history of spectroscopy

"On the Relative Motion of the Earth and the Luminiferous Ether"*

J E A N E T T E G R A S S E L L I Standard Oil, Research & Development, 4440 Warrensville Center Road, Cleveland, Ohio 44128

Index Headings : History of spectroscopy; Albert A. Michelson; Edward W. Morley; Michelson-Morley experiment.

The paper titled "On the Relative Motion of the Earth and the Luminiferous Ether" was published in 1887 by Albert A. Michelson, Professor of Physics, and Edward W. Morley, Professor of Chemistry (Fig. 1), at what is now called Case Western Reserve University. It dem- onstrated that the speed of light is unaffected by the motion of the earth. This finding was contrary to then- current thinking and was an important part of the ex- perimental framework that paved the way for Einstein's theory of relativity, and the birth of modern physics.

The centennial celebration of the famous Michelson- Morley, or "ether drift," experiment is being held in Cleveland, Ohio, from April to October of this year. Its theme is "Light, Space, and Time," and it reflects the broad intellectual and cultural interests of Michelson and Morley and the age of creativity in which they lived. The various centennial programs also show how their work contributed to future developments in pure and applied science and how their experiment helped reshape thinking about man's relationship to a universe in which neither absolute space nor absolute time exists.

In honor of its anniversary celebration, it seems ap- propriate to review the history and background of this unique experiment. Although every scientist is generally aware of the outcome of the experiment, and every as- tronomer and infrared spectroscopist is intimately fa- miliar with a Michelson interferometer, I, for one, must confess that the details of the apparatus and the scientific concepts of the period around 1885-1887 that led to the experiment were pretty fuzzy in my mind. A friend re- ferred me to a biography of Albert M. Michelson, The Matter of Light (The University of Chicago Press), writ- ten by his daughter, Dorothy Michelson Livingston. I was spellbound for several evenings as I entered the world of physics in the late nineteenth century, encountering giants such as Lord Rayleigh, Maxwell, Bunsen, Row- land, Helmholtz, Hertz, Lorentz, Alexander Graham Bell, Stokes, Fresnel, and Lord Kelvin. Michelson, the exact- ing scientist, deraanding teacher, and loving father, is

Received 11 April 1987. * Th i s article appeared, in somewha t al tered form, as a "Le t t e r f rom

America ," in European Spectroscopy News.

warmly drawn; and I followed the details of the aston- ishing experiment with incredulity.

In 1887, when Michelson and Morley performed their famous experiment, it was thought that light was a wave motion in a mysterious substance called the "ether." If this were true, then it should be possible to determine the speed of the earth through the ether by measuring the speed of light in different directions. Light moving against an "ether wind" would travel more slowly than light moving across such a wind. Because light travels exceedingly fast, it was not possible to measure its speed directly in different directions. It was, however, possible to measure small differences in speed using an interfer- ometer, an instrument that Michelson had been im- proving for many years.

Figure 2 shows the mechanics of the equipment and Fig. 3 gives the paths of light in the interferometer-- which traveled a total of 36 feet! (This beautiful pattern is also the logo for the Michelson-Morley Centennial Celebration.) The experiment was so delicate that vi- brations from nearby activity would have made accurate readings impossible. In order that all vibration might be eliminated, the optical equipment that comprised the interferometer was positioned on a five-foot square sand- stone slab, which was mounted on a doughnut-shaped block of wood, which was in turn floated on a pool of mercury in a cast-iron trough. The use of liquid mercury (what would our Environmental Protection Agency say about this!) eliminated virtually all vibrations and en- abled the interferometer to be rotated smoothly on its axis around all points of the compass for the various readings required. A centering pin prevented the float from bumping into the sides of the cast-iron trough but was engaged only while the interferometer was being set into rotation.

Once set in motion, the slab and interferometer would rotate slowly (about once in 6 minutes) for hours at a time. No starting and stopping was necessary, because the slow movement permitted accurate readings while the apparatus rotated. The height of the interferometer was eye level. A single beam of light from the lamp (a, in Fig. 3) is split in two by a half-silvered mirror (b), so that half travels in one direction while the other half moves at right angles to the first. After several reflections at the mirrors (d or d'), the light is recombined and is observed through the telescope (f). If the motion of the

Volume 41, Number 6, 1987 ooo3-vo28/87/4~o6-o93352.oo/o APPLIED SPECTROSCOPY 9:33 © 1987 Society for Applied Spectroscopy

Albert A. Mlchelson (1887) Edward W, Morley (c. 1885)

FIc. 1. (Left) Albert A. Michelson (1887). (Right) Edward W. Morley (ca. 1885).

earth through the ether affects the speed of light in the two directions differently, then the two halves of the light beam will not match when they are brought together. The result may be that no light will be seen in the tel- escope.

The experiment began in April, 1887. The final read- ings were taken at noon on July 8, 9, and 11 and near 6 p.m. on July 8, 9, and 12. They revealed no difference in the paths of light that could be interpreted as having been affected by the movement of the earth through the ether. Reluctantly, Michelson wrote to Rayleigh:

The experiments on relative motion of earth and ether have been completed and the result is decidedly neg- ative. The expected deviation of the interference fringes from the zero should have been 0.40 of a fringe--the maximum displacement was 0.02 and the average much less than 0.01--and not in the right place.

The result was confusing to the world's scientific com- munity because it was exactly opposite from what the experiment was designed to prove: that the rotation of the earth would have an effect on the speed of light.

