In special relativity, space and time can be collected into a four-position vector X = (ct, − r), and in parallel so can energy and momentum which combine into a four-momentum vector P = (E / c, − p). With relativistic quantum mechanics in mind, the time duration and spatial displacement parameters (four in total, one for time and three for space) combine into a spacetime displacement Δ X = (c Δ t, −Δ r), and the energy and momentum operators are inserted in the four-momentum to obtain a four-momentum operator,