18,444 research outputs found
High energy constraints on Lorentz symmetry violations
Lorentz violation at high energies might lead to non linear dispersion
relations for the fundamental particles. We analyze observational constraints
on these without assuming any a priori equality between the coefficients
determining the amount of Lorentz violation for different particle species. We
focus on constraints from three high energy processes involving photons and
electrons: photon decay, photo-production of electron-positron pairs, and
vacuum Cerenkov radiation. We find that cubic momentum terms in the dispersion
relations are strongly constrained.Comment: 7 pages, 1 figure, Talk presented at CPT01; the Second Meeting on CPT
and Lorentz Symmetry, Bloomington, Indiana, 15-18 Aug. 2001. Minor numerical
error corrected, gamma-decay constraint update
Thermodynamic Measurements Using the Knudsen Cell Technique
The Knudsen cell technique has been used for over a century and is a valuable tool for measurement of vapor pressures and thermodynamic properties. It is based on a small enclosure (~1 cm long x 1 cm diameter) in which a condensed phase/vapor equilibria can be established. A small (<1 mm) orifice on the cell allows sampling of the vapor via a variety of techniques including weight loss, torsion effusion, target collection, and mass spectrometry. Many excellent measurements based on these methods have been reported. However in order to obtain reliable measurements, a variety of factors must be considered. They include proper cell material selection, accurate and uniform temperature control and measurement, and proper sampling of the vapor. Each of these factors are discussed in detail in this chapter. Typically these studies are conducted at high temperatures and it is a challenge to select an inert container material. Recommended materials are discussed and in some cases the container may be used as part of the system under study. Temperature control and measurement is perhaps the most important issue. In most systems, the furnace must be compact yet there can be no temperature gradient in the cell. Temperatures are measured with either a thermocouple or pyrometer and the relative advantages of each are discussed. Sampling method considerations depend on the particular technique. It is essential that all of the vapor or a representative portion of the vapor be sampled. The distribution of the effusate from a Knudsen cell is discussed and sampling positions discussed. Mass spectrometry is often used to study the effusing vapor and the relations between ion current and vapor pressure are discussed
Density-altitude data from 150 rocket flights and 26 searchlight probings, 1947 through 1964
Density and altitude data from rocket flights and searchlight probing
The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores
The basic structure of the solar system is set by the presence of low-mass
terrestrial planets in its inner part and giant planets in its outer part. This
is the result of the formation of a system of multiple embryos with
approximately the mass of Mars in the inner disk and of a few multi-Earth-mass
cores in the outer disk, within the lifetime of the gaseous component of the
protoplanetary disk. What was the origin of this dichotomy in the mass
distribution of embryos/cores? We show in this paper that the classic processes
of runaway and oligarchic growth from a disk of planetesimals cannot explain
this dichotomy, even if the original surface density of solids increased at the
snowline. Instead, the accretion of drifting pebbles by embryos and cores can
explain the dichotomy, provided that some assumptions hold true. We propose
that the mass-flow of pebbles is two-times lower and the characteristic size of
the pebbles is approximately ten times smaller within the snowline than beyond
the snowline (respectively at heliocentric distance and
, where is the snowline heliocentric distance), due to ice
sublimation and the splitting of icy pebbles into a collection of
chondrule-size silicate grains. In this case, objects of original sub-lunar
mass would grow at drastically different rates in the two regions of the disk.
Within the snowline these bodies would reach approximately the mass of Mars
while beyond the snowline they would grow to Earth masses. The
results may change quantitatively with changes to the assumed parameters, but
the establishment of a clear dichotomy in the mass distribution of protoplanets
appears robust, provided that there is enough turbulence in the disk to prevent
the sedimentation of the silicate grains into a very thin layer.Comment: In press in Icaru
Origin of the Thermal Radiation in a Solid-State Analog of a Black-Hole
An effective black-hole-like horizon occurs, for electromagnetic waves in
matter, at a surface of singular electric and magnetic permeabilities. In a
physical dispersive medium this horizon disappears for wave numbers with
. Nevertheless, it is shown that Hawking radiation is still emitted if
free field modes with are in their ground state.Comment: 13 Pages, 3 figures, Revtex with epsf macro
Comment on "Accelerated Detectors and Temperature in (Anti) de Sitter Spaces"
It is shown how the results of Deser and Levin on the response of accelerated
detectors in anti-de Sitter space can be understood from the same general
perspective as other thermality results in spacetimes with bifurcate Killing
horizons.Comment: 5 pages, LaTe
Human factors in design of passenger seats for commercial aircraft: A review
Seat comfort and safety research since the early part of the century is reviewed. The approach blends empirical and theoretical human factors and technical knowledge of seated humans under static and dynamic conditions experienced on commercial aircraft
Burner rig corrosion of SiC at 1000 deg C
Sintered alpha-SiC was examined in both oxidation and hot corrosion with a burner rig at 400 kPa (4 atm) and 1000 C with a flow velocity of 310 ft/sec. Oxidation tests for times to 46 hr produced virtually no attack, whereas tests with 4 ppm Na produced extensive corrosion in 13-1/2 hr. Thick glassy layers composed primarily of sodium silicate formed in the salt corrosion tests. This corrosion attack caused severe pitting of the silicon carbide substrate which led to a 32 percent strength decrease below the as-received material. Parallel furnace tests of Na2SO4/air induced attacked yielded basically similar results with some slight product composition differences. The differences are explained in terms of the continuous sulfate deposition which occurs in a burner rig
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