[ft-l] The Speed of Gravity

[lparker@cacaphony.net (L. Parker)]

Most of us have by now heard of the speed of gravity being "measured" a few
weeks ago. As the synopsis below explains this was done by a gravity lensing
experiment utilizing Jupiter and several quasars, an event which occurs only
once every ten years or so.

When I first read the report, I was skeptical, on first read I thought,
"gee, all we have really measured is the speed of light after gravity has
had its effect". Which of course is the same as the speed of light as we
have all come to know and love it. I was tempted to post a message to the
board then, but held my peace.

I just received the latest installment of AIP News, which predictably,
covers the experiment. I have appended the article below, but to summarize,
it seems that a lot of other scientists agree with me. This may not be the
definitive experiment it is being purported to be.

Okay, I am done throwing oil on the waves, here is the article:

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 )

Lee


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