28,749 research outputs found

    Techniques for studying gravity waves and turbulence: Horizontal, vertical and temporal resolution needed

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    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

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    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?

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    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

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    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

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    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

    Extremal Black Hole/CFT Correspondence in (Gauged) Supergravities

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    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

    Curvature Corrections to Dynamics of Domain Walls

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    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 RR and the extrinsic curvature scalar KK. The result is proportional to cK2RcK^2-R with the coefficient given by c2c\simeq 2. 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 c=0c=0 used in previous work.Comment: 19 pages plain TeX, 2 figures include

    Symplectic structure for elastic and chiral conducting cosmic string models

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    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

    Lithostratigraphy of the Late Devonian-Early Carboniferous Horton Group of the Moncton Subbasin, southern New Brunswick

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    The threefold subdivision of the Late Devonian - Early Carboniferous Horton Group in the Moncton Sub-basin of southern New Brunswick conforms to the original description and subsequent adoption of the group elsewhere in the Maritimes Basin. The Horton Group consists of the Memramcook, Albert and Moncton formations, the Moncton Formation having previously been regarded as a group. The Memramcook Formation rests unconformably on pre-Carboniferous basement and is conformably overlain by and in part the lateral facies equivalent of the Albert Formation. The Albert Formation contains five variably developed members (both spatially and, in part, temporally): the Dawson Settlement, Frederick Brook, Hiram Brook, Round Hill and Gautreau members. Of these, the Round Hill and Gautreau members have previously been the subject of much stratigraphic debate but are herein proposed simply as members of the Albert Formation. The formally proposed Moncton Formation is subdivided into a lower Weldon Member and an upper Hillsborough Member; where the contact can actually be defined it is un— conformable. The Albert Formation - Weldon Member contact is conformable and transitional. The Hillsborough Member - Windsor Group contact is marked by an abrupt facies change where Windsor Group strata are marine as distinct from non-marine in origin. RÉSUMÉ Le groupe Horton (Dévonien supéirieur - Carbonifère inférieur) que l'on retrouve dans le sous-bassin de Moncton au sud du Nouveau-Brunswick est divisé en trois formations, conformément à la description originale telle qu'adoptée ailleurs dans le bassin sédimentaire des Maritimes. Le groupe Horton comprend les formations de Memramcook, Albert et Moncton (la formation de Moncton était autrefois considérée comme un groupe). La formation de Memramcook repose en discordance sur un socle pré-Carbonifère et est recouverte de façon concordante par la formation d'Albert. La formation de Memramcook est également en partie un faciès latéral de la formation d'Albert. La formation d'Albert compte cinq membres dont l'emplacement à varié dans l'éspace, et em partie, dans le temps: les membres Dawson Settlement, Frederick Brook, Hiram Brook, Round Hill et Gautreau. Les membres Round Hill et Gautreau, qui par le passe ont soulevé maints débats stratigraphiques, sont ici présentés comme membres de la formation d'Albert. La formation de Moncton, présentée de façon officielle, est divisée en un membre inférieur, Weldon, et un membre supérieur, Hillsborough; la où il peut être indenting, le contact est discordant. Le contact concordant entre la formation d'Albert et le membre Weldon en est Ggalement un de transition. Le contact entre le membre Hillsborough et le groupe Windsor n'est marqué par un brusque changement de faciès que la où les strates du groupe Windsor sont distinctement d'origine marine, plutôt que non-marine. [Traduit par le journal
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