Let \(T(t',t)\), subsection 4.2, be the unitary time development operator for the total system, and assume it is trivial—equal to the identity operator \(I=I_s\otimes I_m\)—for \(t\) and \(t'\) both less than some \(t_1\) or both greater than \(t_2\), whereas for the interval from \(t_1\) to \(t_2\) during which the particle and the apparatus interact,