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

 
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.)