This demonstration illustrates the world line of the spacefaring twin. Use the sliders to choose the velocity of the ship and the duration of the outbound journey as viewed from Earth. Then, use the buttons to fill in details of the spacetime diagram from both perspectives. Notice the jump in time when switching from the ship’s outgoing perspective to its incoming perspective.

The meanings of the $T$-variables are as follows: $T$ is the turnaround time from Earth’s perspective, and $2T$ is the total trip duration from Earth’s perspective; $T^\prime$ is the time-dilated turnaround time, $T/\gamma$, the time that Earth would see the spaceship’s clock read when it turns around; $T_1$ is the time at which the spaceship would say it turned around just before turning around; $T_2$ is the time at which the spaceship would say it turned around just after turning around; and the difference, $T_2-T_1$, is the change in clock readings that occurs when changing from the frame traveling away from Earth to the frame traveling towards Earth.