11,542 research outputs found
Atmospheric effects on remote sensing of non-uniform temperature sources
The equations of transfer, for a plane-parallel scattering atmosphere with a point source of energy on the lower bounding surface, were solved for various values of sensor/point source orientation and optical depths. Applications of this analysis to Skylab and ERTS mission are discussed, and requirements for atmospheric property data and radiation transfer properties are considered
Classical String in Curved Backgrounds
The Mathisson-Papapetrou method is originally used for derivation of the
particle world line equation from the covariant conservation of its
stress-energy tensor. We generalize this method to extended objects, such as a
string. Without specifying the type of matter the string is made of, we obtain
both the equations of motion and boundary conditions of the string. The world
sheet equations turn out to be more general than the familiar minimal surface
equations. In particular, they depend on the internal structure of the string.
The relevant cases are classified by examining canonical forms of the effective
2-dimensional stress-energy tensor. The case of homogeneously distributed
matter with the tension that equals its mass density is shown to define the
familiar Nambu-Goto dynamics. The other three cases include physically relevant
massive and massless strings, and unphysical tahyonic strings.Comment: 12 pages, REVTeX 4. Added a note and one referenc
Speckle-visibility spectroscopy: A tool to study time-varying dynamics
We describe a multispeckle dynamic light scattering technique capable of
resolving the motion of scattering sites in cases that this motion changes
systematically with time. The method is based on the visibility of the speckle
pattern formed by the scattered light as detected by a single exposure of a
digital camera. Whereas previous multispeckle methods rely on correlations
between images, here the connection with scattering site dynamics is made more
simply in terms of the variance of intensity among the pixels of the camera for
the specified exposure duration. The essence is that the speckle pattern is
more visible, i.e. the variance of detected intensity levels is greater, when
the dynamics of the scattering site motion is slow compared to the exposure
time of the camera. The theory for analyzing the moments of the spatial
intensity distribution in terms of the electric field autocorrelation is
presented. It is demonstrated for two well-understood samples, a colloidal
suspension of Brownian particles and a coarsening foam, where the dynamics can
be treated as stationary. However, the method is particularly appropriate for
samples in which the dynamics vary with time, either slowly or rapidly, limited
only by the exposure time fidelity of the camera. Potential applications range
from soft-glassy materials, to granular avalanches, to flowmetry of living
tissue.Comment: review - theory and experimen
Dynamics of test bodies with spin in de Sitter spacetime
We study the motion of spinning test bodies in the de Sitter spacetime of
constant positive curvature. With the help of the 10 Killing vectors, we derive
the 4-momentum and the tensor of spin explicitly in terms of the spacetime
coordinates. However, in order to find the actual trajectories, one needs to
impose the so-called supplementary condition. We discuss the dynamics of
spinning test bodies for the cases of the Frenkel and Tulczyjew conditions.Comment: 11 pages, RevTex forma
Spinning test particles and clock effect in Schwarzschild spacetime
We study the behaviour of spinning test particles in the Schwarzschild
spacetime. Using Mathisson-Papapetrou equations of motion we confine our
attention to spatially circular orbits and search for observable effects which
could eventually discriminate among the standard supplementary conditions
namely the Corinaldesi-Papapetrou, Pirani and Tulczyjew. We find that if the
world line chosen for the multipole reduction and whose unit tangent we denote
as is a circular orbit then also the generalized momentum of the
spinning test particle is tangent to a circular orbit even though and
are not parallel four-vectors. These orbits are shown to exist because the spin
induced tidal forces provide the required acceleration no matter what
supplementary condition we select. Of course, in the limit of a small spin the
particle's orbit is close of being a circular geodesic and the (small)
deviation of the angular velocities from the geodesic values can be of an
arbitrary sign, corresponding to the possible spin-up and spin-down alignment
to the z-axis. When two spinning particles orbit around a gravitating source in
opposite directions, they make one loop with respect to a given static observer
with different arrival times. This difference is termed clock effect. We find
that a nonzero gravitomagnetic clock effect appears for oppositely orbiting
both spin-up or spin-down particles even in the Schwarzschild spacetime. This
allows us to establish a formal analogy with the case of (spin-less) geodesics
on the equatorial plane of the Kerr spacetime. This result can be verified
experimentally.Comment: IOP macros, eps figures n. 2, to appear on Classical and Quantum
gravity, 200
What do implicit measures measure?
