Boosting the accuracy of a relativity test.

Scheduled for launch in 1999, Gravity Probe B represents an ambitious attempt to test the general theory of relativity. By measuring tiny changes in the tilt of four gyroscopes aboard this Earth-orbiting satellite, physicists hope to detect certain subtle gravitational effects predicted by the theory.

Now, a researcher has proposed an alternative design that could make the experiment up to 1,000 times more accurate. Such an improvement would enable scientists not only to test general relativity but also to distinguish between various competing theories of the evolution of the universe.

Benjamin Lange of Stanford University describes his design in the March 13 Physical Review Letters.

According to general relativity, a star or planet warps the geometry of space and time in its vicinity. As it rotates, such a body also "drags" this spacetime distortion with it. These effects cause the spin axis of a gyroscope in orbit around the body to drift slightly.

The original design of Gravity Probe B called for the use of four gyroscopes, each an ultrasmooth quartz ball coated with niobium. The niobium layer becomes a superconductor at liquid helium temperatures, allowing the balls to be suspended in space by electrical forces. Sensitive magnetometers detect any changes in the tilt of a gyroscope's spin axis (SN: 3/3/90, p.143).

Lange's design uses a single gyroscope with a silicon rotor suspended in a rapidly spinning satellite operating at ambient rather than liquid helium temperatures (see diagram). The higher temperature allows use of a more sensitive detection technique. "It is based on existing technology, all of which can be tested prior to flight," Lange contends.

"It's a very interesting idea," says Stanford's Francis Everitt, a principal investigator on the Gravity Probe B project. "But I don't think Lange's approach is necessarily superior. All of the issues haven't been addressed."

Moreover, NASA has already spent more than $230 million on the project, and much of the hardware has been built. "There's no way that we can change horses in midstream at this stage in the program," Everitt insists.

However, continued funding for Gravity Probe B now depends both on the outcome of a review by a National Academy of Sciences committee and on whether NASA can propose budget cuts to offset the $51.5 million designated for the project in fiscal year 1996.

"My proposal offers an alternative way of continuing the program," Lange says.