Albert A. Michelson 1852-1931

In 1878 Michelson began work on what was to be the passion of his life, the accurate measurement of the speed of light. He was able to get useful values with homemade apparatus. Feeling the need to study optics before he could be qualified to make real progress, he travelled to Europe in 1880 and spent two years in Berlin, Heidelberg, and Paris, resigning from the navy in 1881. Upon his return to the United States, he determined the velocity of light to be 299,853 km s^-1, a value that remained the best for a generation. When it was bettered, Michelson bettered it. Albert Michelson, the first chair of Physics Department at Clark University at Worcester, was the first to accurately measure the speed of light. In 1907 he became the first American to win the Nobel Prize in science (Physics). Albert A. Michelson was the first Nobel laureate of the University of Chicago for his measurements of the speed of light.

It was at Clark University that Michelson conducted fundamental research leading to the determination of an international standard of measurement, the meter, in terms of a natural constant, the wave length of cadmium light. His numerous contributions to the physics of light included the invention of optical precision instruments, many of which he used in experiments that marked the beginning of modern physics.

While in Europe, Michelson began constructing an interferometer, a device designed to split a beam of light in two, send the parts along perpendicular paths, then bring them back together. If the light waves had, in the interim, fallen out of step, interfererence fringes of alternating light and dark bands would be obtained. From the width and number of those fringes, unprecedently delicate measurements could be made, comparing the velocity of light rays travelling at right angles to each other. It was Michelson's intention to use the interferometer to measure the Earth's velocity against the "ether" that was then thought to make up the basic substratum of the universe. If the Earth were travelling through the light-conducting ether, then the speed of the light travelling in the same direction would be expected to be equal to the velocity of light plus the velocity of the Earth, whereas the speed of light travelling at right angles to the Earth's path would be expected to travel only at the velocity of light. His earliest experiments in Berlin showed no interference fringes, however, which seemed to signify that there was no difference in the speed of the light rays, and, therefore, no Earth motion relative to the ether. In Cleveland he concentrated his efforts on improving the delicacy of his interferometer experiment. By 1887, with the help of his colleague, American chemist Edward Williams Morley, he was ready to announce the results of what has since come to be called the Michelson-Morley experiment. Those results were still negative; there were no interference fringes and apparently no motion of the Earth relative to the ether. It was perhaps the most significant negative experiment in the history of science. In terms of classical Newtonian physics, the results were paradoxical. Evidently, the speed of light plus any other added velocity was still equal only to the speed of light. To explain the result of the Michelson-Morley experiment, physics had to be recast on a new and more refined foundation, something that resulted, eventually, in Albert Einstein's formulation of the theory of relativity in 1905.

The idea that interferometry might be applied to measure the size of stars was first proposed by a French astronomer, Armand Fizeau, in 1868. But the first real achievement of interferometry was a trailblazing series of experiments by Dr. Albert A. Michelson that began in April 1887. Michelson devised a system using mirrors and semitransparent mirrors (now known as beam splitters) for merging separated beams of light coming from the same object. The optical elements in the system were arranged in such a way that the beams "interfered" with each other; that is, the directions and distances of their light paths were so closely meshed that the beams could interact.

Michelson and his colleague, Dr. Edward W. Morley, used interferometry in a long and futile attempt to detect the existence of "ether," a kind of all-pervasive fluid that many scientists believed extended throughout the universe, providing a medium through which light waves could propagate. The Michelson and Morley experiment consisted of an L-shaped apparatus in which a beam of light was split in two, with the separated beams guided along perpendicular paths of identical length and then recombined. The scientists reasoned that if Earth was moving through a universal ocean of ether, the time needed by the two parts of the split light beam to traverse a sample of the ether current from perpendicular directions should differ slightly and that the difference should be detectable in the resulting pattern of interference fringes. But Michelson never found any difference. This historic null result, as scientists call it, was proof that ether, as it was then imagined, does not exist and that Einstein's special theory of relativity, which offers an alternative explanation for the propagation of light, is correct.

In 1878 Albert A. Michelson first accurately measures the speed of light with $10 worth of apparatus along the seawall.

Mt. San Antonio
In 1923 Michelson returned to the problem of the accurate measurement of the velocity of light. In the California mountains he surveyed a 35-km pathway between two mountain peaks, by sending a beam of light from Mount Wilson to a mirror on Mount San Antonio (the highest mountain in the San Gabriel range) and back again, and determining the distance to an accuracy of less than 2.5 cm. He made use of a special eight-sided revolving mirror and obtained a value of 299,798 km s^-1 for the velocity of light. To refine matters further, he made use of a long, evacuated tube through which a light beam was reflected back and forth until it had travelled 16 km through a vacuum. Michelson died before the results of his final tests could be evaluated, but in 1933 the final figure was announced as 299,774 km s^-1, a value less than 2 km s^-1 higher than the value accepted in the 1970s.

Albert Michelson advocated using some particular wavelength of light as a standard of distance (a suggestion generally accepted in 1960) and, in 1893, measured the standard metre in terms of the red light emitted by heated cadmium. His interferometer made it possible for him to determine the width of heavenly objects by matching the light rays from the two sides and noting the interference fringes that resulted. In 1920, using a 6-metre interferometer attached to a 254-centimetre telescope, he succeeded in measuring the diameter of the star Betelgeuse (Alpha Orionis) as 386,160,000 km (300 times the diameter of the Sun). This was the first substantially accurate determination of the size of a star.

Albert A. Michelson died on May 9, 1931 in Pasadena, California, (U.S.). Albert Einstein, in the same year, publicly paid tribute to Michelson's extensive contributions to science:

THE MASTER OF LIGHT 1984, Cinema Guild VHS, color, 25 mins., 16mm film/video Directed by Steve Michelson Examine the life and scientific career of Albert A. Michelson, a pioneer in the world of physics. His experiments triggered a progressive chain of advances in physics which helped to unravel and unify often opposing theories. It began with the properties of light, destroyed the theory of the luminiferous ether, and laid the groundwork for Einstein's Theory of Relativity as well as contemporary exploration of outer space and atomic energy. In besides its historical and scientific perspective, the film looks at the personal side of Michelson's life. Read more in book of his daughter Dorothy Michelson Livingston (1973): Master of Light: A Biography of Albert A. Michelson. Scribner, New York (376 pp.) or in book of Bernard Jaffe, (1960): Michelson and the Speed of Light. Garden City, N.Y., Doubleday (197 pp.).

Albert A. Michelson, Albert Einstein and Robert A. Millikan at the Califonia Institute of Technology in 1931

Every year, a distinguished scientist is invited to come to the U.S. Naval Academy to present the Michelson Lecture.

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