Gravity Probe B

A telescope aboard the probe will keep the gyroscopes precisely pointed toward the star. Therein lies the test.

Under the laws of motion that Newton formulated, the axis of each spinning gyroscope should stay pointed at the guide star indefinitely. But if the universe works as Einstein predicted, the gyroscopes should be pulled out of alignment ever so imperceptibly as space and time curve around the massive planet that is rotating below.

If Einstein is right, the gyroscopes should tilt slightly as they spin, in a movement called precession that is like the wavering circle described by a child's top as it twirls. The geodetic effect causing the precession is so slight that it would take 200,000 years for the axis of the gyroscope to drift in a complete 360-degree circle.

If the Stanford scientists can even measure the effect, that by itself will be no mean feat because it has to be done to within one-thousandth of an arc-second in a year - an accuracy equivalent to measuring the width of a human hair as seen from 10 miles away. (An arc-second is the unit of measurement based on the angle the sun moves through the sky in 15 seconds of time.)

The gyroscopes should also measure a previously undetected effect predicted by Einstein - the gravitomagnetic field generated by the spinning Earth. That effect should also cause the gyroscopes to tilt at the infinitesimal rate of 44 milli-arc-seconds a year.

With the probe traveling in a polar orbit as planned, the tilt caused by the geodetic effect would be at a right angle to the tilt caused by the gravitomagnetic effect, allowing both to be measured with sufficient precision to form a compelling test of general relativity.

"To say we are not nervous would be foolish," says Buchman. "One of the reasons this program took quite so long was all the tender loving care we put into development and testing to understand the system and get out all the bugs."

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