Number 620, January 13, 2003
by Phil Schewe, James Riordon, and Ben Stein
The Speed of Gravity
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@amolf.nl, 31-20-608-1234; text at www.aip.org/physnews/select
; website at 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 1011,
not 10-11.
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