Maxwell's Demon


Richard Feynman’s explanation of Maxwell’s Demon from his lectures

Sean Carroll, 2010. From Eternity to Here: The Quest for the Ultimate Theory of Time.

Quanta Magazine

Carl Sagan, 1995. The demon-haunted world: Science as a candle in the dark.

Maxwell’s Demon was born 154 years ago in the mind of James Clerk Maxwell who wanted to see if it was possible to violate the second law of thermodynamics. Could an imaginary demon serve as a trapdoor-keeper to direct fast or slow gas molecules to the correct chamber?

Jennifer Rittenhouse West is a Lab postdoctoral fellow in theoretical physics, studying theoretical particle and nuclear physics in the Nuclear Science Division. She discussed the creation of the Gedanken demon and what we can learn from the frantic actions of an imaginary lab assistant.

Q: First, what is the second law of thermodynamics?

A: The second law of thermodynamics says that the universe always tends toward increasing disorder. The measure of disorder in a system is called entropy and that is what we measure. For example, I had eggs for breakfast. I had a very organized egg with a yolk in the center and a shell. I cracked the egg, scrambled it in a pan, and ate it. It is highly unlikely that the universe will take that scrambled egg and turn it back into an uncracked egg (even if I left it on the plate for the age of the universe!). In Maxwell’s time, the second law was formulated as “heat flows from hot to cold”, making the system in question warm. Heat does not flow from cold to hot, making the cold colder and the hot hotter - a very orderly state of affairs. So this is the law of thermodynamics that Maxwell wanted to break.

Q: What was the challenge?

A: Maxwell envisioned two boxes, each full of identical gas molecules. One box was hot, with the molecules moving very fast, and the other was cold with more sluggish molecules. There is a divider that separates the two boxes of molecules. First, let’s remove the barrier ourselves and watch as the fast gas molecules spread, bump into the cold ones, and the system becomes a bunch of warm gas. Now it’s demon time: we position a little demon on top of the box at the reinserted divider. We’ll cut a tiny trap door in the divider and put a tiny handle on it so that the demon can easily open and close it. It could have been a robot or anything that can operate the door, but Maxwell picked a finite being that eventually became known as a demon.

The demon is determined to separate the warm gas molecules into hot and cold again. When it sees a molecule moving faster than average, it opens the trapdoor to allow the molecule to go to the box on the left. When it sees a slower than normal moving molecule, it opens the barrier to allow that molecule into the box on the right. If it can do this successfully, the second law of thermodynamics is broken. The demon took a disordered, high entropy state of warm gas and, by separating hot and cold, ordered it again.

Q: What is the practical implication of returning things to order?

A: The result of this hypothetical scenario is you could create energy from the newly ordered system. The problem is, by wracking its tiny brain in figuring out the molecule speeds and operating the tiny door, the demon becomes more disordered. The amount of order the demon creates by his trapdoor shenanigans is at a minimum balanced by its own growing disorder. You can’t get something for nothing!

Q: So that’s it? It isn’t possible?

A: Researchers are still working on the ideas generated by Maxwell’s demon, especially in information sciences. (In the meantime, there are other demons to learn about. There is Laplace’s demon who also dabbles in thermodynamics. Next Halloween!) If we could break the second law of thermodynamics it could lead to the creation of cheap energy - but sadly, the second law appears quite unbroken. If that ever were to change, we would no longer have Maxwell’s Demon. We would have to call it Maxwell’s Angel.