We identify several ongoing debates related to implicit measures, surveying prominent views and considerations in each debate. First, we summarize the debate regarding whether performance on implicit measures is explained by conscious or unconscious representations. Second, we discuss the cognitive structure of the operative constructs: are they associatively or propositionally structured? Third, we review debates whether performance on implicit measures reflects traits or states. Fourth, we discuss the question of whether a person’s performance on an implicit measure reflects characteristics of the person who is taking the test or characteristics of the situation in which the person is taking the test. Finally, we survey the debate about the relationship between implicit measures and (other kinds of) behavior
Hydro-without-Hydro Framework for Simulations of Black Hole-Neutron Star Binaries
We introduce a computational framework which avoids solving explicitly
hydrodynamic equations and is suitable to study the pre-merger evolution of
black hole-neutron star binary systems. The essence of the method consists of
constructing a neutron star model with a black hole companion and freezing the
internal degrees of freedom of the neutron star during the course of the
evolution of the space-time geometry. We present the main ingredients of the
framework, from the formulation of the problem to the appropriate computational
techniques to study these binary systems. In addition, we present numerical
results of the construction of initial data sets and evolutions that
demonstrate the feasibility of this approach.Comment: 16 pages, 7 figures. To appear in the Classical and Quantum Gravity
special issue on Numerical Relativit
Scattering of Spinning Test Particles by Plane Gravitational and Electromagnetic Waves
The Mathisson-Papapetrou-Dixon (MPD) equations for the motion of electrically
neutral massive spinning particles are analysed, in the pole-dipole
approximation, in an Einstein-Maxwell plane-wave background spacetime. By
exploiting the high symmetry of such spacetimes these equations are reduced to
a system of tractable ordinary differential equations. Classes of exact
solutions are given, corresponding to particular initial conditions for the
directions of the particle spin relative to the direction of the propagating
background fields. For Einstein-Maxwell pulses a scattering cross section is
defined that reduces in certain limits to those associated with the scattering
of scalar and Dirac particles based on classical and quantum field theoretic
techniques. The relative simplicity of the MPD approach and its use of
macroscopic spin distributions suggests that it may have advantages in those
astrophysical situations that involve strong classical gravitational and
electromagnetic environments.Comment: Submitted to Classical and Quantum Gravity. 12 page
On the motion of spinning test particles in plane gravitational waves
The Mathisson-Papapetrou-Dixon equations for a massive spinning test particle
in plane gravitational waves are analysed and explicit solutions constructed in
terms of solutions of certain linear ordinary differential equations. For
harmonic waves this system reduces to a single equation of Mathieu-Hill type.
In this case spinning particles may exhibit parametric excitation by
gravitational fields. For a spinning test particle scattered by a gravitational
wave pulse, the final energy-momentum of the particle may be related to the
width, height, polarisation of the wave and spin orientation of the particle.Comment: 11 page
An axiomatic approach to electromagnetic and gravitational radiation reaction of particles in curved spacetime
The problem of determining the electromagnetic and gravitational
``self-force'' on a particle in a curved spacetime is investigated using an
axiomatic approach. In the electromagnetic case, our key postulate is a
``comparison axiom'', which states that whenever two particles of the same
charge have the same magnitude of acceleration, the difference in their
self-force is given by the ordinary Lorentz force of the difference in their
(suitably compared) electromagnetic fields. We thereby derive an expression for
the electromagnetic self-force which agrees with that of DeWitt and Brehme as
corrected by Hobbs. Despite several important differences, our analysis of the
gravitational self-force proceeds in close parallel with the electromagnetic
case. In the gravitational case, our final expression for the (reduced order)
equations of motion shows that the deviation from geodesic motion arises
entirely from a ``tail term'', in agreement with recent results of Mino et al.
Throughout the paper, we take the view that ``point particles'' do not make
sense as fundamental objects, but that ``point particle equations of motion''
do make sense as means of encoding information about the motion of an extended
body in the limit where not only the size but also the charge and mass of the
body go to zero at a suitable rate. Plausibility arguments for the validity of
our comparison axiom are given by considering the limiting behavior of the
self-force on extended bodies.Comment: 37 pages, LaTeX with style package RevTeX 3.
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