7,626 research outputs found
When Darwin Met Einstein: Gravitational Lens Inversion with Genetic Algorithms
Gravitational lensing can magnify a distant source, revealing structural
detail which is normally unresolvable. Recovering this detail through an
inversion of the influence of gravitational lensing, however, requires
optimisation of not only lens parameters, but also of the surface brightness
distribution of the source. This paper outlines a new approach to this
inversion, utilising genetic algorithms to reconstruct the source profile. In
this initial study, the effects of image degradation due to instrumental and
atmospheric effects are neglected and it is assumed that the lens model is
accurately known, but the genetic algorithm approach can be incorporated into
more general optimisation techniques, allowing the optimisation of both the
parameters for a lensing model and the surface brightness of the source.Comment: 9 pages, to appear in PAS
Topological Hochschild homology of Thom spectra and the free loop space
We describe the topological Hochschild homology of ring spectra that arise as
Thom spectra for loop maps f: X->BF, where BF denotes the classifying space for
stable spherical fibrations. To do this, we consider symmetric monoidal models
of the category of spaces over BF and corresponding strong symmetric monoidal
Thom spectrum functors. Our main result identifies the topological Hochschild
homology as the Thom spectrum of a certain stable bundle over the free loop
space L(BX). This leads to explicit calculations of the topological Hochschild
homology for a large class of ring spectra, including all of the classical
cobordism spectra MO, MSO, MU, etc., and the Eilenberg-Mac Lane spectra HZ/p
and HZ.Comment: 58 page
Mathematical Analysis and Simulations of the Neural Circuit for Locomotion in Lamprey
We analyze the dynamics of the neural circuit of the lamprey central pattern
generator (CPG). This analysis provides insights into how neural interactions
form oscillators and enable spontaneous oscillations in a network of damped
oscillators, which were not apparent in previous simulations or abstract phase
oscillator models. We also show how the different behaviour regimes
(characterized by phase and amplitude relationships between oscillators) of
forward/backward swimming, and turning, can be controlled using the neural
connection strengths and external inputs.Comment: 4 pages, accepted for publication in Physical Review Letter
"Antiscepticism and Easy Justification" - Ch 5 of Seemings and Epistemic Justification
In this chapter I investigate epistemological consequences of the fact that seeming-based justification is elusive, in the sense that the subject can lose this justification simply by reflecting on her seemings. I argue that since seeming-based justification is elusive, the antisceptical bite of phenomenal conservatism is importantly limited. I also contend that since seeming-based justification has this feature, phenomenal conservatism isnât actually afflicted by easy justification problems
Radio Emission and Particle Acceleration in SN 1993J
The radio light curves of SN 1993J are found to be well fit by a synchrotron
spectrum, suppressed by external free-free absorption and synchrotron
self-absorption. A standard r^-2 circumstellar medium is assumed, and found to
be adequate. The magnetic field and number density of relativistic electrons
behind the shock are determined. The strength of the magnetic field argues
strongly for turbulent amplification behind the shock. The ratio of the
magnetic and thermal energy density behind the shock is ~0.14. Synchrotron and
Coulomb cooling dominate the losses of the electrons. The injected electron
spectrum has a power law index -2.1, consistent with diffusive shock
acceleration, and the number density scales with the thermal electron energy
density. The total energy density of the relativistic electrons is, if
extrapolated to gamma ~ 1, ~ 5x10^-4 of the thermal energy density. The
free-free absorption required is consistent with previous calculations of the
circumstellar temperature of SN 1993J, T_e ~ (2-10)x10^5 K. The relative
importance of free-free absorption, Razin suppression, and the synchrotron
self-absorption effect for other supernovae are briefly discussed. Guidelines
for the modeling and interpretation of VLBI observations are given.Comment: accepted for Ap.
