The Michelson-Morley Experiment
The Michelson-Morley Experiment
Measuring Light. American physicist Albert Abraham Michelson (1852-1931) was interested in precise measurement, particularly of the velocity of light. According to the theory current during the second half of the nineteenth century, light traveled in waves through an invisible and imponderable medium called “the ether,” and there were several theories concerning how observations of the velocity of light might be affected by the movement of the Earth, which was thought to drag the ether along with it by gravity through its orbit. Michelson believed that since the ether is at rest and the Earth moves through it, the speed of light as observed on the Earth’s surface should depend on whether it is traveling in the same direction as the Earth’s orbital motion or against it. In 1881, while studying in the physics laboratory of Hermann von Helmholtz in Berlin, Michelson designed an instrument called the interferometer, which used a mirror to split a light beam into two halves traveling in opposite directions. He used it to test his hypothesis with a null result in 1881, but working in partnership with Edward W. Morley (1838-1923) in Cleveland, Ohio, in July 1887, he conducted another experiment with the interferometer that yielded results. He tried to measure the “ether drift,” or effect of the Earth’s motion
through the ether on the speed of light, by observing the extent of the phase shift between the two beams as they rotated through ninety degrees. Although the instrument was highly sensitive, no difference could be detected in the velocity of the two halves of the split beam, thus disproving Michelson’s hypothesis. Deciding that the entire solar system might be moving contrary to the Earth, Michelson decided to repeat the experiment at three-month intervals. In 1897 he even staged the experiment on a mountain top, trying to detect the “ether-wind” as far from Earth’s surface as he could. Again he found no evidence of the ether influencing the speed of light.
The End of the Ether Theory. The Michelson-Morley experiment has been described as “the greatest negative experiment in the history of science” because, although Michelson refused to admit it, the experiment failed to prove that the ether existed. As Albert Einstein (1879-1955) showed in his special theory of relativity (1905), the velocity of light is constant, and, therefore, ether does not exist. Whether Einstein had the Michelson-Morley experiment in mind when he formulated his special theory of relativity, whether he had once heard of it but had forgotten, or whether he was completely unaware of it is still keenly debated by historians of science. Michelson and Morley’s experiment clearly demonstrates how firmly physicists of the late nineteenth century were wedded to a mechanical view of nature that could not admit the existence of action at a distance or through a vacuum. They conceived of the ether only because they thought that light had to travel through a medium. During the first two decades of the twentieth century Einstein and other physicists disproved this idea, developing the quantum theory to show that light is made up of particles, as Michelson believed, but also moves in waves.
Loyd S. Swenson Jr., The Ethereal Aether: A History of the Michelson-Morley-Miller Aether Drift Experiments, 1880-1930 (Austin: University of Texas Press, 1972).