X and Y reside in the 3-sphere.(adsbygoogle = window.adsbygoogle || []).push({});

Y is accelerated to near the speed of light; say, 0.9c. He does not ever change direction. In a little while, he meets X, who happens to be residing on the great circle upon which Y is traveling. When they meet, they give each other high-fives. At that moment, the two are identical twins (Y was younger prior to his acceleration).

Y continues alone his great circle, unaccelerated, and therefore in an inertial frame. X is similiarly in an inertial frame. In a little while, they meet again. When they high-five for a second time, which is younger?

From the moment they high-fived for the first time, neither has undergone any accelerations; yet my understanding is that the reason the Twin Paradox can be resolved in the canonical case is that one of the twins underwent acceleration (when his spaceship turned around), and that is why there is an asymmetry between the two twins. But in this case there seems to be no difference, and it really is as accurate to say that X's time dilates with respect to Y as it is to say that Y's time dilates with respect to X.

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# Twin Paradox in S^3.

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