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Albert Michelson: The pioneer of interferometry
Albert Michelson

Albert Abraham Michelson was born on December 19, 1852 in Strelno, Prussia (today Strzelno, Poland). He came to the United States with his parents when he was two years old. From New York, the family made its way to Virginia City, Nevada and San Francisco.

At 17, Michelson entered the U.S. Naval Academy at Annapolis, Maryland, and graduated in 1873. Early on Michelson was fascinated with the sciences and the problem of measuring the speed of light in particular. After two years of studies in Europe he resigned from the navy in 1881 and set out to determine the speed of light with an unprecedented accuracy using his interferometer. His value remained the best for a generation and when it was improved, Michelson was the one who did it.

In 1883 he accepted a position as professor of physics at the Case School of Applied Science in Cleveland and concentrated on improving his interferometer. By 1887 with the help of his colleague Edward Williams Morley he conducted what was to be known as the Michelson-Morley experiment. Their experiment showed that there was no significant motion of the Earth relative to the ether, the hypothetical medium in which light waves were supposed to travel. This result later became the foundation of Einstein's Theory of Relativity.

After serving as professor at Clark University at Worcester, Massachusetts from 1889, in 1892 Michelson was appointed professor and the first head of the department of physics at the newly organized University of Chicago. In 1907 Michelson became the first American to receive a Nobel prize in physics.

Michelson died on May 9, 1931, in Pasadena, California.

Michelson's interferometer and the ether debate

The nature of light was a matter of intense study during the second half of last century and until the beginning of this century. There was mounting evidence that light is an electromagnetic wave that behaves according to Maxwell's formulas. The strongest and long-known argument was that white light can be split up into a spectrum of colors through a prism or a diffraction grating. The reason that diffraction is not more commonly observed was readily explained by the short wavelength of light. However, so far every wave had to have a medium in which it traveled. Sound waves travel through air, ocean waves on water and so forth. Light was known to travel through the apparent empty space of evacuated laboratory vessels and through the vast distances of interstellar space. This puzzle left physicists to wonder what the medium was that light was traveling in.

At this point scientists had postulated a hypothetical medium that they called the "ether" (also spelled "aether") that was thought to be all pervading, or penetrating any enclosure with ease. However, if that were true, the motion of the Earth around the Sun would result in a noticeable motion of the Earth relative to the ether. Much like a boat cruising over the ocean, this motion should be measurable. Earth's velocity in its orbit is substantial at about 30 kilometers per second (18.6 miles per second), but it is still only 1 ten-thousanth of the speed of light. Therefore, any measurements of this effect had to be extremely accurate.

The situation is much like having two swimmers go the same distance but in different directions or paths. The first swimmer swims the width of a river that is 50 meters wide. The second swimmer swims up-stream, 50 meters along the bank of the river. If both swim at the same speed relative to the water, the swimmer who has to swim up-stream will take longer to return than the swimmer who has to swim the width of the river.

Michelson used the same principle of the swimmers in his first interferometer. He split up one ray of light into two beams and sent them on two equally long, separate paths that are at a right angle to each other. Then he reunited the two light beams into one. He expected any change in the time the two beams would take along their paths to change the relative position of the crests and troughs of the two light waves. This would result in a changing interference pattern that he would be able to observe.

The surprising result of this experiment, and it has since been repeated over and over again with very high accuracy, is that there is no measurable motion of the Earth relative to the ether. This result left physicists stunned for many years and some of them postulated that the ether, while real, was in principle unobservable. Albert Einstein finally took a brave step forward with the publication of his theory of special relativity in 1906. His apparently innocent and reasonable argument was that, if the ether was unobservable, or in other words that there was no experimental proof of it whatsoever, the simplest explanation was that it did not exist.

The surprising consequence of this innocent statement was however, that time itself did pass at different rates along the two paths of Michelson's interferometer. It was this stunning consequence that took scientists years to accept and only in light of the overwhelming experimental proof did they accept as fact what seemingly runs against all intuition.

The first measurement of a star's diameter

The 20-foot beam on top of the 100-inch Hooker Telescope on Mount Wilson in Southern California.

The stars on the night sky appear point-like because the distance to them is so large that our eyes do not resolve their disks. The only star on the sky that we can see as an extended disk is our own star, the Sun. Remember that light from the Sun travels only eight minutes to reach Earth, whereas the light travelling time even from the closest star, Proxima Centauri, is more than 4 years.

This large distance is the reason why we need extremely high resolving telescopes to see and measure the disks of even the closest stars. Here, the resolving power of a telescope is determined by the diameter, or the largest distance between two mirror elements, of a telescope.

Hooker Graphic

In 1919, Albert Michelson enhanced the resolution of the then largest telescope in the world, the 100-inch Hooker telescope on Mt. Wilson, to measure for the first time the diameter of a star. Together with his colleague Pease, Michelson mounted a 20-foot long beam that carried small mirrors on top of the 100-inch telescope. By adding these mirrors, they had increased the effective diameter of the telescope and therefore the resolution of the telescope. The increase in resolution was sufficent to measure for the first time the diameter of the bright, red giant star Betelgeuse. It was also the debut of interferometry in astronomy when it was applied to overcome the limitations of existing instruments.


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