Michelson-Morley Experiment

Designing an experiment that would detect the 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 second (300,000 km/sec) and the speed of the 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 travelled 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 yourself to be at rest with the interferometry. From the instru ment's point of view, it is the ether that moves, creating an ether wind which would push against the light beams. If the 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.