In this expression p is the relative momentum having the form ( p 1 − − p 2 ) / 2 for equal masses. In the c.m. frame ( P 0 = w , P → → = 0 → → ), the time component p 0 of the relative momentum, that is the relative energy, is thus eliminated. in the sense that p 0 Ψ Ψ = 0 .