69 research outputs found
Comments on discrete time in quantum mechanics
The possibility that time can be regarded as a discrete parameter is
re-examined. We study the dynamics of the free particle and find in some cases
superluminal propagation
Information and entropy in quantum Brownian motion: Thermodynamic entropy versus von Neumann entropy
We compare the thermodynamic entropy of a quantum Brownian oscillator derived
from the partition function of the subsystem with the von Neumann entropy of
its reduced density matrix. At low temperatures we find deviations between
these two entropies which are due to the fact that the Brownian particle and
its environment are entangled. We give an explanation for these findings and
point out that these deviations become important in cases where statements
about the information capacity of the subsystem are associated with
thermodynamic properties, as it is the case for the Landauer principle.Comment: 8 pages, 7 figure
Transformation elastodynamics and active exterior acoustic cloaking
This chapter consists of three parts. In the first part we recall the
elastodynamic equations under coordinate transformations. The idea is to use
coordinate transformations to manipulate waves propagating in an elastic
material. Then we study the effect of transformations on a mass-spring network
model. The transformed networks can be realized with "torque springs", which
are introduced here and are springs with a force proportional to the
displacement in a direction other than the direction of the spring terminals.
Possible homogenizations of the transformed networks are presented, with
potential applications to cloaking. In the second and third parts we present
cloaking methods that are based on cancelling an incident field using active
devices which are exterior to the cloaked region and that do not generate
significant fields far away from the devices. In the second part, the exterior
cloaking problem for the Laplace equation is reformulated as the problem of
polynomial approximation of analytic functions. An explicit solution is given
that allows to cloak larger objects at a fixed distance from the cloaking
device, compared to previous explicit solutions. In the third part we consider
the active exterior cloaking problem for the Helmholtz equation in 3D. Our
method uses the Green's formula and an addition theorem for spherical outgoing
waves to design devices that mimic the effect of the single and double layer
potentials in Green's formula.Comment: Submitted as a chapter for the volume "Acoustic metamaterials:
Negative refraction, imaging, lensing and cloaking", Craster and Guenneau
ed., Springe
Mathematical analysis of the two dimensional active exterior cloaking in the quasistatic regime
We design a device that generates fields canceling out a known probing field
inside a region to be cloaked while generating very small fields far away from
the device. The fields we consider satisfy the Laplace equation, but the
approach remains valid in the quasistatic regime in a homogeneous medium. We
start by relating the problem of designing an exterior cloak in the quasistatic
regime to the classic problem of approximating a harmonic function with
harmonic polynomials. An explicit polynomial solution to the problem was given
earlier in [Phys. Rev. Lett. 103 (2009), 073901]. Here we show convergence of
the device field to the field needed to perfectly cloak an object. The
convergence region limits the size of the cloaked region, and the size and
position of the device.Comment: submitted to Analysis and Mathematical Physic
Two-sided Grassmann-Rayleigh quotient iteration
The two-sided Rayleigh quotient iteration proposed by Ostrowski computes a
pair of corresponding left-right eigenvectors of a matrix . We propose a
Grassmannian version of this iteration, i.e., its iterates are pairs of
-dimensional subspaces instead of one-dimensional subspaces in the classical
case. The new iteration generically converges locally cubically to the pairs of
left-right -dimensional invariant subspaces of . Moreover, Grassmannian
versions of the Rayleigh quotient iteration are given for the generalized
Hermitian eigenproblem, the Hamiltonian eigenproblem and the skew-Hamiltonian
eigenproblem.Comment: The text is identical to a manuscript that was submitted for
publication on 19 April 200
Simulation of the Magnetothermal Instability
In many magnetized, dilute astrophysical plasmas, thermal conduction occurs
almost exclusively parallel to magnetic field lines. In this case, the usual
stability criterion for convective stability, the Schwarzschild criterion,
which depends on entropy gradients, is modified. In the magnetized long mean
free path regime, instability occurs for small wavenumbers when (dP/dz)(dln
T/dz) > 0, which we refer to as the Balbus criterion. We refer to the
convective-type instability that results as the magnetothermal instability
(MTI). We use the equations of MHD with anisotropic electron heat conduction to
numerically simulate the linear growth and nonlinear saturation of the MTI in
plane-parallel atmospheres that are unstable according to the Balbus criterion.
