The value of ⟨ ⟨ [ H ^ ^ , B ^ ^ ] ⟩ ⟩ is then substituted in the Robertson uncertainty relation for the energy operator H ^ ^ and B ^ ^ : σ σ H σ σ B ≥ ≥ | 1 2 i ⟨ ⟨ [ H ^ ^ , B ^ ^ ] ⟩ ⟩ | , giving σ σ H σ σ B | d ⟨ ⟨ B ^ ^ ⟩ ⟩ d t | ≥ ≥ ℏ ℏ 2 (whenever the denonminator is nonzero). While this is a universal result, it depends upon the observable chosen and that the deviations σ σ H and σ σ B are computed for a particular state. Identifying Δ Δ E ≡ ≡ σ σ E and the characteristic time τ τ B ≡ ≡ σ σ B | d ⟨ ⟨ B ^ ^ ⟩ ⟩ d t | gives an energy–time relationship Δ Δ E τ τ B ≥ ≥ ℏ ℏ 2 . Although τ τ B has the dimension of time, it is different from the time parameter t that enters the Schrödinger equation. This τ τ B can be interpreted as time for which the expectation value of the observable, ⟨ ⟨ B ^ ^ ⟩ ⟩ , changes by an amount equal to one standard deviation. Examples: