The time evolution of the state vector | ψ ψ ( t ) ⟩ ⟩ is governed by the Schrödinger equation, where H ( t ) is the observable associated with the total energy of the system (called the Hamiltonian) i ℏ ℏ d d t | ψ ψ ( t ) ⟩ ⟩ = H ( t ) | ψ ψ ( t ) ⟩ ⟩