In 19th century physics: Hypothetical medium in which light and other types of electromagnetic radiation propagate as waves. Once the ether is postulated, certain questions arise: Does the earth move through this medium? If yes, how fast? And can this motion be detected by studying the propagation of light? Einstein’s special theory of relativity, in which the value of the speed of light is independent of the motion of light source or observer (more precisely: an inertial observer), the ether turns out to be absolutely undetectable, which has led physicists to abandon the concept.