PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institut

PHYSICS NEWS UPDATE
A digest of physics news items by Phillip F. Schewe, American
Institute of Physics
Number 181  June 3, 1994

ASYMMETRIC SUPERNOVA EXPLOSIONS can impart a "kick"
to the neutron star remnants born during the explosions.  After a new
reassessment, the proper motions (the motions across the sky) of 86
pulsars were found to have a mean velocity of 450 km/sec, which
exceeds the escape velocities for a number of celestial systems, such
as binary stars, globular clusters, and even for our galaxy.  The
University of Manchester (UK) astronomers who performed the study
conclude that more than half of all pulsars will escape from the Milky
Way and that those that stay will assume a larger and more spherical
distribution than was previously thought.  Therefore, the astronomers
assert, this population of old, high-velocity pulsars might be
responsible for more of the gamma bursts seen by the Gamma Ray
Observatory than previously expected.  (A.G. Lyne and D.R.
Lorimer, Nature, 12 May 1994.)

IN A QUANTUM CASCADE (QC) LASER electrons emit light in
a succession of steps as they traverse numerous thin quantum wells
stacked together by epitaxial techniques.  The wavelength of the
output laser light depends on the relative thickness and spacings of the
layers rather than on the specific semiconductor composition of the
device.  Federico Capasso and colleagues at AT&T Bell Labs, who
developed the 500-layer QC laser, believe that the ability to tailor the
output wavelength will make the laser useful for a variety of
applications, such as detecting levels of atmospheric pollution and for
inter-chip telecommunications.  (Jerome Faist et al., Science, 22
April 1994.)

TWO LARGE RINGS LOOMING MYSTERIOUSLY NEAR, but not
exactly around, Supernova 1987A have been imaged by the Hubble
Space Telescope.  One ring stands in front and the other in back of
the supernova in the new pictures.  Together with a much-studied
inner ring centered on the supernova, the three-ring structure
resembles a gigantic hourglass.  A settled explanation for this bizarre
configuration never seen before around a supernova will come only
with additional observations.  (Science News, 28 May 1994.)

ACOUSTIC COLOR is the name for the process by which sound
waves scatter from an object at characteristic frequencies ("colors" in
analogy with optical color) depending on the object's composition,
shape, and other properties.  Nicholas Makris at the Naval Research
Laboratory (202-767-9037) has demonstrated an acoustic-color
imaging technique that can map 200 square kilometers of ocean floor
at a time from a single point at sea.  Using a trio of ships with
underwater instruments, Makris and his colleagues first obtain a
"black and white" image of a region of the Mid Atlantic Ridge, by
radiating a powerful, single-frequency sound wave that "illuminates"
a vast region of the ocean floor.  The echoes received from different
parts of the ocean floor are detected by a 300-meter-long string of
underwater sensors, yielding a rough topographical map.  Then, a
more detailed "color" image is obtained by radiating sound waves
over a spectrum of frequencies from 200-300 Hz.  Whereas
conventional surveying techniques take a month to map 200 square
kilometers of ocean floor, the acoustic color technique can map an
equivalent area in minutes. (Paper to be presented at an upcoming
meeting of the Acoustical Society of America in Cambridge, MA.)