© Blayne Heckel.
At Time t=0, a pulse of light is emitted by a laser on the wall of a rocket. To a stationary observer (or if the rocket is at rest in free space), the light travels in a straight line across the cabin (dashed blue line). At Time t=1, the time it takes for the light to travel halfway across the cabin, the light pulse hits a screen in the center of the cabin. Between t=0 and t=1, the accelerating rocket has traveled upward, so that the position of the laser (solid red dot) is higher than its position at t=0 (pink dot). To an observer in the cabin, the light follows a parabolic path (dashed red line). At Time t=2, the light pulse reaches the opposite side of the cabin. Because gravity and acceleration are equivalent, an observer in a stationary rocket in a vertical gravitation field will see light follow the same parabolic path. (Unit: 3)