The Michelson–Morley experiment found no ether wind in 1887

The Michelson–Morley experiment of 1887 looked for Earth’s motion through the “luminiferous ether”, the medium physicists believed carried light, and found none: any shift in its light pattern was less than a twentieth of what the ether theory predicted.

Albert Michelson of the Case School of Applied Science and chemist Edward Morley of Western Reserve ran their observations in concentrated sessions between April and July 1887, in the basement of a dormitory in Cleveland.

How did the experiment look for the ether?

It raced light against itself. Michelson’s interferometer split a beam of light along two perpendicular arms and recombined it. If Earth were ploughing through an ether, light should take slightly longer along one arm, nudging the interference fringes as the instrument turned. Morley’s cure for vibration was to mount it on a heavy sandstone slab floating in a pool of mercury, so it could rotate smoothly.

It was a rerun. Michelson had tried a smaller version at Potsdam in 1881 and seen nothing, and Hendrik Lorentz had dismissed that result, doubting its accuracy. The Cleveland measurements were the first accurate enough that the null result could not be waved away.

Did the null result inspire Einstein?

Not as neatly as the textbook story, which runs straight from the null result to special relativity in 1905, suggests. FitzGerald and then Lorentz (in 1892) answered the puzzle first, proposing that moving objects shrink along their direction of travel, a patch that kept the ether alive. Einstein, for his part, was always reluctant to say that the experiment, or the fixes others built on it, had shaped his thinking. What relativity did was explain why the answer had to be zero.

Michelson won the 1907 Nobel Prize in Physics, the first American to do so. And the test has never stopped: a 2015 version using ultrastable oscillators, logging data for a year, found the speed of light the same in every direction to better than one part in 10¹⁸.