The general theory of relativity began with a sudden thought. It was late 1907, two years after the “miracle year” in which Albert Einstein had produced his special theory of relativity and his theory of light quanta, but he was still an examiner in the Swiss patent office. The physics world had not yet caught up with his genius. While sitting in his office in Bern, a thought “startled” him, he recalled: “If a person falls freely, he will not feel his own weight.” He would later call it “the happiest thought in my life.”