Three fingers labeled A, B, and C are riding on a train that travels by you at relativistic speed as shown in the animation (position is given in meters and time is given in seconds ). As B passes you, both you and B notice light signals from A and C arriving at the same time. From your point of view, who turned their light on first (A or C)? From B's point of view, who turned their light on first (A or C)?
Hint: If you both observe the signals at the same time, you will not agree on when the emission events occurred. When you and B are at the same position, you see B receiving the signals simultaneously. Since B is moving relative to you, and light signals travel at finite speed, the emission events must occur at different times.
This Physlet was created by Mario Belloni and Wolfgang Christian.