67,328 research outputs found
Quantum dissipation of planar harmonic systems: Maxwell-Chern-Simons theory
The conventional Brownian motion in harmonic systems has provided a deep
understanding of a great diversity of dissipative phenomena. We address a
rather fundamental microscopic description for the (linear) dissipative
dynamics of two-dimensional harmonic oscillators that contains the conventional
Brownian motion as a particular instance. This description is derived from
first principles in the framework of the so-called Maxwell-Chern-Simons
electrodynamics, or also known, Abelian topological massive gauge theory.
Disregarding backreaction effects and endowing the system Hamiltonian with a
suitable renormalized potential interaction, the conceived description is
equivalent to a minimal-coupling theory with a gauge field giving rise to a
fluctuating force that mimics the Lorentz force induced by a particle-attached
magnetic flux. We show that the underlying symmetry structure of the theory
(i.e. time-reverse asymmetry and parity violation) yields an interacting
vortex-like Brownian dynamics for the system particles. An explicit comparison
to the conventional Brownian motion in the quantum Markovian limit reveals that
the proposed description represents a second-order correction to the well-known
damped harmonic oscillator, which manifests that there may be dissipative
phenomena intrinsic to the dimensionality of the interesting system.Comment: 20+11 pages, 3 figures. Comments are welcome. Discussion in Sec. III
and IV improved. Several typos and a misleading remark corrected, and figure
replaced. Close to the published versio
Primordial torsion fields as an explanation of the anisotropy in cosmological electromagnetic propagation
In this note we provide a simple explanation of the recent finding of
anisotropy in electromagnetic (EM) propagation claimed by Nodland and Ralston
(astro-ph/9704196). We consider, as a possible origin of such effect, the
effective coupling between EM fields and some tiny background torsion field.
The coupling is obtained after integrating out charged fermions, it is gauge
invariant and does not require the introduction of any new physics.Comment: 8 pages, LaTeX, one figure, enlarged version with minor correction
Social media censorship in times of political unrest: a social simulation experiment with the UK riots
Following the 2011 wave of political unrest, extending from the Arab Spring to the UK riots, the formation of a large consensus around Internet censorship is underway. The present paper adopts a social simulation approach to show that the decision to “regulate”, filter or censor social media in situations of unrest changes the pattern of civil protest and ultimately results in higher levels of violence. Building on Epstein's (2002) agent-based model, several alternative scenarios are generated. The systemic optimum, represented by complete absence of censorship, not only corresponds to lower levels of violence over time, but allows for significant periods of social peace after each outburst
Kovacs-like memory effect in athermal systems: linear response analysis
We analyse the emergence of Kovacs-like memory effects in athermal systems
within the linear response regime. This is done by starting from both the
master equation for the probability distribution and the equations for the
physically relevant moments. The general results are applied to a general class
of models with conserved momentum and non-conserved energy. Our theoretical
predictions, obtained within the first Sonine approximation, show an excellent
agreement with the numerical results.Comment: 18 pages, 6 figures; submitted to the special issue of the journal
Entropy on "Thermodynamics and Statistical Mechanics of Small Systems
Lattice models for granular-like velocity fields: Hydrodynamic limit
A recently introduced model describing -on a 1d lattice- the velocity field
of a granular fluid is discussed in detail. The dynamics of the velocity field
occurs through next-neighbours inelastic collisions which conserve momentum but
dissipate energy. The dynamics can be described by a stochastic equation in
full phase space, or through the corresponding Master Equation for the time
evolution of the probability distribution. In the hydrodynamic limit, equations
for the average velocity and temperature fields with fluctuating currents are
derived, which are analogous to those of granular fluids when restricted to the
shear modes. Therefore, the homogeneous cooling state, with its linear
instability, and other relevant regimes such as the uniform shear flow and the
Couette flow states are described. The evolution in time and space of the
single particle probability distribution, in all those regimes, is also
discussed, showing that the local equilibrium is not valid in general. The
noise for the momentum and energy currents, which are correlated, are white and
Gaussian. The same is true for the noise of the energy sink, which is usually
negligible
Reliability of Erasure Coded Storage Systems: A Geometric Approach
We consider the probability of data loss, or equivalently, the reliability
function for an erasure coded distributed data storage system under worst case
conditions. Data loss in an erasure coded system depends on probability
distributions for the disk repair duration and the disk failure duration. In
previous works, the data loss probability of such systems has been studied
under the assumption of exponentially distributed disk failure and disk repair
durations, using well-known analytic methods from the theory of Markov
processes. These methods lead to an estimate of the integral of the reliability
function.
Here, we address the problem of directly calculating the data loss
probability for general repair and failure duration distributions. A closed
limiting form is developed for the probability of data loss and it is shown
that the probability of the event that a repair duration exceeds a failure
duration is sufficient for characterizing the data loss probability.
For the case of constant repair duration, we develop an expression for the
conditional data loss probability given the number of failures experienced by a
each node in a given time window. We do so by developing a geometric approach
that relies on the computation of volumes of a family of polytopes that are
related to the code. An exact calculation is provided and an upper bound on the
data loss probability is obtained by posing the problem as a set avoidance
problem. Theoretical calculations are compared to simulation results.Comment: 28 pages. 8 figures. Presented in part at IEEE International
Conference on BigData 2013, Santa Clara, CA, Oct. 2013 and to be presented in
part at 2014 IEEE Information Theory Workshop, Tasmania, Australia, Nov.
2014. New analysis added May 2015. Further Update Aug. 201
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