From Relativity: The Special and General Theory. While working as a patent clerk in 1905, Einstein published four papers in the Annals of Physics, which, among other things, explained the mathematical basis for special relativity and put forth the equation E=mc2. When one aspect of his theory of general relativity was confirmed by the Royal Society in 1919, the London Times exclaimed, “Revolution in Science—New Theory of the Universe—Newton’s Ideas Overthrown— Momentous Pronouncement—Space ‘Warped.’”
Suppose there is a very long train traveling along the rails with the constant velocity v and in the direction indicated in Fig. 1. Lightning has struck the rails on the railway embankment at two places, A and B, far distant from each other.
People traveling in this train will with advantage use the train as a rigid reference-body (coordinate system); they regard all events in reference to the train. Then every event which takes place along the line also takes place at a particular point of the train. As a natural consequence, the following question arises:
Are two events (e.g., the two strokes of lightning at A and B) which are simultaneous with reference to the railway embankment also simultaneous relatively to the train? We shall show directly that the answer must be in the negative.
When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the midpoint M of the length A —> B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the midpoint of the distance A —> B on the traveling train. Just when the flashes of lightning occur, this point M' naturally coincides with the point M, but it moves toward the right in the diagram with the velocity v of the train. If an observer sitting in the position M' in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e., they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening toward the beam of light coming from B, while he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result:
Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference body (coordinate system) has its own particular time; unless we are told the reference body to which the statement of time refers, there is no meaning in a statement of the time of an event.
Now before the advent of the theory of relativity it had always tacitly been assumed in physics that the statement of time had an absolute significance, i.e., that it is independent of the state of motion of the body of reference. But we have just seen that this assumption is incompatible with the most natural definition of simultaneity.