In thermodynamics, we consider two related but substantially different properties of a system: energy and entropy. Both are hard to define - we might say that energy is "the ability to do work", but this isn't really true - consider a hot lump of metal. It certainly has energy, but the second law says we can't extract work from it without a colder reservoir. And even then we need to consider Carnot to tell us how much of this we can actually access. This definition applies better to exergy - the amount of energy available to do work, but let's not complicate things for now. When it comes to entropy though, we struggle even more to find a satisfactory description. Some would describe it as a measure of disorder, some tell us we don't need to define it in words - if we deal with it enough, we'll become comfortable with what it means.

But I find entropy troubling. Depressing, even.

Energy is a nice concept. Energy is always conserved - no matter what you do with it, it will always be there, just converted between its various forms. Entropy, on the other hand, via the second law, essentially states that in the fullness of time everything turns to shit. In everything we do, we make our energy less useful, everything a bit more samey and our toast a lot more burnt.

Entropy says you'll never get back out what you put in.

Entropy says you're on a one way street to a frozen demise.

Entropy says that one day everything you own will count for nothing.

Entropy says your first true love will never take you back.

Entropy says "things have changed, we have to move on".

Entropy says that you'll eventually die cold, unhappy and alone.

Entropy says there's no light at the end of the tunnel, because it's all fucked off into heat.

Entropy is the mistress of the universe - and a cruel mistress she is.

(This rant was adapted from a recent facebook status)