Michelson–Morley experiment

In 1887 Albert Michelson and Edward Morley built an exquisitely sensitive instrument, an interferometer, to catch the aether wind that Earth's motion through space was expected to create. Their device split a beam of light into two perpendicular paths, bounced each off a mirror, and recombined them so that the tiniest difference in travel time would show up as a shift in a pattern of bright and dark fringes. If light moved through a fixed aether, the two arms — one along Earth's motion, one across it — should take slightly different times.

To overcome how small the predicted effect was, they floated the whole apparatus on a pool of mercury and slowly rotated it, watching for the fringes to drift as the arms swapped orientation relative to the supposed wind. They repeated the measurement at different times of day and seasons of the year, when Earth's direction of travel changes. The result was a famous null: no significant fringe shift appeared, far smaller than the aether theory demanded.

This was one of the most important 'failed' experiments in history. It showed that light's measured speed simply did not depend on Earth's motion, a stubborn fact that the aether picture could not honestly accommodate. Einstein later elevated exactly this constancy to a founding postulate of special relativity, turning a baffling negative result into a cornerstone of modern physics.