Starting in one dimension, using the plane wave solution to Schrödinger's equation of a single free particle, ψ ψ ( x , t ) = e i ℏ ℏ ( p x − − E t ) , where p is interpreted as momentum in the x -direction and E is the particle energy. The first order partial derivative with respect to space is ∂ ∂ ψ ψ ( x , t ) ∂ ∂ x = i p ℏ ℏ e i ℏ ℏ ( p x − − E t ) = i p ℏ ℏ ψ ψ .