3,418 research outputs found
Slice Stretching Effects for Maximal Slicing of a Schwarzschild Black Hole
Slice stretching effects such as slice sucking and slice wrapping arise when
foliating the extended Schwarzschild spacetime with maximal slices. For
arbitrary spatial coordinates these effects can be quantified in the context of
boundary conditions where the lapse arises as a linear combination of odd and
even lapse. Favorable boundary conditions are then derived which make the
overall slice stretching occur late in numerical simulations. Allowing the
lapse to become negative, this requirement leads to lapse functions which
approach at late times the odd lapse corresponding to the static Schwarzschild
metric. Demanding in addition that a numerically favorable lapse remains
non-negative, as result the average of odd and even lapse is obtained. At late
times the lapse with zero gradient at the puncture arising for the puncture
evolution is precisely of this form. Finally, analytic arguments are given on
how slice stretching effects can be avoided. Here the excision technique and
the working mechanism of the shift function are studied in detail.Comment: 16 pages, 4 figures, revised version including a study on how slice
stretching can be avoided by using excision and/or shift
Slice Stretching at the Event Horizon when Geodesically Slicing the Schwarzschild Spacetime with Excision
Slice-stretching effects are discussed as they arise at the event horizon
when geodesically slicing the extended Schwarzschild black-hole spacetime while
using singularity excision. In particular, for Novikov and isotropic spatial
coordinates the outward movement of the event horizon (``slice sucking'') and
the unbounded growth there of the radial metric component (``slice wrapping'')
are analyzed. For the overall slice stretching, very similar late time behavior
is found when comparing with maximal slicing. Thus, the intuitive argument that
attributes slice stretching to singularity avoidance is incorrect.Comment: 5 pages, 2 figures, published version including minor amendments
suggested by the refere
Comment on ``Fragmented Condensate Ground State of Trapped Weakly Interacting Bosons in Two Dimensions"
Recently Liu et al. [PRL 87, 030404 (2001)] examined the lowest state of a
weakly-interacting Bose-Einstein condensate. In addition to other interesting
results, using the method of the pair correlation function, they questioned the
validity of the mean-field picture of the formation of vortices and stated that
the vortices are generated at the center of the cloud. This is in apparent
contradiction to the Gross-Pitaevskii approach, which predicts that the
vortices successively enter the cloud from its outer parts as L/N (where N is
the number of atoms in the trap and hbar(L) is the angular momentum of the
system) increases. We have managed to reproduce the results of Liu et al.
however a more careful analysis presented below confirms the validity of the
mean-field approach.Comment: 1 page, RevTex, 2 figure
Size distribution of sputtered particles from Au nanoislands due to MeV self-ion bombardment
Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100)
substrates, were irradiated with 1.5 MeV Au ions up to a fluence of
ions cm and at incidence angles up to
with respect to the surface normal. The sputtered particles were collected on
carbon coated grids (catcher grid) during ion irradiation and were analyzed
with transmission electron microscopy and Rutherford backscattering
spectrometry. The average sputtered particle size and the areal coverage are
determined from transmission electron microscopy measurements, whereas the
amount of gold on the substrate is found by Rutherford backscattering
spectrometry. The size distributions of larger particles (number of
atoms/particle, 1,000) show an inverse power-law with an exponent of
-1 in broad agreement with a molecular dynamics simulation of ion impact
on cluster targets.Comment: 13 pages, 8 figures, Submitted for publication in JA
Dynamical typicality for initial states with a preset measurement statistics of several commuting observables
We consider all pure or mixed states of a quantum many-body system which
exhibit the same, arbitrary but fixed measurement outcome statistics for
several commuting observables. Taking those states as initial conditions, which
are then propagated by the pertinent Schr\"odinger or von Neumann equation up
to some later time point, and invoking a few additional, fairly weak and
realistic assumptions, we show that most of them still entail very similar
expectation values for any given observable. This so-called dynamical
typicality property thus corroborates the widespread observation that a few
macroscopic features are sufficient to ensure the reproducibility of
experimental measurements despite many unknown and uncontrollable microscopic
details of the system. We also discuss and exemplify the usefulness of our
general analytical result as a powerful numerical tool
Vortices in Bose-Einstein condensates - finite-size effects and the thermodynamic limit
For a weakly-interacting Bose gas rotating in a harmonic trap we relate the
yrast states of small systems (that can be treated exactly) to the
thermodynamic limit (derived within the mean-field approximation). For a few
dozens of atoms, the yrast line shows distinct quasi-periodic oscillations with
increasing angular momentum that originate from the internal structure of the
exact many-body states. These finite-size effects disappear in the
thermodynamic limit, where the Gross-Pitaevskii approximation provides the
exact energy to leading order in the number of particles N. However, the exact
yrast states reveal significant structure not captured by the mean-field
approximation: Even in the limit of large N, the corresponding mean-field
solution accounts for only a fraction of the total weight of the exact quantum
state.Comment: Phys Rev A, in pres
Facilitated movement of inertial Brownian motors driven by a load under an asymmetric potential
Based on recent work [L. Machura, M. Kostur, P. Talkner, J. Luczka, and P.
