An interferometer compares round-trip light travel times along perpendicular arms as its orientation changes. The absence of the classical stationary-aether fringe shift at the expected scale constrains orientation dependence of the round-trip speed of light. The result is consistent with special relativity, but alone does not select it uniquely from every alternative theory.[ref]
On the Relative Motion of the Earth and the Luminiferous Ether by Albert Abraham Michelson and Edward W. Morley. 1887 report, interferometer arrangement and comparison of predicted and observed orientation-dependent shifts
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The Michelson-Morley experiment, conducted in 1887 by Albert A. Michelson and Edward W. Morley, is one of the most famous experiments in physics, primarily designed to detect the existence of the luminiferous aether—a hypothetical medium through which light waves were thought to propagate, much like sound waves travel through air. ### Purpose of the Experiment: The experiment aimed to measure the Earth's motion through this aether.
Michelson Interferometer by Amrita Vlab (2013)
Source. Shows the optical controls and alignment in more detail.Michelson Interferometer by TSG Physics (2012)
Source. TSG PHysiQuantum electrodynamics bibliographycs is the channel from the MIT Department of Physics Technical Services Group. In the video they produce a very clear round interference pattern.