How do electromagnetic waves propagate ? The explanation that is normally put forward to this question is that EM propagates through the self sustaining or oscillation of the electric and magnetic fields , which both support each other. As the electric field recedes the magnetic field increases and vice versa , and this can go on almost indefinitely. Yet if we try to duplicate this process in a laboratory by using a capacitance inductance circuit , we find that the oscillations are very far from being self sustaining , coming to an immediate halt once the motivating force has been removed. Surely , given our more or less exact knowledge of how the process supposedly takes place , it should be possible to set up some kind of perpetual motion machine . Always taking into account , of course that Maxwell’s version of the propagation of electromagnetic waves are based in fact .