The Special theory of relativity, which included the concept of space-time, dealt only with relations between measurements made by inertial observers. This restriction to non-accelerated observers was a serious one, and German-American physicist Albert Einstein almost immediately began the search for a more inclusive theory, one that could relate observations made by any observer, regardless of his state of motion.

Any inertial observer perceives a three-dimensional Euclidean space, one in which, for example, the distance between two points satisfies the Pythagorean theorem, and the ratio of the circumference of a circle to its radius is . This is flat space. And, an inertial observer who is at rest with respect to his clocks (no matter where they are located) sees them all run at an identical rate. This is uniform time. In the four-dimensional space-time of special relativity, for any inertial observer space is flat, time is...