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Message: <blockquote data-quote="freyar" data-source="post: 7494578" data-attributes="member: 40227">Let me go back and quote your original mooncar/laser scenario, just so we don't have to skip around the thread to read it.To simplify the problem, I'm going to make it a spaceship coasting past a starbase with a laser.  I will lay down an arbitrary 2D N-S-E-W set of directions.OK, in the starbase frame, the ship is moving north.  The laser is somewhere north-west of the ship.  In the base frame, it fires a laser beam due east (ie, west to east).  It hits the ship.  Let's forget about vaporizing part of the ship, since that introduces unknown amounts of chemical energy, unknown direction of the exhaust, etc.  The light in the laser itself can push the ship.  Let's also assume that the light is perfectly reflected, so we don't have to worry about heating the ship and what the thermal radiation coming off the ship does.  So then the laser hits the car (pushing it east) and reflects off to the west (also pushing the ship to the east).  So, in the end, the ship moves off toward the northeast (at least until the pilot corrects course).  In other words, as you say, the force is to the east.In the frame of the ship, the base and laser are moving south.  While the laser is still north of the ship, it fires.  The laser beam moves southeast and hits the ship, pushing to the southeast.  The beam reflects off the ship in a southwest direction, pushing the ship to the northeast.  The north-south component of the imparted momentum cancels, so the ship now moves to the east.I think we both agree on this.  If the ship is moving at a significant fraction of lightspeed, we have to modify the discussion a bit, particularly if you want to talk about forces and not just momentum conservation.  But the point is that the velocity of the laser beam (particularly its direction of travel) can change without changing the direction of the imparted momentum.Incidentally, the <a href="https://en.wikipedia.org/wiki/Aberration_of_light" target="_blank">aberration of light</a>, the change of the direction of motion of a lightbeam in different frames of reference, is an important effect historically (first observed 1725-6) and was one of the observations that most influenced Einstein in the development of special relativity.</blockquote>

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