Maxwell’s Demon is a thought experiment proposed by James Clerk Maxwell in 1867 to challenge the Second Law of Thermodynamics, which states that entropy (disorder) in a closed system always increases.
The Setup:
Imagine a closed container divided into two chambers, A and B, separated by a door. The system is filled with gas molecules moving at different speeds (which corresponds to different kinetic energies and temperatures).
A tiny, intelligent being—the “Demon”—controls the door between the chambers. The demon can observe individual molecules and selectively open and close the door:
It lets fast-moving (hot) molecules pass from A to B.
It lets slow-moving (cold) molecules pass from B to A.
Otherwise, it keeps the door shut.
The Paradox:
Over time, this process would make chamber B hotter and chamber A colder, creating a temperature gradient without any work being done. This appears to violate the Second Law of Thermodynamics, which states that heat should naturally flow from hot to cold, not the other way around.
Resolution:
Later, physicists resolved the paradox by considering information theory and entropy:
Szilard (1929) and Landauer (1961) argued that the demon itself must process information to decide which molecules to let through. This requires memory storage and erasure, which dissipates energy and increases entropy, restoring compliance with the Second Law.
In essence, the demon doesn’t actually break the Second Law, because the act of measurement and decision-making incurs a hidden entropy cost.
Implications:
Maxwell’s Demon was an early insight into the relationship between information, computation, and thermodynamics, influencing modern fields like statistical mechanics, quantum computing, and information theory.