The fastrunning clock given to the (assumed male) traveler always correctly tells the traveler what the (assumed female) home twin concludes about her current age at any given time in his life. I.e., the fastrunning clock in the spaceship always correctly gives HER conclusion about their respective ages, so it always correctly tells him what HER perspective is. That remains true during his entire round trip.
On the OUTBOUND leg of his trip, the slowrunning clock in the spaceship correctly tells him what her current age is, according to HIS perspective. But when he instantaneously reverses course, his conclusion about her current age instantaneously changes ... according to HIM, she instantaneously gets much older. So at that point, and for the remainder of his trip, the slowrunning clock in the spaceship no longer gives him correct information about her current age ... it continues to show what her current age would have been IF he had never reversed course.
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Mike Fontenot
