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The theory of relativity began as an idea of Galileo, a 17th-century astronomer, who wrote, "Motion exists relatively to things that lack it." This simple concept, when further explored by Albert Einstein, had astounding consequences. Einstein's first step toward a comprehensive theory of relativity established that not only was motion relative, but time and space were as well. He would follow up this discovery with the world's most famous equation, E = mc2. Ten years later, Einstein would again turn the scientific world upside down when he proclaimed that gravity is not a force, but a curvature of space-time. With this declaration, he had rewritten the most fundamental law of science. In Theory of Relativity, discover why some believe that this theory is the greatest scientific discovery ever made.