So, I'm having trouble with the concept of the mass of a "system" being greater than the sum of the masses of the components of that system. Specifically in the case of photons. A system consisting of 1 photon has no mass. A system consisting of 2 photons in the same direction has no mass. However a system consisting of 2 photons moving on different direction has mass, even if neither of the photons do. Just like the idea that heat has mass and can be weighed with precise enough instruments in the future (as in the case of how a system that involves the inelastic collision of two objects seems to end up with more mass after the collision than it had before, which is supposed to come from the heat gained by the objects), does this mean you can theoretically weight two phtons moving in different directions, just not one or the other individual photon?