Einstein's initial work in relativity grew out of the unexpected consequences of an experiment conducted by Abraham Michelson and Edward Morley in 1887. The Michelson-Morley experiment measured the speed of light in two opposite directions: with the direction of the earth through space and against the direction of the earth through space. They expected to find two different velocities, but instead found both velocities of light to be the same, independent of the direction of the reference frame.

Imagine two people throwing a ball back and forth on a train that is moving in a particular direction. When the ball is thrown in the same direction as the train is moving, the velocity of the ball, relative to a stationary observer outside the train, will be the sum of the velocities of the train relative to the ground and of the ball relative to the train. When the ball is thrown in the direction opposite to the direction the train is moving, the velocity of the ball will be the difference between the velocities of the train relative to the ground and of the ball relative to the train.

Michelson and Morley expected the same effect to be true of the velocity of light when measured relative to the direction of the earth going through space. But the speed of light is actually a constant no matter from what frame of reference it is measured. This fact produces many very strange consequences when things move at speeds approaching the speed of light.