What Is Maxwell's Demon?
Maxwell's Demon is a famous thought experiment that imagines a tiny, intelligent being (the 'demon') who controls a door between two chambers of gas. By selectively allowing fast-moving (hot) particles to pass into one chamber and slow-moving (cold) particles into the other, the demon could seemingly violate the second law of thermodynamics by decreasing the total entropy of the system without doing any work.
Section 2: The Setup of the Experiment
The experiment is set in a sealed box filled with gas at a uniform temperature, divided into two equal chambers by a wall. In this wall is a tiny, massless door that the demon can open and shut at will. The demon observes the particles approaching the door from both sides. It only opens the door to let faster-than-average particles move from chamber A to chamber B, and slower-than-average particles move from B to A.
Section 3: The Paradoxical Outcome
Over time, the demon's sorting action would cause chamber B to become hot (filled with fast particles) and chamber A to become cold (filled with slow particles). This creates a temperature difference between the two chambers, seemingly from nothing. This temperature difference could then be used to perform work, effectively creating a perpetual motion machine of the second kind, which violates the established principles of thermodynamics.
Section 4: Resolution and Importance
The paradox is resolved by considering the information the demon must gather and process. To know which particles are fast or slow, the demon must make a measurement. The act of acquiring, storing, and eventually erasing this information has an unavoidable thermodynamic cost. This process generates more entropy in the demon's own system (or the measurement device) than the amount it reduces in the gas, thus preserving the second law of thermodynamics. This thought experiment was crucial in establishing the deep connection between thermodynamics and information theory.