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

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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.)

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