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To be concrete, suppose the event of the rocket leaving Earth is at the point (0,0,0,0) in our coordinate system, and the rocket travels inertially (without acceleration) to the point (5,4,0,0). The rocket immediately turns around, and follows an inertial trajectory back to Earth, arriving at the event (10,0,0,0). The interval between (0,0,0,0) and (5,4,0,0) is, as we have seen, 3. Suppose this corresponds to an elapse of three years according to the onboard clocks. The return trajectory from (5,4,0,0) to (10,0,0,0) also has an interval length 3, corresponding to another three years elapsed. So the astronaut twin arrives back having aged six years, and having had all the experiences that correspond to six years of life.

To be concrete, suppose the event of the rocket leaving Earth is at the point (0,0,0,0) in our coordinate system, and the rocket travels inertially (without acceleration) to the point (5,4,0,0). The rocket immediately turns around, and follows an inertial trajectory back to Earth, arriving at the event (10,0,0,0). The interval between (0,0,0,0) and (5,4,0,0) is, as we have seen, 3. Suppose this corresponds to an elapse of three years according to the onboard clocks. The return trajectory from (5,4,0,0) to (10,0,0,0) also has an interval length 3, corresponding to another three years elapsed. So the astronaut twin arrives back having aged six years, and having had all the experiences that correspond to six years of life.

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