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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?
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?