19,078 research outputs found
Temperature reducing coating for metals subject to flame exposure Patent
Anodizing method for providing metal surfaces with temperature reducing coatings against flame
Development of a magneforming process for the fabrication of thin-wall tungsten cylinders final report
Magneforming process - high energy rate metal forming technique for fabrication of thin wall tungsten cylinder
Superpotentials from variational derivatives rather than Lagrangians in relativistic theories of gravity
The prescription of Silva to derive superpotential equations from variational
derivatives rather than from Lagrangian densities is applied to theories of
gravity derived from Lovelock Lagrangians in the Palatini representation.
Spacetimes are without torsion and isolated sources of gravity are minimally
coupled. On a closed boundary of spacetime, the metric is given and the
connection coefficients are those of Christoffel. We derive equations for the
superpotentials in these conditions. The equations are easily integrated and we
give the general expression for all superpotentials associated with Lovelock
Lagrangians. We find, in particular, that in Einstein's theory, in any number
of dimensions, the superpotential, valid at spatial and at null infinity, is
that of Katz, Bicak and Lynden-Bell, the KBL superpotential. We also give
explicitly the superpotential for Gauss-Bonnet theories of gravity. Finally, we
find a simple expression for the superpotential of Einstein-Gauss-Bonnet
theories with an anti-de Sitter background: it is minus the KBL superpotential,
confirming, as it should, the calculation of the total mass-energy of spacetime
at spatial infinity by Deser and Tekin.Comment: Scheduled to appear in Class. Quantum Grav. August 200
Parametrically excited "Scars" in Bose-Einstein condensates
Parametric excitation of a Bose-Einstein condensate (BEC) can be realized by
periodically changing the interaction strength between the atoms. Above some
threshold strength, this excitation modulates the condensate density. We show
that when the condensate is trapped in a potential well of irregular shape,
density waves can be strongly concentrated ("scarred") along the shortest
periodic orbits of a classical particle moving within the confining potential.
While single-particle wave functions of systems whose classical counterpart is
chaotic may exhibit rich scarring patterns, in BEC, we show that nonlinear
effects select mainly those scars that are locally described by stripes.
Typically, these are the scars associated with self retracing periodic orbits
that do not cross themselves in real space. Dephasing enhances this behavior by
reducing the nonlocal effect of interference
Gravitational energy
Observers at rest in a stationary spacetime flat at infinity can measure
small amounts of rest-mass+internal energies+kinetic energies+pressure energy
in a small volume of fluid attached to a local inertial frame. The sum of these
small amounts is the total "matter energy" for those observers. The total
mass-energy minus the matter energy is the binding gravitational energy.
Misner, Thorne and Wheeler evaluated the gravitational energy of a
spherically symmetric static spacetime. Here we show how to calculate
gravitational energy in any static and stationary spacetime for isolated
sources with a set of observers at rest.
The result of MTW is recovered and we find that electromagnetic and
gravitational 3-covariant energy densities in conformastatic spacetimes are of
opposite signs. Various examples suggest that gravitational energy is negative
in spacetimes with special symmetries or when the energy-momentum tensor
satisfies usual energy conditions.Comment: 12 pages. Accepted for publication in Class. Quantum Gra
The growth of galaxies in cosmological simulations of structure formation
We use hydrodynamic simulations to examine how the baryonic components of
galaxies are assembled, focusing on the relative importance of mergers and
smooth accretion in the formation of ~L_* systems. In our primary simulation,
which models a (50\hmpc)^3 comoving volume of a Lambda-dominated cold dark
matter universe, the space density of objects at our (64-particle) baryon mass
resolution threshold, M_c=5.4e10 M_sun, corresponds to that of observed
galaxies with L~L_*/4. Galaxies above this threshold gain most of their mass by
accretion rather than by mergers. At the redshift of peak mass growth, z~2,
accretion dominates over merging by about 4:1. The mean accretion rate per
galaxy declines from ~40 M_sun/yr at z=2 to ~10 M_sun/yr at z=0, while the
merging rate peaks later (z~1) and declines more slowly, so by z=0 the ratio is
about 2:1. We cannot distinguish truly smooth accretion from merging with
objects below our mass resolution threshold, but extrapolating our measured
mass spectrum of merging objects, dP/dM ~ M^a with a ~ -1, implies that
sub-resolution mergers would add relatively little mass. The global star
formation history in these simulations tracks the mass accretion rate rather
than the merger rate. At low redshift, destruction of galaxies by mergers is
approximately balanced by the growth of new systems, so the comoving space
density of resolved galaxies stays nearly constant despite significant mass
evolution at the galaxy-by-galaxy level. The predicted merger rate at z<~1
agrees with recent estimates from close pairs in the CFRS and CNOC2 redshift
surveys.Comment: Submitted to ApJ, 35 pp including 15 fig
Echo spectroscopy of bulk Bogoliubov excitations in trapped Bose-Einstein condensates
We propose and demonstrate an echo method to reduce the inhomogeneous
linewidth of Bogoliubov excitations, in a harmonically-trapped Bose-Einstein
condensate. Our proposal includes the transfer of excitations with momentum +q
to -q using a double two photon Bragg process, in which a substantial reduction
of the inhomogeneous broadening is calculated. Furthermore, we predict an
enhancement in the method's efficiency for low momentum due to many-body
effects. The echo can also be implemented by using a four photon process, as is
demonstrated experimentally.Comment: 4 pages, 5 figure
What's age got to do with it? Supreme Court appointees and the long run location of the Supreme Court median justice
For approximately the past forty years, Republican Presidents have appointed younger Justices than have Democratic Presidents. Depending on how one does the accounting, the average age difference will vary, but will not go away. This Article posits that Republicans appointing younger justices than Democrats may have caused a rightward shift in the Supreme Court. We use computer simulations to show that if the trend continues the rightward shift will likely increase. We also produce some very rough estimates of the size of the ideological shift, contingent on the size of the age differential. In addition, we show that the Senate’s role in confirming nominated Justices has a significant moderating effect on the shift. Last, we consider the interaction between our results and the oft-proposed eighteen year staggered terms for Supreme Court Justices. We show that such an institutional change would almost completely wipe out the ideological effect of one Party appointing younger Justices
What’s age to do with it? Supreme Court appointees and the long run location of the Supreme Court median justice
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