2,402 research outputs found
Sonoluminescence as a QED vacuum effect: Probing Schwinger's proposal
Several years ago Schwinger proposed a physical mechanism for
sonoluminescence in terms of photon production due to changes in the properties
of the quantum-electrodynamic (QED) vacuum arising from a collapsing dielectric
bubble. This mechanism can be re-phrased in terms of the Casimir effect and has
recently been the subject of considerable controversy. The present paper probes
Schwinger's suggestion in detail: Using the sudden approximation we calculate
Bogolubov coefficients relating the QED vacuum in the presence of the expanded
bubble to that in the presence of the collapsed bubble. In this way we derive
an estimate for the spectrum and total energy emitted. We verify that in the
sudden approximation there is an efficient production of photons, and further
that the main contribution to this dynamic Casimir effect comes from a volume
term, as per Schwinger's original calculation. However, we also demonstrate
that the timescales required to implement Schwinger's original suggestion are
not physically relevant to sonoluminescence. Although Schwinger was correct in
his assertion that changes in the zero-point energy lead to photon production,
nevertheless his original model is not appropriate for sonoluminescence. In
other works (see quant-ph/9805023, quant-ph/9904013, quant-ph/9904018,
quant-ph/9905034) we have developed a variant of Schwinger's model that is
compatible with the physically required timescales.Comment: 18 pages, ReV_TeX 3.2, 9 figures. Major revisions: This document is
now limited to providing a probe of Schwinger's original suggestion for
sonoluminescence. For details on our own variant of Schwinger's ideas see
quant-ph/9805023, quant-ph/9904013, quant-ph/9904018, quant-ph/990503
The role of magnetic anisotropy in spin filter junctions
We have fabricated oxide based spin filter junctions in which we demonstrate
that magnetic anisotropy can be used to tune the transport behavior of spin
filter junctions. Until recently, spin filters have been largely comprised of
polycrystalline materials where the spin filter barrier layer and one of the
electrodes are ferromagnetic. These spin filter junctions have relied on the
weak magnetic coupling between one ferromagnetic electrode and a barrier layer
or the insertion of a nonmagnetic insulating layer in between the spin filter
barrier and electrode. We have demonstrated spin filtering behavior in
La0.7Sr0.3MnO3/chromite/Fe3O4 junctions without nonmagnetic spacer layers where
the interface anisotropy plays a significant role in determining transport
behavior. Detailed studies of chemical and magnetic structure at the interfaces
indicate that abrupt changes in magnetic anisotropy across the
non-isostructural interface is the cause of the significant suppression of
junction magnetoresistance in junctions with MnCr2O4 barrier layers.Comment: 7 pages, 7 figure
Strong predictor-corrector euler methods for stochastic differential equations
This paper introduces a new class of numerical schemes for the pathwise approximation of solutions of stochastic differential equations (SDEs). The proposed family of strong predictor-corrector Euler methods are designed to handle scenario simulation of solutions of SDEs. It has the potential to overcome some of the numerical instabilities that are often experienced when using the explicit Euler method. This is of importance, for instance, in finance where martingale dynamics arise for solutions of SDEs with multiplicative diffusion coefficients. Numerical experiments demonstrate the improved asymptotic stability properties of the proposed symmetric predictor-corrector Euler methods. © 2008 World Scientific Publishing Company
Reply to "Can gravitational dynamics be obtained by diffeomorphism invariance of action?"
In a previous work we showed that, in a suitable setting, one can use
diffeomorphism invariance in order to derive gravitational field equations from
boundary terms of the gravitational action. Standing by our results we reply
here to a recent comment questioning their validity.Comment: Accepted for publication in PR
Lorentz Violation for Photons and Ultra-High Energy Cosmic Rays
Lorentz symmetry breaking at very high energies may lead to photon dispersion
relations of the form omega^2=k^2+xi_n k^2(k/M_Pl)^n with new terms suppressed
by a power n of the Planck mass M_Pl. We show that first and second order terms
of size xi_1 > 10^(-14) and xi_2 < -10^(-6), respectively, would lead to a
photon component in cosmic rays above 10^(19) eV that should already have been
detected, if corresponding terms for electrons and positrons are significantly
smaller. This suggests that Lorentz invariance breakings suppressed up to
second order in the Planck scale are unlikely to be phenomenologically viable
for photons.Comment: 4 revtex pages, 3 postscript figures included, version published in
PR
Gravitational dynamics in Bose Einstein condensates
Analogue models for gravity intend to provide a framework where matter and
gravity, as well as their intertwined dynamics, emerge from degrees of freedom
that have a priori nothing to do with what we call gravity or matter. Bose
Einstein condensates (BEC) are a natural example of analogue model since one
can identify matter propagating on a (pseudo-Riemannian) metric with collective
excitations above the condensate of atoms. However, until now, a description of
the "analogue gravitational dynamics" for such model was missing. We show here
that in a BEC system with massive quasi-particles, the gravitational dynamics
can be encoded in a modified (semi-classical) Poisson equation. In particular,
gravity is of extreme short range (characterized by the healing length) and the
cosmological constant appears from the non-condensed fraction of atoms in the
quasi-particle vacuum. While some of these features make the analogue
gravitational dynamics of our BEC system quite different from standard
Newtonian gravity, we nonetheless show that it can be used to draw some
interesting lessons about "emergent gravity" scenarios.Comment: Replaced with published version. 15 pages, no figures, revtex4.
