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Michelson, Albert Abraham

ether light physics waves

[mik elsn] (1852–1931) US physicist: devised optical measurement methods of great accuracy; and showed that the hypothetical ether probably did not exist.

Born in Strelno (now in Poland), Michelson emigrated with his parents to the USA as a child of 4. At 17 he entered the Annapolis Naval Academy (after an entry appeal in which he saw President Grant) and, following graduation and a tour of duty at sea, was appointed as instructor in physics and chemistry there.

His interest in science was apparently much increased from this time and, when he needed to demonstrate to the midshipmen how the speed of light can be measured, he applied himself to improving the accuracy of the measurement. It is certainly of fundamental importance for physics (and for navigation) and Michelson was to measure it with increasing accuracy throughout his life. The optical devices he used for this, based on his interferometer, were useful for a variety of purposes in physics.

In the early 1880s he visited Europe for 2 years on study leave, and his first interferometer was built in laboratory and paid . It allowed the speed of light to be compared in two pencils of light split from a single beam. One result of this work concerned the so-called ether. Since waves such as sound waves or water waves require a substance or ‘medium’ for their transmission, it had been widely presumed that light and other electromagnetic waves must likewise require a medium, and a hypothetical ether, invisible, universal and weightless, had been invented for the purpose. However, Michelson’s refined results would show the effect on light of the Earth’s motion through the ether; but there was no effect and physicists were forced to doubt if the ether really existed.

In 1881 Michelson left the Navy, and next year became professor of physics in Cleveland, OH. There he continued and improved his optical measurements, and with (the professor of chemistry) confirmed the null result on the ether in 1887. This classical Michelson–Morley experiment, a major result in physics, won him a national prize in 1888, ‘not only for what he has established, but also for what he has unsettled’. In a sense the problem was not ‘settled’ until 1905 when theory of relativity dispensed with the need for ether.

Michelson went on to apply his ingenuity and skill in optics to measure the metre in terms of the wavelength of light and to solve some astronomical problems (he was the first to measure the angular diameter of a star; it was Betelgeuse, and the margin of error was equivalent to a pinhead’s width at a distance of 1000 miles) and to refine his value for the velocity of light (close to 3 × 10 8 m s –1 in air). He also discovered new features of spectra; and he used his interferometer to measure tidal movement due to the Moon’s effect not on the seas, but on the solid Earth. From 1890 until his death he worked at Chicago; in 1907 he became the first American awarded a Nobel Prize.

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