Number 205, December 1, 1994 by Phillip F. Schewe and Ben Stein
COMPOSITE FERMIONS ARE REAL. At low temperature and in a high magnetic
field, a two- dimensional gas of electrons (confined at the interface between
two semiconductors) can exhibit the quantum Hall effect: as the magnetic
field is increased, the Hall resistance (the voltage across the sample
divided by the current) rises not linearly but in a series of discrete
steps. In one manifestation of the phenomenon, the fractional quantum Hall
effect, the strongly interacting electrons behave in a particularly complicated
way. In an effort to avoid each other, they form a fluid which consists
of electrons that have amalgamated with magnetic flux lines. That is, the
electrons each assimilate an even number of magnetic vortices. This process
has the effect of partially or totally "using up" the magnetic
field. Furthermore, the resultant particles, called composite fermions,
interact with each other very little. Recent experiments have shown that
composite fermions have many of the properties of true particles: they
have mass, execute cyclotron motions in the remaining magnetic field, and
have definite energy levels. (J.K. Jain, Science, 18 November 1994.)
THE HEARD ISLAND FEASIBILITY TEST (HIFT) was developed to study the
propagation of underwater sound waves across whole oceans, possibly as
a means of monitoring ocean temperatures on a large scale. (The speed of
sound in water depends on the water temperature.) In January 1991 a series
of coded sound waves was transmitted from an underwater station near Heard
Island in the southern Indian Ocean. The waves were later detected across
a network of 16 sites (some on board vessels at sea) positioned along great-circle
sightlines (or perhaps soundlines would be a more appropriate term) around
the world. One example: the signal took 2.95 hours to reach the Bermuda
station 16,000 km away. The researchers were generally satisfied with the
quality of the received signals, as measured by the signal-to-noise ratio
and the stability of the waveform over time. They suggested, however, that
for monitoring ocean climate variability acoustic thermometry should be
integrated with satellite measurements. On another point of concern, the
scientists found no evidence that the sound transmissions produced any
distress among local marine mammals. (17 papers in the Journal of the Acoustical
Society of America, October 1994; introduction by Walter R. Munk of Scripps
and Arthur Baggeroer of MIT.)
MUON-MUON COLLIDERS are not even on the drawing boards yet. But some
physicists, such as Robert Palmer of Brookhaven, believe muon machines
might be an economical alternative to building long linear accelerators
for generating TeV-energy beams of electrons and positrons. Linacs are
popular with those who contemplate future accelerators because linacs avoid
the grave loss of energy to synchrotron radiation suffered by circular
facilities such as CERN's LEP collider. Muons are leptons, like electrons,
but with a mass 200 times greater. Muons would shed much less energy than
electrons in making myriad trips around a circular accelerator. This would
allow machine engineers to retain the circular design, and along with it
the ability for stored beams to interact with each other over and over,
which is not the case with linacs. Major drawbacks to using muons are the
difficulty in generating muons in the first place (they are produced from
the decay of pions created when a proton beam is smashed into a target)
and the fact that they are unstable, with a lifetime of only a millionth
of a second. (CERN Courier, October 1994.)
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