Because the spin matrices 1 are only 3 × × 3 to calculate the commutator in the same space we approximate the spin matrices by angular momentum matrices of the particle with the length 3 / 2 ≈ ≈ 1 while dropping the multiplying 1 / 2 since the resulting Maxwell equations in 4 dimensions would look too artificial to the original (alternatively we can keep the original 1 / 2 factors but normalize the new 4-spinor to 2 as 4 scalar particles normalized to 1/2):