PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Publ

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PHYSICS NEWS UPDATE A digest of physics news items prepared by Phillip F. Schewe, AIP Public Information Number 161 January 24, 1994 GAMMA RAY BURSTS MAY BE EXHIBITING TIME DILATION owing to the expansion of the universe. The powerful gamma ray bursts studied over the past few years by the Gamma Ray Observatory (GRO) are sprinkled uniformly across the sky, suggesting that they come not from our galaxy but from beyond, perhaps in some cases from the distant edge of the universe. At the recent meeting of the American Astronomical Society in Virginia, Jay Norris of NASA Goddard announced that from among the more than 700 bursts seen so far dim bursts are typically twice as long as brighter bursts and, furthermore, that the dim bursts lie more toward the "red" end of the gamma-ray range. The cosmological explanation of this pattern would proceed as follows: the theory of relativity holds that a time interval measured in one frame of reference will be different for an observer in another frame of reference. The difference (or time dilation) will increase as the relative velocity of the two frames increases. Thus gamma sources near the edge of the universe would be receding from Earth at a greater velocity than closer sources, and consequently the length of a far-out burst would appear to be longer than for near-in bursts. Radiation from the cosmic microwave background (which is presumably even more redshifted) streams in on us unabated and therefore, unlike the gamma bursts, has no beginning, middle, or end. Norris admits that the cosmological hypothesis is still tentative because the relation between burst energy and duration is not yet calibrated. PICOKELVIN TEMPERATURES have been achieved in rhodium nuclei. The temperature of a physical system, whether it be an ice cube or a collection of nuclei, can be defined as the amount of disorder, or entropy, in the system. If the spins in a group of nuclei are distributed over a wide range of directions, then the system's disorder (and its temperature) is high; in a low temperature system the spins would tend to be organized in a single direction corresponding to a low-energy state. Pertti Hakonen and his colleagues at the Helsinki University of Technology in Finland aligned the spins of rhodium nuclei in an external magnetic field, dropping them to picokelvin temperatures. Then the researchers quickly reversed the direction of the field in a way that kept the amount of entropy constant. The result was that most of the nuclear spins were opposed to the field, and therefore in a high-energy state; since the spin distribution was now exactly the inverse of what it was in the previous case, the sample of nuclei was considered to have a "negative temperature." Hakonen believes that femtokelvin temperatures (positive and negative) may be possible in future experiments. (Scientific American, January 1994.) THE CLEMENTINE SPACECRAFT, to be launched in a few weeks, will perform the first global survey of the Moon's surface. Indeed it is the first extensive lunar venture since the 1970s. The new map, to be constructed from overlapping image tracks, will be comparable to Magellan's mosaic picture of Venus' surface. Clementine will then leave its lunar orbit and make the first rendezvous (in August 1994) with an Earth-orbit-crossing asteroid, Geographos. The relatively cheap probe ($75 million) will also be carrying out some detector tests for the Ballistic Missile Defense Organization, the new incarnation of the Strategic Defense Initiative. (Astronomy, Feb. 1994.)

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