# Space gyroscopes for testing relativity.

Space gyroscopes for testing relativity

Scientists at Stanford University are starting to assemble a crucial experiment designed to test Einstein's general theory of relativity to an unprecedented level of precision. Known as Gravity Probe-B, the satellite-based experiment will attempt to detect two specific physical effects predicted by the theory but never before measured.

In Einstein's theory, the force of gravity is a manifestation of the curvature of space and time caused by the warping effects of concentrations of masS. For example, the Earth orbits the sun not because it's attracted to the sun but because it follows the shortest possible path in space-time distorted by the sun's mass. Although the general theory of relativity now plays a central role in astrophysics, much of the theory has never been tested or verified.

The Stanford experiment focuses on what should happen to a freely spinning gyroscope in orbit around the Earth. According to Newton's laws of motion, the axis of such a gyroscope should always stay pointed in the same direction. However, because an orbiting gyroscope would be moving through curved space-time around the Earth, Einstein's theory predicts the gyroscope should precess, or tilt, slightly as it spins. Known as the geodetic effect, the tilt would amount to 6.6 arc-seconds a year, or 360[degrees] in 200,000 years.

At the same time, the theory predicts the gyroscope should feel a second effect. As the Earth rotates, it drags space and time around with it. Just as a moving electric charge generates a magnetic field in addition to the electric field already present, the Earth's motion ought to generate a completely new kind of field -- a gravitomagnetic field -- different from the ordinary gravitational field surrounding a body. That effect would also cause a gyroscope to tilt. In a polar orbit the tilt due to the geodetic effect would be at a right angle to the tilt caused by the gravitomagnetic effect, making both effects detectable. However, the gravitomagnetic effect amounts to a minuscule 44 milli-arc-seconds a year.

To measure such tiny effects, the orbiting gyroscopes must be as free as possible from any interference. The satellite will contain four gyroscopes, each a ball of pure quartz 1.5 inches in diameter, ground so smoothly that deviations from roundness measure less than one-millionth of an inch. Each ball has a niobium coating, which becomes a superconductor at liquid-helium temperatures and thus permits the balls to be suspended electrically and any tilt to be measured. Jets of helium gas will set them spinning at 10,000 rotations per minute in a near-perfect vacuum.

Stanford researchers hope to test the assembled gyroscopes aboard the space shuttle in 1993 in preparation for a rocket launch three or four years later--the culmination of more than 20 years of thought and effort.