The search for an invisible wind
In the late 19th century, physicists believed that light, like sound, needed a medium to travel through. They called this invisible, weightless substance the "luminiferous ether". It was thought to fill the entire universe, remaining stationary as planets, stars, and galaxies moved through it. If this ether existed, then the Earth, orbiting the sun at approximately 30 kilometers per second, should experience an "ether wind". Detecting this wind was a primary goal of physics.
Two scientists at Cleveland's Case School of Applied Science and Western Reserve University decided to find it. Albert A. Michelson, a physicist, and Edward W. Morley, a chemist, devised an ingenious experiment between April and July 1887. They worked in the stone basement of Adelbert Dormitory to ensure stability for their sensitive equipment. The original building, later renamed Pierce Hall, was demolished in 1962.
An elegant machine finds nothing
Michelson designed a device called an interferometer. It split a beam of light, sending the two halves along perpendicular paths of equal length. The beams bounced off mirrors and were then recombined. If an ether wind existed, it should slow one beam more than the other, creating a measurable shift in the resulting interference pattern.
To eliminate vibrations, the entire apparatus was mounted on a massive sandstone slab that floated in a trough of liquid mercury. Despite its bulk, the slab could be rotated smoothly to align the interferometer's arms with and against the supposed ether wind. The expected shift in the light pattern was small—just 0.4 of a fringe—but well within the device's ability to detect.
They ran the experiment repeatedly, at different times of day and in different seasons, to account for the Earth's changing direction of travel. The result was always the same: no significant shift in the interference pattern. The observed fringe shift was consistently less than 0.01. The ether wind could not be found.
This "null result" was considered a failure. It deeply puzzled the scientific community and directly contradicted the established theory of light propagation. Yet this perplexing outcome became one of the most important "failures" in the history of science. It provided strong evidence against the existence of the luminiferous ether, allowing for a new understanding of physics. In 1905, Albert Einstein's theory of special relativity would posit that the speed of light is a universal constant for all observers, making the ether an unnecessary concept.