We were taught in the vibrations and waves lecture course that the solution to the wave equation for traveling waves is of the form ψ(z,t) = Acos(wt-kz). In the Electromagnestism course we learnt that EM waves are traveling waves and have the solution E = E0cos(kz-wt). I know that changing the sign of the z term determines the direction of propagation along the z axis, but does that mean that the "-wt" term correspond to moving back in time? Or does the sign of the wt term not matter? Thanks!