5,326 research outputs found
Generalization of the Schott energy in electrodynamic radiation theory
We discuss the origin of the Schott energy in the Abraham-Lorentz version of
electrodynamic radiation theory and how it can be used to explain some apparent
paradoxes. We also derive the generalization of this quantity for the
Ford-O'Connell equation, which has the merit of being derived exactly from a
microscopic Hamiltonian for an electron with structure and has been shown to be
free of the problems associated with the Abraham-Lorentz theory. We emphasize
that the instantaneous power supplied by the applied force not only gives rise
to radiation (acceleration fields), but it can change the kinetic energy of the
electron and change the Schott energy of the velocity fields. The important
role played by boundary conditions is noted
A quantum violation of the second law?
An apparent violation of the second law of thermodynamics occurs when an atom
coupled to a zero-temperature bath, being necessarily in an excited state, is
used to extract work from the bath. Here the fallacy is that it takes work to
couple the atom to the bath and this work must exceed that obtained from the
atom. For the example of an oscillator coupled to a bath described by the
single relaxation time model, the mean oscillator energy and the minimum work
required to couple the oscillator to the bath are both calculated explicitly
and in closed form. It is shown that the minimum work always exceeds the mean
oscillator energy, so there is no violation of the second law
Experiences of food poverty among undocumented parents with children in three European countries: a multi-level research strategy
A growing literature addresses undocumented migrants in different countries, with governmental exclusion from welfare and health services a common theme. However, little is known comparatively about the difference social context makes to the resources available to these migrants in different circumstances or how they manage and experience material deprivation and social exclusion. Adopting a realist approach, this paper draws on a comparative study that examined food poverty in low-income families with children aged 11–15 years in the UK, Portugal and Norway following the 2008 financial crisis. It shows the ways in which the study’s multi-tiered research design enabled the analysis of the complex conditions in which parents sought to sustain and feed their families. Undocumented migrants living in extreme conditions constitute ‘test cases’ for examining the specific resources available (or not) to households in different layers of context and the consequences for the ways in which food and food poverty were experienced by children and parents in these contexts. The paper thus contributes to the methodological literature on comparative research, in particular to research design in the field of migration and to knowledge about an under-researched group in an increasingly hostile Europe
Balmer and Metal Absorption Feature Gradients in M32
Spectra from MDM Observatory are used to assess Lick/IDS feature strength
gradients inside the half-light radius of the compact Local Group elliptical
galaxy M32. All but a few (of 24 measured) indices show a statistically
significant gradient. Comparing with models, the index gradients indicate a
mean age and abundance gradient in the sense that the nucleus is a factor of
2.5 younger and a factor of 0.3 dex more metal-rich than at 1 effective radius.
This conclusion is only weakly dependent on which index combinations are used
and is robust to high accuracy. Stars near the M32 nucleus have a mean age and
heavy element abundance [M/H] of (4.7 Gyr, +0.02), judging from models by
Worthey with variable abundance ratios. This result has very small formal
random errors, although, of course, there is significant age-metallicity
degeneracy along an (age, abundance) line segment from (5.0 Gyr, 0.00) to (4.5
Gyr, +0.05). An abundance pattern of [C/M]=+0.077, [N/M]=-0.13, [Mg/M]=-0.18,
[Fe/M]~0.0, and [Na/M]=+0.12 is required to fit the feature data, with a
fitting precision of about 0.01 dex. Model uncertainties make the accuracies of
these values at least twice the magnitude of the precision. Forcing
scaled-solar abundances does not change the age very much, but it increases the
rms goodness of model-data fit by a factor of 3 and broadens the allowed range
of age to Gyr. The overall abundance pattern contrasts with larger
elliptical galaxies, in which all measurable lighter elements are enhanced
relative to iron and calcium.Comment: 23 pages, 9 figures, Astronomical Journal, in pres
Rotation and Spin in Physics
We delineate the role of rotation and spin in physics, discussing in order
Newtonian classical physics, special relativity, quantum mechanics, quantum
electrodynamics and general relativity. In the latter case, we discuss the
generalization of the Kepler formula to post-Newtonian order )
including spin effects and two-body effects. Experiments which verify the
theoretical results for general relativistic spin-orbit effects are discussed
as well as efforts being made to verify the spin-spin effects
Does the Third Law of Thermodynamics hold in the Quantum Regime?
