From: LARRY KLAES (ljk4_at_msn.com)
Date: Wed Sep 17 2003 - 11:32:52 PDT
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From: physnews_at_aip.org
Sent: Wednesday, September 17, 2003 2:22 PM
To: ljk4_at_MSN.COM
Subject: Physics News Update 654
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 654 September 17, 2003 by Phillip F. Schewe, Ben Stein, and
James Riordon
A SINGLE-ATOM LASER, a device employing a single trapped atom to
resonantly emit light back and forth between two reflective mirrors,
has been created by Jeffrey Kimble at Caltech. Although single-atom
lasers have been demonstrated before
(http://www.aip.org/enews/physnews/1994/split/pnu204-3.htm ),
Kimble's is the first to use a single atom nearly at rest, and not a
parade of atoms in a dilute beam entering a reflective cavity one at
a time. The singleness of the source means that the number of
photons emitted by the laser over a certain time interval is, while
not exactly predictable (which would be outlawed by Heisenberg's
uncertainty principle), much less jittery than emission from
multi-atom lasers. The emission is weak by laser standards---only
about 100,000 photons per second---but this quiet, more controllable
form of photons should aid future quantum information schemes.
(McKeever et al., Nature, 18 September 2003.)
THE LASER INTERFEROMETRY SPACE ANTENNA (LISA) is not due for launch
until 2012 but tests of components are of course going forward now.
LISA will search for gravity waves passing the sun's vicinity by
watching how the distance between two test masses changes. A
gravity wave can be thought of as a traveling disturbance in
spacetime itself; such a wave would temporarily shorten and then
lengthen the path between the test masses. In this case the masses
would be 5 million km apart, an interval that would be monitored
every instant by the interference of laser beams traveling back and
forth between the masses. Actually three pairs of test masses would
be mounted on three far-flung satellites, spread out in space in an
equilateral triangle where each leg is 5 million km long, with all
three craft in independent orbit around the sun (see LISA websites
at http://sci.esa.int/science-e/www/area/index.cfm?fareaid=27 and
http://lisa.jpl.nasa.gov/ ). While the spaceborne LISA would look
for waves with very low frequencies (.001-.1 Hz), the earthbound
detector LIGO would search for gravity waves in a higher frequency
range (100-1000 Hz).
As an interim step toward deploying LISA, the European Space Agency
(ESA) plans to launch in 2007 its Pathfinder mission, a craft
serving as a miniature version of LISA, two free-floating test
masses 35 cm apart (small thruster rockets will be used to
reposition the spacecraft so its sides do not come in contact with
the masses), will be tried out. The test, watching that the masses
move along in parallel trajectories, is not unlike the famous (or
apocryphal) experiment conducted by Galileo Galilei to affirm that
two objects, one light and one heavy, would fall at the same rate
from the Leaning Tower of Pisa. And to perform the test in 2007
some terrestrial tests have now been carried out in 2003.
Basically, scientists at the Universita' di Trento (Italy) are
attempting to understand all the possible forces, in addition to
gravity, that could influence the motion of the test mass. In an
ideal experiment, the test mass (2 kg or, in units of weight, about
20 newtons) would be hung from a thin wire and surrounded by all the
apparatus that will accompany it into space, including the motion
sensor needed to reorient the spacecraft, and all extraneous forces
on the mass, down to a precision of a femto-newton (10^-15 newtons)
would have to be accounted for if the desired levels of precision
needed for LISA were to be achieved. Such precision is not possible
with ground-based detectors, so the experimenters used not the full
test mass, but a hollow facsimile. At this early stage in
understanding, the Trento physicists
found a satisfactorily "quiet" force environment, but there are
still a fact of 10 away from the precision needed for Pathfinder and
a factor of 100 away from the precision needed for LISA. (Carbone et
al., Physical Review Letters, upcoming article; Stefano Vitale,
vitale@science,unitn.it, 39-0461-881568 )
THE FRACTION OF PHYSICS GRADUATE STUDENTS coming to the US from
abroad has declined since the 2000/2001 academic year, reversing a
steady climb that had been in effect since 1970. The fraction of
non-US first year grad students grew from about 20% in 1970 to a
peak of 55% in 2001. However, in the past two years the fraction
has eased back to less than 50%, a new report shows. Two-thirds of
the PhD-granting physics departments in the US say that at least
some of their admitted students from abroad have been unable to
attend owing to visa problems. Students from China and from the
Middle East seem to have had the most trouble entering the US.
("Physics Students from Abroad in the Post-9/11 Era," report
prepared by the Statistical Research Center at the American
Institute of Physics; contact Patrick J. Mulvey, pmulvey_at_aip.org; a
copy of the reported can be obtained at
http://www.aip.org/statistics/trends/undtrends.htm )
***********
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