The Practical Significance of Relativity

Einstein's Theories of Relativity and Gravitation by Albert Einstein, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. The Practical Significance of Relativity

THE PRACTICAL SIGNIFICANCE OF RELATIVITY

The Best Discussion of the Special Theory Among All the Competing Essays

BY PROFESSOR HENRY NORRIS RUSSELL, PRINCETON UNIVERSITY

Can a small child catch a baseball moving sixty miles an hour without getting hurt? We should probably answer “No”—but suppose that the boy and his father were sitting side by side in an express train, and the ball was tossed lightly from one to the other. Then there would be no trouble about it, whether the train was standing still, or going at full speed. Only the relative motion of ball and boy would count.

This every-day experience is a good illustration of the much discussed Principle of Relativity, in its simplest form. If there were no jolting, the motion of the train, straight ahead at a uniform speed, would have no effect at all upon the relative motions of objects inside it, nor on the forces required to produce or change these motions. Indeed, the motion of the earth in its orbit, which is free from all jar, but a thousand times faster, does not influence even the most delicate apparatus. We are quite unconscious of it, and would not know that the earth [307]was moving, if we could not see other bodies outside it. This sort of relativity has been recognized for more than two centuries and lies at the bottom of all our ordinary dynamical reasoning, upon which both science and engineering are based.

But there are other things in nature besides moving bodies,—above all, light, which is intimately related to electricity and magnetism, and can travel through empty space, between the stars. It moves at the enormous speed of 186,000 miles per second, and behaves exactly like a series of vibrations or “waves.” We naturally think of it as travelling through some medium, and call this thing, which carries the light, the “ether.”

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 [309]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.

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