The longest experiment in space physics began with three men in a university swimming pool arguing about Einstein. It ended Wednesday, after 52 years and $750 million, with scientists affirming his theory of relativity after studying the most perfect spheres ever made as they orbit around the Earth.

Called Gravity Probe B, the exotic experiment measured how the revolving mass of Earth imperceptibly twists the fabric of space in a test of Einstein's general theory of relativity. By one finding, the distortion amounts to 1.1...