Einstein did not show that. The principal you are referring to is, ironically, Galilean relativity, and attributable to Galileo. Einstein's contribution was com

Einstein did not show that. The principal you are referring to is, ironically, Galilean relativity, and attributable to Galileo. Einstein's contribution was combining the principle of relativity with a constant speed of light. Additionally, under both systems, relativity only applies to intertial reference frames. Rotating reference frames are detectable. You can detect Earth's rotation without any external observations using a pendulum. Although such an experiment would not be done until Léon Foucault about 2 centuries after Galileo.