But however reluctant physicists of the day were to accept the result, none of them thought of questioning either the experiment or the reliability of the verdict. This was because of the immense respect that was ac- corded the two scientists and the exquisite instrumen- tation they had built to measure slight variations in the

FIG. 3. The paths of light in the interferometer.

speed of light traveling at 186,284 miles per second. Ac- cording to the English astronomer Arthur Stanley Ed- dington, the interferometer could detect "a lag of one- ten-thousand-billionth of a second in the arrival of a light wave."

Shortly before Michelson's death, he attended a gath- ering in honor of Einstein (Fig. 4). Einstein began his prepared remarks by turning to Michelson and saying:

From far away I have come to you; but not to strangers. I have come from among men who for many years have been true comrades with me in my labors. You, my honored Dr. Michelson, began with this work when I was only a little youngster, hardly three feet high. It was you who led the physicists into new paths, and through your marvelous experimental work paved the way for the development of the theory of relativity.

You might be wondering why the Cleveland Centen- nial Celebration of the Michelson-Morley experiment in- cludes a significant number of artistic as well as scientific

Light source Adjustable mirror • M rrors

l Mirrors .,. I Slivered glass plate Mirrors

FIG. 2. The Cleveland interferometer of 1887, mounted on a stone floating in mercury.

Albert Mlchelson. left. and Albert Einstein f1931)

FIG. 4. Albert Michelson, left, and Albert Einstein (1931).

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events. Let me give you a few personal notes about the two men.

Edward Williams Morley (1838-1923) was a major fig- ure in the field of science--both chemistry and physics-- during the late 1800s and early 1900s. As a chemist, Morley determined the densities of the gases hydrogen and oxygen, as well as the proportions of each that, in combination, form water. From this data he calculated the relative densities of hydrogen and oxygen. The im- portance of his results lies in the fact that all of chemistry is based on accurate knowledge of the weights of the elements, and Morley's determinations achieved a new standard of accuracy and reliability. He also conducted intensive experimentation that led to his determination of the atomic weight of oxygen. Although essentially a self-taught scientist, he was regarded as one of the great- est chemists of his era.

Morley, son of a Congregational minister, was born in New Jersey and grew up in Connecticut. He was tutored by his father, then attended Williams College and An- dover Theological Seminary. He came to Ohio in 1868 as a Congregational minister in Twinsburg. Almost im- mediately, he joined the faculty of Western Reserve Uni- versity, then located in Hudson, Ohio. When the Uni- versity moved to Cleveland in 1880, Morley followed. He retired in 1906 and moved to his boyhood hometown, West Hartford, Connecticut, where he died in 1923.

Morley received seven honorary degrees and three gold medal awards in the sciences, including the Davy Medal of the Royal Society in London, England. Highly re- spected by his peers, he served as president of the Amer- ican Association for the Advancement of Science and of the American Chemical Society.

Throughout his life, Morley loved music and was an avid concert goer. In his youth, he had taught himself to play the organ, and for many years he served as a church organist. He was devoted to the study of languages-- both English and foreign--and was known among those he taught as a strict grammarian. Thoroughly proficient in Greek, Latin, Hebrew, French, and German, he also developed a good reading knowledge of Chaldean and Russian.

Of his teaching, it was said that he was always inspiring and that he not only insisted on a mastery of the prin- ciples of chemistry but also inculcated the proper use of the English language, good manners, and clear methods of expression.

Albert Abraham Michelson (1852-1931), the first American to win a Nobel Prize in the sciences, is best known for his contributions to the physics of light and for the elegance and precision of his many experiments. In 1907 he received the Nobel Prize in Physics for his pioneering efforts in measuring the exact speed of light

and for his inventions of precision scientific optical in- struments. In 1920, he invented the first stellar inter- ferometer and used it to make the first accurate mea- surement of a star's diameter. The star was Betelgeuse.

Michelson also developed a standard unit of length by measuring the meter in terms of a light wave. In 1925, the International Committee on Weights and Measures adopted this unit, which has remained the standard length of a meter ever since.

Michelson was born in 1852 in Strzelno, German-oc- cupied Poland, and came to the United States at the age of four. He grew up in the rugged gold and silver mining towns of California and Nevada and later attended high school in San Francisco. When Michelson was 16, Pres- ident Ulysses S. Grant awarded him a special appoint- ment to the U.S. Naval Academy. In 1882, he was ap- pointed the first professor of physics at the newly formed Case School of Applied Science in Cleveland, Ohio.

In 1889, Michelson was appointed to the first chair in physics at Clark University, Worcester, Massachusetts, and in 1894 he accepted the first chair in physics at the University of Chicago. He retired from the University of Chicago in 1929 and moved to Pasadena, California, where his experiments with the velocity of light continued until his death in 1931.

Michelson received eleven honorary doctorates and nine prizes of international prestige in addition to the Nobel Prize. He served as president of the American Physical Society and of the National Academy of Sciences.

In his youth, Michelson studied the violin, and he loved fine music all his life. In addition, he was a gifted artist whose watercolors were considered of professional qual- ity.

So, as you see, the Cleveland Michelson-Morley Cen- tennial Celebration has good reason to salute both the scientific and the artistic interests and talents of Mich- elson and Morley.

Albert A. Michelson wrote, in "Light Waves and Their Uses":

If a poet could at the same time be a physicist, he might convey to others the pleasure, the satisfaction, almost the reverence, which the subject inspires. The aesthetic side of the subject is, I confess, by no means the least attractive to me. Especially is its fascination felt in the branch which deals with light.

As a spectroscopist, I too have felt that fascination shape and influence my life. The history of spectroscopy is our intellectual heritage, and it is a pleasure to ac- knowledge, from time to time, the people who made that history. This year we honor Albert A. Michelson and Edward W. Morley.

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