Number 122, April 6, 1993 by Phillip F. Schewe and Ben Stein
THE CASIMIR-POLDER FORCE , the force between a neutral atom and a metal
plate, has been observed for the first time by Edward Hinds and his co-workers
at Yale (Phys. Rev. Lett., 1 Feb. 1993). They measured the force on neutral
sodium atoms traveling through a pair of gold plates spaced only microns
apart. Predicted as long ago as 1948, the Casimir-Polder force is a consequence
of the fact that the presence of a conducting plate (even with no electric
field applied) modifies the vacuum around the atom, which in turn affects
the quantum behavior of the atom, such as its ability to emit light of
a certain wavelength. A force of the Casimir-Polder type is believed to
play a major role in binding together the atoms of the recently discovered
He2 molecule. In another example of cavity quantum electrodynamics, as
the physics of atoms confined by reflecting walls in micron-scale volumes
is called, an experiment currently underway in Paris is exploiting the
wavelike nature of atoms in an attempt to count precisely the number of
photons trapped in a cavity without perturbing the atom-photon system.
Such a "quantum nondemolition" technique might lead to the development
of more sensitive light sensors. (Scientific American, April 1993; Science
News, 13 February 1993; Physics Today, April 1993; Nature, 18 Mar. 1993.)
THE FORMATION OF THE INNER PLANETS from low-mass planetesimals may have
happened quickly, in as little as a million years, new simulations show.
These results were reported by Douglas Lin of UC Santa Cruz and his colleagues
in the 20 January Astrophysical Journal. The whole process of rocky-planet
building, beginning with the first solid-material grains in the solar system
about 4.56 billion years ago (and still preserved in meteorites), and continuing
with the growth and repeated collisions of larger and larger objects in
the extended circumstellar disk, took perhaps 100 million years. This model
in which planet condensation was quick and rocky seems to be displacing
the older model of the solar system in which planets precipitated slowly
from a more gaseous disk (Science News, 20 Mar.). As for the outer planets,
a separate study by George Wetherwill of the Carnegie Institution indicates
that the formation of planets as large as Saturn and Jupiter should be
rare since there would have been only a short time for them to fashion
cores and then to accumulate such massive atmospheres from matter in the
circumstellar disk before the disk dissipated. (Science News, 27 Mar.)
IN THE STARBURST MODEL , quasar energy derives from the concerted birth
and then death (by supernova) of supermassive stars, first with a generation
of 25-60-solar-mass stars and later with the explosion of more numerous
but somewhat smaller stars, all within a 10-million year period. First
proposed in 1960s but eclipsed by the model which holds that black holes
fuel quasar emissions, the starburst model is making a comeback, according
to Roberto Terlevich of the Royal Greenwich Observatory and Brian Bayle
of Cambridge (UK), who have been making a survey of quasar brightnesses.
Opponents of the theory, such as Martin Rees of Cambridge, argue that star
energy alone could not account for the brightness of quasars. (Science,
19 Mar.)
SCIENCE WRITERS wishing to receive Physics News Update should contact
Phillip Schewe by electronic mail at this address: pfs2@aip.org
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