sentences:  The expectation value of the Hamiltonian is conserved under the time evolution that the Hamiltonian generates.If the Hamiltonian itself has an intrinsic time dependence, as occurs when interaction strengths or other parameters vary over time, then computing the family of unitary operators becomes more complicated (see Dyson series).In the Schrödinger picture, the unitary operators are taken to act upon the system's quantum state, whereas in the Heisenberg picture, the time dependence is incorporated into the observables instead.