[Retric]

We have some great models but what actually happens doesn't quite fit into any one model. For example all photons are constantly redshifted as they travel through space because space is expanding. That’s not really relevant on human timescales but it is an effect that takes place between your monitor and your eyes.

When you really dig into this stuff your realise stuff like the density of air is really an abstraction that doesn't quite fit what is actually going on.