The open future, bivalence and assertion
It is highly intuitive that the future is open and the past is closedâwhereas it is unsettled whether there will be a fourth world war, it is settled that there was a first. Recently, it has become increasingly popular to claim that the intuitive openness of the future implies that contingent statements about the future, such as âthere will be a sea battle tomorrow,â are non-bivalent (neither true nor false). In this paper, we argue that the non-bivalence of future contingents is at odds with our pre-theoretic intuitions about the openness of the future. These are revealed by our pragmatic judgments concerning the correctness and incorrectness of assertions of future contingents. We argue that the pragmatic data together with a plausible account of assertion shows that in many cases we take future contingents to be true (or to be false), though we take the future to be open in relevant respects. It follows that appeals to intuition to support the non-bivalence of future contingents is untenable. Intuition favours bivalence
The Computational Complexity of the Lorentz Lattice Gas
The Lorentz lattice gas is studied from the perspective of computational
complexity theory. It is shown that using massive parallelism, particle
trajectories can be simulated in a time that scales logarithmically in the
length of the trajectory. This result characterizes the ``logical depth" of the
Lorentz lattice gas and allows us to compare it to other models in statistical
physics.Comment: 9 pages, LaTeX, to appear in J. Stat. Phy
Circular Polarization of Water Masers in the Circumstellar Envelopes of Late Type Stars
We present circular polarization measurements of circumstellar H_2O masers.
The circular polarization detected in the (6_{16}-5_{23}) rotational transition
of the H_{2}O maser can be attributed to Zeeman splitting in the intermediate
temperature and density regime. The magnetic fields are derived using a
general, LTE Zeeman analysis as well as a full radiative transfer method
(non-LTE), which includes a treatment of all hyperfine components
simultaneously as well as the effects of saturation and unequal populations of
the magnetic substates. The differences and relevances of these interpretations
are discussed extensively. The field strengths are compared with previous
detections of the magnetic field on the SiO and OH masers. We show that the
magnetic pressure dominates the thermal pressure by a factor of 20 or more.Comment: 15 pages, 15 figures; accepted for publication in A&A; (Abstract
Abridged
An infrared study of galactic OH/IR stars. II. The `GLMP sample' of red oxygen-rich AGB stars
We present optical and near-infrared finding charts taken from the DSS and
2MASS surveys of 94 IRAS sources selected from the GLMP catalogue
(Garc\'{\i}a-Lario 1992), and accurate astrometry (~0.2") for most of them.
Selection criteria were very red IRAS colours representative for OH/IR stars
with optically thick circumstellar shells and the presence of variability
according to the IRAS variability index (VAR>50). The main photometric
properties of the stars in this `GLMP sample' are presented, discussed and
compared with the correspondent properties of the `Arecibo sample' of OH/IR
stars studied in Jim\'enez-Esteban et al. (2005a). We find that 37% of the
sample (N=34) have no counterpart in the 2MASS, implying extremely high optical
depths of their shells. Most of the sources identified in the 2MASS are faint
(K>~8) and of very red colour in the near-infrared, as expected. The brightest
2MASS counterpart (K=5.3mag) was found for IRAS 18299--1705. Its blue colour
H--K=1.3 suggests that IRAS 8299--1705 is a post-AGB star. A couple of GLMP
sources have faint but relatively blue counterparts. They might be misidentifed
field stars or stars hich experienced recently a drop of their mass loss rates.
The `GLMP sample' in general is made of oxygen-rich AGB stars, which are highly
obscured by their circumstellar shells. They belong to the same population as
the reddest OH/IR stars in the `Arecibo sample'.Comment: 15 pages, 9 figures, for associated finding charts see:
http://www.hs.uni-hamburg.de/DE/Ins/Per/Jimenez-Esteban/Thesis/Atlas/glmp_ind
ex.html accepted for publicating by A&A on 21/07/200
Relating imperatives to action
The aim of this chapter is to provide an analysis of the use of logically complex imperatives, in particular, imperatives of the form Do A1 or A2 and Do A, if B. We argue for an analysis of imperatives in terms of classical logic which takes into account the influence of background information on imperatives. We show that by doing so one can avoid some counter-intuitive results which have been associated with analyses of imperatives in terms of classical logic. In particular, I address Hamblin's observations concerning rule-like imperatives and Ross' Paradox. The analysis is carried out within an agent-based logical framework. This analysis explicates what it means for an agent to have a successful policy for action with respect to satisfying his or her commitments, where some of these commitments have been introduced as a result of imperative language use
- âŠ