The first in a long series of papers by John T. Lewis,
G. W. Ford and the present author, considered the problem of the most general
coupling of a quantum particle to a linear passive heat bath, in the course of
which they derived an exact formula for the free energy of an oscillator
coupled to a heat bath in thermal equilibrium at temperature T. This formula,
and its later extension to three dimensions to incorporate a magnetic field,
has proved to be invaluable in analyzing problems in quantum thermodynamics.
Here, we address the question raised in our title viz. Nernst's third law of
thermodynamics
Signatures of the Youngest Starbursts: Optically-thick Thermal Bremsstrahlung Radio Sources in Henize 2-10
VLA radio continuum imaging reveals compact (<8 pc) ~1 mJy radio sources in
the central 5" starburst region of the blue compact galaxy Henize 2-10. We
interpret these radio knots as extremely young, ultra-dense HII regions. We
model their luminosities and spectral energy distributions, finding that they
are consistent with unusually dense HII regions having electron densities, 1500
cm^-3 < n_e < 5000 cm^-3, and sizes of 3-8 pc. Since these H II regions are not
visible in optical images, we propose that the radio data preferentially reveal
the youngest, densest, and most highly obscured starforming events. Energy
considerations imply that each of the five \HII regions contains ~750 O7V
equivalent stars, greater than the number found in 30 Doradus in the LMC. The
high densities imply an over-pressure compared to the typical interstellar
medium so that such objects must be short-lived (<0.5 Myr expansion
timescales). We conclude that the radio continuum maps reveal the very young
(<0.5 Myr) precursors of ``super starclusters'' or ``proto globular clusters''
which are prominent at optical and UV wavelengths in He 2-10. If the
ultra-dense HII regions are typical of those which we predict will be found in
other starbursting systems, then super starclusters spend 15% of their lifetime
in heavily-obscured environments, similar to Galactic ultra-compact HII
regions. This body of work leads us to propose that massive extragalactic star
clusters (i.e. proto globular clusters) with ages <10^6 yr may be most easily
identified by finding compact radio sources with optically-thick thermal
bremsstrahlung spectral signatures.Comment: AASTeX, 8 figures 2 included with psfig in text; other 6 in jpeg
format; Postscript versions of figures may be found at
http://zem.ucolick.org/chip/Research/young_clusters.html -- Accepted for
publication in the Astrophysical Journa
Two Meson Systems with Ginsparg-Wilson Valence Quarks
Unphysical effects associated with finite lattice spacing and partial
quenching may lead to the presence of unphysical terms in chiral extrapolation
formulae. These unphysical terms must then be removed during data analysis
before physical predictions can be made. In this work, we show that through
next-to-leading order, there are no unphysical counterterms in the
extrapolation formulae, expressed in lattice-physical parameters, for meson
scattering lengths in theories with Ginsparg-Wilson valence quarks. Our work
applies to most sea quark discretization, provided that chiral perturbation
theory is a valid approximation. We demonstrate our results with explicit
computations and show that, in favorable circumstances, the extrapolation
formulae do not depend on the unknown constant C_Mix appearing at lowest order
in the mixed action chiral Lagrangian. We show that the I=1 KK scattering
length does not depend on C_Mix in contrast to the I=3/2 K-pi scattering
length. In addition, we show that these observables combined with f_K / f_pi
and the I=2 pi-pi scattering length share only two linearly independent sets of
counterterms, providing a means to test the mixed action theory at one lattice
spacing. We therefore make a prediction for the I=1 KK scattering length.Comment: 21 pages, 2 figures, 2 tables. Version to be published in PRD.
Improved discussion in Sec. III B. Added reference
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