Pardon my elementary questions, but: Why is it that photons can only travel at the speed of light? I know because they have zero rest-mass it is only possible for them to travel at c, but is there some mathematical reasoning to this through the mass and momentum equivalence equation? Lastly, is there some mathematical reasoning as to why anything with non-zero rest mass cannot reach light speed? I understand it would take infinite energy, but can that be reasoned mathematically, or is it solely physics? Thanks, once again.