Four-momentum
In special relativity, four-momentum is the generalization of the classical three-dimensional momentum to four-dimensional spacetime. Momentum is a vector in three dimensions; similarly four-momentum is a four-vector in spacetime. The contravariant four-momentum of a particle with energy E and three-momentum p = (px, py, pz) = mu = γmv, where v is the particle's 3-velocity and γ the Lorentz factor, is