Pressure = energy flux / speed of light Taking an energy flux of 1000 watts per meter sq (typical flux of sunlight on earths surface) We gain, pressure = 1,000/300,000,000 = 1/300,000 Pascals. If it is reflected, we gain twice that = 2/300,000 pascals Now my question, if I had two flat mirrors of 1m^2 face to face one meter apart and put a beam of light in between them of energy 1000j (basically a beam of sunlight for 1 second). I would get my 2/300,000 Pascal’s of force per "collision" of the light on the mirrors. Now since they are bouncing back and fourth. I would get 300,000/2 collisions per second on each mirror. That gives me one Newton of force on each mirror? Is that correct or have I done something wrong? If it is correct, why isn’t this method used to propel spacecraft? or even earth craft? that is assuming we can get near perfect mirrors (which we have already). It would also not violate any physics laws I can see. It conserves momentum. It conserves energy (since the light will be red Doppler shifted, losing its energy and giving it to the KE of the mirrors). So where have I gone wrong?