27,874 research outputs found
The Equivalence Principle as a Stepping Stone from Special to General Relativity: A Socratic Dialog
In this paper we show how the student can be led to an understanding of the
connection between special relativity and general relativity by considering the
time dilation effect of clocks placed on the surface of the Earth. This paper
is written as a Socratic dialog between a lecturer Sam and a student Kim.Comment: 8 pages, 1 figure, uses the revtex4 documentclass. Submitted to the
American Journal of Physics. Minor modification and corrections following
referees' comment
Teleological Essentialism
Placeholder essentialism is the view that there is a causal essence that holds category members together, though we may not know what the essence is. Sometimes the placeholder can be filled in by scientific essences, such as when we acquire scientific knowledge that the atomic weight of gold is 79. We challenge the view that placeholders are elaborated by scientific essences. On our view, if placeholders are elaborated, they are elaborated Aristotelian essences, a telos. Utilizing the same kinds of experiments used by traditional essentialists—involving superficial change (study 1), transformation of insides (study 2), acquired traits (study 3) and inferences about offspring (study 4)—we find support for the view that essences are elaborated by a telos. And we find evidence (study 5) that teleological essences may generate category judgments
Mapping of serotype-specific, immunodominant epitopes in the NS-4 region of hepatitis C virus (HCV):use of type-specific peptides to serologically differentiate infections with HCV types 1, 2, and 3
The effect of sequence variability between different types of hepatitis C virus (HCV) on the antigenicity of the NS-4 protein was investigated by epitope mapping and by enzyme-linked immunosorbent assay with branched oligopeptides. Epitope mapping of the region between amino acid residues 1679 and 1768 in the HCV polyprotein revealed two major antigenic regions (1961 to 1708 and 1710 to 1728) that were recognized by antibody elicited upon natural infection of HCV. The antigenic regions were highly variable between variants of HCV, with only 50 to 60% amino acid sequence similarity between types 1, 2, and 3. Although limited serological cross-reactivity between HCV types was detected between peptides, particularly in the first antigenic region of NS-4, type-specific reactivity formed the principal component of the natural humoral immune response to NS-4. Type-specific antibody to particular HCV types was detected in 89% of the samples from anti-HCV-positive blood donors and correlated almost exactly with genotypic analysis of HCV sequences amplified from the samples by polymerase chain reaction. Whereas almost all blood donors appeared to be infected with a single virus type (97%), a higher proportion of samples (40%) from hemophiliacs infected from transfusion of non-heat-inactivated clotting factor contained antibody to two or even all three HCV types, providing evidence that long-term exposure may lead to multiple infection with different variants of HCV
The role of angular momentum in the construction of electromagnetic multipolar fields
Multipolar solutions of Maxwell's equations are used in many practical
applications and are essential for the understanding of light-matter
interactions at the fundamental level. Unlike the set of plane wave solutions
of electromagnetic fields, the multipolar solutions do not share a standard
derivation or notation. As a result, expressions originating from different
derivations can be difficult to compare. Some of the derivations of the
multipolar solutions do not explicitly show their relation to the angular
momentum operators, thus hiding important properties of these solutions. In
this article, the relation between two of the most common derivations of this
set of solutions is explicitly shown and their relation to the angular momentum
operators is exposed.Comment: 13 pages, 2 figure
Parametrization of the angular correlation and degree of linear polarization in two-photon decays of hydrogen-like ions
The two-photon decay in hydrogen-like ions is investigated within the
framework of second order perturbation theory and Dirac's relativistic
equation. Special attention is paid to the angular correlation of the emitted
photons as well as to the degree of linear polarization of one of the two
photons, if the second is just observed under given angles. Expressions for the
angular correlation and the degree of linear polarization are expanded in terms
of -polynomials, whose coefficients depend on the atomic number and
the energy sharing of the emitted photons. The effects of including higher
(electric and magnetic) multipoles upon the emitted photon pairs beyond the
electric-dipole approximation are also discussed. Calculations of the
