NP and NC (the efficiently parallelizable functions)
Helicopter tail rotor orthogonal blade vortex interaction
The aerodynamic operating environment of the helicopter is particularly complex and,
to some extent, dominated by the vortices trailed from the main and tail rotors. These
vortices not only determine the form of the induced flow field but also interact with
each other and with elements of the physical structure of the flight vehicle. Such
interactions can have implications in terms of structural vibration, noise generation
and flight performance. In this paper, the interaction of main rotor vortices with the
helicopter tail rotor is considered and, in particular, the limiting case of the orthogonal
interaction. The significance of the topic is introduced by highlighting the operational
issues for helicopters arising from tail rotor interactions. The basic phenomenon is
then described before experimental studies of the interaction are presented. Progress
in numerical modelling is then considered and, finally, the prospects for future
research in the area are discussed
Consistent histories, the quantum Zeno effect, and time of arrival
We present a decomposition of the general quantum mechanical evolution
operator, that corresponds to the path decomposition expansion, and interpret
its constituents in terms of the quantum Zeno effect (QZE). This decomposition
is applied to a finite dimensional example and to the case of a free particle
in the real line, where the possibility of boundary conditions more general
than those hitherto considered in the literature is shown. We reinterpret the
assignment of consistent probabilities to different regions of spacetime in
terms of the QZE. The comparison of the approach of consistent histories to the
problem of time of arrival with the solution provided by the probability
distribution of Kijowski shows the strength of the latter point of view
Slow relaxation due to optimization and restructuring: Solution on a hierarchical lattice
Motivated by the large strain shear of loose granular materials we introduced
a model which consists of consecutive optimization and restructuring steps
leading to a self organization of a density field. The extensive connections to
other models of statistical phyics are discussed. We investigate our model on a
hierarchical lattice which allows an exact asymptotic renormalization
treatment. A surprisingly close analogy is observed between the simulation
results on the regular and the hierarchical lattices. The dynamics is
characterized by the breakdown of ergodicity, by unusual system size effects in
the development of the average density as well as by the age distribution, the
latter showing multifractal properties.Comment: 11 pages, 7 figures revtex, submitted to PRE see also:
cond-mat/020920
Maximally incompressible neutron star matter
Relativistic kinetic theory, based on the Grad method of moments as developed
by Israel and Stewart, is used to model viscous and thermal dissipation in
neutron star matter and determine an upper limit on the maximum mass of neutron
stars. In the context of kinetic theory, the equation of state must satisfy a
set of constraints in order for the equilibrium states of the fluid to be
thermodynamically stable and for perturbations from equilibrium to propagate
causally via hyperbolic equations. Application of these constraints to neutron
star matter restricts the stiffness of the most incompressible equation of
state compatible with causality to be softer than the maximally incompressible
equation of state that results from requiring the adiabatic sound speed to not
exceed the speed of light. Using three equations of state based on experimental
nucleon-nucleon scattering data and properties of light nuclei up to twice
normal nuclear energy density, and the kinetic theory maximally incompressible
equation of state at higher density, an upper limit on the maximum mass of
neutron stars averaging 2.64 solar masses is derived.Comment: 8 pages, 2 figure
Neutron star properties in the quark-meson coupling model
The effects of internal quark structure of baryons on the composition and
structure of neutron star matter with hyperons are investigated in the
quark-meson coupling (QMC) model. The QMC model is based on mean-field
description of nonoverlapping spherical bags bound by self-consistent exchange
of scalar and vector mesons. The predictions of this model are compared with
quantum hadrodynamic (QHD) model calibrated to reproduce identical nuclear
matter saturation properties. By employing a density dependent bag constant
through direct coupling to the scalar field, the QMC model is found to exhibit
identical properties as QHD near saturation density. Furthermore, this modified
QMC model provides well-behaved and continuous solutions at high densities
relevant to the core of neutron stars. Two additional strange mesons are
introduced which couple only to the strange quark in the QMC model and to the
hyperons in the QHD model. The constitution and structure of stars with
hyperons in the QMC and QHD models reveal interesting differences. This
suggests the importance of quark structure effects in the baryons at high
densities.Comment: 28 pages, 10 figures, to appear in Physical Review
Post-Newtonian SPH calculations of binary neutron star coalescence. I. Method and first results
We present the first results from our Post-Newtonian (PN) Smoothed Particle
Hydrodynamics (SPH) code, which has been used to study the coalescence of
binary neutron star (NS) systems. The Lagrangian particle-based code
incorporates consistently all lowest-order (1PN) relativistic effects, as well
as gravitational radiation reaction, the lowest-order dissipative term in
general relativity. We test our code on sequences of single NS models of
varying compactness, and we discuss ways to make PN simulations more relevant
to realistic NS models. We also present a PN SPH relaxation procedure for
constructing equilibrium models of synchronized binaries, and we use these
equilibrium models as initial conditions for our dynamical calculations of
binary coalescence. Though unphysical, since tidal synchronization is not
expected in NS binaries, these initial conditions allow us to compare our PN
work with previous Newtonian results.
We compare calculations with and without 1PN effects, for NS with stiff
equations of state, modeled as polytropes with . We find that 1PN
effects can play a major role in the coalescence, accelerating the final
inspiral and causing a significant misalignment in the binary just prior to
final merging. In addition, the character of the gravitational wave signal is
altered dramatically, showing strong modulation of the exponentially decaying
waveform near the end of the merger. We also discuss briefly the implications
of our results for models of gamma-ray bursts at cosmological distances.Comment: RevTeX, 37 pages, 17 figures, to appear in Phys. Rev. D, minor
corrections onl
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