Michelson Measures the Diameter of a Star

Date December 13, 1920

Albert A. Michelson accomplished the first precise measurement of a star other than the Sun.

Locale Mount Wilson Observatory, California

Key Figures

• Albert A. Michelson (1852-1931), German American physicist
• Edward Williams Morley (1838-1923), American chemist
• Arthur Stanley Eddington (1882-1944), English astronomer and physicist
• Edwin Powell Hubble (1889-1953), American astronomer
• George Ellery Hale (1868-1938), American astronomer
• Henrietta Swan Leavitt (1868-1921), American astronomer

Summary of Event

Albert A. Michelson ranks as a distinguished scientist who was one of the finest instrument makers and experimental physicists of his time. Over several centuries, a body of scientific experiments had accumulated that demonstrated that light travels in a wave pattern. Because other wave motions, such as sound waves and water waves, travel in particular media, scientists assigned to light a medium called “ether.” One method of proving the existence of ether was to demonstrate the effect produced by the medium. For example, the sound of a train whistle traveling toward an observer has a higher pitch than one moving away from the observer. This happens because the speed of sound added to the speed of the train results in a higher velocity as it is carried toward the observer and a lower velocity while it is moving away. As scientists believed that the earth and all celestial objects move in an ether substance, they concluded that the speed of light traveling in different directions in this ether must have different values.

In 1878, Michelson became interested in measuring the speed of light. While working on this problem, he developed an instrument called an interferential refractometer, which he used to measure the effect of the ether. By splitting a beam of light so that two sections traveled at right angles to each other, with the movement of the earth through the ether, one of the two beams would be affected by the ether. Although this instrument was extremely sensitive, it produced no measurable results. Michelson’s attempts to measure the effect of the ether faced two major problems: first, a debate on whether the ether itself was stationary or moving, hence producing different results; second, the possibility that the instrument itself produced experimental errors.

Undaunted by these setbacks, Michelson teamed up with Edward Williams Morley in 1885 to redesign the experiment. In the process of developing instruments for the experiment, Michelson and Morley undertook the challenge of determining the length of the international meter bar in Paris. By providing an exact length of the meter bar in terms of the light wavelength of cadmium, Michelson transformed light into a standard of measurement. As a result of the precise nature of the experiment, Michelson achieved international recognition.

During this period, Michelson developed another instrument called an interferometer. This device showed the interference patterns for light waves when light waves intersect each other. For example, when light waves hit a screen with two small pinholes, the pattern behind the screen shows alternating patterns of light and dark areas. Similarly, light waves that arrive at different times also will show an interference fringe. No such fringe was found, however.

In 1905, Albert Einstein published his theory of special relativity, which holds that the speed of light is a constant in a vacuum. There was no longer a need for the ether. Over the course of his life, however, Michelson attempted to refine his experiment and prove the existence of the ether.

In 1918, at the end of World War I, Michelson was sixty-six years old but still filled with ideas on new experiments and instruments. Even though he was saddled with increasing administrative responsibilities at the University of Chicago, he began to travel to California for the summers. He divided his time between the California Institute of Technology and the nearby Mount Wilson Observatory. One of his experiments was a return to the ether problem; in another project he used the interferometer to measure the size of a star. Earlier, George Ellery Hale had obtained the funds for the building of the Mount Wilson Observatory and had overseen its construction. By 1908, Hale had constructed a 60-inch (152-centimeter) reflecting telescope at Mount Wilson. Not satisfied with this achievement, he constructed the 100-inch (254-centimeter) Hooker telescope, which was the largest telescope in the world until Hale built the Palomar Observatory in 1929. Michelson saw the possibility of using the Hooker telescope, along with a redesigned interferometer, to measure the diameter of a star. He chose Betelgeuse (Orionis) in the constellation of Orion.

In 1920, the British astronomerArthur Stanley Eddington had made a calculation of the size of Betelgeuse based on surface radiation. This figure, however, was based on theoretical deductions rather than direct instrumental measurement. Consequently, Michelson’s measurement served not only to corroborate Eddington’s figure but also to substantiate part of the theoretical foundations of astronomy. For the experiment, Michelson blacked out all of the star except for the extreme edges; his interferometer picked up the light of these edges through slits. On December 13, 1920, while Michelson was in Chicago, his assistant, Francis Gladheim Pease, reported the final readings that Betelgeuse had a diameter of 386 million kilometers (239.8 million miles). This figure turned out to be very close to the one calculated by Eddington.

