In the usual derivation of time dilation in special relativity, we have two frames of reference, A and B, moving relative to each other with velocity v. In A's frame, a light beam is directed vertically upward toward a mirror and reflected vertically back down. In B's frame, the light follows a triangular path due to the relative motion. If we were instead talking about a ball being thrown up and down vertically in A's frame, B could explain the triangular path by saying that the ball initially had velocity v before it was thrown, there are no horizontal forces acting and therefore due to its inertia it continues to have the velocity v hence a triangular path according to B. (We're assuming we're out in deep space; on earth the ball would follow a parabolic path). So returning to the case of the light beam, how does B explain the triangular path of the light from a physical point of view given that light doesn't have inertia. It's obvious that a vertical path in from A will be a triangular path in frame B, but what, according to B, is causing it to continue to keep pace with A's frame once it is emitted?