Special Relativity Redux

The Missing Ingredient People have been pushing back on my assertion that the rest frame for a Lorentz transformation cannot be dealer’s choice. It boggled my mind for the longest time because it seemed perfectly obvious to me. Why did Hafele and Keating start with a non-rotating Earth for example? Because that context has no angular momentum. The cosmic rest frame has no net linear momentum so the same logic should apply. The universe has no net momentum so the cosmic rest frame is the best context for distinguishing between hitting the gas and hitting the brakes. But momentum doesn’t factor into the equations. Time dilation is agnostic to the direction of motion and proper time intervals are invariant so who cares about the cosmic rest frame? We can transform events into the cosmic rest frame before calculating proper time intervals: Elapsed cosmic time from launch to reunion is Tb  =  t / sqrt(1- Vb ^2/c^2) Elapsed traveller time from launch to waypoint is To  = ( Tb /2 )  * sqrt(1