I'm debating whether this should go in the GR & SR section, but anyways, I heard both claims and both of them are supported by calculations and observations. The ∞ mass is proved by equations from SR (and the fact that a certain amount of energy pushing an object to c will always be converted to mass to add on to the existing mass, thus, requiring even more energy to push it and so on...) and the zero rest-mass is proved because a photon is the only thing in the universe that has zero rest-mass (I think), which is why light (aka photons) is fastest thing in the universe, but how can both results be possible? Wouldn't it be either zero mass or ∞ mass? Or is it really both? Thanks in advance.