The second law of thermodynamics — the law of entropy — is a fascinating thing. It’s the law that makes the past different from the future; it’s the law that predicts an effective end to the Universe, yet it’s the law that makes life possible. It’s also a deeply mysterious law. It took well over a century for true meaning of entropy to be understood (in fact arguments on the subject still rage today), and we still don’t understand, on a cosmological level, exactly why it was so low in the past.
One of the things that’s often said about entropy is that it means “disorder”. This post is about that idea. It’s worth discussing for two reasons: firstly, it’s wrong. It’s close to the truth, in the same sort of way that a spectrum is close to a rainbow, but not the same. Secondly, the real truth is much more interesting.