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George, you wrote:
> However, the link to the Gravity B experiment came from Paul Hirose, not
> Frank. And it leads to a further question-
>
> That article, http://einstein.stanford.edu/sitemap.html  , was written in
> the future tense, describing a proposed experiment which was intended to
> launch in 2003, and last for two years. So by now it should have
> been-and-gone. Can anyone tell us whether it actually happened, an if so,
> what the results were?

Yes, that was Paul.  :-)

I have been following the evolution of the plans for this mission since the
1980s (the original funding for research on this project goes back to the
early 1960s). The mission ran from launch in April 2004 up until the
evaporation of the last liquid helium in September 2005. As for the results,
the principal investigators are still working and they have not released
their final results (that's due in just a couple of months). But so far, the
results have been somewhat inconclusive. There was more noise in the system
than expected. The gyros were torqued by patches on their surfaces with
different electrostatic properties. Although the so-called "geodetic"
precession has been detected, some questions have been raised about the
level of confidence. And at least so far, it has not been possible to model
enough of the remaining noise to see the smaller frame-dragging effect (the
idea is that if you can model it reliably, you can subtract it from the
observed data). Here's a press release from last spring:
http://einstein.stanford.edu/content/press_releases/SU/pr-aps-041807.pdf

Incidentally, there's no connection between GP-B and the earlier "Gravity
Probe A" which was a high-altitude, sub-orbital flight with an atomic clock
which provided detailed confirmation of special relativistic and general
relativistic time dilation (the gravitational redshift).

 -FER


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