PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 620 January 10, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

CAN THE SPEED OF GRAVITY be measured directly through the observation of
gravitational lensing effects?  Two scientists who monitored the deflection
of quasar light as it passed very near Jupiter argue that they have derived
an experimental value for the speed of gravity equal to 1.06 times the speed
of light (with an uncertainty of 20%).  But two other scientists claim that
the lensing experiment only served as a crude measurement of the speed of
light itself.
Physicists have long taken for granted that the effect of gravitational
force, like the effect of electromagnetic force, is not instantaneous but
should travel at a finite velocity.  A familiar example of this delay is the
fact that when we see the sun, we see it as it was 8 minutes ago.  Many
believe that gravity also travels at the speed of light.  The trouble is,
while it is relatively easy to gauge the strength of gravity (one can
measure gravity even near a black hole, where orbiting matter emits telltale
x rays), it is difficult to study the propagation of gravity.
Although not as heavy as a star, Jupiter still has considerable gravity,
and when on September 8, 2002, it swept very near the position of quasar
J0842 + 1835, the theory of general relativity suggests that the apparent
quasar position on the sky would execute a small loop over the course of
several days owing to the lensing of quasar light by the passing planet.
Sergei Kopeiken (University of Missouri) and Ed Fomolont (National Radio
Astronomy Observatory, or NRAO) have now seen just such a loop, as they
reported this week at the meeting of the American Astronomical Society (AAS)
in Seattle.  For this purpose they employed the Very Long Baseline Array
(VLBA) of radio telescopes, a configuration of dish detectors providing an
angular resolution of 10 micro-arcseconds.  Actually the observed lensing
loop was slightly displaced from what one would expect if gravity propagated
instantaneously.   Kopeiken and Fomolont interpret this slight displacement
as providing an experimental handle on the speed of gravity itself, and
thereby calculate the value of 1.06 times c.
Other scientists disagree with this interpretation, and say that the radio
lensing data can do little more than provide a measurement of the speed of
light, not gravity.  Two such opinions, by scientists who did not report at
the AAS meeting, are as follows: Clifford Will of Washington University in
the US (preprint at (www.arxiv.org/abs/astro-ph/0301145 ) and Hideki Asada
of Hirosaki University in Japan (www.arxiv.org/abs/astro-ph/0206266 )

BEC ENDS GLOBALLY BUT STARTS LOCALLY. Bose Einstein condensations (BEC),
essentially dilute gas clouds in which millions of atoms enter into a
single, corporate coherent object, have proven to be a versatile testbed for
numerous quantum effects. But having attained the critical conditions
necessary for making BEC in the first place, physicists have not paid much
attention to the collapse process itself. Now an experiment conducted by
scientists from the FOM Institute for Atomic and Molecular Physics
(Netherlands) and the Kurchatov Institute (Russia) look at the collapse more
closely and find something surprising while analyzing cigar shaped samples.
In their experiment atoms enter the BEC state through the use of "shock
cooling," in which radio-frequency waves used to cool atoms are provided in
a single one millisecond burst rather than in a sustained way as in
conventional evaporative cooling.  The work shows that BEC is a local effect
with local coherence (atoms acting in concert) and that coherence over the
whole of a condensate occurs only later. In other words, the condensation
has happened so fast that not all atoms are in the ground state; that is,
the atoms are not all in equilibrium. Instead, the cloud is much elongated,
with warmer atoms near the center and cooler atoms toward the ends of a
cigar shaped condensate. While coming to eventual equilibrium, the
condensate undergoes oscillations in its shape. This is observed by
absorption imaging after switching-off the trap (a figure will posted soon
at www.aip.org/mgr/png ). Usually this release gives rise to a cloud
expanding in all directions.  But in this case oscillating condensates
released at the proper moment contract axially while expanding radially.
The axial size reaches a minimum value as the sample drops under the
influence of gravity. This is equivalent to focusing of a cavity dumped atom
laser. The size of the focus is determined by the distribution of axial
momenta among the condensate atoms and therefore contains valuable
information on the phase fluctuation in the condensate at the moment of
release. (Shvarchuck et al., Physical Review Letters, 30 December 2002;
contact Jook Walraven, walraven{at}amolf.nl, 31-20-608-1234; text at
www.aip.org/physnews/select ;  website at http://www.amolf.nl/)


CORRECTION.  In last week's Update (619), the stability or uncertainty in
several frequency measurements was incorrectly reported because of a stray
negative sign in the exponent.  Thus, for example, the stability of the
Mossbauer radiation emission line at a wavelength of 0.086 nm is at the
level of one part in 10^11, not 10^-11.

***********
PHYSICS NEWS UPDATE is a digest of physics news items arising
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