You can set c to 1 if you want, then energy == mass. As a matter of fact this is pretty common in particle physics where all masses are given in electronvolts.

You can set c to 1 if you want, then energy == mass. As a matter of fact this is pretty common in particle physics where all masses are given in electronvolts. https://en.wikipedia.org/wiki/Geometrized_unit_system https://en.wikipedia.org/wiki/Geometrized_unit_system