Number 194, September 13, 1994 by Phillip F. Schewe and Ben Stein
CRITICAL EXPONENTS IN NUCLEAR PHYSICS: In the 1970s physicists found
that a variety of physical systems exhibited a universal behavior near
a certain critical point. Examples include ferromagnets near their Curie
temperature (below which atomic magnetic moments line up) or fluids near
to those conditions of temperature and pressure where the gaseous and liquid
phases coexist. These systems were observed to be characterized by "critical
exponents" which typified the exponential temperature dependence of
the system (e.g., the magnet's susceptibility or the fluid's density) near
the critical point. Scientists at the LBL Bevalac have now demonstrated
that the fragmentation of a gold nucleus (accelerated up to energies of
1 GeV per nucleon) upon striking a carbon target acts like a critical system.
They detected the collision debris and catalogued events according to the
size and multiplicity of the fragments. Using the multiplicity as the equivalent
of "temperature," the nuclear physicists were able to plot various
fragment distributions and to extract the same sort of critical exponents
studied by condensed matter physicists. The calculated values are very
close to those which characterize liquid-gas systems. (M.L. Gilkes et al.,
Physical Review Letters, 19 Sept. 1994.)
THE SOLAR SYSTEM SITS IN AN INTERSTELLAR CLOUD. The existence of the
cloud and its geometry can be deduced from its effect on the spectra of
nearby stars and cosmic rays. Priscilla Frisch of the University of Chicago
calculates, further, that our solar system first encountered the cloud
(moving at right angles to it) between 2000 and 8000 years ago. (Priscilla
C. Frisch, Science, 2 Sept.)
ARTIFICIAL NEURAL NETWORKS , logic programs which can be "trained"
to do certain types of pattern recognition, have been used to extract information
from complex observational data sets in a number of research areas, such
as atomic physics, chemistry, and geophysics. Neural network algorithms
will soon be used by scientists at Livermore to deduce the properties of
laser- produced plasmas (such as temperature and density) form the complex
spectrum emitted by the plasmas. Currently the neural network is being
trained and tried out on spectra associated with known plasma properties.
The Livermore scientists hope eventually to establish a library of network
responses which can be accessed in future plasma spectroscopic work. (A.L.
Osterheld et al., Phys. Rev. Lett., 12 Sept.)
THE TEMPERATURE OF THE COSMIC MICROWAVE BACKGROUND (CMB) , a uniform
2.7 K (not counting tiny fluctuations at the mK level) across the sky,
is in effect the ambient temperature of the universe. According to the
big bang model, this temperature would have been higher in the past; in
fact, it should increase linearly with redshift as we look farther back
in time. Using the Keck Telescope, astronomers have indirectly measured
the CMB temperature at a redshift of 1.776 by looking at the fine features
in the absorption spectrum of carbon atoms in an interstellar cloud sitting
in front of a more distant quasar. The measured CMB temperature, 7.4 K,
is very close to the big bang prediction of 7.6 K. (A. Songalia et al.,
Nature, 1 Sept.)
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