In the half century and more of Einstein’s work in science, one discovery stands above all as his greatest achievement. It is his general theory of relativity. In it, Einstein found a new way to think of the gravity that pulls apples from their trees and keeps the moon in orbit around our earth. There are no forces pulling on them, he saw. They are merely responding to a curvature in the geometrical fabric of space and time.

This discovery of this theory is somehow more than mere science. It is not the fitting of a formula to a set of data or succumbing to the weight of unanswerable evidence. General relativity was an achievement of creative imagination. Through it, Einstein found the boundary of science and art. There he wrote equations linking space, time, matter, and gravity every bit as beautiful as Shakespeare’s sonnets, but written in the universal language of mathematics. The evidence that favors general relativity is nowhere near as strong or thorough as that which speaks for quantum theory. Yet we favor general relativity simply because no conception this beautiful should be wrong. And it survives because no theorist in the many decades since 1915 has been imaginative enough to find a theory that does better than general relativity. Every time a new test is devised Einstein’s theory wins.