''Whereas in classical physics mass and energy are distinct concepts, on relativistic scales, they become indistiguishable and are replaced by mass-energy.
In higher-energy physics, mass and energy eventually become meaningless as individual concepts; a particle's so-called "mass" is actually mass-energy and is often given equivalently in units of either mass (GeV/c2) or energy (GeV).
''For equations normalized with Planck units, the quantities of mass and energy become numerically identical, revealing their true nature as mass-energy.
