16,035 research outputs found
On the parameters of the Kerr-NUT-(anti-)de Sitter space-time
Different forms of the metric for the Kerr-NUT-(anti-)de Sitter space-time
are being widely used in its extension to higher dimensions. The purpose of
this note is to relate the parameters that are being used to the physical
parameters (mass, rotation, NUT and cosmological constant) in the basic four
dimensional situation.Comment: 4 pages. To appear as a Note in Classical and Quantum Gravit
Energy of gravitational radiation in plane-symmetric space-times
Gravitational radiation in plane-symmetric space-times can be encoded in a
complex potential, satisfying a non-linear wave equation. An effective energy
tensor for the radiation is given, taking a scalar-field form in terms of the
potential, entering the field equations in the same way as the matter energy
tensor. It reduces to the Isaacson energy tensor in the linearized,
high-frequency approximation. An energy conservation equation is derived for a
quasi-local energy, essentially the Hawking energy. A transverse pressure
exerted by interacting low-frequency gravitational radiation is predicted.Comment: 7 REVTeX4 page
Quantum probe and design for a chemical compass with magnetic nanostructures
Magnetic fields as weak as Earth's may affect the outcome of certain
photochemical reactions that go through a radical pair intermediate. When the
reaction environment is anisotropic, this phenomenon can form the basis of a
chemical compass and has been proposed as a mechanism for animal
magnetoreception. Here, we demonstrate how to optimize the design of a chemical
compass with a much better directional sensitivity simply by a gradient field,
e.g. from a magnetic nanostructure. We propose an experimental test of these
predictions, and suggest design principles for a hybrid metallic-organic
chemical compass. In addition to the practical interest in designing a
biomimetic weak magnetic field sensor, our result shows that gradient fields
can server as powerful tools to probe spin correlations in radical pair
reactions.Comment: 8 pages, 6 figures, comments are welcom
Gambling in Great Britain:a response to Rogers
A recent issue of Practice: Social Work in Action featured a paper by Rogers that examined whether the issue of problem gambling was a suitable case for social work. Rogers’ overview was (in various places) out of date, highly selective, contradictory, presented unsupported claims and somewhat misleading. Rogers’ paper is to be commended for putting the issue of problem gambling on the social work agenda. However, social workers need up-to-date information and contextually situated information if they are to make informed decisions in helping problem gamblers
Asymptotic solutions of glass temperature profiles during steady optical fibre drawing
In this paper we derive realistic simplified models for the high-speed drawing of glass optical fibres via the downdraw method, that capture the fluid dynamics and heat transport in the fibre via conduction, convection and radiative heating. We exploit the small aspect ratio of the fibre and the relative orders of magnitude of the dimensionless parameters that characterize the heat transfer to reduce the problem to one- or two-dimensional systems via asymptotic analysis. The resulting equations may be readily solved numerically and in many cases admit exact analytic solutions. The systematic asymptotic breakdown presented is used to elucidate the relative importance of furnace temperature profile, convection, surface radiation and conduction in each portion of the furnace and the role of each in controlling the glass temperature.\ud
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The models derived predict many of the qualitative features observed in the real industrial process, such as the glass temperature profile within the furnace and the sharp transition in fibre thickness. The models thus offer a desirable route to quick scenario testing, providing valuable practical information into the dependencies of the solution on the parameters and the dominant heat-transport mechanism
The Exact Ground State of the Frenkel-Kontorova Model with Repeated Parabolic Potential: II. Numerical Treatment
A procedure is described for efficiently finding the ground state energy and
configuration for a Frenkel-Kontorova model in a periodic potential, consisting
of N parabolic segments of identical curvature in each period, through a
numerical solution of the convex minimization problem described in the
preceding paper. The key elements are the use of subdifferentials to describe
the structure of the minimization problem; an intuitive picture of how to solve
it, based on motion of quasiparticles; and a fast linear optimization method
with a reduced memory requirement. The procedure has been tested for N up to
200.Comment: 9 RevTeX pages, using AMS-Fonts (amssym.tex,amssym.def), 3 Postscript
figures, accepted by Phys.Rev.B to be published together with
cond-mat/970722
Contributions to the Science of Environmental Impact Assessment: Three Papers on the Arctic Cisco (Coregonus autumnalis) of Northern Alaska
Editor's Introduction -- D. W. Norton; An Assessment of the Colville River Delta Stock of Arctic Cisco--Migrants from Canada? -- B. J. Gallaway, W. B. Griffiths, P. C. Craig, W. J. Gazey, and J. W. Helmericks; Temperature Preference of Juvenile Arctic Cisco (Coregonus autumnalis) From the Alaskan Beaufort Sea -- R. G. Fechhelm, W. H. Neill, and B. J. Gallaway; Modeling Movements and Distribution of Arctic Cisco (Coregonus autumnalis) Relative to Temperature-Salinity Regimes of the Beaufort Sea Near the Waterflood Causeway, Prudhoe Bay, Alaska. -- W. H. Neill, R. G. Fechhelm, B. J. Gallaway, J. D. Bryan, and S. W. Anderson; Notice to Author
Optimal Eavesdropping in Quantum Cryptography. II. Quantum Circuit
It is shown that the optimum strategy of the eavesdropper, as described in
the preceding paper, can be expressed in terms of a quantum circuit in a way
which makes it obvious why certain parameters take on particular values, and
why obtaining information in one basis gives rise to noise in the conjugate
basis.Comment: 7 pages, 1 figure, Latex, the second part of quant-ph/970103
Constraints on the distance to SGR 1806-20 from HI absorption
The giant flare detected from the magnetar SGR 1806-20 on 2004 December 27
had a fluence more than 100 times higher than the only two other SGR flares
ever recorded. Whereas the fluence is independent of distance, an estimate for
the luminosity of the burst depends on the source's distance, which has
previously been argued to be ~15 kpc. The burst produced a bright radio
afterglow, against which Cameron et al. (2005) have measured an HI absorption
spectrum. This has been used to propose a revised distance to SGR 1806-20 of
between 6.4 and 9.8 kpc. Here we analyze this absorption spectrum, and compare
it both to HI emission data from the Southern Galactic Plane Survey and to
archival 12-CO survey data. We confirm ~6 kpc, as a likely lower limit on the
distance to SGR 1806-20, but argue that it is difficult to place an upper limit
on the distance to SGR 1806-20 from the HI data currently available. The
previous value of ~15 kpc thus remains the best estimate of the distance to the
source.Comment: 3 pages, 1 embedded EPS figure. Added sentences to end of Abstract
and Conclusion, clarifying that most likely distance is 15 kpc. ApJ Letters,
in pres
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