In 1887, two American scientists, Polish-born physicist Albert Abraham Michelson (1852–1931) and physical chemist Edward Williams Morley (1838–1923), performed an experiment that was designed to detect the motion of Earth through a hypothetical medium known as the luminiferous ether. This ether was thought by scientists of this age to be present throughout space. Michelson and Morley made their measurements with a very sensitive optical instrument now called a Michelson interferometer. Their observations showed no indication of movement through the predicted ether. This outcome was unexpected and has become one of the fundamental experimental results in support of the theory of special relativity, developed by German–American physicist Albert Einstein (1879–1955) in 1905. In 1907, Michelson was awarded the Nobel Prize in physics for his work in the field of interferometry, but not specifically for his work with this experiment.
During the 1800s, scientists had become convinced that light was composed of waves, as opposed to a theory that light was made up of particles proposed more than a century earlier by English physicist and mathematician Sir Isaac Newton (1642–1727). They based their belief on experiments that demonstrated phenomena such as interference—the change in intensity caused by mixing two or more beams of light; and diffraction—the fact that beams of light do not always travel in straight lines.
But if light was a wave, what medium did it travel through? Earthquakes produce seismic waves that are transmitted by Earth’s crust and a clanging bell makes sound waves carried by air. Scientists were certain that light had to be transmitted by something, so it was hypothesized that there existed a luminiferous ether. The term luminiferous means light-bearing, but the word ether was not so specific. No substance could be associated with the ether, especially in space where sunlight and starlight travel in what otherwise appears to be a vacuum. The ether was predicted, but had not been observed.
Scientists thought that the ether should be everywhere and that it must be stationary, at rest with respect to absolute space which, following Newton, was believed to exist independently of the objects in it. It was thought that by measuring the motion of Earth relative to the ether it would be possible to observe the latter.
Designing an experiment that would detect Earth’s movement through the ether was a formidable task, requiring the comparison of the speed of light, which was already known to be about 186,300 miles per sececond (300,000 km/sec) and the speed of Earth (almost 18 mi or 30 km per sec). Michelson, who excelled in the art and science of measurement, built an instrument to do the job.
He made use of the interference that occurs between light waves. Light waves are transverse waves, which means that they vibrate perpendicularly to the direction in which they travel. If two waves of light of a single color (monochromatic light) arrive at a screen with their crests and troughs aligned, they will interfere constructively, adding up to make higher crests and lower troughs. If, on the other hand, the waves arrive so that crests coincide with troughs, they will cancel with each other, leaving the screen in darkness.
In Michelson’s apparatus, monochromatic light from a source was sent toward a beam splitter—a partially silvered mirror—where half of the beam continued on to mirror #2 while the other half was reflected along a perpendicular path toward mirror #1. A compensating plate placed in path #1 assured that both beams passed through equal thicknesses of glass. Following reflections at the mirrors the beams returned to the beam splitter where they joined and traveled to the telescope. Because the two rays are not exactly parallel and the wavefronts are not exactly plane the observer would not see all light or all dark, but rather a set of interference fringes—alternating dark and light parallel lines.
With his interferometer Michelson would have been able to measure movement through the ether by noting the change in the position of the fringes as the apparatus was rotated. To understand this, first think of oneself to be at rest with the interferometry. From the instrument’s point of view, it is the ether that moves, creating an ether wind, which would push against the light beams. If Earth moves in the direction of path #2, then the ether wind will be felt in the opposite direction. Beam #2 will act like a sailboat sailing first against the wind and then with it. It will travel slower when it opposes the ether wind but faster when the wind is at its back. In contrast, Beam #1 travels perpendicular to the ether wind on both parts of its trip. Because the ether wind affects each beam by different amounts there is a difference in the times it takes the beams to travel along their respective paths. That difference shows up as a fringe pattern.
The sole presence of the fringe pattern, however, does not allow measurement of Earth’s motion. That is accomplished by rotating the entire instrument. As the two beams change their orientation with respect to the ether wind, their travel times change. That causes the fringes to move or shift from their initial position. By measuring the fringe shift as the interferometer rotated, it should have been possible to measure Earth’s velocity through a stationary luminiferous ether; or, from the laboratory perspective, the velocity of an ether wind across a stationary interferometer.
Michelson performed the experiment for the first time in Germany in 1881. Contrary to his expectations, no fringe shift could be observed. He repeated the experiment in 1887 in the United States, this time in collaboration with Morley. They placed their optical elements on a granite slab, and the slab on a vat full of liquid mercury. They lengthened the path each beam had to travel, and took good care to control the temperature in their laboratory to avoid thermal distortions.
According to their calculations the Michelson interferometer should have registered a fringe shift of about four-tenths (0.4) of a fringe. Instead, no fringe shift was observed. They were forced to conclude that their experiment had shown that the hypothesis of a stationary, luminiferous ether was not correct.
The Michelson-Morley experiment is a perfect example of a null experiment, one in which something that was expected to happen is not observed. The consequences of their observations for the development of physics were profound. Having proven that there could be no stationary ether, physicists tried to advance new theories that would save the ether concept. Michelson himself suggested that the ether might
Absolute space— The concept that space exists independently of the objects that occupy it.
Diffraction— The deviation from a straight path that occurs when a wave passes through an aperture.
Interference— The change in intensity caused by mixing two or more beams of light.
Interference fringes— Alternating dark and bright lines produced by the mixing of two beams of light in an interferometer.
Luminiferous ether— A hypothetical medium proposed to explain the propagation of light. The Michelson-Morley experiment made it necessary to abandon this hypothesis.
Michelson interferometer— An instrument designed to divide a beam of visible light into two beams which travel along different paths until they recombine for observation of the interference fringes that are produced. Interferometers are used to make precision measurements of distances.
Special relativity— The part of Einstein’s theory of relativity that deals only with nonaccelerating (inertial) reference frames.
move, at least near Earth. Others studied the possibility that rigid objects might actually contract as they traveled. But it was Einstein’s theory of special relativity that finally explained their results.
The significance of the Michelson-Morley experiment was not assimilated by the scientific community until after Einstein presented his theory. In fact, when Michelson was awarded the Nobel Prize in physics in 1907, the first American to receive that honor, it was for his measurements of the standard meter using his interferometer. The ether wind experiment was not mentioned.
There has also been some controversy as to how the experiment affected the development of special relativity. Einstein commented that the experiment had only a negligible effect on the formulation of his theory. Clearly, it was not a starting point for him. Yet the experiment has been repeated by others over many years, upholding the original results in every case. Even if special relativity did not spring directly from its results, the Michelson-Morley experiment has convinced many scientists of the accuracy of Einstein’s theory and has remained one of the foundations upon which relativity stands.
See also Relativity, special.
Brooker, Geoffrey. Modern Classical Optics. Oxford, UK: Oxford University Press, 2003.
Chartier, Germain. Introduction to Optics. New York: Springer, 2005.
Mermin, N. David. It’s About Time: Understanding Einstein’s Relativity. Princeton, NJ: Princeton University Press, 2005.
“Special Issue: Michelson-Morley Centennial.” Physics Today (May 1987).
American Institute of Physics. “Michelson-Morley Experiment.” <http://www.aip.org/history/einstein/emc1.htm> (accessed March 20, 2007).