It Took Me 10 Years to Understand Entropy, Here is What I Learned.

From the Big bang to the Heat death of the universe

You are probably already somewhat familliar with the notions of entropy and the second law of thermodynamics. These are key concepts in thermodynamics classes, but entropy is a notion that we all struggled with at some points in our studies. Like my statistical physics teacher used to say:

There are only 4 or 5 persons in this world that truly understand entropy, and I don’t belong to them.

— Nicolas Sator —

In fact, in contrast to other quantity in physics such as mass or energy, it seems that entropy is a subjective quantity that depends on the macroscopic variables that an observer chose to define, or has access to. Yet, the second law seems to be a fundamental law driving our universe, on the same level as convervation of Energy. And everytime I was digging deeper into these matters, I was just getting more and more confused…

So, what is entropy ?

Entropy was originally introduced by Clausius in the early 1850s in order to describe energy loss in irreversible processes, which turned very useful to predict the spontaneous…