The momentum four-vector is defined by
where is the velocity four-vector. Writing out,
where is relativistic gamma, is the rest mass, c is the speed of light, and is the velocity vector. The norm of is Lorentz invariant since
where the primed quantities are Lorentz transformed. Thus, since c is constant, the momentum four-vector encodes the fact that the rest mass is also a constant independent of the reference frame.
Energy, Lorentz Invariant, Momentum, Velocity Four-Vector