Reference adde
A mixed-methods study of challenges experienced by clinical teams in measuring improvement
Objective: Measurement is an indispensable element of most quality improvement (QI) projects, but it is undertaken to variable standards. We aimed to characterise challenges faced by clinical teams in undertaking measurement in the context of a safety QI programme that encouraged local selection of measures. Methods: Drawing on an independent evaluation of a multi-site improvement programme (Safer Clinical Systems), we combined a qualitative study of participating teams’ experiences and perceptions of measurement with expert review of measurement plans and analysis of data collected for the programme. Multidisciplinary teams of frontline clinicians at nine UK NHS sites took part across the two phases of the programme between 2011 and 2016. Results: Developing and implementing a measurement plan against which to assess their improvement goals was an arduous task for participating sites. The operational definitions of the measures that they selected were often imprecise or missed important details. Some measures used by the teams were not logically linked to the improvement actions they implemented. Regardless of the specific type of data used (routinely collected or selected ex-novo), the burdensome nature of data collection was underestimated. Problems also emerged in identifying and using suitable analytical approaches. Conclusion: Measurement is a highly technical task requiring a degree of expertise. Simply leveraging individual clinicians’ motivation is unlikely to defeat the persistent difficulties experienced by clinical teams when attempting to measure their improvement efforts. We suggest that more structural initiatives and broader capability-building programmes should be pursued by the professional community. Improving access to, and ability to use repositories of validated measures, and increasing transparency in reporting measurement attempts, is likely to be helpful
Hawking-like radiation does not require a trapped region
We discuss the issue of quasi-particle production by ``analogue black holes''
with particular attention to the possibility of reproducing Hawking radiation
in a laboratory. By constructing simple geometric acoustic models, we obtain a
somewhat unexpected result: We show that in order to obtain a stationary and
Planckian emission of quasi-particles, it is not necessary to create a trapped
region in the acoustic spacetime (corresponding to a supersonic regime in the
fluid flow). It is sufficient to set up a dynamically changing flow
asymptotically approaching a sonic regime with sufficient rapidity in
laboratory time.Comment: revtex4, 4 pages, 1 figur
A multi-point distributed random variable accelerator for Monte Carlo simulation in finance
The pricing and hedging of complex derivative securities via Monte Carlo simulations of stochastic differential equations constitutes an intensive computational task. To achieve "real time" execution, as often required by financial institutions, one needs highly efficient implementations of the multi-point distributed random variables underlying the simulations. In this paper a fast and flexible dedicated hardware solution is proposed. A comparative performance analysis demonstrates that the hardware solution is bottleneck-free and flexible, and significantly increases the computational efficiency of the software solution. © 2005 IEEE
Sonoluminescence as a QED vacuum effect. II: Finite Volume Effects
In a companion paper [quant-ph/9904013] we have investigated several
variations of Schwinger's proposed mechanism for sonoluminescence. We
demonstrated that any realistic version of Schwinger's mechanism must depend on
extremely rapid (femtosecond) changes in refractive index, and discussed ways
in which this might be physically plausible. To keep that discussion tractable,
the technical computations in that paper were limited to the case of a
homogeneous dielectric medium. In this paper we investigate the additional
complications introduced by finite-volume effects. The basic physical scenario
remains the same, but we now deal with finite spherical bubbles, and so must
decompose the electromagnetic field into Spherical Harmonics and Bessel
functions. We demonstrate how to set up the formalism for calculating Bogolubov
coefficients in the sudden approximation, and show that we qualitatively retain
the results previously obtained using the homogeneous-dielectric (infinite
volume) approximation.Comment: 23 pages, LaTeX 209, ReV-TeX 3.2, five figure
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