What Is Entropy?

Podcast of the Day

Melvyn Bragg and guests discuss the Second Law of Thermodynamics which can be very simply stated like this: "Energy spontaneously tends to flow from being concentrated in one place to becoming diffused and spread out". It was first formulated – derived from ideas first put forward by Lord Kelvin - to explain how a steam engine worked, it can explain why a cup of tea goes cold if you don't drink it and how a pan of water can be heated to boil an egg. But its application has been found to be rather grander than this. The Second Law is now used to explain the big bang, the expansion of the cosmos and even suggests our inexorable passage through time towards the 'heat death' of the universe. It's been called the most fundamental law in all of science, and CP Snow in his Two Cultures wrote: "Not knowing the Second Law of Thermodynamics is like never having read a work of Shakespeare".What is the Second Law? What are its implications for time and energy in the universe, and does it tend to be refuted by the existence of life and the theory of evolution?

Listen to the In Our Time episode on The Second Law of Thermodynamics

Video of the Day

Short Article of the Day

...Entropy, which in rough terms is the measure of a system's disorder, creeps up over time, as dictated by the second law of thermodynamics. To illustrate entropy's inexorable growth, Carroll takes us to the breakfast table—you can't unscramble an egg, he points out, and you can't unstir the milk out of your coffee. These systems invariably proceed to disordered, or high-entropy, arrangements. Each of these examples shows how the continual growth of entropy fills the world with irreversible processes that divide the past from the future: The making of an omelet and the mixing of milk into a cup of coffee are events that work in only one temporal direction.

But why should entropy always increase? This is where Carroll turns to cosmology, which must explain why the universe began in a uniquely low-entropy state...

Continue reading John Matson's article: What Keeps Time Moving Forward? Blame It on the Big Bang

Further Reading

The thermodynamic time asymmetry is one of the most salient and consequential features of the physical universe. Heat flows from hot to cold, never the reverse. The smell of coffee spreads throughout its available volume, never the reverse. Car engines convert fuel energy into work and thermal energy, never the reverse. And so on. The science of thermodynamics is able to capture these generalizations as consequences of its claim that systems spontaneously evolve to future equilibrium states but do not spontaneously evolve away from equilibrium states. This generalization covers an amazing amount of macroscopic physics and is rightly celebrated as one of the great laws of physics.

Despite its familiarity, however, the thermodynamic arrow of time raises many deep questions relevant to both philosophy and the foundations of physics...

Continue reading the Stanford Encyclopedia of Philosophy article: Thermodynamic Asymmetry in Time by Craig Callender

Bonus Webcomic

Elementary - Saturday Morning Breakfast Cereal

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