This adds the proof for the relativistic mass effects from motion, proving that gravity causes it.
Proving the Relativistic Energy-Momentum relation, the hard way and the easier way.
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.