1,443 research outputs found
Mechanics of extended masses in general relativity
The "external" or "bulk" motion of extended bodies is studied in general
relativity. Compact material objects of essentially arbitrary shape, spin,
internal composition, and velocity are allowed as long as there is no direct
(non-gravitational) contact with other sources of stress-energy. Physically
reasonable linear and angular momenta are proposed for such bodies and exact
equations describing their evolution are derived. Changes in the momenta depend
on a certain "effective metric" that is closely related to a non-perturbative
generalization of the Detweiler-Whiting R-field originally introduced in the
self-force literature. If the effective metric inside a self-gravitating body
can be adequately approximated by an appropriate power series, the
instantaneous gravitational force and torque exerted on it is shown to be
identical to the force and torque exerted on an appropriate test body moving in
the effective metric. This result holds to all multipole orders. The only
instantaneous effect of a body's self-field is to finitely renormalize the
"bare" multipole moments of its stress-energy tensor. The MiSaTaQuWa expression
for the gravitational self-force is recovered as a simple application. A
gravitational self-torque is obtained as well. Lastly, it is shown that the
effective metric in which objects appear to move is approximately a solution to
the vacuum Einstein equation if the physical metric is an approximate solution
to Einstein's equation linearized about a vacuum background.Comment: 39 pages, 2 figures; fixed equation satisfied by the Green function
used to construct the effective metri
A conjugate gradient minimisation approach to generating holographic traps for ultracold atoms
Direct minimisation of a cost function can in principle provide a versatile
and highly controllable route to computational hologram generation. However, to
date iterative Fourier transform algorithms have been predominantly used. Here
we show that the careful design of cost functions, combined with numerically
efficient conjugate gradient minimisation, establishes a practical method for
the generation of holograms for a wide range of target light distributions.
This results in a guided optimisation process, with a crucial advantage
illustrated by the ability to circumvent optical vortex formation during
hologram calculation. We demonstrate the implementation of the conjugate
gradient method for both discrete and continuous intensity distributions and
discuss its applicability to optical trapping of ultracold atoms.Comment: 11 pages, 4 figure
Electromagnetic self-forces and generalized Killing fields
Building upon previous results in scalar field theory, a formalism is
developed that uses generalized Killing fields to understand the behavior of
extended charges interacting with their own electromagnetic fields. New notions
of effective linear and angular momenta are identified, and their evolution
equations are derived exactly in arbitrary (but fixed) curved spacetimes. A
slightly modified form of the Detweiler-Whiting axiom that a charge's motion
should only be influenced by the so-called "regular" component of its
self-field is shown to follow very easily. It is exact in some interesting
cases, and approximate in most others. Explicit equations describing the
center-of-mass motion, spin angular momentum, and changes in mass of a small
charge are also derived in a particular limit. The chosen approximations --
although standard -- incorporate dipole and spin forces that do not appear in
the traditional Abraham-Lorentz-Dirac or Dewitt-Brehme equations. They have,
however, been previously identified in the test body limit.Comment: 20 pages, minor typos correcte
Self-forces from generalized Killing fields
A non-perturbative formalism is developed that simplifies the understanding
of self-forces and self-torques acting on extended scalar charges in curved
spacetimes. Laws of motion are locally derived using momenta generated by a set
of generalized Killing fields. Self-interactions that may be interpreted as
arising from the details of a body's internal structure are shown to have very
simple geometric and physical interpretations. Certain modifications to the
usual definition for a center-of-mass are identified that significantly
simplify the motions of charges with strong self-fields. A derivation is also
provided for a generalized form of the Detweiler-Whiting axiom that pointlike
charges should react only to the so-called regular component of their
self-field. Standard results are shown to be recovered for sufficiently small
charge distributions.Comment: 21 page
On the multifractal statistics of the local order parameter at random critical points : application to wetting transitions with disorder
Disordered systems present multifractal properties at criticality. In
particular, as discovered by Ludwig (A.W.W. Ludwig, Nucl. Phys. B 330, 639
(1990)) on the case of diluted two-dimensional Potts model, the moments
of the local order parameter scale with a set
of non-trivial exponents . In this paper, we revisit
these ideas to incorporate more recent findings: (i) whenever a multifractal
measure normalized over space occurs in a random
system, it is crucial to distinguish between the typical values and the
disorder averaged values of the generalized moments , since
they may scale with different generalized dimensions and
(ii) as discovered by Wiseman and Domany (S. Wiseman and E. Domany, Phys Rev E
{\bf 52}, 3469 (1995)), the presence of an infinite correlation length induces
a lack of self-averaging at critical points for thermodynamic observables, in
particular for the order parameter. After this general discussion valid for any
random critical point, we apply these ideas to random polymer models that can
be studied numerically for large sizes and good statistics over the samples. We
study the bidimensional wetting or the Poland-Scheraga DNA model with loop
exponent (marginal disorder) and (relevant disorder). Finally,
we argue that the presence of finite Griffiths ordered clusters at criticality
determines the asymptotic value and the minimal value of the typical multifractal spectrum
.Comment: 17 pages, 20 figure
Palaeontological site conservation and the law in Britain
The legal situation regarding palaeontological site conservation in Britain is unclear. There is no modern review of the law. Five main areas of concern are identified. Most exsisting laws do not specifically consider the needs of palaeontological conservation. Legislation empowers the Nature Conservancy Council upon policy decisions. The NCC is primarily concerned with nationally important sites, and responsibility for recording other sites therefore falls upon voluntary National Scheme for Geological Site Documentation. Local authorities have potentially useful powers. Site occupiers are disadvantaged by the damage caused by, and to some extent the liability due to visitiors, but they can forbid access to almost all sites on private land. The ownership of in situ fossils may be presumed to go with the mineral rights in the land, and collecting them without permission may involve criminal damage and theft. Loose fossils may in some cases be legally collected without express permission. If the landowner has not exerted rights of controil of access or ownership. This is potentially important for coastal exposures. The compulsary public ownership of fossils is not likely to be a successful strategy in geological conservation. Resources are on the whole better spent in education and popularization than on compulsion
Linearity and Scaling of a Statistical Model for the Species Abundance Distribution
We derive a linear recursion relation for the species abundance distribution
in a statistical model of ecology and demonstrate the existence of a scaling
solution
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