[Mentors' note - this post has been edited to remove some discussion of the EM-drive. See https://www.physicsforums.com/threads/nasas-em-drive.884753/ for our current policy with regard to em-drive discussions] Hello. Imagine a closed box with reflecting walls. Inside you have two antennas emiting electromagnetic waves of certain wavelength. Both antennas are separated 1/4 of the wavelength (or some N wavelengths + 1/4) and one antenna is 90º phase-shifted (1/4 wavelength delayed) with respect to the other. I attach a picture of the superposition we will get inside the box. We will get one side of destructive interference, one region between the antennas where a standing wave develops, and another side of constructive interference. You can see that the wall on the left is on the destructive interference region, so it almost doesn't receive any photons, but the wall on the right will receive a wave created by constructive interference: I also attach a top-view animation of the box and the different regions and waves that will develop. You can see there'll be more photons impacting the right wall: So my question is: if we have more photons impacting one wall than the other, why shouldn't the box start moving to that direction?