In APS, the spacetime position is represented as the paravector x = x 0 + x 1 e 1 + x 2 e 2 + x 3 e 3 , where the time is given by the scalar part x = t, and e 1, e 2, e 3 are the standard basis for position space. Throughout, units such that c = 1 are used, called natural units. In the Pauli matrix representation, the unit basis vectors are replaced by the Pauli matrices and the scalar part by the identity matrix. This means that the Pauli matrix representation of the space-time position is x → → ( x 0 + x 3 x 1 − − i x 2 x 1 + i x 2 x 0 − − x 3 )