Longitudinal EM waves can (and most often do) move at light speed c.

In space, light waves are NOT force field waves, because force fields exist only in mass. Mass is a component of force by F = d/dt(mv). When m = 0, as in massless space, then F = 0.

So as Nobelist Feynman pointed out in his 1964 three volumes of sophomore physics,

"…in dealing with force the tacit assumption is always made that the force is equal to zero unless some physical body is present… One of the most important characteristics of force is that it has a material origin…" [Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, Addison-Wesley, Reading, MA, Vol. 1, 1964, p. 12-2].

"…the existence of the positive charge, in some sense, distorts, or creates a "condition" in space, so that when we put the negative charge in, it feels a force. This potentiality for producing a force is called an electric field." [Richard P. Feynman, Robert B. Leighton, and Matthew Sands, The Feynman Lectures on Physics, Addison-Wesley, Reading, MA, Vol. 1, 1964, p. 2-4].

"We may think of E(x, y, z, t) and B(x, y, z, t) as giving the forces that would be experienced at the time t by a charge located at (x, y, z), with the condition that placing the charge there did not disturb the positions or motion of all the other charges responsible for the fields." [ibid, vol. II, p. 1-3.]

Hope this helps,