There is a light bulb in the center of the rod. In the reference frame in which the rod is at rest, the light from the light bulb will reach the ends of the rod at the same time, and in the reference frame in which the rod is moving longitudinally at a speed of v, the light will arrive at the far end c2lv1−c2v2 later than at the near end; l is the length of the rod in the reference frame in which the rod is stationary. Prove it.