Let's say a light beam is sent towards a planet from far away. On the surface of the planet no measurement can ever tell that the light beam is approaching, until the light beam has arrived, because information can not travel faster than the light beam. Now at the moment when the light beam is 10 m above the planet surface, let's magically reverse all velocities. Now the light beam is traveling upwards, and nothing below 10 m has any idea that the light beam is there. Particularly instruments that measure gravitational pull do not measure any pull from the light beam. So, is the following conclusion correct: No momentum exchange happens between a mass and a light beam that moves towards the mass or away from the mass? Note: When all velocities were magically reversed, I was using "time reversal symmetry law", or what ever the correct term for that kind of thing is. I hope I used it correctly.