 Moreover, taking into account the way that the conditional wave function depends upon time \(t\)   \[\psi_t (x) = \Psi_t (x, Y_t)\]   via the time dependence of \(Y\) as well as that of \(\Psi\), it is not difficult to see (Dürr, Goldstein, &amp; Zanghì 1992a) the following two things about the evolution of the conditional wave: First, that it obeys Schrödinger’s equation for the subsystem when that system is suitably decoupled from its environment. Part of what is meant by this decoupling is that \(\Psi\) has a special form, what might be called an effective product form (similar to but more general than the superposition produced in an “ideal quantum measurement”), in which case the conditional wave function of the subsystem is also called its effective wave function. Second, using the  quantum equilibrium hypothesis,  that it randomly collapses according to the usual quantum mechanical rules under precisely those conditions on the interaction between the subsystem and its environment that define an ideal quantum measurement.