Gravity Probe B

Definitive tests of general relativity were beyond the science of his day, but his equations were so compelling to other scientists that his theory won almost immediate acceptance. So confident was he in the rightness of his insight that in a divorce settlement with his first wife he promised to split the proceeds from the Nobel Prize he felt certain it would help him earn. Indeed, in 1921 - three years after the divorce became final - he was awarded the Nobel Prize in physics.

In the years since, researchers have labored diligently in experiments of such bewildering complexity that it is easy to conclude that, in the matter of relativity, Einstein had the easy part.

To test general relativity, scientists have, among other things, pondered variations in the orbit of the planet Mercury, measured microwaves as they warp around the sun, bounced signals off Mars and the moon, monitored the radioactivity of superdense stars and studied the attractions between the stellar partners of a binary pulsar.

The Stanford experiment - decades in the making due to its unprecedented precision - entails the first direct measurements of several relativity effects.


On one point at least, the project's backers and detractors agree: Gravity Probe B is science at its most fundamental.

The relativity mission entails some of the most sophisticated aerospace engineering and applied physics of the past two decades, yet at the center it is no more complicated than a spinning crystal ball.

At the heart of the relativity experiment are four supersensitive, superchilled gyroscopes, which fit into a block of fused quartz. Engineered to be almost completely free from disturbances, the gyroscopes will form an almost perfect space-time reference system as they orbit Earth within the supercooled probe. The rotor of each gyroscope is a polished sphere of electrically neutral quartz crystal about the size of a ping-pong ball.

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