other is a spheroid, a squashed sphere. The observable proper physical shape of the bodies remains the same in all frames. The non-rotating-spheroid frame has physics whose cause lies outside the system, responsible for the oblateness of the spheroid. The non-rotating-sphere frame does not, which makes it privileged in that it doesn't require external causes. This applies to all inertial frames, who are privileged in the same regard. Einstein went on to develop general relativity and the equivalence principle, in which inertial-gravitational frames are no longer privileged, because the geodesics of spacetime explain these inertial-gravitational effects without an external cause. See also Tests of special relativity Modern searches for Lorentz violation Cosmic microwave background Test theories of special relativity References Further reading Einstein (1954) Relativity, the special and the general theories Special relativity Aether theories Frames of reference