Michelson–Morley experiment

The 1887 interferometer experiment compared light traveling along perpendicular paths to detect Earth’s motion through the hypothesized luminiferous ether. It found no detectable ether wind.

Michelson interferometer: An optical instrument that splits a light beam into separate paths and recombines them to measure differences in travel time or distance. Its perpendicular arms formed the experiment’s core measurement apparatus.
Luminiferous aether: The hypothetical medium once proposed to carry light waves through otherwise empty space. Its presumed rest frame supplied the motion the experiment was designed to detect.
Special relativity: Einstein’s 1905 theory in which the laws of physics are the same in all inertial frames and light has invariant vacuum speed. The experiment became a landmark in the history of the theory, though its null result alone did not establish it.
1887 ether-drift measurements: The series of interferometer observations made by Michelson and Morley in Cleveland in 1887. These observations produced the reported absence of the expected large fringe shift.
Emission theory of light: A family of theories in which light’s speed is determined by the motion of its source. It offered a different account of light propagation from wave motion through a stationary ether.
Interference: The superposition of waves that produces reinforcement or cancellation according to their relative phase. Changes in fringe positions would have revealed a difference between the beams’ travel times.
Albert A. Michelson: An American physicist known for precision optical measurements and the first American Nobel laureate in physics. He designed the interferometer method and led the 1887 experiment.
Lorentz transformation: A mathematical transformation relating space and time coordinates between inertial reference frames in special relativity. Lorentz developed these transformations while addressing electrodynamics and the ether-drift problem.
Dayton Miller ether-drift experiments: A series of interferometer measurements by Dayton Miller, who reported a small residual ether-drift signal in the 1920s. Miller’s claimed positive signal reopened debate over how to interpret the 1887 result.
Balloonist fallacy: A mistaken inference that source-dependent light-speed theories must produce a detectable effect in a particular experiment. It cautions against treating this experiment alone as a decisive test of every emission theory.

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