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."