In this post Matt Strassler talks about proportionalities and symmetry in the context of massless photons.
I asked this question
«x and y are not invariant, but the equation which relates them is invariant! » This sounds like the definition of a proportionality, ratios making up the equality of ratios vary but the rule of the proportionality stays same. What is the relationship?
Matt Strassler replied but I did not really understand the relationship between proportionalities and symmetry. I did not want to discuss the issue in comments. Can anybody explain this in more simple terms? Is symmetry more fundamental then proportionalities? What is their relation?