Can light have lateral momentum, and if so as a particle or wave or both? If a person on a train throws a ball up, then from the train's frame of reference the ball goes directly up then down (ignoring air resistance). From the ground's frame of reference, the ball goes at an angle upward and forward, then downward and forward. This is because relative to the ground, the ball has forward (lateral) momentum. Next, consider a light clock oriented perpendicular to the forward direction of a rocket traveling close to c. From the rocket's frame of reference, the light in the clock goes directly up then down (after reflecting off the mirror at the top of the clock). From the ground's frame of reference, the light goes just like the ball, upward and forward, then downward and forward. Since we should treat relativity events as "what you see is what you get" (rather than trusting our instincts), does this mean that the light in the clock has lateral momentum, just like the ball? Can light have this lateral momentum as a particle? As a wave?