For de Broglie matter waves the frequency dispersion relation is non-linear: ω ω ( k ) ≈ ≈ m 0 c 2 ℏ ℏ + ℏ ℏ k 2 2 m 0 . The equation says the matter wave frequency ω ω in vacuum varies with wavenumber ( k = 2 π π / λ λ ) in the non-relativistic approximation. The variation has two parts: a constant part due to the de Broglie frequency of the rest mass ( ℏ ℏ ω ω 0 = m 0 c 2 ) and a quadratic part due to kinetic energy.