Einstein's first postulate of Special Relativity is concerned almost entirely with physical processes observed from reference frames moving at constant velocities with respect to each other. So-called "inertial frames of reference in relative motion". However, relative motion produces the controversial Clock or Twin Paradox. But there /is no real paradox/. Which of two clocks runs slower depends on which has been moved, yet the relative motion of the clocks does not depend on which has been moved.

The Twin Paradox results from Einstein's invalid postulate that the inertial frames are symmetrical. In the real universe, where gravitational force connects all matter in real time, the inertial frames are asymmetric. The clock that is moving fastest with respect to the fixed stars has more energy than that of a slower clock. So according to e=mc2 (mass is an energetic variable and it's matter combined with energy which does work), the masses of all subatomic particles in the moving clock will increase and slow the atomic clock rate. Time does not dilate for the moving clock. The clock simply counts fewer ticks in universal time. And this interpretation is fully verified by the global positioning system which uses orbiting clocks that have their slower clock-rates adjusted by counting fewer oscillations per tick before the launch, so when in orbit they tick at the rate of the faster ground clocks. Note that a ground clock /never/ runs slower than an orbiting clock, /nor/ does it even appear to run slower. So, there is no Einstein or Lorentz relativistic symmetry.