An automatic occurrence in the Dirac equation (and the Weyl equation) is the projection of the spin ⁠ 1 / 2 ⁠ operator on the 3-momentum (times c), σ · c p, which is the helicity (for the spin ⁠ 1 / 2 ⁠ case) times E 2 − − ( m 0 c 2 ) 2 .