Word: ethers
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...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...
...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...
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...
...difficult concept, which even today few laymen and not all scientists fully comprehend. Furthermore, measuring the speed of light is so difficult that the Michelson-Morley experiment and its successors left a nagging possibility that when better apparatus was developed, it might yet detect some trace of an ether wind...
Maser Measure. Then, in 1954, Professor Charles H. Townes of Columbia University invented the ammonia maser. The maser is a device in which ammonia molecules are subjected to electrical excitation, giving off radio microwaves of accurately known frequency. If any sort of ether exists, these waves (which move at the same speed as light) should seem to change their frequency slightly when they are moving against a wind of ether caused by the earth's motion...