Number 190, August 16, 1994 by Phillip F. Schewe and Ben Stein
DIRECT IMAGES OF THE SHOEMAKER-LEVY IMPACT at Jupiter were recorded
by the Galileo spacecraft last month and are just now being processed,
owing partly to the antenna problems plaguing the Galileo mission. An image
of fragment W's impact on July 22 will be released soon and is expected
to clearly show an explosion. Galileo's detector was the only telescope
in a position to take such direct pictures. A NASA meeting this fall will
bring together observations and theories from many institutions. (The New
York Times, 16 August 1994.)
PHOTON-ELECTRON AND PHOTON-PHOTON COLLIDERS could be fashioned from
linear electron-positron colliders. High-energy beams (up to hundreds of
GeV) of photons would be produced by backscattering intense laser beams
from oncoming bunches of electrons or positrons. Such high-energy photon
beams, which could add a new avenue for studying the standard model of
particle physics, are one ingredient in discussions now underway among
US, European, and Japanese accelerator designers contemplating a next generation
electron accelerator. Some of these ideas could be tried out on the present
Stanford Linear Collider. (Physics Today, July; and CERN Courier, June.)
TIME REVERSED SOUND , a phase-conjugate mirror version of an acoustic
signal, has been demonstrated by Mathias Fink at the University of Paris.
In Fink's experiment sound waves fall on an array of rodlike piezoelectric
elements (a sort of "acoustic retina") which transduces the sound
into electric signals, where they are computer processed, and re-composed
as time- reversed signals and re-broadcast in the opposite direction. Phase-conjugate
mirrors for light waves have been known for many years but this is the
first acoustic analog. Fink has been using his device in sharpening ultrasound
medical images and in the search for defects in metal alloys. (Science,
22 July.)
BOSE-EINSTEIN CONDENSATION is a hypothetical state of matter in which
a gas of atoms is cooled to such a low temperature that the atoms begin
to overlay and eventually assume a single ensemble quantum state. This
is a manifestation of the uncertainty principle: near absolute zero the
uncertainty in the atoms' momentum is reduced so there must be a consequential
increase the uncertainty as to their whereabouts. Atoms in the Bose-Einstein
condition hold great interest for scientists since they would constitute
an exotic sort of "coherent" matter with potentially novel thermodynamic
and optical properties. In general, condensation requires that the participating
atoms be bosons (particles with an integer-valued spin) and that they be
very densely packed. The main problems here are achieving the requisite
density and temperature. Recently several groups have come close to success
with traps that cool atoms to 100 microkelvins or less and hold them with
either crossed laser beams, or magnetic fields that can be eased gradually
to allow warmer atoms to evaporate away, or with a combination of these
approaches (16 May, Physical Review Letters). MIT has the density, 10**14
atoms/cm**3, and needs to go down a factor of 3 in temperature. NIST, working
with cesium atoms, has the temperature, 700 nK (the coldest temperature
in a sample of atoms ever recorded), but not the density. Groups at Colorado
and Stanford are also in the running. (Science, 8 July.)
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