Key points are not available for this paper at this time.
Because the time parameter in the Schr\"odinger equation is not observable, energy apparently obeys a superselection rule in the same sense that charge does. That is, observables must all commute with the Hamiltonian and hence be stationary. This means that it is consistent with all observations to assume that any closed system such as the Universe is in a stationary state. We show how the observed dynamic evolution of a system can be described entirely in terms of stationary observables as a dependence upon internal clock readings.
Page et al. (1983) studied this question.