Luminiferous aether
The hypothetical all-pervasive medium that nineteenth-century physics posited as the substrate in which light waves propagate. Searched for via interferometry from 1881 onward; never detected. Discarded by Einstein's 1905 special-relativity paper.
Definition
The luminiferous aether was the hypothetical all-pervasive medium in which light waves were thought to propagate, by analogy with sound waves in air or water waves in water. The wave theory of light (Young 1801, Fresnel 1816, confirmed via Maxwell's electromagnetic-wave prediction in 1862) seemed to require a medium: a wave is a periodic disturbance of something, and electromagnetic waves were no exception. The aether was therefore postulated as a continuous, perfectly elastic, massless solid filling all of space — solid because transverse waves (which light is, per polarisation experiments) require shear restoring forces, and rigid enough to support oscillations at light's frequencies, yet offering no measurable drag on planetary motion. It was an uncomfortable theoretical object from the start.
If the aether existed, the Earth's orbital motion at 30 km/s should produce an "aether wind" detectable by precise comparison of light-propagation times along orthogonal arms. The 1887 Michelson-Morley experiment did exactly this and got a null result — no aether wind to within their precision (a few parts per million of 30 km/s). Lorentz and FitzGerald independently proposed length contraction as a real-material aether effect to explain the null; Einstein's 1905 paper On the Electrodynamics of Moving Bodies discarded the aether outright, replacing it with two postulates (the principle of relativity and the constancy of c) from which length contraction and time dilation followed as kinematic consequences requiring no medium. The aether was never detected. The word survives in physics today only as the historical name for the conceptual landscape that special relativity replaced.