States with definite energy evolve in time with phase e − − i E t / ℏ ℏ , ( | Ψ Ψ ( t ) ⟩ ⟩ = | Ψ Ψ ( 0 ) ⟩ ⟩ e − − i E t / ℏ ℏ ) where E is the energy of the state, since the probability of finding the system in state | ⟨ ⟨ x | Ψ Ψ ( t ) ⟩ ⟩ | 2 = | ⟨ ⟨ x | Ψ Ψ ( 0 ) ⟩ ⟩ | 2 is independent of time. Such states are termed stationary states, so if a system is prepared in a stationary state, (i.e. one of the eigenstates of the Hamiltonian operator), then P (t) = 1, i.e. it remains in that state indefinitely. This is the case only for isolated systems. When a system in a stationary state is perturbed, its state changes, so it is no longer an eigenstate of the system's complete Hamiltonian. This same phenomenon happens in magnetic resonance for a spin 1 2 system in a magnetic field.