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鈥檚 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鈥檚 predictions about massive bodies 鈥� measurements of the precession of Mercury鈥檚 orbit around the sun.
Gravity Probe B, a $600-million NASA spacecraft, is intended to test relativity further by measuring two previously unseen interactions between matter and space-time. Launched this year, it was actually conceived in the 1960s. In the intervening time, the two phenomena have already made an appearance. In 1988, the geodetic effect, which describes the extent to which matter warps space-time, was detected by measuring how the sun鈥檚 mass influences the spin of the Earth-moon system. Now an Italian team says it has found the much smaller Lense-Thirring effect, by which the Earth drags space-time with it as it spins (see 鈥淩elativity tied up on a shoestring鈥�).
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Does this mean that Gravity Probe B is a gigantic waste of time and money? Fortunately not. First, its $600-million price tag is as much a reflection of the delays and uncertainties inflicted by NASA鈥檚 troubled history as of the cost of physics. More important is the question of precision. While this week鈥檚 result measures the Lense-Thirring effect to within 10 per cent, Gravity Probe B will do it to better than 1 per cent. And its measurement of the geodetic effect should be correct to within one part in 10,000, relativity鈥檚 sternest test yet.
Gravity Probe B is designed for one purpose: testing the predictions of relativity. Physicists will have more confidence in its results than those of any other experiment, and if it should return unexpected answers, they are more likely to consider rethinking relativity. The theory is so central to our understanding of physics that we need to be sure of it. Einstein, who worried so much about the precarious foundations of his theory, would surely agree.