May 24, 2000

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.


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Scientists 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


"We've tried very hard to design an absolutely perfect gyroscope," said Dr. Francis Everitt, the Principal Investigator at Stanford University. Even in an age of exquisite measurements, nothing is perfect. The GP-B gyros, though, are about as close as humans can get. The gyros and their support system are so precise that non-relativity effects will cause them to drift by no more than 1/3 milli-arc-second during a year.

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.