This is the expression for the canonical momentum. For a charged particle q in an electromagnetic field, during a gauge transformation, the position space wave function undergoes a local U(1) group transformation, and p ^ ^ ψ ψ = − − i ℏ ℏ ∂ ∂ ψ ψ ∂ ∂ x will change its value. Therefore, the canonical momentum is not gauge invariant, and hence not a measurable physical quantity.