It’s a fact of life: Some things are absolute, and some are relative. For me, the teapot on the table is to the left of my cup. From the point of view of an observer sitting directly opposite, it’s the other way around: My cup is to the left of the teapot. “Left” and “right” are relative. Whether or not an object is located to the left or to the right of another depends on the observer. On the other hand, if the cup is filled to the brim with coffee, all observers should agree to the fact, regardless of where they sit. That, it would seem, is an absolute statement, independent of who makes the observation.

Einstein’s special theory of relativity (special relativity) is all about what’s relative and what’s absolute about time, space, and motion. Some of Einstein’s conclusions are rather surprising. They are nonetheless correct, as numerous physics experiments have shown. And they have forced physicists to revise the way they think about some of their science’s most basic concepts.