The photon, "time" and entropy (ignore original title, in fact id appreciate if a mod would change it. It was not my intention to have a conversation about time, lol!) Can we view a closed system of just photons, as ever, or usually, being subject to entropy (defined by the whole 2nd law, rather than merely just quantum statistical mechanics) 1. Say for example, the light from the big bang at the "edge of the universe", travelling in "vacuum" 2. Or a laser that we point out into space that does not encounter any objects or mass-bearing particles? I have heard it said that photons do not naturally decay on their own (not sure if thats right)? And that redshift or blueshift is not a loss in energy, that light/the photon generally does not lose energy (not sure if thats right)? Light travels in a straight line (well a waveshaped one), so without any interaction with a mass bearing object or particle, a group of photons, it would not lose or gain order? (Although if the photons were heading in different directions, they would get larger/more distributed, but retain the same shape I am thinking, so not more statistically distributed) Is it appropriate to think of photons as a system? (I believe they can interact, even if they cant collide?) Are there any systems that do not have entropy? (I guess the center of a black hole might be one, although its not closed, so thats not the best example - perhaps if a black hole tore away from all other mass during expansion?) .... Oh, and one last final question, if the photon is massless, how does it push a solar sail, or light windmill?