The flow velocity is defined by u ( x , t ) = 1 m ∇ ∇ S , from which we also find that 1 m ρ ρ m u = j = ℏ ℏ 2 m i [ ψ ψ ∗ ∗ ( ∇ ∇ ψ ψ ) − − ψ ψ ( ∇ ∇ ψ ψ ∗ ∗ ) ] , where j is the probability current of standard quantum mechanics.