
Perhaps the key experiment which led to Einstein postulating his theory of relativity in 1905, in particular the invariance of the speed of light and the complete absence of a medium (the “aether”) for light being a wave, was the Michelson-Morley Interferometer Experiment of 1887 (see image above).
Despite noticing some potential dependence of light speed with direction, this was considered far enough below the expected result to declare the result “null” – namely there is no aether medium for light as originally proposed by Maxwell and others.















