This experiment on light waves, first carried out in 1881 by physicists Albert A. Michelson (1852–1931) and E.W. Morley (1838–1923) in the United States, is one of the most historically significant experiments in physics and led to the development of Einstein’s theory of relativity. The original experiment, using the Michelson interferometer, attempted to detect the velocity of Earth with respect to the hypothetical “luminiferous ether,” a medium in space proposed to carry light waves. The procedure measured the speed of light in the direction of Earth and the speed of light at right angles to Earth’s motion. No difference was found. This result discredited the ether theory and ultimately led to the proposal by Albert Einstein (1879–1955) that the speed of light is a universal constant.