Parable #4: The twin paradox.

But what about round trips? Can’t either side say the other’s clock is running slower?
Suppose Betty and Barney are twins and one of them travels to a star 4 light years away at a speed that would result in a time and length contraction of a factor of two (0.866*c)

But what about round trips? Can’t either side say the other’s clock is running slower?