Finding the luminiferous aether a useless hypothesis, Einstein in 1905 "a priori" unified all inertial reference frames to state special "principle" of relativity, which, by omitting aether, converted space and time into "relative" phenomena whose relativity aligned electrodynamics with the Newtonian principle Galilean relativity or invariance. Originally epistemic or instrumental, this was interpreted as ontic or realist—that is, a causal mechanical explanation—and the "principle" became a "theory", refuting Newtonian gravitation. By predictive success in 1919, general relativity apparently overthrew Newton's theory, a revolution in science resisted by many yet fulfilled around 1930.