Yes the word space in its fundamental meaning is a cubic area that has nothing in it. No energy of any kind.
the phrase 'in its fundamental meaning' seems to take us out of the realm of physics and into philosophy or metaphysics .. someone like Kant makes interesting reading on this level.
But we're talking here about physical models which firstly say: no, space is never utterly empty, and not only because of cosmic radiation, but also for 2 other reasons:
- because of a postulated 'vacuum energy' called 'zero point energy' which arises from Heisenberg's uncertainty principle
The truth however is this is still largely speculative - here's a good exposition by John Baez
- the energy density of empty space according to General Relativity. In fact it's the Cosmological Constant which determines this energy .. Wikipedia puts it
The term can be positive, negative, or zero. It is the energy density of empty space: it can be thought of as the "cost" of having space. Because the cosmological constant has negative pressure, according to general relativity a positive cosmological constant--which means empty space has positive energy--causes the expansion of empty space to accelerate (see dark energy for details).
At this point it's time for me to say, these answers come out of my own googling of the subject, for instance wikipedia. Really, the only advantage I have over you at this stage is, my experience helps me to know which keywords are the best for the search. My point is that you should start doing this yourself if you are truly interested. I can still comment occasionally but I can't keep doing all the basic work, all the time.
<b?So far I have not found the words "permittivity" or "permeability" in that book, they could be implied in the mathematics, but without having the complete convention available for the meaning of some of the symbols, it becomes very difficult to decipher the context.
try some of these links, including wikipedia
But then the permittivity and/or permeability changes with the density of the medium, such as glass or water or the photon concentration of space it self.
Well yes, these are definitions which apply to other materials, not just empty space, and have different values (or sizes) for different media. They are only constant for a specific single medium.
BTW, this is also related to why light is refracted (bent) from one medium to another, for instance from water to air, or from glass to air.