Counterintuitive Facts (47): The Demon Who Died But Is Still Charging You

I. In 1871, physicist Maxwell proposed a thought experiment that puzzled physics for over 100 years. Imagine a box divided into left and right chambers, filled with randomly flying gas molecules. In the middle is a door. Beside the door stands a little demon. Whenever a fast moving molecule (hot molecule) flies by, he opens the door to let it into the left side. Whenever a slow moving molecule (cold molecule) flies by, he blocks it on the right side. As long as this demon keeps working, the left gets hotter and hotter, the right gets colder and colder.

II. The problem is: If you can create temperature difference (order) without consuming energy, you've broken the Second Law of Thermodynamics. We could build a perpetual motion machine. But this is obviously impossible.