Hi all! I've checked Wikipedia and a related thread regarding experimentally measuring an upper limit for the photon rest mass: https://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass https://www.physicsforums.com/threads/photon-rest-mass-0-wasnt-proven-experimentally.792583/ I was thinking of another experiment which may be possible to set up in laboratory conditions. As I understand the specific polarization properties of the photon are related to its spin axis being oriented in the direction of its propagation, and this is only possible due to the photon having zero rest mass. Based on this I was thinking of an experiment in which we first point a laser beam to a polarizing sheet and measure the intensity behind. Then we place another polarizing sheet some relevant long distance away (maybe we could simulate long distance with mirrors) and measure the intensity behind. If they differ (after other factors have been accounted for) that would indicate that the spin axis is not exactly aligned with the direction of the propagation, and therefore the photon indeed has mass. If they don't (as I would expect), the parameters (distance...) of the experiment could be used to determine the upper limit to the photon mass. I have not done a detailed analysis of the parameters, but I would expect that if the principles in this experiment are right, the parameters could be designed to be able to make very fine measurements. As a matter of fact I can imagine that the actual experiment would require some fine calibration and maybe some slight redesign (e.g. to reduce the contribution of other effects to the dimming of the intensity, we could devise the experiment such as the second polariser can be rotated with high precision. In this case we would just need to find the angle, relative to the first polariser, at which the intensity is the max, instead of comparing intensities. It comes without saying that this would require to be able to precisely measure the angle at which both polarisers are set with high precision). My question is: is anyone aware of such experiment (or a similar one) having been performed? Specifically the paper in the other thread https://www.princeton.edu/~romalis/PHYS312/Coulomb Ref/TuCoulomb.pdf says in 3.2 "All those effects [polarization] are useful approaches for laboratory experiments", but in table 1 there is not mention of such experiment having been performed. So, do you know if it has been done? Thanks!