Note that for massless particles, in which case m 0 = 0, we have: ( ω ω c ) 2 = k ⋅ ⋅ k , or ‖ k ‖ = ω / c . Note this is consistent with the above case; for photons with a 3-wavevector of modulus ω / c , in the direction of wave propagation defined by the unit vector n ^ ^ .