In 1912, Albert Einstein wrote down an equation that describes the structure of the universe. But even he didn—t recognize its full meaning. Aczel (Probability 1, 1998, etc.) has made a career of explaining the frontiers of mathematics. Here he tackles Einstein’s field equation of general relativity not only in the context of modern physics, but in the history of mathematics. When Einstein began to incorporate gravity into his theories, he realized that it must have certain effects on light. In particular, light leaving a massive object would be red-shifted; its frequency would become longer, as if the object were moving away. Space was curved, and that curvature could be described in terms of non-Euclidean geometry—built on alterations of Euclid’s fifth postulate, which after trying unsuccessfully to prove for two millennia, mathematicians decided to treat as an arbitrary and unprovable assumption. The curvature of space and its effect on light made possible experimental verifications of relativity: for example, the positions of stars seen near the sun in an eclipse should differ from their positions when the sun was in another part of the sky. In 1919, a British expedition led by Arthur Eddington measured those star positions and proved Einstein’s theories correct. Meanwhile, Einstein had been exploring the cosmological implications of his theory, in particular the question of whether the universe expands, contracts, or remains the same size. Here, for the first time, he did not believe his own calculations and felt it necessary to add a “cosmological constant” to his field equation—a fudge factor he later described as his greatest blunder when astronomers demonstrated that the universe was in fact expanding. More recent theorists suspect that the cosmological constant was needed, after all—but until another Einstein comes along, the field equation remains the closest thing we have to a divine blueprint for the universe. While the actual math is heavy going, Aczel gives a very readable account of the science and the scientists involved.