Although astronomers expected this result, the information brought an immediate reaction from the public. Most people found the idea that a star has a diameter equal to the orbit of Mars almost beyond comprehension. For scientists, this heralded a new era in which an instrument could measure with precision an object 150 light-years away. Michelson formally announced his findings at a joint meeting of the American Physical Society and the American Association for the Advancement of Science on December 29, 1920.

Significance

When Hale erected the Hooker telescope, it was his intention to foster a revolution in astronomy. Although the science of astronomy extends thousands of years back to the ancient Babylonians, astronomers in the early twentieth century knew little about the evolution of stars, the form and shape of the universe, or the matter within it. Michelson’s measurement of the diameter of a star was one step in the realization of Hale’s vision. In 1928, the members of the Optical Society of America named their annual meeting after Michelson in celebration of the fiftieth anniversary of his first paper on the speed of light. In dedicating the meeting to him, the Society said that Michelson had established light as a standard or “measuring rod” of the physical sciences. If light traveled in a medium, then its speed would depend on the relative motion of its source. Therefore, the absolute value of the velocity of light in a vacuum can be used as a device to measure the universe.

The task of measuring the universe occupied the talents and activities of astronomers for the first half of the twentieth century. In 1912, while studying a cluster of stars (Cepheids) in the constellation Cepheus, Henrietta Swan Leavitt discovered that it is possible to determine distances by comparing the true brightness of the stars. In 1920, while using this Cepheid method, Harlow Shapley used the Hooker telescope and determined the size and shape of our galaxy. As Hale had promised during the construction of the telescope, the revolution in astronomy was about to begin. In 1924, Edwin Powell Hubble led the charge by measuring the distance to the Andromeda nebula (soon to be classified as a galaxy). By 1929, he theorized Hubble’s law, which stated that galaxies recede from Earth in proportion to their distance from Earth. These discoveries ended the notion that had existed for thousands of years that Earth was not only at the center of our galaxy but also at the center of the universe.

The Michelson interferometer played an important role in this new age of astronomy. The resolving power of this instrument was limited only by the size of the rigid and vibration-free grid that can be constructed. In addition, during adverse sighting conditions, the instrument can continue to separate the light source distinctly. However, Michelson’s contributions to more precise measurements of the universe did not end a series of debates on the theoretical foundations of astronomy, such as the rate of expansion and the age and formation of the universe. Toward the end of the 1940’s, the “big bang” theory was one resolution of these problems.

Bibliography

Clark, David H., and Matthew D. H. Clark. Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe. New Brunswick, N.J.: Rutgers University Press, 2004. Relates the stories of the scientists who have contributed to current knowledge about the size, mass, and age of the universe. Chapters 4 and 5 include discussion of the work of Hubble and Leavitt. Features glossary, bibliography, and index.

Jaffe, Bernard. Michelson and the Speed of Light. 1960. Westport, Conn.: Greenwood Press, 1979. Solid source of information on a number of Michelson’s experiments, accessible to the general reader. Details the scientific debates involving the speed of light.

Livingston, Dorothy Michelson. The Master of Light: A Biography of Albert A. Michelson. New York: Charles Scribner’s, 1973. Amusing, gossipy, and highly readable account of Michelson’s life by his daughter. Includes her collection of papers and memorabilia.

Millikan, Robert A. “Biographical Memoirs of Albert Abraham Michelson, 1852-1931.” Biographical Memoirs of the National Academy of Sciences 19 (1938): 120-147. Covers Michelson’s career and offers some observations on his contributions to science. Millikan, a highly respected scientist, was both a student and colleague of Michelson.

Swenson, Lloyd S., Jr. The Ethereal Aether: A History of the Michelson-Morley-Miller Aether-Drift Experiments, 1880-1930. Austin: University of Texas Press, 1972. Focuses less on Michelson than on the resolve of solid and creative scientists to prove a theory. Includes some technical discussion best suited for readers with science background.

Watson, Fred. Stargazer: The Life and Times of the Telescope. New York: Da Capo Press, 2005. History of the telescope’s development includes discussion of the impacts on society of the discoveries the instrument has made possible. Presents the stories of the astronomers and other scientists responsible for advances in telescope technology.

Wright, Helen. Explorer of the Universe: A Biography of George Ellery Hale. 1966. Reprint. Melville, N.Y.: American Institute of Physics Press, 1994. Describes the life and activities of one of the pioneers in the development of large-scale, high-technology science. Includes drawings and photographs.