2,488 research outputs found
Large reflector antenna study
In some applications, the wires used to construct the grids are plated over with highly conducting materials such as gold or silver. In those cases, depending on the frequency of operation, the coating may not be thick enough to prevent currents from flowing in the substrate. The conjugate gradient method, in conjunction with the fast Fourier transform is employed to solve the problem of scattering from such rectangular grids. An internal impedance is utilized to account for the effects of the substrate conductivity on the induced current densities. Calculated values of the reflection coefficient and induced currents from different coating thicknesses, angles of incidence and polarizations are presented and discussed
Coarse Brownian Dynamics for Nematic Liquid Crystals: Bifurcation Diagrams via Stochastic Simulation
We demonstrate how time-integration of stochastic differential equations
(i.e. Brownian dynamics simulations) can be combined with continuum numerical
bifurcation analysis techniques to analyze the dynamics of liquid crystalline
polymers (LCPs). Sidestepping the necessity of obtaining explicit closures, the
approach analyzes the (unavailable in closed form) coarse macroscopic
equations, estimating the necessary quantities through appropriately
initialized, short bursts of Brownian dynamics simulation. Through this
approach, both stable and unstable branches of the equilibrium bifurcation
diagram are obtained for the Doi model of LCPs and their coarse stability is
estimated. Additional macroscopic computational tasks enabled through this
approach, such as coarse projective integration and coarse stabilizing
controller design, are also demonstrated
How to detect an anti-spacetime
Is it possible, in principle, to measure the sign of the Lapse? We show that
fermion dynamics distinguishes spacetimes having the same metric but different
tetrads, for instance a Lapse with opposite sign. This sign might be a physical
quantity not captured by the metric. We discuss its possible role in quantum
gravity.Comment: Article awarded with an "Honorable Mention" from the 2012 Gravity
Foundation Award. 6 pages, 8 (pretty) figure
On learning time delays between the spikes from different input neurons in a biophysical model of a pyramidal neuron.
Biological systems are able to recognise temporal sequences of stimuli or compute in the temporal domain. In this paper we are exploring whether a biophysical model of a pyramidal neuron can detect and learn systematic time delays between the spikes from different input neurons. In particular, we investigate whether it is possible to reinforce pairs of synapses separated by a dendritic propagation time delay corresponding to the arrival time difference of two spikes from two different input neurons. We examine two subthreshold learning approaches where the first relies on the backpropagation of EPSPs (excitatory postsynaptic potentials) and the second on the backpropagation of a somatic action potential, whose production is supported by a learning-enabling background current. The first approach does not provide a learning signal that sufficiently differentiates between synapses at different locations, while in the second approach, somatic spikes do not provide a reliable signal distinguishing arrival time differences of the order of the dendritic propagation time. It appears that the firing of pyramidal neurons shows little sensitivity to heterosynaptic spike arrival time differences of several milliseconds. This neuron is therefore unlikely to be able to learn to detect such differences
Critical collapse of collisionless matter - a numerical investigation
In recent years the threshold of black hole formation in spherically
symmetric gravitational collapse has been studied for a variety of matter
models. In this paper the corresponding issue is investigated for a matter
model significantly different from those considered so far in this context. We
study the transition from dispersion to black hole formation in the collapse of
collisionless matter when the initial data is scaled. This is done by means of
a numerical code similar to those commonly used in plasma physics. The result
is that for the initial data for which the solutions were computed, most of the
matter falls into the black hole whenever a black hole is formed. This results
in a discontinuity in the mass of the black hole at the onset of black hole
formation.Comment: 22 pages, LaTeX, 7 figures (ps-files, automatically included using
psfig
Final fate of spherically symmetric gravitational collapse of a dust cloud in Einstein-Gauss-Bonnet gravity
We give a model of the higher-dimensional spherically symmetric gravitational
collapse of a dust cloud in Einstein-Gauss-Bonnet gravity. A simple formulation
of the basic equations is given for the spacetime with a perfect fluid and a cosmological constant. This is a
generalization of the Misner-Sharp formalism of the four-dimensional
spherically symmetric spacetime with a perfect fluid in general relativity. The
whole picture and the final fate of the gravitational collapse of a dust cloud
differ greatly between the cases with and . There are two
families of solutions, which we call plus-branch and the minus-branch
solutions. Bounce inevitably occurs in the plus-branch solution for ,
and consequently singularities cannot be formed. Since there is no trapped
surface in the plus-branch solution, the singularity formed in the case of
must be naked. In the minus-branch solution, naked singularities are
massless for , while massive naked singularities are possible for
. In the homogeneous collapse represented by the flat
Friedmann-Robertson-Walker solution, the singularity formed is spacelike for , while it is ingoing-null for . In the inhomogeneous collapse with
smooth initial data, the strong cosmic censorship hypothesis holds for and for depending on the parameters in the initial data, while a
naked singularity is always formed for . These naked
singularities can be globally naked when the initial surface radius of the dust
cloud is fine-tuned, and then the weak cosmic censorship hypothesis is
violated.Comment: 23 pages, 1 figure, final version to appear in Physical Review
Self-Similar Collapse of Conformally Coupled Scalar Fields
A massless scalar field minimally coupled to the gravitational field in a
simplified spherical symmetry is discussed. It is shown that, in this case, the
solution found by Roberts, describing a scalar field collapse, is in fact the
most general one. Taking that solution as departure point, a study of the
gravitational collapse for the self-similar conformal case is presented.Comment: 9 pages, accepted for publication, Classical and Quantum Gravity.
