What proof is there that light has zero mass?? I often come across the assertion that light has zero mass. i was wondering is this a proven fact or just an assumption because we dont currently have the technology to measure it?? i have thought of an experiment that may determine if light has mass. you wold need an exactly balanced set of scales housed within a sealed box covered in mirrors. on one half of the scales, the scales and balance weight are covered in mirrors, on the other they are balck as that absorbs most light i think. if a light is shone within this box for a period of time any absorbing light would register on the black side if it had mass. the scales wopuld need to be extremely precise however and set up to be vibration free and various other measures to eliminate any other contributory effect that could cause scale variation. only an idea, but i personally think it has to have a mass, though i know this is an amateurs opinion, and it's commonly believed to be massless.