Number 244, October 11, 1995 by Phillip F. Schewe and Ben Stein
THE PHYSICS NOBEL PRIZE GOES TO MARTIN PERL OF SLAC AND FREDERICK REINES
OF UC IRVINE for their discoveries of elementary particles. Perl led the
team of scientists that found the tau lepton in electron-positron collisions
in 1975. In his experiment at the SPEAR collider at SLAC, high energy electrons
and positrons were smashed together head on; among the particles created
out of the energy of collision were pairs of new particles, later identified
as the tau and antitau, each with a mass of about 1.8 GeV. The tau and
its associated neutrino are the fifth and sixth (and perhaps last) of a
family of particles known as leptons. The six known leptons, along with
the six quarks, are the basic alphabet from which all the other constituents
of ordinary atomic matter are made. Reines, working with Clyde Cowan (who
died in 1974) made the first experimental detection of a neutrino (to be
exact, the electron antineutrino), another member of the lepton family.
The existence of neutrinos had been predicted in 1930 by Wolfgang Pauli
as a way of accounting for the energy that seemed to be missing from reactions
in which neutrons decayed into protons. In the early 1950s Reines and Cowan
successfully sought evidence for the neutrino in an experiment, at a reactor
at Savannah River, SC, in which a neutrino interacted with a proton to
create a neutron plus a positron.
BRAGG SCATTERING FROM ATOMS IN OPTICAL LATTICES: In 1912 William and
Lawrence Bragg used x rays to elucidate the structure of crystalline solids.
The x rays, whose wavelengths were well matched to the lattice spacing
of the atoms, scattered from many planes of atoms and produced a characteristic
interference pattern. Now artificial crystals consisting of no more than
a tenuous swarm of atoms held in midair by a web of intersecting laser
beams can be studied using analogous techniques. In such an "optical
lattice" the atom spacing is more like a micron rather than an angstrom,
and so laser light at optical wavelengths is an appropriate probe for performing
a Bragg-like experiment. Scientists at NIST (Gaithersburg, MD) create an
optical lattice with a density of about 10**10 cesium atoms per cu.cm.
Even at that density only a small fraction of lattice sites is filled with
atoms, but the periodic structure of the lattice is still evident; for
example, the Bragg-scattered light beam has a Gaussian profile when the
atoms are in their optical lattice formation, whereas the profile is flat
when the atoms are in a disordered state. The NIST physicists see Bragg
scattering as an important new tool for measuring optical lattice properties,
such as its density or index of refraction. (G. Birkl et al., Physical
Review Letters, 9 October 1995 and Physical Review Letters, 18 December
1995.)
FLIPPING EARTH'S MAGNETIC FIELD has been accomplished, at least inside
a computer. Gary Glatzmaier of Los Alamos and Paul Roberts of UCLA used
2000 CPU hours on a Cray C- 90 to simulate 40,000 years in the life of
our planet's interior. The job included solving numerically the nonlinear
equations describing the movements of Earth's fluid outer core repeatedly
for 2 million time intervals. Some geologists believe the motion of the
electrically conducting core constitutes the dynamo that generates terrestrial
magnetism. Signs of gigantic shifts in the geodynamo, including complete
reversals, are present in the geologic record, and so it was reassuring
to see such a reversal during the computer simulation. The researchers
expect to run longer simulations and hope to see several reversals. (Nature,
21 September 1995.)
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