Michelson-Morley Experiment (1887): The Search for the Ether

The Michelson-Morley experiment, carried out by Albert A. Michelson and Edward W. Morley in Cleveland, Ohio, between April and July 1887, tried to detect the Earth’s motion through the “luminiferous ether”, the invisible medium that physicists believed carried light. Using a highly sensitive interferometer, they found almost no effect. This famous null result undermined the ether theory and later became a cornerstone of the evidence for Albert Einstein’s special theory of relativity.

Background: light and the ether

By the late nineteenth century, physicists knew that light behaves as a wave, and James Clerk Maxwell had shown it to be an electromagnetic wave (see Maxwell’s electromagnetic theory). Sound waves need air and water waves need water, so most scientists assumed light also needed a medium. They called it the luminiferous (“light-bearing”) ether and imagined it filling all of space.

If the ether was at rest, the Earth, moving around the Sun at about 30 km per second, should be travelling through it. To an observer on Earth this should produce an “ether wind”, making light move at slightly different speeds in different directions. The difference was tiny, about one part in 100 million when compared over a return journey, so detecting it required extraordinary precision.

Michelson’s interferometer

Albert Michelson, a former United States Navy officer and expert on measuring the speed of light, invented an instrument for the task. His interferometer split a beam of light into two beams at right angles, sent each to a mirror and back, and then recombined them. When the beams met, they produced a pattern of light and dark bands called interference fringes. If one beam took slightly longer than the other, the fringes would shift.

Michelson first tried the experiment in 1881 while studying in Germany, at the Astrophysical Observatory in Potsdam. He found no clear effect, but his apparatus was not sensitive enough to be conclusive, and there were errors in his calculations.

The 1887 experiment

By the mid-1880s Michelson was professor of physics at the Case School of Applied Science in Cleveland, and he joined forces with Edward Morley, a chemist at the neighbouring Western Reserve University known for careful measurements. They built a much improved apparatus in a basement room at Case.

The optical parts were mounted on a square slab of sandstone about 1.5 metres across, which floated on a trough of mercury so that it could be turned smoothly and slowly without disturbing the alignment. Mirrors reflected the beams back and forth several times, lengthening each path to about 11 metres. As the slab rotated, the two arms would face the ether wind in turn, and the fringes should have shifted back and forth.

The main observations were made in July 1887, with readings taken around noon and in the evening as the slab rotated. Based on the Earth’s orbital speed, Michelson and Morley expected a shift of about 0.4 of a fringe. What they saw was far smaller: according to their paper, the shift was certainly less than a twentieth of a fringe and probably less than a fortieth, within the range of experimental error. They concluded that any relative motion between the Earth and the ether was probably less than one-sixth of the Earth’s orbital speed. Their paper appeared in the American Journal of Science in November 1887.

Attempts to explain the result

The result was puzzling, and Michelson himself considered it a failure. Several explanations were proposed. One idea was that the Earth dragged the ether along with it, but this conflicted with other observations of starlight.

In 1889 the Irish physicist George FitzGerald, and independently in 1892 the Dutch physicist Hendrik Lorentz, suggested that objects moving through the ether contract slightly in the direction of motion, just enough to hide the effect. Lorentz developed this into a more complete theory, including mathematical rules now called the Lorentz transformations.

In 1905 Albert Einstein offered a more radical answer in his special theory of relativity. He proposed that the speed of light in a vacuum is the same for all observers moving uniformly, and that no ether is needed at all. The null result then follows naturally.

Myths and facts

• Myth: Einstein built relativity to explain this experiment. Einstein’s own accounts differ, and historians generally think he was led mainly by theoretical problems in Maxwell’s electromagnetism, though he knew of the ether-drift experiments.
• Myth: one experiment immediately killed the ether. Many physicists kept the ether for decades. In the 1920s Dayton Miller, who had worked with Morley, claimed to detect a small ether drift, but later analysis and many more precise experiments did not confirm it.

Timeline

• 1881: Michelson performs his first interferometer experiment at Potsdam.
• 1887, April–July: Michelson and Morley build and run their improved apparatus in Cleveland.
• 1887, July: The main series of observations is made.
• 1887, November: Their paper is published in the American Journal of Science.
• 1889: George FitzGerald proposes length contraction.
• 1892: Hendrik Lorentz independently proposes contraction.
• 1905: Einstein publishes the special theory of relativity.
• 1907: Michelson receives the Nobel Prize in Physics.

Why it matters

The Michelson-Morley experiment is often called the most famous failed experiment in history. Its null result helped overturn the ether theory, and the interferometer it used remains a basic tool of physics, from measuring tiny distances to detecting gravitational waves.