Testing Einstein : in 1959, just two years after the launch of Sputnik I, investigators began work on a space-based experiment to verify the general theory of relativity. Their efforts are about to come to fruition

Natural History, March, 2005 by Arthur Fisher

Frame-dragging was derived in one form from the general theory of relativity in 1918 by two Austrian physicists, Josef Lense and Hans Thirring, and thus is also known as the Lense-Thirring effect. Their calculations concerned the effect of frame-dragging on a moon's orbit. Schiff, instead, calculated the effect of frame-dragging on the axis of a gyroscope. To return to the analogy of the soccer ball spinning in honey, the effect would show up as a gradual change in direction of a pointer immersed in the honey [see illustration at left].

In both cases the effect of flame-dragging is very small. For a gyroscope in polar orbit, it works out to be about 0.041 arc second per year. If you walked "up" a slope at that angle, you would have to walk nearly eighty miles to climb an inch. But in 1959 Schiff calculated that an ideally constructed gyroscope would be able to detect not only the geodetic effect but also flame-dragging. Aboard Gravity Probe B flame-dragging should cause a gyroscope to turn, or yaw, in the same direction as Earth's rotation.

Conducting the experiment, however, requires much more than a near-perfect, drift-free gyroscope. Every force that might affect the gyroscope, except gravity, must be understood and excluded. Minute changes in the gyroscope's spin angle have to be measured without disturbing the gyroscope. A point of reference is needed, in the form of a bright, nearby star whose motion is known, and an onboard telescope is needed to keep track of the star. Each of those challenges at first seemed insurmountable.

The heart of GP-B is a twenty-one-inch-long block of fused quartz bonded to a quartz telescope [see illustration on pages 52 and 53]. For redundancy, the block of quartz houses four gyroscopes, each a gemlike sphere the size of a Ping-Pong ball, also made of fused quartz. The four gyroscopes are the most perfectly round objects ever manufactured, honed to within forty atomic layers, or 0.6 millionth of an inch from the highest "peak" to the deepest "valley." Each sphere is coated with a uniform layer of niobium, a metallic element that becomes a superconductor at 9.3 degrees Kelvin (-442.9 degrees Fahrenheit).

To reduce friction on the spheres, they are levitated by voltages applied to saucer-shaped electrodes. Jets of helium gas spin the gyroscopes up to a speed of 5,000 revolutions per minute. Then, to further minimize friction, the gas is evacuated from around the spheres. The gyroscopes are isolated from external magnetic fields by a succession of magnetic shields.

The telescope itself is kept rigorously aimed at the guide star, IM Pegasi, in the constellation Pegasus. Because the satellite loses sight of the star when it passes behind the Earth, the telescope must be able to reliably reacquire, or locate, the star after every orbit.

The entire gyroscope-telescope assembly is maintained at high vacuum within a nine-foot-long chamber, known as the "probe." The probe in turn is enclosed within a kind of large thermos bottle known as a Dewar. Filled with 645 gallons of liquid helium, the Dewar maintains the probe at a temperature of only 1.8 degrees Kelvin (-456.4 degrees Fahrenheit). The temperature is kept from rising by gradually venting helium from the Dewar.