The Michelson-Morley experiment
Motivated by the idea that the Aether existed, physicists undertook experiments to detect its influence on the speed of light. One of the most famous experiments in physics is the Michelson-Morley experiment, first conducted in 1881 and repeated subsequently many times [Michelson & Morley, 1887]. In that experiment, conducted in a laboratory on Earth, light from a source was split by a beam-splitter into two transverse directions then reflected back onto a detector. Depending on the time spent in transit, there would be a pattern of interference fringes observable in the detector: electromagnetic waves arriving in phase at the same point would add constructively whilst those arriving out out phase would add destructively.
In addition, if the light travelling in one direction had a different speed to the other, then when the two returning components were superposed, there would be additional interference effects observable. It was calculated that if the Aether were stationary relative to the Sun, then the Earth’s motion during the course of a year would produce an observable effect: an easily measurable interference fringe shift of 1/25 of a fringe was predicted.
Michelson and Morley’s equipment was good enough to detect effects of the order 1/100 of a fringe. Their results did not show any expected effects: their experimental errors were consistent with a fringe shift of zero. There appeared to be no effect of motion relative to the Aether
Subsequently various other experiments were done but generally the results were the same. No Aether could be detected. This is the situation to date.