13,882 research outputs found
Semiclassical Gravity Theory and Quantum Fluctuations
We discuss the limits of validity of the semiclassical theory of gravity in
which a classical metric is coupled to the expectation value of the stress
tensor. It is argued that this theory is a good approximation only when the
fluctuations in the stress tensor are small. We calculate a dimensionless
measure of these fluctuations for a scalar field on a flat background in
particular cases, including squeezed states and the Casimir vacuum state. It is
found that the fluctuations are small for states which are close to a coherent
state, which describes classical behavior, but tend to be large otherwise. We
find in all cases studied that the energy density fluctuations are large
whenever the local energy density is negative. This is taken to mean that the
gravitational field of a system with negative energy density, such as the
Casimir vacuum, is not described by a fixed classical metric but is undergoing
large metric fluctuations. We propose an operational scheme by which one can
describe a fluctuating gravitational field in terms of the statistical behavior
of test particles. For this purpose we obtain an equation of the form of the
Langevin equation used to describe Brownian motion.Comment: In REVTEX. 20pp + 4 figures(not included, available upon request)
TUTP-93-
Letters from L. L. Lewis, Barton O. Aylesworth, and B. C. Buffum
Letters of recommendation for Elmer D. Ball
Large-area CCD imagers for spacecraft applications
Backside illuminated CCD imagers with 100 x 160 resolution elements have been fabricated using double level metal technology. Detailed study of the optical performance of such arrays has been performed between 24 C and -40 C using data rates from 10 kHz to 1 MHz. A 400 x 400 array is presently being fabricated
JABAWS 2.2 Distributed Web Services for Bioinformatics:Protein Disorder, Conservation and RNA Secondary Structure
Novel features of the energy momentum tensor of a Casimir apparatus in a weak gravitational field
The influence of the gravity acceleration on the regularized energy-momentum
tensor of the quantized electromagnetic field between two plane parallel
conducting plates is derived. A perturbative expansion, to first order in the
constant acceleration parameter, of the Green functions involved and of the
energy-momentum tensor is derived by means of the covariant geodesic point
splitting procedure. The energy-momentum tensor is covariantly conserved and
satisfies the expected relation between gauge-breaking and ghost parts.Comment: 8 pages, based on a talk given by Luigi Rosa at the QFEXT07
Conference, Leipzig. Equation (13) and the formulae for rho and energy E
stored in the Casimir device have been amended, jointly with related
discussio
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Release from UNC93B1 reinforces the compartmentalized activation of select TLRs
Nucleic acid-sensing Toll-like receptors (TLRs) are subject to complex regulation to facilitate the recognition of microbial DNA and RNA while limiting the recognition of an organism's own nucleic acids1. Failure to properly regulate these TLRs can lead to autoimmune and autoinflammatory diseases2-6. Intracellular localization of these receptors is thought to be crucial for the discrimination between self and non-self7, but the molecular mechanisms that reinforce compartmentalized activation of intracellular TLRs remain poorly understood. Here we describe a mechanism that prevents the activation of TLR9 from locations other than endosomes. This control is achieved through the regulated release of the receptor from its trafficking chaperone UNC93B1, which occurs only within endosomes and is required for ligand binding and signal transduction. Preventing release of TLR9 from UNC93B1, either by mutations in UNC93B1 that increase affinity for TLR9 or through an artificial tether that impairs release, results in defective signalling. Whereas TLR9 and TLR3 are released from UNC93B1, TLR7 does not dissociate from UNC93B1 in endosomes and is regulated by distinct mechanisms. This work defines a checkpoint that reinforces the compartmentalized activation of TLR9, and provides a mechanism by which activation of individual endosomal TLRs may be distinctly regulated
Fluctuations of the vacuum energy density of quantum fields in curved spacetime via generalized zeta functions
For quantum fields on a curved spacetime with an Euclidean section, we derive
a general expression for the stress energy tensor two-point function in terms
of the effective action. The renormalized two-point function is given in terms
of the second variation of the Mellin transform of the trace of the heat kernel
for the quantum fields. For systems for which a spectral decomposition of the
wave opearator is possible, we give an exact expression for this two-point
function. Explicit examples of the variance to the mean ratio of the vacuum energy density of a
massless scalar field are computed for the spatial topologies of and , with results of , and
respectively. The large variance signifies the importance
of quantum fluctuations and has important implications for the validity of
semiclassical gravity theories at sub-Planckian scales. The method presented
here can facilitate the calculation of stress-energy fluctuations for quantum
fields useful for the analysis of fluctuation effects and critical phenomena in
problems ranging from atom optics and mesoscopic physics to early universe and
black hole physics.Comment: Uses revte
Semiclassical Casimir Energies at Finite Temperature
We study the dependence on the temperature T of Casimir effects for a range
of systems, and in particular for a pair of ideal parallel conducting plates,
separated by a vacuum. We study the Helmholtz free energy, combining
Matsubara's formalism, in which the temperature appears as a periodic Euclidean
fourth dimension of circumference 1/T, with the semiclassical periodic orbital
approximation of Gutzwiller. By inspecting the known results for the Casimir
energy at T=0 for a rectangular parallelepiped, one is led to guess at the
expression for the free energy of two ideal parallel conductors without
performing any calculation. The result is a new form for the free energy in
terms of the lengths of periodic classical paths on a two-dimensional cylinder
section. This expression for the free energy is equivalent to others that have
been obtained in the literature. Slightly extending the domain of applicability
of Gutzwiller's semiclassical periodic orbit approach, we evaluate the free
energy at T>0 in terms of periodic classical paths in a four-dimensional cavity
that is the tensor product of the original cavity and a circle. The validity of
this approach is at present restricted to particular systems. We also discuss
the origin of the classical form of the free energy at high temperatures.Comment: 17 pages, no figures, Late
Mesospheric Density Climatologies Determined at Midlatitudes Using Rayleigh Lidar
The original Rayleigh-scatter lidar that operated at the Atmospheric Lidar Observatory (ALO; 41.7°N, 111.8°W) in the Center for Atmospheric and Space Sciences (CASS) on the campus of Utah State University (USU), collected 11 years of data between 1993 and 2004. From Rayleigh lidar photon-count returns, relative densities throughout the mesosphere, from 45 to 90 km, were determined. Using these relative densities, three climatologies are derived, each using a different density normalization method at 45 km: the first method normalized the relative densities to a constant; the second normalized them to the NRLMSISe00 empirical model; and the third normalized them to the CPC analyses, a first principles, assimilative, meteorological model. From there, the average density profile for each night of the composite year is found by averaging the nighttime density profiles in a multi-year, 31-day window centered on that particular night. From these three density climatologies, some different and many common features in the mesospheric densities are evident. In the future, with improvements to the lidar, it will be possible to provide an absolute normalization for the density profiles
The Energy Density in the Casimir Effect
We compute the expectations of the squares of the electric and magnetic
fields in the vacuum region outside a half-space filled with a uniform
dispersive dielectric. We find a positive energy density of the electromagnetic
field which diverges at the interface despite the inclusion of dispersion in
the calculation. We also investigate the mean squared fields and the energy
density in the vacuum region between two parallel half-spaces. Of particular
interest is the sign of the energy density. We find that the energy density is
described by two terms: a negative position independent (Casimir) term, and a
positive position dependent term with a minimum value at the center of the
vacuum region. We argue that in some cases, including physically realizable
ones, the negative term can dominate in a given region between the two
half-spaces, so the overall energy density can be negative in this region.Comment: 16 pages, 4 figures; 3 references and some new material in Sect. 4.4
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