A Pop Quiz for Einstein
May 24, 2000: Crystal balls rarely have anything to do with science, but a special set of four soon may provide an answer to one of the last, untested portions of Einstein's General Theory of Relativity. Rather than peering into a crystal ball, scientists will put four - each a manmade quartz gyroscope - in orbit on a year-long Relativity Mission measuring how they spin to see if the Earth's rotating mass distorts time and space.
often compare the fabric of space to a rubber sheet, with the
Earth as a marble denting the surface, thus curving the paths
of passing objects. If Einstein is correct, then the rotation
of a planet or star also twists the sheet and distorts time,
just a little, in an effect called frame dragging that will slightly
repoint the gyros.
The gyros also will measure an effect called geodetic precession, a miniscule compression of space caused by the Earth's physical presence. Both the frame-dragging and geodetic precession effects are so small that they require near-perfection in the design and construction of the instrument, Gravity Probe B (or GP-B).
Right: Under the glow of a green light, a scientist at Stanford University checks for traces of dust on the quartz block assembly that contains the four quartz gyroscopes at the heart of the Relativity Mission. Credit: Stanford
It's a number that Everitt knows well. He once took a loose hair from his scalp and measured it in the machine shop at Stanford University. At a distance of 32 km (20 miles), that hair would appear to be a half milli-arc-second wide.
"That means the gyros can measure frame dragging to about 1 part in 150, and geodetic precession to about 1 part in 100,000," Everitt explained. But while he's confident about the accuracy and precision of the answer, he won't predict the answer itself.