The square of the norm is: ‖ ‖ K ‖ ‖ 2 = K μ μ K μ μ = ( ω ω c ) 2 − − k ⋅ ⋅ k , and by the de Broglie relation: ‖ ‖ K ‖ ‖ 2 = 1 ℏ ℏ 2 ‖ ‖ P ‖ ‖ 2 = ( m 0 c ℏ ℏ ) 2 , we have the matter wave analogue of the energy–momentum relation: ( ω ω c ) 2 − − k ⋅ ⋅ k = ( m 0 c ℏ ℏ ) 2 .