Michelson , Albert Abraham

(1852–1931) American physicist

Michelson, who was born at Strelno (now Strzelno in Poland), came to America with his parents when he was two years old. He graduated from the US Naval Academy in 1873 and remained there to teach physics and chemistry. Some five years later he began his work on measuring the speed of light and to this end he traveled to Europe to study optics at the Collège de France, Heidelberg, and Berlin. When he returned to America he left the navy to become professor of physics at the Case School, Cleveland. In 1882 he estimated the speed of light as 186,320 miles per second. This was the most accurate value then available and remained so for another ten years, when Michelson made an even more accurate measurement.

In the course of this work Michelson developed an interferometer, an instrument that can divide a beam of light in two, send the beams in different directions, and then unite them again. If the two beams traveled the same distance at different speeds (or different distances at the same speed) then, on being brought together again, the waves would be out of step and produce interference fringes on a screen. Michelson used the interferometer to test whether light travelng in the same direction as the Earth moves more slowly than light traveling at right angles to the Earth's surface. This was effectively testing the presence of the ‘ether’ – a substance that was supposed to exist in all space beyond the Earth's atmosphere. Because the ether was thought to be motionless and the Earth moved through it, it followed that light traveling in the same direction as the Earth would be more impeded than light going at right angles to it.

Michelson first conducted this experiment in 1881 in Berlin and got a negative result, that is there were no interference fringes and thus no evidence that the two beams were traveling at different speeds. He repeated the procedure several times under increasingly elaborate conditions until, in 1887, with Edward Morley (1838–1923), the experiment was made under near perfect conditions at the Case School. Again the ether could not be detected and physicists had seriously to consider that the ether did not exist. This result questioned much orthodox physical theory, and it remained for Einstein to develop the special theory of relativity to explain the constancy of the speed of light. Michelson was awarded the 1907 Nobel Prize in physics for this work.

Others, however, continued to report that they had found a measurable difference. Thus Dayton Miller (1866–1941), professor of physics at the Case School of Applied Science, Cleveland, Ohio took his equipment in 1925 to the 6000-ft summit of Mount Wilson in California. He claimed to have detected a difference of 6 miles per second for light travelling at right angles to the Earth's orbit. It was later established, however, that Miller's results were caused by different temperature conditions.

Michelson also applied interference techniques to astronomical measurements and was able to measure the diameters of various heavenly bodies by contrasting the light emitted from both sides. He also continued to make increasingly more accurate estimates of the speed of light and he suggested that the wavelength of light waves should be used as the length standard rather than the platinum–iridium meter in Paris. This suggestion was taken up in 1960 when light waves from the inert gas krypton became the standard measure.