The above definition of the relative momentum forces the orthogonality of the total momentum and the relative momentum, P ⋅ ⋅ p = 0 , which follows from taking the scalar product of either equation with P . From Eqs.(1) and (2), this relative momentum can be written in terms of p 1 and p 2 as p = ε ε 2 − − P 2 p 1 − − ε ε 1 − − P 2 p 2