17,369 research outputs found
Negative Group Velocity
The group velocity for pulses in an optical medium can be negative at
frequencies between those of a pair of laser-pumped spectral lines. The gain
medium then can amplify the leading edge of a pulse resulting in a time advance
of the pulse when it exits the medium, as has been recently demonstrated in the
laboratory. This effect has been called superluminal, but, as a classical
analysis shows, it cannot result in signal propgation at speeds greater than
that of light in vacuum.Comment: v3 adds discussion of "rephasing", and adds a figure. v4 adds
references to the early history of negative group velocity, and adds a
figure; thanks to Alex Grani
Letter from W. T. McDonald
Letter concerning a position in animal husbandry at Utah Agricultural College
The interplanetary acceleration of energetic nucleons
Co-rotating proton and electron streams are the dominant type of low-energy (0.1-10 MeV/nucleon) particle event observed at 1 A.U. The radial dependence of these events was studied between 1 and 4.6 A.U. using essentially identical low-energy detector systems on IMP 7, Pioneer 10 and Pioneer 11. It was expected that at a given energy, the intensity of these streams would decrease rapidly with heliocentric distance due to the effects of interplanetary adiabatic deceleration. Instead it was found that from event to event the intensity either remains roughly constant or increases significantly (more than an order of magnitude) between 1 and 3 A.U. It appears that interplanetary acceleration processes are the most plausible explanation. Several possible acceleration models are explored
Measurement of the fluxes of galactic cosmic ray H-2 and He-3 in 1972 - 1973
If a nearby source of low-energy helium is present, which has traversed a relatively small amount of matter and thus has not caused the production of a significant amount of H-2 or He-3, then these abundance ratios will be suppressed, particularly at low energies. This seems to be the most likely explanation for low ratios
Ubiquitous energy storage
This paper presents a vision of a future power system with "ubiquitous energy storage", where storage would be utilized at all levels of the electricity system. The growing requirement for storage is reviewed, driven by the expansion of distributed generation. The capabilities and existing applications of various storage technologies are presented, providing a useful review of the state of the art. Energy storage will have to be integrated with the power system and there are various ways in which this may be achieved. Some of these options are discussed, as are commercial and regulatory issues. In two case studies, the costs and benefits of some storage options are assessed. It is concluded that electrical storage is not cost effective but that thermal storage offers attractive opportunities
Precise study of asymptotic physics with subradiant ultracold molecules
Weakly bound molecules have physical properties without atomic analogues,
even as the bond length approaches dissociation. In particular, the internal
symmetries of homonuclear diatomic molecules result in formation of two-body
superradiant and subradiant excited states. While superradiance has been
demonstrated in a variety of systems, subradiance is more elusive due to the
inherently weak interaction with the environment. Here we characterize the
properties of deeply subradiant molecular states with intrinsic quality factors
exceeding via precise optical spectroscopy with the longest
molecule-light coherent interaction times to date. We find that two competing
effects limit the lifetimes of the subradiant molecules, with different
asymptotic behaviors. The first is radiative decay via weak magnetic-dipole and
electric-quadrupole interactions. We prove that its rate increases
quadratically with the bond length, confirming quantum mechanical predictions.
The second is nonradiative decay through weak gyroscopic predissociation, with
a rate proportional to the vibrational mode spacing and sensitive to
short-range physics. This work bridges the gap between atomic and molecular
metrology based on lattice-clock techniques, yielding new understanding of
long-range interatomic interactions and placing ultracold molecules at the
forefront of precision measurements.Comment: 12 pages, 6 figure
Second Epoch Global VLBI Observations of Compact Radio Sources in the M82 Starburst Galaxy
We have presented the results of a second epoch of global Very Long Baseline
Interferometry observations, taken on 23 February 2001 at a wavelength of 18
cm, of the central kiloparsec of the nearby starburst galaxy Messier 82. These
observations were aimed at studying the structural and flux evolution of some
of the compact radio sources in the central region that have been identified as
supernova remnants. The objects 41.95+575 and 43.31+592 have been studied,
expansion velocities of 2500 +/- 1200 km/s and 7350 +/- 2100 km/s respectively
have been derived. Flux densities of 31.1 +/- 0.3 mJy and 17.4 +/- 0.3 mJy have
been measured for the two objects. These results are consistent with
measurements and predictions from previous epochs.Comment: 5 pages, 3 figures. To be published on the accompanying CD of the
Proceedings of IAU Colloquium 192: Supernova
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