Yet Another Run at the Twin “Paradox”

What’s with the “if” in this question? And what do you mean by the word “co-moving” other than perhaps stationary with respect to one another?
The standard version of the Twin “Paradox” starts with two twins, who are obviously “co-moving” in that sense at birth, and a distant star which is also “co-moving” (ie stationary with respect to the twins). Then at some point one of the twins travels to the star and back (usually with unspecified periods of acceleration and mostly constant speed in both directions).
Any correct application of Special Relativity predicts that when they re-unite the traveller is younger. The age difference can be calculated in terms of any frame of reference and (for any specified acceleration history – including that of instantaneous speed jumps) the answer is always the same so there is no real paradox.
The alleged “paradox” arises only in the mind of someone who notices that the traveller perceives the homie to have been ageing more slowly during the constant-speed legs of the trip and then just ignores the fact that the traveller also perceives a sudden rapid ageing of the homie during the turn-around. (During that turn-around the traveller feels the force of acceleration and so is aware of being in a non-inertial frame, whereas the homie feels no such forces. So the situation is definitely NOT symmetrical.)

Source: (1001) Alan Cooper’s answer to How do you solve the Twin Paradox if everyone (including the waypoint) is co-moving at the get-go? – Quora

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