In three dimensions, this generalizes to j = ℏ ℏ 2 m i ( Ψ Ψ ∗ ∗ ∇ ∇ Ψ Ψ − − Ψ Ψ ∇ ∇ Ψ Ψ ∗ ∗ ) = ℏ ℏ m ℜ ℜ { Ψ Ψ ∗ ∗ ∇ ∇ i Ψ Ψ } = ℏ ℏ m ℑ ℑ { Ψ Ψ ∗ ∗ ∇ ∇ Ψ Ψ } , where ∇ ∇ denotes the del or gradient operator. This can be simplified in terms of the kinetic momentum operator, p ^ ^ = − − i ℏ ℏ ∇ ∇ to obtain j = 1 2 m ( Ψ Ψ ∗ ∗ p ^ ^ Ψ Ψ − − Ψ Ψ p ^ ^ Ψ Ψ ∗ ∗ ) .