219 research outputs found
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
Possible direct method to determine the radius of a star from the spectrum of gravitational wave signals
We computed the spectrum of gravitational waves from a dust disk star of
radius R inspiraling into a Kerr black hole of mass M and specific angular
momentum a. We found that when R is much larger than the wave length of the
quasinormal mode, the spectrum has several peaks and the separation of peaks
is proportional to irrespective of M and a. This
suggests that the radius of the star in coalescing binary black hole - star
systems may be determined directly from the observed spectrum of gravitational
wave. This also suggests that the spectrum of the radiation may give us
important information in gravitational wave astronomy as in optical astronomy.Comment: 4 pages with 3 eps figures, revtex.sty, accepted for publication in
Phys. Rev. Let
Observation of critical phenomena and self-similarity in the gravitational collapse of radiation fluid
We observe critical phenomena in spherical collapse of radiation fluid. A
sequence of spacetimes is numerically computed, containing
models () that adiabatically disperse and models () that
form a black hole. Near the critical point (), evolutions develop a
self-similar region within which collapse is balanced by a strong,
inward-moving rarefaction wave that holds constant as a function of a
self-similar coordinate . The self-similar solution is known and we show
near-critical evolutions asymptotically approaching it. A critical exponent
is found for supercritical () models.Comment: 10 pages (LaTeX) (to appear in Phys. Rev. Lett.), TAR-039-UN
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Intelligent Signal Processing for Detection System Optimization
A wavelet-neural network signal processing method has demonstrated approximately tenfold improvement over traditional signal-processing methods for the detection limit of various nitrogen and phosphorus compounds from the output of a thermionic detector attached to a gas chromatograph. A blind test was conducted to validate the lower detection limit. All fourteen of the compound spikes were detected when above the estimated threshold, including all three within a factor of two above the threshold. In addition, two of six spikes were detected at levels of 1/2 the concentration of the nominal threshold. Another two of the six would have been detected correctly if we had allowed human intervention to examine the processed data. One apparent false positive in five nulls was traced to a solvent impurity, whose presence was subsequently identified by analyzing a solvent aliquot evaporated to 1% residual volume, while the other four nulls were properly classified. We view this signal processing method as broadly applicable in analytical chemistry, and we advocate that advanced signal processing methods should be applied as directly as possible to the raw detector output so that less discriminating preprocessing and post-processing does not throw away valuable signal
The Dynamics of a Meandering River
We present a statistical model of a meandering river on an alluvial plane
which is motivated by the physical non-linear dynamics of the river channel
migration and by describing heterogeneity of the terrain by noise. We study the
dynamics analytically and numerically. The motion of the river channel is
unstable and we show that by inclusion of the formation of ox-bow lakes, the
system may be stabilised. We then calculate the steady state and show that it
is in agreement with simulations and measurements of field data.Comment: Revtex, 12 pages, 2 postscript figure
Head-on collisions of black holes: the particle limit
We compute gravitational radiation waveforms, spectra and energies for a
point particle of mass falling from rest at radius into a
Schwarzschild hole of mass . This radiation is found to lowest order in
with the use of a Laplace transform. In contrast with numerical
relativity results for head-on collisions of equal-mass holes, the radiated
energy is found not to be a monotonically increasing function of initial
separation; there is a local radiated-energy maximum at . The
present results, along with results for infall from infinity, provide a
complete catalog of waveforms and spectra for particle infall. We give a
representative sample from that catalog and an interesting observation: Unlike
the simple spectra for other head-on collisions (either of particle and hole,
or of equal mass holes) the spectra for show a series of
evenly spaced bumps. A simple explanation is given for this. Lastly, our energy
vs. results are compared with approximation methods used elsewhere, for
small and for large initial separation.Comment: 15 pages, REVTeX, 25 figure
Collapse to Black Holes in Brans-Dicke Theory: I. Horizon Boundary Conditions for Dynamical Spacetimes
We present a new numerical code that evolves a spherically symmetric
configuration of collisionless matter in the Brans-Dicke theory of gravitation.
In this theory the spacetime is dynamical even in spherical symmetry, where it
can contain gravitational radiation. Our code is capable of accurately tracking
collapse to a black hole in a dynamical spacetime arbitrarily far into the
future, without encountering either coordinate pathologies or spacetime
singularities. This is accomplished by truncating the spacetime at a spherical
surface inside the apparent horizon, and subsequently solving the evolution and
constraint equations only in the exterior region. We use our code to address a
number of long-standing theoretical questions about collapse to black holes in
Brans-Dicke theory.Comment: 46 pages including figures, uuencoded gz-compressed postscript,
Submitted to Phys Rev
Atomic micromotion and geometric forces in a triaxial magnetic trap
Non-adiabatic motion of Bose-Einstein condensates of rubidium atoms arising
from the dynamical nature of a time-orbiting-potential (TOP) trap was observed
experimentally. The orbital micromotion of the condensate in velocity space at
the frequency of the rotating bias field of the TOP was detected by a
time-of-flight method. A dependence of the equilibrium position of the atoms on
the sense of rotation of the bias field was observed. We have compared our
experimental findings with numerical simulations. The nonadiabatic following of
the atomic spin in the trap rotating magnetic field produces geometric forces
acting on the trapped atoms.Comment: 4 pages, 4 figure
Evolving Einstein's Field Equations with Matter: The ``Hydro without Hydro'' Test
We include matter sources in Einstein's field equations and show that our
recently proposed 3+1 evolution scheme can stably evolve strong-field
solutions. We insert in our code known matter solutions, namely the
Oppenheimer-Volkoff solution for a static star and the Oppenheimer-Snyder
solution for homogeneous dust sphere collapse to a black hole, and evolve the
gravitational field equations. We find that we can evolve stably static,
strong-field stars for arbitrarily long times and can follow dust sphere
collapse accurately well past black hole formation. These tests are useful
diagnostics for fully self-consistent, stable hydrodynamical simulations in 3+1
general relativity. Moreover, they suggest a successive approximation scheme
for determining gravitational waveforms from strong-field sources dominated by
longitudinal fields, like binary neutron stars: approximate quasi-equilibrium
models can serve as sources for the transverse field equations, which can be
evolved without having to re-solve the hydrodynamical equations (``hydro
without hydro'').Comment: 4 postscript figures. Submitted to Phys. Rev. D15 as a Brief Repor
Nonadiabatic Dynamics of Atoms in Nonuniform Magnetic Fields
Dynamics of neutral atoms in nonuniform magnetic fields, typical of
quadrupole magnetic traps, is considered by applying an accurate method for
solving nonlinear systems of differential equations. This method is more
general than the adiabatic approximation and, thus, permits to check the limits
of the latter and also to analyze nonadiabatic regimes of motion. An unusual
nonadiabatic regime is found when atoms are confined from one side of the
z-axis but are not confined from another side. The lifetime of atoms in a trap
in this semi-confining regime can be sufficiently long for accomplishing
experiments with a cloud of such atoms. At low temperature, the cloud is
ellipsoidal being stretched in the axial direction and moving along the z-axis.
The possibility of employing the semi-confining regime for studying the
relative motion of one component through another, in a binary mixture of gases
is discussed.Comment: 1 file, 17 pages, RevTex, 2 table
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