>But under the laws of special relativity, mass simply couldn’t be the ultimate conserved quantity, since different observers would disagree about what the ener

>But under the laws of special relativity, mass simply couldn’t be the ultimate conserved quantity, since different observers would disagree about what the energy of a system was. Maybe I'm being pedantic, but not only does classical mechanics already tell us that the energy of a system different for different for observers, but this by itself doesn't mean it's not conserved, merely that it's not a universal quantity. Furthermore E = mc^2 doesn't break conservation of mass in any way, it's more accurate to say that it equates energy with mass, meaning conservation of energy and conservation of mass are one and the same. And in fact both still hold, when the energy and mass are replaced with their relativistic counterparts (E = mc^2 is obviously false using the classical notions of mass and energy).