I don't understand why you say this is without acceleration?!? Gracie's spaceship accelerates from 0 c (relative to George) to 12/13 c (relative to George) when she flies away from Earth.

HIs language was a little sloppy. He is referring to the acceleration needed to cause Gracie to turn around and come back.

But how do we know we can't trust Gracie until she does her dance at the waypoint? Her antics over there can't retroactively resolve the clock rate ambiguity in the outgoing leg. The answer is, establish the centre of momentum. That's the context with the fastest clock (in terms of proper time.)

But how do we know we can't trust Gracie until she does her dance at the waypoint? Her antics over there can't retroactively resolve the clock rate ambiguity in the outgoing leg. The answer is, establish the centre of momentum. That's the context with the fastest clock. (Oops, sorry. I created a duplicate. The reply button on the main page is confusing.)

instantaneously synching clocks while going 12/13c on a clock that's coming at you at 12/13c no stop to adjust the clock and no time for re-accelerating, yep I'm getting used to ignoring reality., excellent brain eats though. and my mind is so hungry atm.