November 1887: Michelson and Morley report their failure to detect the luminiferous ether
By the late 1800s, physicists generally believed that light was a wave. Therefore, it was thought, it had to travel through some sort of medium, just as sound waves are vibrations in air. Scientists had believed for centuries that a mysterious, ghostly substance, known as the luminiferous ether, must permeate the universe and serve as a medium for the light waves. Various scientists attempted to detect the ether, with no success. Finally, in 1887 Albert Michelson and Edward Morley carried out their famous experiment, which provided strong evidence against the ether. They reported the results in November of that year, but both thought their experiment a failure and continued to cling to their belief in the ether.
Albert Abraham Michelson was born in Strelno, Germany in 1852. When he was two years old his family moved to the US, and he grew up in the rough mining towns of Murphy’s Camp, California and Virginia City, Nevada. As a youngster, he showed some aptitude for science, and at age 16 he obtained a special appointment to the U.S. Naval Academy from President U.S. Grant.
As a student at the Naval Academy, he excelled at optics and other sciences, and clearly had an aptitude for precision instruments and measurements. He graduated in 1873, and then became an instructor of physics and chemistry at the Naval Academy. In 1877, while conducting a classroom demonstration of Foucault’s measurement of the speed of light, he realized he could make significant improvements on the method. Within the next two years, Michelson managed to measure the speed of light with much greater precision than ever before. The measurement brought him some recognition as a scientist, and settled him on pursuing a career in physics research. He then headed to Europe to study for the next two years.
Working in Berlin, he invented the device known as the Michelson interferometer. He realized he could use the setup to detect the Earth’s velocity through the ether. The basic design is simple and elegant. A beam of light is split and sent down two perpendicular paths. Then, after bouncing off mirrors, the two beams are recombined, producing an interference pattern. If the Earth was indeed traveling through the ether, the speed of light would differ depending on its direction with respect to the Earth’s motion through the ether, and Michelson’s interferometer would pick up a slight shift in the interference fringes. However, these early efforts found no evidence of the Earth’s movement with respect to the ether. Michelson was disappointed by the result and considered the experiment a failure. Nonetheless, he continued his effort to detect the ether when he returned to the United States.
In 1882 Michelson took a position at the Case School of Applied Science in Cleveland, Ohio. There he teamed up with chemist Edward Morley, who helped make some improvements in the experiments Michelson had begun in Berlin. The new apparatus was similar in basic design to his previous ones, but much more sensitive. It used extra mirrors to allow the light beams to bounce back and forth, creating a much longer path length. Michelson and Morley conducted the experiments in a basement lab, and to minimize vibrations, the setup rested atop a huge stone block, which floated in a pool of mercury that allowed the entire apparatus to rotate.
Even with this exquisitely sensitive design, Michelson and Morley couldn’t detect evidence of motion through the ether. They reported their null result in November 1887 in the American Journal of Science, in a paper titled “On the Relative Motion of the Earth and the Luminiferous Ether.” (The paper is online at www.aip.org/history/gap/Michelson/Michelson.html
Though disappointing to Michelson and Morley, the experiment revolutionized physics. Some scientists initially tried to explain the results while keeping the ether concept. For instance, George FitzGerald and Hendrik Lorentz independently proposed that moving objects contract along their direction of motion, making the speed of light appear the same for all observers. Then in 1905 Albert Einstein, with his groundbreaking theory of special relativity, abandoned the ether and explained the Michelson-Morley result, though it is uncertain whether Einstein was actually influenced by their experiment.
Michelson and Morley nonetheless both continued to believe that light must be a vibration in the ether, though Michelson did acknowledge the importance of Einstein’s work on relativity.
Although it couldn’t detect the non-existent ether, the Michelson interferometer proved useful for other measurements. Michelson used his interferometer to measure the length of the international standard meter in terms of wavelengths of cadmium light, and in 1920 he was the first to measure the angular diameter of a distant star, also using an interferometer. In 1901 Michelson was the second president of the APS, and he became the first American to win the Nobel Prize in 1907, for his precision optical instruments and measurements made with them. In 1889 Michelson moved to Clark University in Worcester, Massachusetts, and then in 1892 to the University of Chicago. He returned to his work refining measurements of the speed of light, and continued making more and more precise measurements right up to his death in 1931.