28,129 research outputs found
Techniques for studying gravity waves and turbulence: Horizontal, vertical and temporal resolution needed
One of the most important atmospheric measurements that is needed is a measure of the gravity-wave spectrum. The MST radar has been investigated as means to measure the temporal resolution required to determine gravity-wave oscillations. The required vertical and horizontal resolution is dependent on the particular part of the gravity wave spectrum that is analyzed. Horizontal spacing is also discussed
Entrainment coefficient and effective mass for conduction neutrons in neutron star crust: II Macroscopic treatment
Phenomena such as pulsar frequency glitches are believed to be attributable
to differential rotation of a current of ``free'' superfluid neutrons at
densities above the ``drip'' threshold in the ionic crust of a neutron star.
Such relative flow is shown to be locally describable by adaption of a
canonical two fluid treatment that emphasizes the role of the momentum
covectors constructed by differentiation of action with respect to the
currents, with allowance for stratification whereby the ionic number current
may be conserved even when the ionic charge number Z is altered by beta
processes. It is demonstrated that the gauge freedom to make different choices
of the chemical basis determining which neutrons are counted as ``free'' does
not affect their ``superfluid'' momentum covector, which must locally have the
form of a gradient (though it does affect the ``normal'' momentum covector
characterising the protons and those neutrons that are considered to be
``confined'' in the nuclei). It is shown how the effect of ``entrainment''
(whereby the momentum directions deviate from those of the currents) is
controlled by the (gauge independent) mobility coefficient K, estimated in
recent microscopical quantum mechanical investigations, which suggest that the
corresponding (gauge dependent) ``effective mass'' m* of the free neutrons can
become very large in some layers. The relation between this treatment of the
crust layers and related work (using different definitions of ``effective
mass'') intended for the deeper core layers is discussed.Comment: 21 pages Latex. Part II of article whose Part I (Simple microscopic
models) is given by nucl-th/0402057. New version extended to include figure
How Rare Are Extraterrestrial Civilizations and When Did They Emerge?
It is shown that, contrary to an existing claim, the near equality between
the lifetime of the sun and the timescale of biological evolution on earth does
not necessarily imply that extraterrestrial civilizations are exceedingly rare.
Furthermore, on the basis of simple assumptions it is demonstrated that a near
equality between these two timescales may be the most probable relation. A
calculation of the cosmic history of carbon production which is based on the
recently determined history of the star formation rate suggests that the most
likely time for intelligent civilizations to emerge in the universe, was when
the universe was already older then about 10 billion years (for an assumed
current age of about 13 billion years).Comment: 11 pages (including 2 figures), accepted for publication in
Astrophys. Journa
Resonant electric dipole-dipole interactions between cold Rydberg atoms in a magnetic field
Laser cooled Rb atoms were optically excited to 46d Rydberg states. A
microwave pulse transferred a fraction of the atoms to the 47p Rydberg state.
The resonant electric dipole-dipole interactions between atoms in these two
states were probed using the linewidth of the two-photon microwave transition
46d-47d. The presence of a weak magnetic field (approximately 0.5 G) reduced
the observed line broadening, indicating that the interaction is suppressed by
the field. The field removes some of the energy degeneracies responsible for
the resonant interaction, and this is the basis for a quantitative model of the
resulting suppression. A technique for the calibration of magnetic field
strengths using the 34s-34p one-photon transition is also presented.Comment: Accepted for publication in Physical Review
Quantum Algorithm for the Collision Problem
In this note, we give a quantum algorithm that finds collisions in arbitrary
r-to-one functions after only O((N/r)^(1/3)) expected evaluations of the
function. Assuming the function is given by a black box, this is more efficient
than the best possible classical algorithm, even allowing probabilism. We also
give a similar algorithm for finding claws in pairs of functions. Furthermore,
we exhibit a space-time tradeoff for our technique. Our approach uses Grover's
quantum searching algorithm in a novel way.Comment: 8 pages, LaTeX2
Curvature Corrections to Dynamics of Domain Walls
The most usual procedure for deriving curvature corrections to effective
actions for topological defects is subjected to a critical reappraisal. A
logically unjustified step (leading to overdetermination) is identified and
rectified, taking the standard domain wall case as an illustrative example.
Using the appropriately corrected procedure, we obtain a new exact (analytic)
expression for the corresponding effective action contribution of quadratic
order in the wall width, in terms of the intrinsic Ricci scalar and the
extrinsic curvature scalar . The result is proportional to with the
coefficient given by . The resulting form of the ensuing dynamical
equations is obtained in terms of the second fundamental form and the
Dalembertian of its trace, K. It is argued that this does not invalidate the
physical conclusions obtained from the "zero rigidity" ansatz used in
previous work.Comment: 19 pages plain TeX, 2 figures include
Extremal Black Hole/CFT Correspondence in (Gauged) Supergravities
We extend the investigation of the recently proposed Kerr/CFT correspondence
to large classes of rotating black hole solutions in gauged and ungauged
supergravities. The correspondence, proposed originally for four-dimensional
Kerr black holes, asserts that the quantum states in the near-horizon region of
an extremal rotating black hole are holographically dual to a two-dimensional
chiral theory whose Virasoro algebra arises as an asymptotic symmetry of the
near-horizon geometry. In fact in dimension D there are [(D-1)/2] commuting
Virasoro algebras. We consider a general canonical class of near-horizon
geometries in arbitrary dimension D, and show that in any such metric, the
[(D-1)/2] central charges each imply, via the Cardy formula, a microscopic
entropy that agrees with the Bekenstein-Hawking entropy of the associated
extremal black hole. In the remainder of the paper we show for most of the
known rotating black hole solutions of gauged supergravity, and for the
ungauged supergravity solutions with four charges in D=4 and three charges in
D=5, that their extremal near-horizon geometries indeed lie within the
canonical form. This establishes that in all these examples, the microscopic
entropies of the dual CFTs agree with the Bekenstein-Hawking entropies of the
extremal rotating black holes.Comment: 32 pages, references added and minor typos fixe
The dental labour force in Australia: the position and policy directions
The practice of dentistry in Australia is changing. One substantial change is a decrease in visits per year supplied by dentists. At present dental graduate numbers, there will be a widening gap between the capacity of the dental labour force and the population?s demand for dental visits and services. This publication presents an overview of the aggregate shortage of the dental labour force and considers the policy directions to close the supply-demand gap. While both short-term and long-term directions are presented, the focus is on longer term directions for Australia to develop a sustainable self-sufficiency in its dental labour force.John Spencer, Dana Teusner, Knute Carter and David Brenna
Symplectic structure for elastic and chiral conducting cosmic string models
This article is based on the covariant canonical formalism and corresponding
symplectic structure on phase space developed by Witten, Zuckerman and others
in the context of field theory. After recalling the basic principles of this
procedure, we construct the conserved bilinear symplectic current for generic
elastic string models. These models describe current carrying cosmic strings
evolving in an arbitrary curved background spacetime. Particular attention is
paid to the special case of the chiral string for which the worldsheet current
is null. Different formulations of the chiral string action are discussed in
detail, and as a result the integrability property of the chiral string is
clarified.Comment: 18 page
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