In this demonstration of the twin paradox, each twin sends out a light signal at a rate of one flash per year according to their clock. The total duration of the journey from the perspective of the twin who stays on Earth has been set to 20 years. You can see the trajectory of the light beams sent by either or both twins. The key thing to notice is that immediately after heading back toward Earth, the spacefaring twin receives light flashes at a faster rate than during the outbound portion of the journey. The twin who stays on Earth only receives flashes at a faster rate after sufficient time has elapsed—enough time for the light signal sent by the twin at the turnaround location to reach Earth.
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