1,735 research outputs found
Numerical Integration of Nonlinear Wave Equations for General Relativity
A second-order numerical implementation is given for recently derived
nonlinear wave equations for general relativity. The Gowdy T cosmology is
used as a test bed for studying the accuracy and convergence of simulations of
one-dimensional nonlinear waves. The complete freedom in space-time slicing in
the present formulation is exploited to compute in the Gowdy line-element.
Second-order convergence is found by direct comparison of the results with
either analytical solutions for polarized waves, or solutions obtained from
Gowdy's reduced wave equations for the more general unpolarized waves. Some
directions for extensions are discussed.Comment: 19 pages (LaTex), 3 figures (ps
Uniqueness in MHD in divergence form: right nullvectors and well-posedness
Magnetohydrodynamics in divergence form describes a hyperbolic system of
covariant and constraint-free equations. It comprises a linear combination of
an algebraic constraint and Faraday's equations. Here, we study the problem of
well-posedness, and identify a preferred linear combination in this divergence
formulation. The limit of weak magnetic fields shows the slow magnetosonic and
Alfven waves to bifurcate from the contact discontinuity (entropy waves), while
the fast magnetosonic wave is a regular perturbation of the hydrodynamical
sound speed. These results are further reported as a starting point for
characteristic based shock capturing schemes for simulations with
ultra-relativistic shocks in magnetized relativistic fluids.Comment: To appear in J Math Phy
Scattering Lens Resolves sub-100 nm Structures with Visible Light
The smallest structures that conventional lenses are able to optically
resolve are of the order of 200 nm. We introduce a new type of lens that
exploits multiple scattering of light to generate a scanning nano-sized optical
focus. With an experimental realization of this lens in gallium phosphide we
have succeeded to image gold nanoparticles at 97 nm optical resolution. Our
work is the first lens that provides a resolution in the nanometer regime at
visible wavelengths.Comment: 4 pages, 3 figure
Entropic force in black hole binaries and its Newtonian limits
We give an exact solution for the static force between two black holes at the
turning points in their binary motion. The results are derived by Gibbs'
principle and the Bekenstein-Hawking entropy applied to the apparent horizon
surfaces in time-symmetric initial data. New power laws are derived for the
entropy jump in mergers, while Newton's law is shown to derive from a new
adiabatic variational principle for the Hilbert action in the presence of
apparent horizon surfaces. In this approach, entropy is strictly monotonic such
that gravity is attractive for all separations including mergers, and the
Bekenstein entropy bound is satisfied also at arbitrarily large separations,
where gravity reduces to Newton's law. The latter is generalized to point
particles in the Newtonian limit by application of Gibbs' principle to
world-lines crossing light cones.Comment: Accepted for publication in Phys. Rev.
Measurement of the ÎS=-ÎQ Amplitude from K_(e3)^0 Decay
We have measured the time distribution of the Ï^+e^-Îœ and Ï^-e^+Îœ modes from initial K^0's in a spark-chamber experiment performed at the Bevatron. From 1079 events between 0.2 and 7 K_S^0 lifetime, we find ReX=-0.069±0.036, ImX=+0.108_(-0.074)^(+0.092). This result is consistent with X=0 (relative probability = 0.25), but more than 4 standard deviations from the existing world average, +0.14 -0.13i
Theory and astrophysical consequences of a magnetized torus around a rapidly rotating black hole
(Abbrev.) We analyze the topology, lifetime, and emissions of a torus around
a black hole formed in hypernovae and black hole-neutron star coalescence. The
torus is ab initio uniformly magnetized, represented by two counter oriented
current-rings, and develops a state of suspended accretion against a "magnetic
wall" around the black hole. Magnetic stability of the torus gives rise to a
new fundamental limit EB/Ek<0.1 for the ratio of poloidal magnetic field
energy-to-kinetic energy. The lifetime of rapid spin of the black hole is
effectively defined by the timescale of dissipation of black hole-spin energy
in the horizon, and satisfies T= 40s (MH/7MSun)(R/6MH)^4(0.03MH/MT) for a black
hole of mass MH surrounded by a torus of mass MT and radius R. The torus
converts a major fraction Egw/Erot=0.1 into gravitational radiation through a
finite number of multipole mass-moments, and a smaller fraction into MeV
neutrinos and baryon-rich winds. At a source distance of 100Mpc, these
emissions over N=2e4 periods give rise to a characteristic strain amplitude
\sqrt{N}hchar=6e-21. We argue that torus winds create an open magnetic
flux-tube on the black hole, which carries a minor and standard fraction
Ej/Erot=1e-3 in baryon-poor outflows to infinity. We identify this baryon poor
output of tens of seconds with GRBs with contemporaneous and strongly
correlated emissions in gravitational radiation, conceivably at multiple
frequencies. Ultimately, this leaves a black hole binary surrounded by a
supernova remnant.Comment: To appear in ApJ (44p
Validity of an instrument that assesses functional abilities in people with profound intellectual and multiple disabilities:Look what I can do!
Background: Research about the psychometric properties of the Behavioural Appraisal Scales (BAS) in people with profound intellectual and multiple disabilities (PIMD) is limited. This study evaluates invariance in factor structure, item bias and convergent validity of the BAS. Methods: Data on the BAS from two studies (nâ=â25; nâ=â52) were analysed using the oblique multiple group method. The scale structure and item ordering were compared in the two groups. Convergent validity was assessed by correlating scores on the BAS with scores on two other instruments. Results: Of all items, 16â18% correlated stronger with other subscales of the BAS than the subscale they were originally assigned to. Scale structure and order of difficulty differed between groups. Correlations between the BAS and two other instruments varied from low to excellent (râ=â.48â.85). Conclusions: The results support the construct validity of the BAS. Removing, reassigning and adapting items may enhance construct validity
On the detectability of gravitational waves background produced by gamma ray bursts
In this paper we discuss a new strategy for the detection of gravitational
radiation likely emitted by cosmological gamma ray burst. Robust and
conservative estimates lead to the conclusion that the uncorrelated
superimposition of bursts of gravitational waves can be detected by
interferometric detectors like VIRGO or LIGO. The expected signal is predicted
to carry two very distinctive signatures: the cosmological dipole anisotropy
and a characteristic time scale in the auto correlation spectrum, which might
be exploited, perhaps with ad hoc modifications and/or upgrading of the planned
experiments, to confirm the non-instrumental origin of the signal.Comment: 9 pages, 2 figures, LATEX2e, Accepted for pubblications as a Letter
to the Editor in Journal of Physics G: Nuclear and Particle Physic
Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO
Gamma-ray bursts are believed to originate in core-collapse of massive stars.
This produces an active nucleus containing a rapidly rotating Kerr black hole
surrounded by a uniformly magnetized torus represented by two counter-oriented
current rings. We quantify black hole spin-interactions with the torus and
charged particles along open magnetic flux-tubes subtended by the event
horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of
frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with
GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii)
aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich
et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al.
2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating
LIGO/Virgo detectors enables searches for nearby events and their spectral
closure density 6e-9 around 250Hz in the stochastic background radiation in
gravitational waves. At current sensitivity, LIGO-Hanford may place an upper
bound around 150MSolar in GRB030329. Detection of Egw thus provides a method
for identifying Kerr black holes by calorimetry.Comment: to appear in PRD, 49
Low Loss Transmission in Negative Curvature Optical Fibers With Elliptical Capillary Tubes
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