It still makes me sad that physics doesn't think about things in the simpler way that Feynman presented. Two names for the same thing are redundant, so physici

It still makes me sad that physics doesn't think about things in the simpler way that Feynman presented. Two names for the same thing are redundant, so physicists decided to make a meaningful distinction: By convention, 'mass' denotes the invariant but not conserved scalar quantity (rest mass/energy), whereas 'energy' denotes the non-invariant but conserved quantity, the time component of energy-momentum (total mass/energy).