 There is, however, a seemingly even deeper problem posed by the possibility of black-hole evaporation, one that raises doubts about the possibility of describing black holes using any standard formulation of quantum theory. According to standard quantum theory, the entropy of a closed system never changes; this is captured formally by the nature of the evolution of a quantum system, by the technical property of unitarity. Unitary evolution guarantees that the initial conditions, together with the Schrödinger equation (the equation governing the temporal evolution of quantum systems), will fix the future state of the system. Likewise, a reverse application of the Schrödinger equation will take us from the later state back to the original initial state. In other words, the states at each time contain enough information to fix the states at all other times, given the unitarity of dynamical evolution for quantum systems. Thus there is a sense in which the completeness of the state is maintained by the standard time evolution in quantum theory. (See the entry  Quantum Theory.)  