Maxwell's Demon

I happen to know that James Clerk Maxwell was 48 years old when he died.

In that relatively short lifespan, Maxwell contributed as much to our understanding of the universe as almost anyone else, ever. When asked if he stood on the shoulders of Newton, Einstein said no, “I stood on the shoulders of Maxwell.”

He meant it, too: relativity came to be thanks to Maxwell’s foundational work with electromagnetism. Even beyond that, virtually all modern physics—both relativity and quantum mechanics—is built on Maxwell’s ideas and discoveries.

Like Einstein, Maxwell enjoyed a good thought experiment. These little games envisioned things that were difficult or impossible to set up in an actual experiment, but if you imagined what would happen if they played out, you could sometimes come to understand nature better.

One of these thought experiments came to be called Maxwell’s Demon.

You start with a box full of a gas. It’s not really too important what kind of gas is in there, but what matters i…