How do EM waves travel in a vacuum?

Question

Apart from that electric and magnetic field variation thing, is there any other explanation? Can photons simply pass through vacuum?

Answer 1

The real underlying basis of an electromagnetic wave is a synergy of zillions of photons. In this sense it is only macroscopically that the classical theory applies. The way the build up happens, photons into an electromagnetic wave, is not simple but an example can be seen here.

Hand waving: the photon as an elementary particle is a quantum mechanical entity and its wave function obeys a quantized Maxwell's equation. This wave function contains the elements that build up, when in an ensemble of photons. the classical wave . Elementary particles have no problem in traversing a vacuum. As the photons do not interact in a vacuum they keep their synergy in building a particular wave as it goes through the vacuum.

Answer 2

How do EM waves travel in a vacuum?

Like waves travel through other things. Electromagnetic waves are comprised of photons, which have an energy E=hf or E=hc/λ where f is frequency and λ is wavelength. The frequency and wavelength are there because photons have a wave nature, not a billiard-ball nature. And as per any wave, the speed depends on the properties of the medium. Space is such a medium, see the Robert B Laughlin quote here:

"It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed".

Space has properties such as permittivity and permeability, wherein the speed of light is given as $c_0={1\over\sqrt{\mu_0\varepsilon_0}}$, see Wikipedia. This expression is somewhat similar to shear wave velocity $v_s = \sqrt{\frac {G} {\rho} }$, again see Wikipedia.

Note that electromagnetic waves don't propagate because the electric wave E induces the magnetic wave B which creates the electric wave E in some dozy-doh fashion. This is often touted as the reason why electromagnetic waves don't need a medium, but I'm afraid it's a popscience myth. See electromagnetic radiation on Wikipedia and note this: "the curl operator on one side of these equations results in first-order spatial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field, is first order in time”. The electric wave and the magnetic wave are just two different derivates, they're just two different aspects of the electromagnetic wave. If it was an ocean wave and you were in a canoe, E denotes the slope of your canoe and B denotes the rate of change of slope. One is not creating the other.