What Is Entropy?

People say many things about entropy: entropy increases with time, entropy is disorder, entropy increases with energy, entropy determines the arrow of time, etc.. But I have no idea what entropy is, and from what I find, neither do most other people. This is the introduction I wish I had when first told about entropy, so hopefully you find it helpful. My goal is that by the end of this long post we will have a rigorous and intuitive understanding of those statements, and in particular, why the universe looks different when moving forward through time versus when traveling backward through time. This journey begins with defining and understanding entropy. There are multiple formal definitions of entropy across disciplines—thermodynamics, statistical mechanics, information theory—but they all share a central idea: entropy quantifies uncertainty. The easiest introduction to entropy is through Information Theory, which will lead to entropy in physical systems, and then finally to the rela ... Read full article.