Maxwell's Demon

Maxwell's Demon is a thought experiment proposed by physicist James Clerk Maxwell in 1867 that seems to violate the second law of thermodynamics.

Imagine a container of gas divided by a partition with a tiny door, controlled by a hypothetical "demon." Gas molecules naturally move at different speeds - some fast (hot), some slow (cold). The demon observes individual molecules and operates the door intelligently: it lets fast molecules pass one way and slow molecules the other way.

Over time, this sorting would create a temperature difference without expending work - one side gets hotter, the other colder. This appears to decrease entropy (increase order) spontaneously, violating the second law which says entropy in an isolated system always increases.

The resolution came much later: the demon must measure and remember which molecules are which, and this information processing itself increases entropy. When Rolf Landauer showed in 1961 that erasing information has a minimum thermodynamic cost, it became clear the demon's memory erasure dissipates at least as much energy as the sorting saves. The second law survives.

The thought experiment became foundational for understanding the deep connection between information theory and thermodynamics - a link that's crucial in modern physics and quantum computing.