Caption: A diagram illustrating the luminiferous aether---which was a 19th century hypothesis that the medium of electromagnetic radiation (EMR) (i.e., light).

Features:

1. The luminiferous aether was hypothesized to be blowing past the Solar System in an aether wind.

2. In hypothesizing the luminiferous aether, people were assuming that EMR was a mechanical wave and needed a medium like other mechanical waves (e.g., sound waves).

3. But if there was a luminiferous aether, then the vacuum light speed should vary with the Earth's velocity relative to the luminiferous aether---just as the speed of sound varies with velocity relative to the air.

4. No such variations in the vacuum light speed have ever been detected.

5. The most famous experiment to detect motion with respect to the luminiferous aether was the Michelson-Morley experiment (1887) done by Albert A. Michelson (1852--1931) and Edward W. Morley (1838--1923) in Cleveland, Ohio at what is now Case Western Reserve University. This experiment's to detect such motion eventually caused people to take seriously the hypothesis of the invariance of vacuum light speed.

6. The conclusion from all direct measurements of the vacuum light speed and theory of relativity (and all its numerous and extremely robust verifications) is that the luminiferous aether does NOT exist---at least as conceived of in the 19th century---and EMR does NOT need a medium (of matter) and vacuum light speed is invariant relative to all local inertial frames (i.e., inertial frames right where the vacuum light speed is being measured) at least in the ideal limit which is easily approached very closely.

7. Since nature has given us object with an invariant speed, but NO macroscopic scale object with invariant length, modern metrology decided to define as exact vacuum light speed c = 2.99792458*10**8 m/s ≅ 3*10**8 m/s = 3*10**5 km/s ≅ 1 ft/ns (where the value was set by historical precedent) and then define the meter as the distance light travels in a vacuum in 1/(2.99792458*10**8) s. The second has its own modern definition based on an invariant duration in the ideal limit as guaranteed by quantum mechanics (see Wikipedia: Second: "Atomic" second).

8. Invariance of vacuum light speed is, in fact, an axiom of special relativity first published by Albert Einstein (1879--1955) in 1905.

   It is interesting to note that Einstein in later years made contradictory statements about the Michelson-Morley experiment---some said it did influence his thinking and others that it didn't---maybe he could NOT quite remember.