I have come accross this question in another forum and thought to post it here, hopefully will get some interesting views. This is a thought experiment so the setting might not be achivable with todays technology and knowledge, instead try to look at the implications of this set up rather then the possibility of achiving such set up. Imagine if we have a enclosed vacuum space, nothing can go in or out. We keep adding photons inside the enclosed space. (assume that whatever material or field we are using to enclose it will not absorb the photons, as well as the material used to generate the photons will not absorbe them). What would happen if we keep adding photons? What would happen if we increase the density of the photons in an enclosed vacuum space? Would they interact with each other? would they form some other form of particle that can carry more energy? What would happen? Thanks to all in advance.