The linear growth rates measured from the simulations are in excellent
agreement with the weak field dispersion relation. The addition of isotropic
conduction, e.g. radiation, or strong magnetic fields can damp the growth of
the MTI and affect the nonlinear regime. The instability saturates when the
atmosphere becomes isothermal as the source of free energy is exhausted. By
maintaining a fixed temperature difference between the top and bottom
boundaries of the simulation domain, sustained convective turbulence can be
driven. MTI-stable layers introduced by isotropic conduction are used to
prevent the formation of unresolved, thermal boundary layers. We find that the
largest component of the time-averaged heat flux is due to advective motions as
opposed to the actual thermal conduction itself. Finally, we explore the
implications of this instability for a variety of astrophysical systems, such
as neutron stars, the hot intracluster medium of galaxy clusters, and the
structure of radiatively inefficient accretion flows.Comment: Accepted for publication in Astrophysics and Space Science as
proceedings of the 6th High Energy Density Laboratory Astrophysics (HEDLA)
Conferenc
Observational Constraints on the Modified Gravity Model (MOG) Proposed by Moffat: Using the Magellanic System
A simple model for the dynamics of the Magellanic Stream (MS), in the
framework of modified gravity models is investigated. We assume that the galaxy
is made up of baryonic matter out of context of dark matter scenario. The model
we used here is named Modified Gravity (MOG) proposed by Moffat (2005). In
order to examine the compatibility of the overall properties of the MS under
the MOG theory, the observational radial velocity profile of the MS is compared
with the numerical results using the fit method. In order to obtain
the best model parameters, a maximum likelihood analysis is performed. We also
compare the results of this model with the Cold Dark Matter (CDM) halo model
and the other alternative gravity model that proposed by Bekenstein (2004), so
called TeVeS. We show that by selecting the appropriate values for the free
parameters, the MOG theory seems to be plausible to explain the dynamics of the
MS as well as the CDM and the TeVeS models.Comment: 14 pages, 3 Figures, accepted in Int. J. Theor. Phy
Quantum Fluctuation Relations for the Lindblad Master Equation
An open quantum system interacting with its environment can be modeled under
suitable assumptions as a Markov process, described by a Lindblad master
equation. In this work, we derive a general set of fluctuation relations for
systems governed by a Lindblad equation. These identities provide quantum
versions of Jarzynski-Hatano-Sasa and Crooks relations. In the linear response
regime, these fluctuation relations yield a fluctuation-dissipation theorem
(FDT) valid for a stationary state arbitrarily far from equilibrium. For a
closed system, this FDT reduces to the celebrated Callen-Welton-Kubo formula
Decoherence and CPT Violation in a Stringy Model of Space-Time Foam
I discuss a model inspired from the string/brane framework, in which our
Universe is represented as a three brane, propagating in a bulk space time
punctured by D0-brane (D-particle) defects. As the D3-brane world moves in the
bulk, the D-particles cross it, and from an effective observer on D3 the
situation looks like a ``space-time foam'' with the defects ``flashing'' on and
off (``D-particle foam''). The open strings, with their ends attached on the
brane, which represent matter in this scenario, can interact with the
D-particles on the D3-brane universe in a topologically non-trivial manner,
involving splitting and capture of the strings by the D0-brane defects. Such
processes are described by logarithmic conformal field theories on the
world-sheet. Physically, they result in effective decoherence of the string
matter on the D3 brane, and as a result, of CPT Violation, but of a type that
implies an ill-defined nature of the effective CPT operator. Due to electric
charge conservation, only electrically neutral (string) matter can exhibit such
interactions with the D-particle foam. This may have unique, experimentally
detectable, consequences for electrically-neutral entangled quantum matter
states on the brane world, in particular the modification of the pertinent EPR
Correlation of neutral mesons in a meson factory.Comment: 41 pages Latex, five eps figures incorporated. Uses special macro
Use of SMS texts for facilitating access to online alcohol interventions: a feasibility study
A41 Use of SMS texts for facilitating access to online alcohol interventions: a feasibility study
In: Addiction Science & Clinical Practice 2017, 12(Suppl 1): A4
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