2,021 research outputs found
Spherically Symmetric Gravitational Collapse of Perfect Fluids
Formulating a perfect fluid filled spherically symmetric metric utilizing the
3+1 formalism for general relativity, we show that the metric coefficients are
completely determined by the mass-energy distribution, and its time rate of
change on an initial spacelike hypersurface. Rather than specifying
Schwarzschild coordinates for the exterior of the collapsing region, we let the
interior dictate the form of the solution in the exterior, and thus both
regions are found to be written in one coordinate patch. This not only
alleviates the need for complicated matching schemes at the interface, but also
finds a new coordinate system for the Schwarzschild spacetime expressed in
generalized Painleve-Gullstrand coordinates.Comment: 3 pages, To appear in the proceedings of the eleventh Marcel
Grossmann meeting on general relativity (MGXI), 23-29 July, 2006, Berli
Cooperative Beamforming for Wireless Ad Hoc Networks
Via collaborative beamforming, nodes in a wireless network are able to
transmit a common message over long distances in an energy efficient fashion.
However, the process of making available the same message to all collaborating
nodes introduces delays. In this paper, a MAC-PHY cross-layer scheme is
proposed that enables collaborative beamforming at significantly reduced
collaboration overhead. It consists of two phases. In the first phase, nodes
transmit locally in a random access time-slotted fashion. Simultaneous
transmissions from multiple source nodes are viewed as linear mixtures of all
transmitted packets. In the second phase, a set of collaborating nodes, acting
as a distributed antenna system, beamform the received analog waveform to one
or more faraway destinations. This step requires multiplication of the received
analog waveform by a complex weight, which is independently computed by each
cooperating node, and which allows packets bound to the same destination to add
coherently at the destination node. Assuming that each node has access to
location information, the proposed scheme can achieve high throughput, which in
certain cases exceeds one. An analysis of the symbol error probability
corresponding to the proposed scheme is provided.Comment: 5 pages, 4 figures. To appear in the Proceedings of the IEEE Global
Communications Conference (GLOBECOM), Washington, DC, November 26 - 30, 200
Gravitational collapse of plasmas in General Relativity
We provide a covariant derivation of plasma physics coupled to gravitation by
utilizing the 3+1 formulation of general relativity, including a discussion of
the Lorentz force law. We then reduce the system to the spherically symmetric
case and show that all regions of the spacetime can be represented in a single
coordinate system, thus revoking the need for junction conditions. We further
show that the region exterior to the collapsing region is naturally described
by the charged Vaidya spacetime in non-null coordinates.Comment: Talk given at the Spanish Relativity Meeting, Tenerife, September
200
Universal velocity distributions in an experimental granular fluid
We present experimental results on the velocity statistics of a uniformly
heated granular fluid, in a quasi-2D configuration. We find the base state, as
measured by the single particle velocity distribution , to be universal
over a wide range of filling fractions and only weakly dependent on all other
system parameters. There is a consistent overpopulation in the distribution's
tails, which scale as . More
importantly, the high probability central region of , at low velocities,
deviates from a Maxwell-Boltzmann by a second order Sonine polynomial with a
single adjustable parameter, in agreement with recent theoretical analysis of
inelastic hard spheres driven by a stochastic thermostat. To our knowledge,
this is the first time that Sonine deviations have been measured in an
experimental system.Comment: 13 pages, 15 figures, with minor corrections, submitted to Phys. Rev.
Energy non-equipartition in systems of inelastic, rough spheres
We calculate and verify with simulations the ratio between the average
translational and rotational energies of systems with rough, inelastic
particles, either forced or freely cooling. The ratio shows non-equipartition
of energy. In stationary flows, this ratio depends mainly on the particle
roughness, but in nonstationary flows, such as freely cooling granular media,
it also depends strongly on the normal dissipation. The approach presented here
unifies and simplifies different results obtained by more elaborate kinetic
theories. We observe that the boundary induced energy flux plays an important
role.Comment: 4 pages latex, 4 embedded eps figures, accepted by Phys Rev
Studies on women's sexuality in China since 1980: A critical review
Over the last 20 years, women's sexuality has become one of the most written about and intensely debated subjects in sexology, sociology, and women's studies in mainland China. From these studies, one can trace the development of academic discourse on women's sexuality through three distinct phases: from a predominantly medical discourse, to the use of survey methods to investigate women's real experiences, and finally to a phase in which Western theories have become increasingly influential. Each phase features a distinct stereotype of woman. The implications of this review for sexual research in China will also be discussed.published_or_final_versio
Gravitational collapse of spherically symmetric plasmas in Einstein-Maxwell spacetimes
We utilize a recent formulation of a spherically symmetric spacetime endowed
with a general decomposition of the energy momentum tensor [Phys. Rev. D, 75,
024031 (2007)] to derive equations governing spherically symmetric
distributions of electromagnetic matter. We show the system reduces to the
Reissner-Nordstrom spacetime in general, spherically symmetric coordinates in
the vacuum limit. Furthermore, we show reduction to the charged Vaidya
spacetime in non-null coordinates when certain equations of states are chosen.
A model of gravitational collapse is discussed whereby a charged fluid resides
within a boundary of finite radial extent on the initial hypersurface, and is
allowed to radiate charged particles. Our formalism allows for the discussion
of all regions in this model without the need for complicated matching schemes
at the interfaces between successive regions. As further examples we consider
the collapse of a thin shell of charged matter onto a Reissner-Nordstrom black
hole. Finally, we reduce the entire system of equations to the static case such
that we have the equations for hydrostatic equilibrium of a charged fluid.Comment: Accepted for publication in Phys. Rev.
Dynamics of inelastically colliding rough spheres: Relaxation of translational and rotational energy
We study the exchange of kinetic energy between translational and rotational
degrees of freedom for inelastic collisions of rough spheres. Even if
equipartition holds in the initial state it is immediately destroyed by
collisions. The simplest generalisation of the homogeneous cooling state allows
for two temperatures, characterizing translational and rotational degrees of
freedom separately. For times larger than a crossover frequency, which is
determined by the Enskog frequency and the initial temperature, both energies
decay algebraically like with a fixed ratio of amplitudes, different
from one.Comment: 5 pages, RevTeX, 2 eps figures, slightly expanded discussion, new
figures with dimensionless units, added references, accepted for publication
in PRE as a Rapid Com
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