Two reference frames in motions are not symmetrical. A system at rest on the surface of a planet moves faster in time than a moving system. This is not the case using just Special Relativity. This is proven just using the gravitational metric field in General Relativity.
The Dirac Equation is Relativistically Invariant.
Proof attached.
Proving the Relativistic Energy-Momentum relation, the hard way and the easier way.