Suppose a ball were thrown forward at a velocity of 20 kilometers per hour by a man on top of a flatcar that is moving forward at 20 kilometers an hour. To someone watching from the roadside the ball would appear to be travelling at 40 kilometers an hour. The velocity of the thrower is added to the velocity of the ball.
If the ball were thrown forward at 20 kilometers an hour by a man on top of a flatcar that is moving backward at 20 kilometers an hour, then the ball (to someone watching from the roadside) would seem to be not moving at all after it left the hand of the thrower. It would just drop to the ground.
There seemed no reason in the 19th century to suppose that light didn’t behave in the same fashion. It was known to travel at the enormous speed of just a trifle under 300,000 kilometers per second, while earth moved in its orbit about the sun at a speed of about 30 kilometers per second. Surely if a beam of light beginning at some earth-bound source shone in the direction of earth’s travel, it ought to move at a speed of 300,030 kilometers per second. If it shone in the opposite direction, against earth’s motion, it ought to move at a speed of 299,970 kilometers per second.
Could such a small difference in an enormous speed be detected?
Albert A. Michelson
The German-American physicist Albert Abraham Michelson (1852-1931) had invented a delicate instrument, the interferometer, that could compare the velocities of different beams of light with great precision. In 1887 he and a co-worker, the American chemist Edward Williams Morley (1838-1923), tried to measure the comparative speeds of light, using beams headed in different directions. Some of this work was performed at the U. S. Naval Academy and some at the Case Institute.
The results of the Michelson-Morley experiment were unexpected. It showed no difference in the measured speed of light. No matter what the direction of the beam—whether it went in the direction of the earth’s movement, or against it, or at any angle to it—the speed of light always appeared to be exactly the same.
To explain this, the German-Swiss-American scientist Albert Einstein (1879-1955) advanced his “special theory of relativity” in 1905. According to Einstein’s view, speeds could not merely be added. A ball thrown forward at 20 kilometers an hour by a man moving at 20 kilometers an hour in the same direction would not seem to be going 40 kilometers an hour to an observer at the roadside. It would seem to be going very slightly less than 40 kilometers an hour; so slightly less that the difference couldn’t be measured.
However, as speeds grew higher and higher, the discrepancy in the addition grew greater and greater (according to a formula Einstein derived) until, at velocities of tens of thousands of kilometers per hour, that discrepancy could be easily measured. At the speed of light, which Einstein showed was a limiting velocity that an observer would never reach, the discrepancy became so great that the speed of the light source, however great, added or subtracted zero to or from the speed of light.