Luminiferous aether

Luminiferous aether was a hypothetical medium thought to fill space and carry light waves. Experiments and special relativity showed that light does not require such a medium.

Wave theory of light: The theory that light propagates as waves rather than as a stream of particles. Wave phenomena suggested that light, like sound, might need a medium in which to travel.
Michelson–Morley experiment: An 1887 interferometer experiment that searched for changes in light travel time caused by Earth's motion through aether. Its null result undermined the expected variation in light's speed through a stationary aether.
Special relativity: Einstein's theory of space and time in which the laws of physics are the same in all inertial frames and light's speed is invariant. It explains the relevant observations without assigning light a preferred stationary medium.
Lorentz transformation: A transformation relating space and time coordinates measured in different inertial frames. Its role in aether models helped lead to the relativistic account of moving observers.
Thomas Young: An English physician and physicist whose experiments established the wave nature of light. His interference experiments strengthened the wave picture that motivated a light-bearing medium.
Aether drag: The proposed entrainment of luminiferous aether by moving matter. Partial drag was one proposed way to reconcile aether with observations of stellar aberration.
Emission theory of light: A theory that treats light as particles emitted by sources, with their speeds dependent on the source's motion. It offered a non-wave alternative to aether theories, though observations later contradicted it.
Michelson–Morley experiment in relativity: The interpretation of the Michelson–Morley null result as evidence against detecting Earth's motion relative to aether. The experiment became a landmark in the shift away from a measurable aether frame.
Augustin-Jean Fresnel: A French physicist who developed a mathematical wave theory of light, including diffraction and polarization. His wave theory made the aether a setting for light's mechanical vibrations.
Stellar aberration: The apparent shift in a star's position caused by the combination of its light's travel and Earth's motion. Its observed pattern constrained whether Earth or nearby matter could drag the aether.

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