The idea of "non-Hertzian" EM energy goes back to the 19th century. I'll try to give a short summary.
At the end of the 19th century, physicists believed that light was a mechanical oscillation in a mechanical medium, the aether. But such a medium could in principle sustain two kinds of waves: transversal and longitudinal. Those readers who are unfamiliar with physics should think of water for illustration: transversal waves are the surface waves which wave in a fashion perpendicular to their axis of propagation, while longitudinal waves are the sounds waves or pressure waves that propagate underneath the surface. The latter oscillate in the direction of propagation.
Transversal waves generally move at a different speed than longitudinal waves.
