Abstract

Einstein expounded his notion that simultaneity is relative to the observer by describing a conceivable experience in which lightning strikes at both ends of a rapidly moving and very long train. Someone stands at the center of the train, so that (as an outside observer might view it) he is moving toward the signal traveling from the lightning stroke at the locomotive end. He is also traveling away from the signal from the other stroke, which must pursue and catch up with him. (This is what the outsider would say.) Because of this he declares that there was a stroke at the locomotive before the stroke at the other end. The signals traveled equal distances, he says, when they went from the ends to the middle, and he saw one before he saw the other. On the track, however, stands an observer at the center of the distance between the two strokes (which leave marks on both track and train). He sees the signals from both ends simultaneously and disagrees with the tiem order claimed by the observer on the train. If motion is relative, said Einstein, who is to say that one of these observers is right because he was "standing still" and the other wrong?...