Assuming that H ^ ^ is time-independent (unchanging in time), this equation holds for any time t. Therefore, this is a differential equation describing how | Ψ Ψ ⟩ ⟩ varies in time. Its solution is | Ψ Ψ ( t ) ⟩ ⟩ = e − − i E Ψ Ψ t / ℏ ℏ | Ψ Ψ ( 0 ) ⟩ ⟩ .