Maxwell's Demon
A demon sorts fast and slow molecules — apparently violating the second law of thermodynamics. Until you count the cost of its memory.
The Setup
In 1867, James Clerk Maxwell imagined a box divided in two, filled with gas. At the dividing wall sits a tiny demon. It can see individual molecules and operate a small frictionless gate. Fast molecules go left. Slow molecules stay right.
After a while: the left side is hot, the right side is cold. Temperature difference — without doing any apparent work. The demon seems to have decreased the entropy of the system without spending energy. A perpetual motion machine of the second kind.
The Paradox
The second law of thermodynamics says entropy in a closed system can only stay the same or increase. Maxwell's demon appears to break it. The puzzle remained open for nearly a century.
The Resolution
In 1961, Rolf Landauer showed that the demon has to remember each decision. When the demon's memory fills up, it must erase it to continue operating.
Erasing information is not free. Landauer's principle states that erasing one bit releases at least kT ln 2 of heat — exactly enough to restore the entropy the demon collected. The second law survives. Not because the demon is clumsy, but because information has thermodynamic weight.
Landauer's principle was experimentally confirmed in 2012 by Éric Lutz and colleagues, using a single colloidal particle as a one-bit memory device.