5,631 research outputs found
Sub-Microarcsecond Astrometry and New Horizons in Relativistic Gravitational Physics
Attaining the limit of sub-microarcsecond optical resolution will completely
revolutionize fundamental astrometry by merging it with relativistic
gravitational physics. Beyond the sub-microarcsecond threshold, one will meet
in the sky a new population of physical phenomena caused by primordial
gravitational waves from early universe and/or different localized astronomical
sources, space-time topological defects, moving gravitational lenses, time
variability of gravitational fields of the solar system and binary stars, and
many others. Adequate physical interpretation of these yet undetectable
sub-microarcsecond phenomena can not be achieved on the ground of the
"standard" post-Newtonian approach (PNA), which is valid only in the near-zone
of astronomical objects having a time-dependent gravitational field. We
describe a new, post-Minkowskian relativistic approach for modeling astrometric
observations having sub-microarcsecond precision and briefly discuss the
light-propagation effects caused by gravitational waves and other phenomena
related to time-dependent gravitational fields. The domain of applicability of
the PNA in relativistic space astrometry is explicitly outlined.Comment: 5 pages, the talk given at the IAU Colloquium 180 "Towards Models and
Constants for Sub-Microarcsecond Astrometry", Washington DC, March 26 - April
2, 200
Spatial and temporal characteristics of electrically fixed holograms in photorefractive strontium-barium niobate
We report measurements of the holographic diffraction efficiency (HDE) of single photorefractive gratings fixed by an externally applied voltage pulse in Sr0.75Ba0.25Nb2O6. The polarization grating HDE and decay rate are strongly dependent on the grating spacing and also depend on the intensity of the light beams during the recording and erasing/reconstruction process. Knowledge of the dependence of the fixing process on spatial frequency may be useful for the creation of physical models of local polarization switching in ferroelectrics
Irreducible wavelet representations and ergodic automorphisms on solenoids
We focus on the irreducibility of wavelet representations. We present some
connections between the following notions: covariant wavelet representations,
ergodic shifts on solenoids, fixed points of transfer (Ruelle) operators and
solutions of refinement equations. We investigate the irreducibility of the
wavelet representations, in particular the representation associated to the
Cantor set, introduced in \cite{DuJo06b}, and we present several equivalent
formulations of the problem
Electrical fixing of photorefractive holograms in Sr0.75Ba0.25Nb2O6
Photorefractive holograms stored in Sr0.75Ba0.25Nb2O6 crystals are electrically fixed at room temperature. The fixed holograms can be read out directly or after a positive-voltage pulse is applied that can dramatically enhance the diffraction efficiency. Single gratings as well as images are recorded and fixed
- …