Maxwell's demon
A tiny demon opens a door and makes heat flow backwards. Free energy forever?
James Clerk Maxwell, letter of 1867; solved by Szilard, Landauer and Bennett
The doorkeeper
The second law of thermodynamics is one of the toughest laws in physics: heat flows from hot to cold, never the other way, unless you do work. A cup of chai cools down; it never spontaneously heats up by stealing warmth from the room.
In 1867 the Scottish physicist James Clerk Maxwell imagined a way to cheat. Take a box of gas, split it in two with a wall, and cut a tiny door in the wall. Put a tiny, clever creature at the door: later called a demon.
Gas molecules zoom around at different speeds. Whenever a fast molecule heads towards the door from the left, the demon opens it and lets it through to the right. Whenever a slow one heads from right to left, it lets that through. Otherwise the door stays shut.
After a while, the right side is full of fast molecules, which means hot, and the left is full of slow ones, which means cold. The demon has made heat flow from cold to hot just by opening a door. You could run an engine off the difference. Forever.
The puzzle
A demon that only sorts molecules seems to break the second law of thermodynamics without doing any work.
See it for yourself
What's really going on
For about a hundred years nobody could quite pin down the flaw. The answer turned out to be information. The demon has to measure every molecule and remember what it saw. In 1929 Leo Szilard linked information with entropy, and in 1961 Rolf Landauer showed that erasing a bit of information always costs a tiny amount of energy as heat.
The demon's memory eventually fills up. To keep going it must erase it, and erasing releases at least as much entropy as the demon removed from the gas. Charles Bennett completed the argument in 1982. The second law survives, and information turns out to be physical.
Think about it
Your computer's processor gets warm when it works. How much of that do you think comes from erasing information?
More physics paradoxes
- Schrödinger's catA cat in a box is both alive and dead until someone looks. Really?
- The black hole information paradoxThrow a book into a black hole. When the hole evaporates, is the book's information gone forever?
- The double-slit experimentFire particles one at a time through two slits. They make a wave pattern. Watch them and the pattern vanishes.