> most people understand photons to be massless particles. I suspect that most people have no opinion at all and are generally unaware of the properties of lig

> most people understand photons to be massless particles. I suspect that most people have no opinion at all and are generally unaware of the properties of light. That being said, a (perfectly reflective or otherwise steady state, e.g. black body at equilibrium) box of photons has mass due to those photons. You can stick it on a scale or a pendulum and measure it. It attracts other nearby objects according to GR or, in the appropriate limit, Newtonian gravity, in accordance with the relativistic mass of those photons.