text: The four-momentum is defined as: formula_53 where formula_54 is the formula_55 component of the "relativistic" momentum, formula_56 is the total energy of the system: formula_57 The "length" of the vector is the mass times the speed of light, which is invariant across all reference frames: formula_58 Momentum of massless objects Objects without a rest mass, such as photons, also carry momentum