I just looked at the URL (http://www.ligo.caltech.edu/) for the Laser
Interferometer Gravitational-Wave Observatory. It is an excellent site and
may be of interest to some of you who are following gravity-wave
measurments. I heard about LIGO and it's URL from an article in the Dec. 99
R&D Magazine, p11. One part of the system consists of a large Sapphire
mass. Here is it's description from the web site: 

"First, we plan to use sapphire instead of fused quartz for the test mass
material. Sapphire has very low mechanical losses, and also a high speed of
sound and a high density. These are all advantageous
for the thermal noise (and for the radiation pressure noise). However, to
obtain sapphire in the size required for a LIGO test mass
(order of 25 cm diameter, 10 cm thickness) and of an optical quality
sufficient for the interferometric sensing, requires a significant
development effort, but will be rewarded with a much reduced thermal noise ..."
George

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