EINSTEIN once made a telling remark about the equation at the heart of general relativity. He said the left-hand side of the equation, which describes the curvature of space-time, was built on granite. But the right-hand side, which links matter, gravity and space-time, was built on sand.
Which is why any chance to test the theory should not be sniffed at. As one of the cornerstones of modern physics, scientists place huge faith in relativity. But that faith owes more to admiration for the theory's elegance than to mountains of experimental data supporting it. In fact, it has been tested in only a handful of experiments. Tests of relativity fall into three categories based on timing anomalies, electromagnetic waves and massive bodies. Until recently, only one experimental result confirmed relativity's predictions about massive bodies - measurements of the precession of Mercury's orbit around the sun.
Gravity Probe B, a $600-million ...
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