Pierre007080 said:
Thanks for the advice. Yes Dave, My common sense does shout out that a wave needs something to propagate through. My understanding of the current theory is that it involves "virtual" photons and the original question was based on the dilemma that I have that it is necessary to theorise anything virtual when gillions of photons are already present to do the job (by the way, that IS a question, but I understand if you say it could be leading ... but I don't know where else to involk such dialectic. Perhapa all questions are leading in a way)
Virtual photons is a different set of ideas. First, it is encompassed within relativistic quantum electrodynamic theory, like quantum field theory, and is not used in classical electrodynamics or even non-relativistic quantum mechanics. The latter here would be able to predict the background energy and black body radiators. Virtual photons arise only when we allow relativity. One way of thinking about it is that in relativity, we can go between matter and energy due to the equivalence principle (but this is not to say that they are both at the same time, I would not equate matter as being energy and vice-versa as you seem to be promulgating by stating that the background medium is the background radiation). This allows fields of high enough energy to create particles, like photons or electrons. An additional effect from quantum theory is that the energy of a field can vary due to the Heisenberg Uncertainty Principle with respect to a given length of time. Thus, over short time periods, a field can have a high enough energy to create a particle that is short lived. This particle is created and annihilated back to energy over very short time spans with accordance to the Heisenberg Uncertainty Principle. These are virtual particles. They do not persist in their existence long enough (or to any degree of certainty let's say) to be considered to be real. However, they do interact with a system and leave their mark in such a way.
So a static field is non-radiating. In a way, we can thus simulate it as interacting via virtual photons. These virtual photons will propagate an electric/magnetic field, but since they are virtual in a sense they will not create a radiating field.
The background radiation however is not made up of virtual photons. Virtual photons are not radiating waves nor are they meant to be taken as true physical phenomenon. Most of the time when we talk about virtual particles in quantum field theory, the processes that we apply them to are not meant to be taken as actual physical processes. Instead they are just a means of describing the problem mathematically.
Probably the biggest thing to note though is that photons are indistinguishable boson particles. They do not interact with each other like a fermion (ie: electron) would. If I have a bottle with a photon in the energy eigenstate E_1, I can place another photon in the bottle with energy eigenstate E_1 and they both will be happy. This cannot happen with fermions like electrons because of the Pauli Exclusion Principle. So classically, electromagnetic waves follow linear superposition. If there is a background radiation field, this field does not interact in any way with other radiation outside of just being an additive field (though it will affect any sources but we are strictly speaking of a source free region here). If the electromagnetic waves required this background radiation to propagate, it would stand to reason that the behavior of the background radiation should impact the propagation. But I have yet to hear of differences in the propagation of electromagnetic waves as a function of background radiation. In addition, there could be other effects similar to what would arise with the aether of yesteryear, like the what Dave has mentioned previously, which have not been observed.
