Einstein's theory of relativity was one of the giant leaps of the human intellect. So impressive was it that for years most physicists have accepted it as the fundamental law of the universe, even though no one had devised methods or machines sensitive enough to verify it completely. Last week its most basic concept triumphantly passed the most rigorous test yet.
Crucial in the theory of relativity is the nature and behavior of light. In the 19th century, the prevailing idea was that light is a wave motion, and therefore needs a medium to travel in, as sound waves travel in air. Since light passes unhindered through vacuums, including the vacuum between the stars and the earth, 19th century scientists were driven to postulate a "luminiferous ether." which filled all space. It offered no resistance to the motions of stars or planets, but carried light waves with perfect efficiency.
Ether Wind. The ether had another useful property: it was presumed to be motionless, and therefore it provided the basic frame of reference from which all motions were measured. A star, for instance, could be said to be moving so many miles per hour through the ether. When the earth swung around its orbit, it moved through the ether too, creating an "ether wind" blowing past it.
In 1887 Albert A. Michelson and Edward W. Morley tried to measure this ether wind. Their idea was to measure the speed of light (186,000 miles per second) against the speed of the earth's motion on its orbit around the sun (18.5 miles per second). They set up their apparatus, an affair of many mirrors, in a lab in a downtown Cleveland building. Once a day the earth's rotation aligned the apparatus with the earth's path around the sun. If there were an ether wind, the light flashing across their lab floor should be slowed by 18.5 miles per second. Twelve hours later, the apparatus would be moving in the direction opposite to the earth's orbital motion, and light's speed should have been increased by the same amount. The appa ratus was crude by modern standards, but theoretically sensitive enough to detect this degree of change. No such effect was found. Light seemed to move at the same speed in all directions, regardless of the ether wind. So scientists had to abandon the concept of the luminiferous ether. Light had nothing to travel in, and therefore could not be simple waves. Worse yet. the scientists were left with nothing by which they could measure absolute motion. For awhile man seemed to have lost his bearings in the universe.
Rescue by Theory. In 1905 Albert Einstein announced his Special Theory of Relativity, and rescued physics from the confusion into which it was thrown by loss of the ether. In the new world of Einstein, the speed of light itself was established as the only dependable constant. Thus, to an observer, the speed of light would remain the same, whether the observer was approaching the light's source or speeding away from it. With light doing duty as the universe's basic constant, the ether was no longer needed as a theoretical frame of reference.