19,309 research outputs found
Elimination sequence optimization for SPAR
SPAR is a large-scale computer program for finite element structural analysis. The program allows user specification of the order in which the joints of a structure are to be eliminated since this order can have significant influence over solution performance, in terms of both storage requirements and computer time. An efficient elimination sequence can improve performance by over 50% for some problems. Obtaining such sequences, however, requires the expertise of an experienced user and can take hours of tedious effort to affect. Thus, an automatic elimination sequence optimizer would enhance productivity by reducing the analysts' problem definition time and by lowering computer costs. Two possible methods for automating the elimination sequence specifications were examined. Several algorithms based on the graph theory representations of sparse matrices were studied with mixed results. Significant improvement in the program performance was achieved, but sequencing by an experienced user still yields substantially better results. The initial results provide encouraging evidence that the potential benefits of such an automatic sequencer would be well worth the effort
Collisions of Shock Waves in General Relativity
We show that the Nariai-Bertotti Petrov type D, homogeneous solution of
Einstein's vacuum field equations with a cosmological constant describes the
space-time in the interaction region following the head-on collision of two
homogeneous, plane gravitational shock waves each initially traveling in a
vacuum containing no cosmological constant. A shock wave in this context has a
step function profile in contrast to an impulsive wave which has a delta
function profile. Following the collision two light-like signals, each composed
of a plane, homogeneous light-like shell of matter and a plane, homogeneous
impulsive gravitational wave, travel away from each other and a cosmological
constant is generated in the interaction region. Furthermore a plane,
light-like signal consisting of an electromagnetic shock wave accompanying a
gravitational shock wave is described with the help of two real parameters, one
for each wave. The head-on collision of two such light-like signals is examined
and we show that if a simple algebraic relation is satisfied between the two
pairs of parameters associated with each incoming light-like signal then the
space-time in the interaction region following the collision is a Bertotti
space-time which is a homogeneous solution of the vacuum Einstein-Maxwell field
equations with a cosmological constant.Comment: Latex file, 10 page
On The Interaction of Gravitational Waves with Magnetic and Electric Fields
The existence of large--scale magnetic fields in the universe has led to the
observation that if gravitational waves propagating in a cosmological
environment encounter even a small magnetic field then electromagnetic
radiation is produced. To study this phenomenon in more detail we take it out
of the cosmological context and at the same time simplify the gravitational
radiation to impulsive waves. Specifically, to illustrate our findings, we
describe the following three physical situations: (1) a cylindrical impulsive
gravitational wave propagating into a universe with a magnetic field, (2) an
axially symmetric impulsive gravitational wave propagating into a universe with
an electric field and (3) a `spherical' impulsive gravitational wave
propagating into a universe with a small magnetic field. In cases (1) and (3)
electromagnetic radiation is produced behind the gravitational wave. In case
(2) no electromagnetic radiation appears after the wave unless a current is
established behind the wave breaking the Maxwell vacuum. In all three cases the
presence of the magnetic or electric fields results in a modification of the
amplitude of the incoming gravitational wave which is explicitly calculated
using the Einstein--Maxwell vacuum field equations.Comment: 15 pages, Latex file, accepted for publication in Physical Review
Coupling Rydberg atoms to microwave fields in a superconducting coplanar waveguide resonator
Rydberg helium atoms traveling in pulsed supersonic beams have been coupled
to microwave fields in a superconducting coplanar waveguide (CPW) resonator.
The atoms were initially prepared in the 1s55s S Rydberg level by
two-color two-photon laser excitation from the metastable 1s2s S level.
Two-photon microwave transitions between the 1s55s S and 1s56s
S levels were then driven by the 19.556 GHz third-harmonic microwave
field in a quarter-wave CPW resonator. This superconducting microwave resonator
was fabricated from niobium nitride on a silicon substrate and operated at
temperatures between 3.65 and 4.30 K. The populations of the Rydberg levels in
the experiments were determined by state-selective pulsed electric field
ionization. The coherence of the atom-resonator coupling was studied by
time-domain measurements of Rabi oscillations.Comment: 6 pages, 5 figure
Excitation and characterization of long-lived hydrogenic Rydberg states of nitric oxide
High Rydberg states of nitric oxide (NO) with principal quantum numbers
between 40 and 100 and lifetimes in excess of 10 s have been prepared by
resonance enhanced two-color two-photon laser excitation from the X
ground state through the A intermediate state.
Molecules in these long-lived Rydberg states were detected and characterized
126 s after laser photoexcitation by state-selective pulsed electric field
ionization. The laser excitation and electric field ionization data were
combined to construct two-dimensional spectral maps. These maps were used to
identify the rotational states of the NO ion core to which the observed
series of long-lived hydrogenic Rydberg states converge. The results presented
pave the way for Rydberg-Stark deceleration and electrostatic trapping
experiments with NO, which are expected to shed further light on the decay
dynamics of these long-lived excited states, and are of interest for studies of
ion-molecule reactions at low temperatures.Comment: 12 pages, 10 figure
Scattering of High Speed Particles in the Kerr Gravitational Field
We calculate the angles of deflection of high speed particles projected in an
arbitrary direction into the Kerr gravitational field. This is done by first
calculating the light-like boost of the Kerr gravitational field in an
arbitrary direction and then using this boosted gravitational field as an
approximation to the gravitational field experienced by a high speed particle.
In the rest frame of the Kerr source the angles of deflection experienced by
the high speed test particle can then easily be evaluated.Comment: 10 pages, Latex file, accepted for publication in Phys. Rev.
Collision of Shock Waves in Einstein-Maxwell Theory with a Cosmological Constant: A Special Solution
Post-collision space-times of the Cartesian product form M'xM'', where M' and
M'' are two-dimensional manifolds, are known with M' and M'' having constant
curvatures of equal and opposite sign (for the collision of electromagnetic
shock waves) or of the same sign (for the collision of gravitational shock
waves). We construct here a new explicit post-collision solution of the
Einstein-Maxwell vacuum field equations with a cosmological constant for which
M' has constant (nonzero) curvature and M'' has zero curvature.Comment: Latex file, 7 page
Colliding Impulsive Gravitational Waves and a Cosmological Constant
We present a space--time model of the collision of two homogeneous, plane
impulsive gravitational waves (each having a delta function profile)
propagating in a vacuum before collision and for which the post collision
space--time has constant curvature. The profiles of the incoming waves are
and where are real constants and are intersecting null hypersurfaces. The cosmological constant
in the post collision region of the space--time is given by .Comment: 12 pages, Latex file, published pape
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