609 research outputs found
Should You Be Allowed to Use Your Cellular Phone While Driving?
Regulation of the use of cellular phones by individuals while driving is now commonplace outside the United States and has been proposed in a number of jurisdictions in the United States. There is growing concern that using cellular phones while driving leads to increases in accidents and fatalities. This paper provides an economic analysis of regulatory options for addressing cellular phone usage by drivers of vehicles. While large uncertainties surrounding both benefits and costs exist, a key conclusion is that banning drivers from using cellular phones is a bad idea. Our best estimate is that the costs of a ban are likely to exceed benefits by about $20 billion annually. Less intrusive regulation, such as requiring the use of a hands-free device that would allow a driver to use both hands for steering also is not likely to be economically justified.
A general variational principle for spherically symmetric perturbations in diffeomorphism covariant theories
We present a general method for the analysis of the stability of static,
spherically symmetric solutions to spherically symmetric perturbations in an
arbitrary diffeomorphism covariant Lagrangian field theory. Our method involves
fixing the gauge and solving the linearized gravitational field equations to
eliminate the metric perturbation variable in terms of the matter variables. In
a wide class of cases--which include f(R) gravity, the Einstein-aether theory
of Jacobson and Mattingly, and Bekenstein's TeVeS theory--the remaining
perturbation equations for the matter fields are second order in time. We show
how the symplectic current arising from the original Lagrangian gives rise to a
symmetric bilinear form on the variables of the reduced theory. If this
bilinear form is positive definite, it provides an inner product that puts the
equations of motion of the reduced theory into a self-adjoint form. A
variational principle can then be written down immediately, from which
stability can be tested readily. We illustrate our method in the case of
Einstein's equation with perfect fluid matter, thereby re-deriving, in a
systematic manner, Chandrasekhar's variational principle for radial
oscillations of spherically symmetric stars. In a subsequent paper, we will
apply our analysis to f(R) gravity, the Einstein-aether theory, and
Bekenstein's TeVeS theory.Comment: 13 pages; submitted to Phys. Rev. D. v2: changed formatting, added
conclusion, corrected sign convention
The fate of the unexpected positive intraoperative cultures after revision total knee arthroplasty
Of a consecutive series of 692 revision total knees at 3 centers, intraoperative cultures were unexpectedly found to be positive in 41 cases (5.9%). Of the 41, 29 (71%) cases had a single positive intraoperative culture and were determined to be a probable false positive based on absence of any other evidence of infection, of which 5 were treated with extended course of intravenous antibiotics after hospital discharge and the remaining 24 received no further treatment. None of these 24 patients manifested any sign of infection at follow-up, averaging 46 months (range, 24-74 months). Twelve patients were determined to have probable type 1 periprosthetic infection, 11 of which were treated with a course of antibiotics. Two of these patients became reinfected within a year. A single positive intraoperative culture after revision total knee arthroplasty does not mandate further treatment in the absence of any other signs of infection
Elemental abundances of major elements in the solar wind as measured in Genesis targets and implications on solar wind fractionation
The UCLA ion microprobe facility is partially supported by a grant from the NSF Instrumentation and Facilities program. V. S. Heber thanks NASA for financial support. This work was supported by grants from the NASA Laboratory Analysis of Returned Samples (LARS) program (NASA LARS 80NSSC17K0025 to D. S. Burnett and A. J. G. Jurewicz). R. Wieler acknowledges the hospitality of Caltech's Division of Geologial and Planetary Sciences during his stay in Pasadena.We present elemental abundance data of C, N, O, Na, Mg, Al, Ca, and Cr in Genesis silicon targets. For Na, Mg, Al, and Ca, data from three different SW regimes are also presented. Data were obtained by backside depth profiling using Secondary Ion Mass Spectrometry. The accuracy of these measurements exceeds those obtained by in-situ observations; therefore the Genesis data provide new insights into elemental fractionation between Sun and solar wind, including differences between solar wind regimes. We integrate previously published noble gas and hydrogen elemental abundances from Genesis targets, as well as preliminary values for K and Fe. The abundances of the solar wind elements measured display the well-known fractionation pattern that correlates with each element's First Ionization Potential (FIP). When normalized either to spectroscopic photospheric solar abundances or to those derived from CI-chondritic meteorites, the fractionation factors of low-FIP elements (K, Na, Al, Ca, Cr, Mg, Fe) are essentially identical within uncertainties, but the data are equally consistent with an increasing fractionation with decreasing FIP. The elements with higher FIPs between ~11 and ~16 eV (C, N, O, H, Ar, Kr, Xe) display a relatively well-defined trend of increasing fractionation with decreasing FIP, if normalized to modern 3D photospheric model abundances. Among the three Genesis regimes, the Fast SW displays the least elemental fractionation for almost all elements (including the noble gases) but differences are modest: for low-FIP elements the precisely measured Fast-Slow SW variations are less than 3%.PostprintPeer reviewe
The prognostic significance of trisomy 4 in acute myeloid leukaemia is dependent on age and additional abnormalities
Using institutional theory to analyse hospital responses to external demands for finance and quality in five European countries
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