Michelson-Morley experiment

The Michelson-Morley experiment was an 1887 interferometry experiment conducted by Albert Michelson and Edward Morley at the Case School of Applied Science in Cleveland, Ohio, designed to detect the motion of Earth through the luminiferous aether — the hypothetical medium in which light was supposed to propagate. It detected nothing. The null result was one of the most consequential experimental failures in the history of physics: a measurement so precise, and so empty, that it forced a reconstruction of the foundations of space and time.

The Aether and the Expected Result

Newtonian mechanics held that waves required a medium. Sound travels through air; water waves travel through water. If light was a wave — which Maxwell's equations proved — it should travel through something. That something was the luminiferous aether: an invisible, rigid, all-pervading medium filling all of space. The Earth, orbiting the Sun at 30 km/s, should be moving through this aether. Light traveling in the direction of Earth's motion should appear slightly faster; light traveling perpendicular to it, slightly slower. Michelson's interferometer was precise enough to detect this difference.

It was not there. The speed of light was the same in both directions.

The Null Result and Its Aftermath

Several explanations were proposed. Hendrik Lorentz and George FitzGerald independently suggested that matter physically contracts in the direction of motion — an ad hoc hypothesis that explained the null result mathematically without explaining why the contraction would occur. The Lorentz transformation emerged from this effort, predating Einstein's derivation of the same mathematics from his two postulates.

Einstein, who may or may not have been aware of the Michelson-Morley result when he wrote his 1905 paper, took a different approach. Rather than explaining the null result as a compensation between aether wind and material contraction, he discarded the aether entirely. The speed of light is constant not because the apparatus compensates for its own motion, but because there is no absolute motion to compensate for. Special relativity was the only framework in which the null result was not surprising — in which it was, in fact, inevitable.

The Michelson-Morley experiment did not discover what it was looking for. It discovered, instead, that what it was looking for could not exist. This is among the highest achievements available to a scientific experiment: to eliminate a category, not merely a candidate.