In frame F1 there are two BB pellets at rest spaced 10 meters apart as measured in F1. F0, per Einstein, measures the separation between the two BB pellets to be 5 meters. At time t0 in frame F0, frame F0 simultaneously starts both BB pellets accelerating in the positive x direction. The two BB pellets each accelerate in the identical pattern until they have zero velocity relative to frame F2 at which time F0 simultaneously stops the accelerations.

An observer travels with one of the BB pellets accelerating identically as the adjacent BB pellet accelerates. That observer measures that the two BB pellets were initially 10 meters apart. When the BB pellets and the traveling observer have zero velocity with respect to frame F0, the traveling observer measures the BB pellets to be only 5 meters apart, just as every other observer in frame F0 does. When the BB pellets and traveling observer reach frame F2 and stop accelerating, the traveling observer once again measures the separation between the two BB pellets to be 10 meters.

Since the acceleration is always in the positive x direction, and since the pattern is identical for the two BB pellets (we can say make the acceleration constant as measured by the traveling observer), why does the accelerating observer say that the identical acceleration in the positive direction sometime causes the two BB pellets to move toward each other (the first part of the journey) and sometimes causes the two BB pellets to move away from each other (the second part of the journey)?

Thanks

David Seppala

Bastrop TX