Hanggi, Phys. Rev. Lett. 98, 040601 (2007)], we extend the study of inertial
Brownian motors to the case of an asymmetric potential. It is found that some
transport phenomena appear in the presence of an asymmetric potential. Within
tailored parameter regimes, there exists two optimal values of the load at
which the mean velocity takes its maximum, which means that a load can
facilitate the transport in the two parameter regimes. In addition, the
phenomenon of multiple current reversals can be observed when the load is
increased.Comment: 7 pages, 3 figure
Phase transitions in optimal unsupervised learning
We determine the optimal performance of learning the orientation of the
symmetry axis of a set of P = alpha N points that are uniformly distributed in
all the directions but one on the N-dimensional sphere. The components along
the symmetry breaking direction, of unitary vector B, are sampled from a
mixture of two gaussians of variable separation and width. The typical optimal
performance is measured through the overlap Ropt=B.J* where J* is the optimal
guess of the symmetry breaking direction. Within this general scenario, the
learning curves Ropt(alpha) may present first order transitions if the clusters
are narrow enough. Close to these transitions, high performance states can be
obtained through the minimization of the corresponding optimal potential,
although these solutions are metastable, and therefore not learnable, within
the usual bayesian scenario.Comment: 9 pages, 8 figures, submitted to PRE, This new version of the paper
contains one new section, Bayesian versus optimal solutions, where we explain
in detail the results supporting our claim that bayesian learning may not be
optimal. Figures 4 of the first submission was difficult to understand. We
replaced it by two new figures (Figs. 4 and 5 in this new version) containing
more detail
Hexagons become second if symmetry is broken
Pattern formation on the free surface of a magnetic fluid subjected to a
magnetic field is investigated experimentally. By tilting the magnetic field
the symmetry can be broken in a controllable manner. When increasing the
amplitude of the tilted field, the flat surface gives way to liquid ridges. A
further increase results in a hysteretic transition to a pattern of stretched
hexagons. The instabilities are detected by means of a linear array of magnetic
hall sensors and compared with theoretical predictions.Comment: accepted for publication by Physical Review E/Rapid Communicatio
Circular 78
Historically, sales of exotic meats have been limited only by supply. As supply has increased in recent years, national
and international exotic game markets have grown rapidly. In the United States, growth
has occurred primarily in the restaurant section, although over-the-counter sales
have also increased.
The Alaskan reindeer industry is exploring the potential of expanding its meat sales as well as antler sales. Meat production increased from 320,000 pounds in 1987 to 432,000 pounds in 1988. This production increase is reflected in a 27
percent increase in dollar value (Alaska Crop and Livestock Reporting Service, 1989). Under current management procedures, potential meat production has been estimated at 500,000 pounds (Pearson and Lewis, 1988). Any future market expansion
is likely to occur in urban Alaska and in areas outside the state (Jones, 1988)
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