Available at http://dft.if.uerj.br/preprint/e-17.tex or at
ftp://dft.if.uerj.br/preprint/e-17.tex . Figures can be obtained on request
at [email protected]
Simulations of the Poynting--Robertson Cosmic Battery in Resistive Accretion Disks
We describe the results of numerical "2.5--dimensional" MHD simulations of an
initially unmagnetized disk model orbiting a central point--mass and responding
to the continual generation of poloidal magnetic field due to a secular source
that emulates the Poynting--Robertson (PR) drag on electrons in the vicinity of
a luminous stellar or compact accreting object. The fluid in the disk and in
the surrounding hotter atmosphere has finite electrical conductivity and allows
for the magnetic field to diffuse freely out of the areas where it is
generated, while at the same time, the differential rotation of the disk twists
the poloidal field and quickly induces a substantial toroidal--field component.
The secular PR term has dual purpose in these simulations as the source of the
magnetic field and the trigger of a magnetorotational instability (MRI) in the
disk. The MRI is especially mild and does not destroy the disk because a small
amount of resistivity dampens the instability efficiently. In simulations with
moderate resistivities (diffusion timescales up to 16 local dynamical
times) and after 100 orbits, the MRI has managed to transfer outward
substantial amounts of angular momentum and the inner edge of the disk, along
with azimuthal magnetic flux, has flowed toward the central point--mass where a
new, magnetized, nuclear disk has formed. The toroidal field in this nuclear
disk is amplified by differential rotation and it cannot be contained; when it
approaches equipartition, it unwinds vertically and produces episodic jet--like
outflows. The poloidal field in the inner region cannot diffuse back out if the
characteristic diffusion time is of the order of or larger than the dynamical
time; it continues to grow linearly in time undisturbed and without saturation,
as the outer sections of many poloidal loops are being drawn radially outward.Comment: 27 pages, 55 figure
Black Hole Entropy from Loop Quantum Gravity
We argue that the statistical entropy relevant for the thermal interactions
of a black hole with its surroundings is (the logarithm of) the number of
quantum microstates of the hole which are distinguishable from the hole's
exterior, and which correspond to a given hole's macroscopic configuration. We
compute this number explicitly from first principles, for a Schwarzschild black
hole, using nonperturbative quantum gravity in the loop representation. We
obtain a black hole entropy proportional to the area, as in the
Bekenstein-Hawking formula.Comment: 5 pages, latex-revtex, no figure
Virtual laboratory instruments and simulations remotely controlled via the internet
Abstract One of the problems of long distance education in engineering is performing laboratory experiments without actually being in the lab. That is perhaps the biggest limitation on the quality of any engineering program that wants to go the long-distance way. Most programs depend on simulations. Although this is possible, the student does not get the opportunity to get a very practical experience during the process. In this paper, we demonstrate how a student, from anywhere in the world, can operate a laboratory equipment such as a network or spectrum analyzer, residing in a remote laboratory, via a regular internet browser. This new approach allows several students, from different parts in the world, to run an experiment together, simultaneously. It also, can allow the sharing of equipment and resources among universities and other laboratories without the physical movement of equipment or researchers
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