The four-momentum contracted with the gamma matrices is an important case in relativistic quantum mechanics and relativistic quantum field theory. In the Dirac equation and other relativistic wave equations, terms of the form: P / = P α α γ γ α α = P 0 γ γ 0 + P 1 γ γ 1 + P 2 γ γ 2 + P 3 γ γ 3 = E c γ γ 0 − − p x γ γ 1 − − p y γ γ 2 − − p z γ γ 3 appear, in which the energy E and momentum components (p x, p y, p z) are replaced by their respective operators.