You guys ask great questions—I’ve missed replying to them! But I love talking about entropy so prepare yourself.
The Second Law of Thermodynamic is part of a set of three fundamental, beautifully simple physical laws of a thermodynamic system.
- First Law: Energy is not created or destroyed.
- Second Law: In an isolated system, entropy must always increase.
- Third Law: Absolute zero cannot be achieved.
Most people understand the Second Law to mean that “In an isolated system, disorder must always increase”, as in things always tend from order to disorder, but entropy does not automatically equate to disorder. It’s just a measure of disorder.
For those who are unfamiliar with entropy, the basic concept is pretty simple because we see it all around us everyday—objects break, relationships disintegrate, people age. To understand the terms of “high” and “low” entropy means, think of it like this: if a library is highly-organised and has an efficient indexing system, it could be called a low entropy library, whereas if a library has jumbled shelving, a disorganised indexing system and books all over the place, it could be called a high entropy library.
By referring to an ‘isolated system’, the Second Law is referring to a system where no energy is being added to it or taken from it, and entropy is a gauge of the energy in such a system that can’t be used anymore. This unavailable energy hasn’t left the system—it’s just become irretrievably disordered—but even though this will increase, it doesn’t mean that elsewhere in the system other energy can’t become more ordered.
Sure, a system (for example, the universe) contains unavailable energy, but the rest of the universe’s energy needs to be accounted for. It’s still out there, still doing stuff, and can still behave in a bunch of different ways depending on a variety of forces. It’s completely possible for a closed system to create even complex and elaborate order, just as long as there’s a balance and there’s an increase of disorder elsewhere in the system—importantly, you can only create order by increasing disorder too, because creating order involves expending energy, which is inevitably inefficient and so it adds to the unavailable energy.
True to the Second Law of Thermodynamics, the total amount of order in the universe is always decreasing, but that doesn’t mean parts of the universe can’t continue to become ordered too.
It’s like a struggle against the tide on a cosmic-scale. Structures, stars and organisms are created as low-entropy systems, but it’s fruitless because disorder is a byproduct of order and will triumph in the end—and yet, the universe keeps striving for order all the same.