In the 1887 Michelson–Morley experiment, what did the investigators fail to detect?

The 1887 Michelson–Morley experiment failed to detect Earth’s motion through the ether.

Albert A. Michelson and Edward W. Morley used an optical interferometer to compare the speed of light traveling in perpendicular directions. If Earth moved through a proposed light-carrying medium called the luminiferous ether, the apparatus should have detected a measurable difference as Earth rotated.

The result was effectively null: the expected fringe shift was not observed. The experiment did not immediately prove special relativity, but it seriously weakened the ether hypothesis. Einstein’s 1905 theory instead treated the speed of light in vacuum as invariant for inertial observers.

A common mix-up is saying the experiment showed that light has no speed. It showed that the predicted directional difference in light’s speed could not be detected. Later, improved tests reached much greater precision and continued to support Lorentz symmetry.