Trick or Treat

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You know, what with the recent passing of Halloween, I thought I’d try my hand at a little horror story-telling of my own. It is a chilling tale about a demon, passed down through the ages in hushed tone and bated breath. Few dare to become too invested, and even the most intrepid minds find their dreams being haunted by this eerie creature. Those that are feint of heart, be warned…

In the mid 1800s, physicist James Clerk Maxwell devised a thought experiment involving a cunning demon. This demon has the ability to detect and identify the properties of any particle within a contained gas (I know, scary stuff). It guards a trapdoor between two separated parts of a container, which have both been filled with the same gas at equal temperatures, meticulously tracking all the particles which approach this door. When a faster than average molecule from one chamber moves towards the trapdoor, the demon allow it to pass, and likewise, when a slower than average molecule moves towards the trapdoor from the other chamber, the demon will let it through, like a somewhat backwards traffic warden.

maxwells_demon
Image: Htkym

The result of this repeated process is that the average speed of molecules in one chamber will have increased, whilst in the other chamber the molecules will have, on average, slowed down. Since temperature is a measure of the average molecular speed, the temperature decreases in one, and increases in the other. However, the second law of thermodynamics, which states that the total entropy of an isolated system can never decrease, implies that heat can never pass from a colder to a warmer body. How is this demon causing this apparently physics-defying paradox? Are the laws of thermodynamics inherently wrong?

This thought experiment, which has been coined the spooky name Maxwell’s Demon, disturbed the scientific world with the prospect of offering endless free energy. We all know, however, that nothing in this life comes free (unless you’re trick-or-treating). The best response came 60 years after the first proposal, offering a solution to the terrifying problem. Fellow physicist Leo Szilard considered the entropy that would be generated when the demon made an observation. Since the demon is making a measurement on the particles and must therefore be interacting with them, the demon itself is an inherent part of the system and so we must consider its entropy too. When the demon measures the speed of a molecule, it is acquiring information, and this requires an expenditure of energy, causing its entropy to increase by a larger amount than the entropy of the gas is decreased. This argument was later extended, but the general idea behind why this paradox is in fact not paradoxical stems from the idea that in a modern interpretation, information – including how we obtain it, store it and delete it – is itself a form of entropy. Recent experiments have been conducted which confirm the theoretical predictions, and I urge you to take a look if you feel so inclined.

I hope that this post has left you sufficiently spooked, pleasantly treated and most of all, not feeling tricked…

Harvey

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