This seems simple and easy enough to understand. But the consequences which follow from it are extraordinary, and at first acquaintance seem almost absurd.
In the first place, if an observer measures the velocity of light, he must always get the same result, no matter how fast he and his apparatus are moving, or in what direction (so long as the motion is uniform and rectilinear). This sounds harmless; but let us go back to the Michelson-Morley experiment where the light came back in exactly the same time from the two mirrors. If the observer supposes himself to be at rest, he will say that the distances
and
were equal. But if he fancies that the whole universe is moving in the direction
, he will conclude that M is nearer to O than N is—for if they were equidistant, the round-trip would take longer in the first case, as we have proved. If once more he fancies that the universe is moving in the direction
, he will conclude that N is nearer to O than M is. His answer to the question which of the two distances,