Causation and radiation

Question

Electromagnetic radiation phenomena exhibit a temporal asymmetry: we observe radiation coherently diverging from a radiating source, such the light emitted by a star, but we do not observe radiation coherently converging into a source, unless we delicately set up such a system. What can explain this asymmetry? And how is the asymmetry related to the causal asymmetry, on the one hand, and the thermodynamic asymmetry, on the other?

Answer 1

This is simply a manifestation of thermodynamic irreversiblitly. Diverging radiation spreads energy around, increasing entropy, whilst converging radiation concentrates energy in one place, decreasing entropy. There are far more configurations for a given amount of energy to be spread out than concentrated, so if we put that energy in initial configuration and let it go, it is most likely to move into a spread out configuration than a concentrated one, simply because there are far more of them. We would have to find very specific initial conditions to have the energy spontaneously reconcentrate itself, as you observed.

Answer 2

Seems to me that the asymmetry in radiation you cite is merely a side-effect of the asymmetrry of gravity, i.e. that (in most cases) it attracts and never repels. This creates compact structures like stars, which then radiate in all directions, and would fly apart into a formless radiating plasma if not for gravity maintaining the spherical shape.

There doesn't seem to be any asymmetry inherent to radiation itself, as we observe the Cosmic Microwave Background radiation very nearly equally in all directions. Indeed it may be the most symmetrical thing in nature that we know of.

As for thermodynamic arrow of time and its relation to gravity, there is a lot published on how entropy is defined in gravitating systems that you may want to read.