2,978 research outputs found
Limitations of the Standard Gravitational Perfect Fluid Paradigm
We show that the standard perfect fluid paradigm is not necessarily a valid
description of a curved space steady state gravitational source. Simply by
virtue of not being flat, curved space geometries have to possess intrinsic
length scales, and such length scales can affect the fluid structure. For modes
of wavelength of order or greater than such scales eikonalized geometrical
optics cannot apply and rays are not geodesic. Covariantizing thus entails not
only the replacing of flat space functions by covariant ones, but also the
introduction of intrinsic scales that were absent in flat space. In principle
it is thus unreliable to construct the curved space energy-momentum tensor as
the covariant generalization of a geodesic-based flat spacetime energy-momentum
tensor. By constructing the partition function as an incoherent average over a
complete set of modes of a scalar field propagating in a curved space
background, we show that for the specific case of a static, spherically
symmetric geometry, the steady state energy-momentum tensor that ensues will in
general be of the form
where the
anisotropic is a symmetric, traceless rank two tensor which
obeys . Such a type term is absent for an
incoherently averaged steady state fluid in a spacetime where there are no
intrinsic length scales, and in principle would thus be missed in a
covariantizing of a flat spacetime . While the significance of such
type terms would need to be evaluated on a case by case basis,
through the use of kinetic theory we reassuringly find that the effect of such
type terms is small for weak gravity stars where perfect fluid
sources are commonly used.Comment: Final version to appear in General Relativity and Gravitation (the
final publication is available at http://www.springerlink.com). 29 pages, 1
figur
Nondegenerate Two-Photon Absorption in GaAs/AlGaAs Multiple Quantum Well Waveguides
We present femtosecond pump-probe measurements of the nondegenerate
(1960nm excitation and 1176--1326nm probe) two-photon absorption
spectra of 8 nm GaAs/12nm AlGaAs quantum well
waveguides. Experiments were performed with light pulses co-polarized normal
and tangential to the quantum well plane. The results are compared to
perturbative calculations of transition rates between states determined by the
method with an 8 or 14 band basis. We find
excellent agreement between theory and experiment for normal polarization, then
use the model to support predictions of orders-of-magnitude enhancement of
nondegenerate two-photon absorption as one constituent photon energy nears an
intersubband resonance.Comment: 13 pages, 8 figure
Sensing Art and Artifacts: Explorations in Sensory Museology
This article proposes a sensory studies methodology for the interpretation of museum objects. The proposed method unfolds in two phases: virtual encounter via an on-line catalog and actual exposure in the context of a handling workshop. In addition to exploring the écart between image and object, the “Sensing Art and Artifacts” exercise articulates a framework for arriving at a multisensory, cross-cultural, interactive understanding of aesthetic value. The case studies presented here involve four objects from the collection of the Hunterian Museum as sensed and interpreted by scholars of psychology, philosophy, sociology, and anthropology. It is proposed that aesthetic judgment in the expanded (cross-cultural) sense contemplated here involves apprehending the museum object through multiple sensory modalities in place of the conventional Western fixation on visible form
Analysis of sounding data from Porto Santo island during ASTEX
November 7, 1992.Includes bibliographical references.We present an analysis of the ASTEX radiosonde data obtained on Porto Santo Island during 1-28 June 1992. The data consists of 203 soundings and is available in two forms-Level I and Level II. The Level I data is the raw data produced by the real time software of the AIR radiosonde system. This data is at irregular pressure and height levels. Level II data consists of processed thermodynamic and wind data at a uniform vertical resolution of 10m, which essentially retains the highest possible vertical resolution in the original raw data. Plots of temperature and dew point temperature are presented for each of the 203 soundings taken during ASTEX. Plots of the 28 day means and of the diurnal variations are also presented. Accessibility of this data via anonymous ftp is also discussed
Epidemic Levels of Drug Resistant Tuberculosis (MDR and XDR-TB) in a High HIV Prevalence Setting in Khayelitsha, South Africa
BACKGROUND: Although multidrug-resistant tuberculosis (MDR-TB) is emerging as a significant threat to tuberculosis control in high HIV prevalence countries such as South Africa, limited data is available on the burden of drug resistant tuberculosis and any association with HIV in such settings. We conducted a community-based representative survey to assess the MDR-TB burden in Khayelitsha, an urban township in South Africa with high HIV and TB prevalence. METHODOLOGY/PRINCIPAL FINDINGS: A cross-sectional survey was conducted among adult clinic attendees suspected for pulmonary tuberculosis in two large primary care clinics, together constituting 50% of the tuberculosis burden in Khayelitsha. Drug susceptibility testing (DST) for isoniazid and rifampicin was conducted using a line probe assay on positive sputum cultures, and with culture-based DST for first and second-line drugs. Between May and November 2008, culture positive pulmonary tuberculosis was diagnosed in 271 new and 264 previously treated tuberculosis suspects (sample enriched with previously treated cases). Among those with known HIV status, 55% and 71% were HIV infected respectively. MDR-TB was diagnosed in 3.3% and 7.7% of new and previously treated cases. These figures equate to an estimated case notification rate for MDR-TB of 51/100,000/year, with new cases constituting 55% of the estimated MDR-TB burden. HIV infection was not significantly associated with rifampicin resistance in multivariate analyses. CONCLUSIONS/SIGNIFICANCE: There is an extremely high burden of MDR-TB in this setting, most likely representing ongoing transmission. These data highlight the need to diagnose drug resistance among all TB cases, and for innovative models of case detection and treatment for MDR-TB, in order to interrupt transmission and control this emerging epidemic
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Simulating economic effects of disruptions in the telecommunications infrastructure.
CommAspen is a new agent-based model for simulating the interdependent effects of market decisions and disruptions in the telecommunications infrastructure on other critical infrastructures in the U.S. economy such as banking and finance, and electric power. CommAspen extends and modifies the capabilities of Aspen-EE, an agent-based model previously developed by Sandia National Laboratories to analyze the interdependencies between the electric power system and other critical infrastructures. CommAspen has been tested on a series of scenarios in which the communications network has been disrupted, due to congestion and outages. Analysis of the scenario results indicates that communications networks simulated by the model behave as their counterparts do in the real world. Results also show that the model could be used to analyze the economic impact of communications congestion and outages
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