67 research outputs found
A Note on Lie-Lorentz Derivatives
The definition of ``Lie derivative'' of spinors with respect to Killing
vectors is extended to all kinds of Lorentz tensors. This Lie-Lorentz
derivative appears naturally in the commutator of two supersymmetry
transformations generated by Killing spinors and vanishes for Vielbeins. It can
be identified as the generator of the action of isometries on supergravity
fields and its use for the calculation of supersymmetry algebras is revised and
extended.Comment: Latex2e, 9 pages, AMS font
The Sun's position in the sky
We express the position of the Sun in the sky as a function of time and the
observer's geographic coordinates. Our method is based on applying rotation
matrices to vectors describing points on the celestial sphere. We also derive
direct expressions, as functions of date of the year and geographic latitude,
for the duration of daylight, the maximum and minimum altitudes of the Sun, and
the cardinal directions to sunrise and sunset. We discuss how to account for
the eccentricity of the earth's orbit, the precessions of the equinoxes and the
perihelion, the size of the solar disk, and atmospheric refraction. We
illustrate these results by computing the dates of "Manhattanhenge" (when
sunset aligns with the east-west streets on the main traffic grid for
Manhattan, in New York City), by plotting the altitude of the Sun over
representative cities as a function of time, and by showing plots ("analemmas")
for the position of the Sun in the sky at a given hour of the day.Comment: 19 pages, 16 figures. v3: Replaced to match published version and to
re-package Mathematica notebook as an ancillary fil
Maxwell's theory on a post-Riemannian spacetime and the equivalence principle
The form of Maxwell's theory is well known in the framework of general
relativity, a fact that is related to the applicability of the principle of
equivalence to electromagnetic phenomena. We pose the question whether this
form changes if torsion and/or nonmetricity fields are allowed for in
spacetime. Starting from the conservation laws of electric charge and magnetic
flux, we recognize that the Maxwell equations themselves remain the same, but
the constitutive law must depend on the metric and, additionally, may depend on
quantities related to torsion and/or nonmetricity. We illustrate our results by
putting an electric charge on top of a spherically symmetric exact solution of
the metric-affine gauge theory of gravity (comprising torsion and
nonmetricity). All this is compared to the recent results of Vandyck.Comment: 9 pages, REVTeX, no figures; minor changes, version to be published
in Class. Quantum Gra
Coherent methods in the X-ray sciences
X-ray sources are developing rapidly and their coherent output is growing
extremely rapidly. The increased coherent flux from modern X-ray sources is
being matched with an associated rapid development in experimental methods.
This article reviews the literature describing the ideas that utilise the
increased brilliance from modern X-ray sources. It explores how ideas in
coherent X-ray science are leading to developments in other areas, and vice
versa. The article describes measurements of coherence properties and uses this
discussion as a base from which to describe partially-coherent diffraction and
X-ray phase contrast imaging, with its applications in materials science,
engineering and medicine. Coherent diffraction imaging methods are reviewed
along with associated experiments in materials science. Proposals for
experiments to be performed with the new X-ray free-electron-lasers are briefly
discussed. The literature on X-ray photon correlation spectroscopy is described
and the features it has in common with other coherent X-ray methods are
identified. Many of the ideas used in the coherent X-ray literature have their
origins in the optical and electron communities and these connections are
explored. A review of the areas in which ideas from coherent X-ray methods are
contributing to methods for the neutron, electron and optical communities is
presented.Comment: A review articel accepted by Advances in Physics. 158 pages, 29
figures, 3 table
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Seasonal migration and homing of channel catfish in the lower Wisconsin River, Wisconsin
A multiyear tag and recapture study was conducted to determine whether channelcatfish (Icalurus punctatus) were migratory and if they had strong homing tendencies. Over10,000 channel catfish were tagged from the lower Wisconsin River and adjacent waters of theupper Mississippi River during the 3-year sampling period. Data on movements were obtainedfrom study recaptures and through tag returns and harvest information provided by sport anglersand commercial fishers. Channel catfish occupied relatively small home ranges during summer,migrated downstream to the upper Mississippi River in autumn, then migrated back up theWisconsin River in spring to spawn and to occupy the same summer home sites they had used inprevious summers. Fish size was a factor in the degree of fidelity to summer home sites, withlarger fish showing greater fidelity
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