83,822 research outputs found
Construction of the factorized steady state distribution in models of mass transport
For a class of one-dimensional mass transport models we present a simple and
direct test on the chipping functions, which define the probabilities for mass
to be transferred to neighbouring sites, to determine whether the stationary
distribution is factorized. In cases where the answer is affirmative, we
provide an explicit method for constructing the single-site weight function. As
an illustration of the power of this approach, previously known results on the
Zero-range process and Asymmetric random average process are recovered in a few
lines. We also construct new models, namely a generalized Zero-range process
and a binomial chipping model, which have factorized steady states.Comment: 6 pages, no figure
Random access-random release relay switching matrix
XY relay switching matrix provides complete random access and random release of 400 points. A mercury-wetted bistable relay with independent set and reset coils is the unique feature associated with each point
Cold cathode gauge experiment (ALSEP)
Cold cathode ionization gages were left on the lunar surface as part of ALSEP (Apollo Lunar Surface Experiment Package) on Apollo missions 12, 14, and 15. An instrument prepared for Apollo 13 did not reach the surface because of the abort of that mission. The gages that reached the lunar surface measured the amounts of gas present in the vicinity of the ALSEP sites. The observed daytime gas concentrations were initially about two orders of magnitude greater than the nighttime observations; this was due to contamination of the landing area by the Apollo operations and equipment, and the daytime measurements showed a decrease with time characterized by a time constant of a few months
Proposed Distributed Feedback Crystal Cavities for X-Ray Lasers
The strong interest in the coherent generation and guiding
of x rays is well known. Many papers have recently
appeared treating different concepts of stimulated x-ray
emissions, and x-ray guiding in thin films was achieved.
Crystals were suggested as end reflectors to generate feedback.
Here we suggest a different type of cavity using zeolite
crystals that would guide the emitted x-rays and at
the same time generate the necessary feedback for self-sustained
oscillation
Non-Standard Fermion Propagators from Conformal Field Theory
It is shown that Weyl spinors in 4D Minkowski space are composed of primary
fields of half-integer conformal weights. This yields representations of
fermionic 2-point functions in terms of correlators of primary fields with a
factorized transformation behavior under the Lorentz group. I employ this
observation to determine the general structure of the corresponding Lorentz
covariant correlators by methods similar to the methods employed in conformal
field theory to determine 2- and 3-point functions of primary fields. In
particular, the chiral symmetry breaking terms resemble fermionic 2-point
functions of 2D CFT up to a function of the product of momenta. The
construction also permits for the formulation of covariant meromorphy
constraints on spinors in 3+1 dimensions.Comment: 15 pages, Latex, LMU-TPW 94-1
TRUTH – A Conversation between P F Strawson and Gareth Evans (1973)
This is a transcript of a conversation between P F Strawson and Gareth Evans in 1973, filmed for The Open University. Under the title 'Truth', Strawson and Evans discuss the question as to whether the distinction between genuinely fact-stating uses of language and other uses can be grounded on a theory of truth, especially a 'thin' notion of truth in the tradition of F P Ramsey
Linear multistep methods for integrating reversible differential equations
This paper studies multistep methods for the integration of reversible
dynamical systems, with particular emphasis on the planar Kepler problem. It
has previously been shown by Cano & Sanz-Serna that reversible linear
multisteps for first-order differential equations are generally unstable. Here,
we report on a subset of these methods -- the zero-growth methods -- that evade
these instabilities. We provide an algorithm for identifying these rare
methods. We find and study all zero-growth, reversible multisteps with six or
fewer steps. This select group includes two well-known second-order multisteps
(the trapezoidal and explicit midpoint methods), as well as three new
fourth-order multisteps -- one of which is explicit. Variable timesteps can be
readily implemented without spoiling the reversibility. Tests on Keplerian
orbits show that these new reversible multisteps work well on orbits with low
or moderate eccentricity, although at least 100 steps/radian are required for
stability.Comment: 31 pages, 9 figures, in press at The Astronomical Journa
A "Baedecker" for the Dark Matter Annihilation Signal
We provide a ``Baedecker'' or travel guide to the directions on the sky where
the dark matter annihilation signal may be expected. We calculate the flux of
high energy gamma-rays from annihilation of neutralino dark matter in the
centre of the Milky Way and the three nearest dwarf spheroidals (Sagittarius,
Draco and Canis Major), using realistic models of the dark matter distribution.
Other investigators have used cusped dark halo profiles (such as the
Navarro-Frenk-White) to claim a significant signal. This ignores the
substantial astrophysical evidence that the Milky Way is not dark-matter
dominated in the inner regions. We show that the annihilation signal from the
Galactic Centre falls by two orders of magnitude on substituting a cored dark
matter density profile for a cusped one. The present and future generation of
high energy gamma-ray detectors, whether atmospheric Cerenkov telescopes or
space missions like GLAST, lack the sensitivity to detect any of the
monochromatic gamma-ray annihilation lines. The continuum gamma-ray signal
above 1 GeV and above 50 GeV may however be detectable either from the dwarf
spheroidals or from the Milky Way itself. If the density profiles of the dwarf
spheroidals are cusped, then the best prospects are for detecting Sagittarius
and Canis Major. However, if the dwarf spheroidals have milder, cored profiles,
then the annihilation signal is not detectable. For GLAST, an attractive
strategy is to exploit the wide field of view and observe the Milky Way at
medium latitudes, as suggested by Stoehr et al. This is reasonably robust
against changes in the density profile.Comment: 13 pages, 3 figures, version in press at The Physical Review
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