Einstein’s second postulate, sometimes called the light postulate, drew from nineteenth-century electromagnetic theory. It states that “light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body,” as measured from any inertial frame of reference. The problem was how to reconcile the two postulates. Consider two observers who are in non-accelerated motion with respect to each other. If one observer, Sarah, stands on a train platform, while another observer, Joe, sits in a train traveling east at 50 miles per hour, they will measure the velocity of a second train moving westward differently. Albert Einstein
Albert Einstein photographed in 1921,
a few years after he finished his
general theory of relativity.
(Photograph by Ferdinand Schmutzer, via Wikimedia)Let’s say that the westward train moves toward Sarah and the platform — her frame of reference — at 100 miles an hour. According to classical mechanics, Joe will perceive the train to be moving toward his train — his frame of reference — at 150 miles per hour. So far, so classical. But now consider how they will each measure a beam of light traveling