coefficients are performed for the transitions ,
and , along the
entire hydrogen isoelectronic sequence ()
The limits of process: On (re)reading Henri Bergson
This article offers a reading of the work of Henri Bergson as it pertains to organizations through the lens of ideas drawn from critical realism. It suggests an alternative to interpretations based on a stark division between process and realist perspectives. Much of the existing literature presents a rather partial view of Bergson’s work. A review suggests some interesting parallels with themes in critical realism, notably the emergence of mind. Critical realism has a focus on process at its heart, but is also concerned with how the products of such processes become stabilized and form the conditions for action. This suggests that attention might usefully be paid to the relationship between organizational action and the sedimented practices grouped under the heading of ‘routines’. More attention to Bergson’s account of the relationship between instinct, intuition and intelligence provides a link to the social character of thought, something which can be mapped on to Archer’s work on reflexivity and the ‘internal conversation’. This suggests that our analyses need to pay attention to both memory and history, to building and dwelling, rather than the one-sided focus found in some process theory accounts
Nonlinearity in a dynamo
Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.Comment: Accepted to Geophys. Astrophys. Fluid Dyna
Intensity enhancement of O VI ultraviolet emission lines in solar spectra due to opacity
Opacity is a property of many plasmas, and it is normally expected that if an
emission line in a plasma becomes optically thick, its intensity ratio to that
of another transition that remains optically thin should decrease. However,
radiative transfer calculations undertaken both by ourselves and others predict
that under certain conditions the intensity ratio of an optically thick to thin
line can show an increase over the optically thin value, indicating an
enhancement in the former. These conditions include the geometry of the
emitting plasma and its orientation to the observer. A similar effect can take
place between lines of differing optical depth. Previous observational studies
have focused on stellar point sources, and here we investigate the
spatially-resolved solar atmosphere using measurements of the I(1032 A)/I(1038
A) intensity ratio of O VI in several regions obtained with the Solar
Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the
Solar and Heliospheric Observatory (SoHO) satellite. We find several I(1032
A)/I(1038 A) ratios observed on the disk to be significantly larger than the
optically thin value of 2.0, providing the first detection (to our knowledge)
of intensity enhancement in the ratio arising from opacity effects in the solar
atmosphere. Agreement between observation and theory is excellent, and confirms
that the O VI emission originates from a slab-like geometry in the solar
atmosphere, rather than from cylindrical structures.Comment: 17 pages, 4 figures, ApJ Letters, in pres
Relativistic total cross section and angular distribution for Rayleigh scattering by atomic hydrogen
We study the total cross section and angular distribution in Rayleigh
scattering by hydrogen atom in the ground state, within the framework of Dirac
relativistic equation and second-order perturbation theory. The relativistic
states used for the calculations are obtained by making use of the finite basis
set method and expressed in terms of B-splines and B-polynomials. We pay
particular attention to the effects that arise from higher (non-dipole) terms
in the expansion of the electron-photon interaction. It is shown that the
angular distribution of scattered photons, while it is symmetric with respect
to the scattering angle =90 within the electric dipole
approximation, becomes asymmetric when higher multipoles are taken into
account. The analytical expression of the angular distribution is parametrized
in terms of Legendre polynomials. Detailed calculations are performed for
photons in the energy range 0.5 to 10 keV. When possible, results are compared
with previous calculations.Comment: 8 pages, 5 figure
A simple description of the states and in
A sixth-order quadrupole boson Hamiltonian is used to describe 26 states
and 67 states which have been recently identified in .
Two closed expressions are alternatively used for energy levels. One
corresponds to a semi-classical approach while the other one represents the
exact eigenvalue of the model Hamiltonian. The semi-classical expression
involves four parameters, while the exact eigenvalue is determined by five
parameters. In each of the two descriptions a least square fit procedure is
adopted.
Both expressions provide a surprisingly good agreement with the experimental
data.Comment: 9 pages, 5 figure
- …