 Complete the following phrase:  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 realistthat is, a causal mechanical explanationand the principle became a theory, refuting Newtonian gravitation. By predictive success 