, and the time for the round trip will be correspondingly increased. A simple calculation shows that it is
.
The increase above the time required when the system was at rest is less in this case than the preceding. Hence, if the apparatus is moving through the ether, the flashes reflected from M and N will not return at the same instant.
For such velocities as are attainable—even the 18 miles per second of the earth in its orbit—the difference is less than a hundred-millionth of the elapsed time. Nevertheless, Michelson and Morley tried to detect it in their famous experiment.
A beam of light was allowed to fall obliquely upon a clear glass mirror (placed at O in the diagram) which reflected part of it toward the mirror, M, and let the rest pass through to the mirror N. By reuniting the beams after their round trips, it was possible to tell whether one had gained upon the other by even a small fraction of the time of vibration of a single light wave. The apparatus was so sensitive that the predicted difference, though amounting to less than a millionth part of a billionth of a second, could easily have been measured; but they actually found no difference at all—though the earth is certainly in motion.
Other optical experiments, more intricate, and even more delicate, were attempted, with the same object of detecting the motion of the earth through the ether; and they all failed.
The Special Theory and Its Surprising Consequences
It was upon these facts that Einstein based his original, or “special” theory of Relativity. He assumed boldly that the universe is so constituted that uniform straight-ahead motion of an observer and all his apparatus will not produce any difference whatever in the result of any physical process or experiment of any kind. Granting this, it follows that if all objects in the visible universe were moving uniformly together in any direction, no matter how fast, we could not find this out at all. We cannot determine whether the universe, as a whole, is at rest or in motion, and may as well make one guess as another. Only the relative motions of its parts can be detected or studied.