Maxwell's demon is a thought experiment proposed by the physicist James Clerk Maxwell in 1867. It involves a hypothetical creature that can sort molecules of gas into hot and cold reservoirs without expending energy, seemingly violating the second law of thermodynamics.
In the thought experiment, Maxwell's demon controls a small door between two chambers of gas. By only allowing fast-moving (hot) molecules to pass in one direction and slow-moving (cold) molecules to pass in the other, the demon creates a temperature difference between the two chambers, which could theoretically be used to do work, without any energy expenditure by the demon itself.
The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time. Maxwell's demon seems to violate this law by decreasing the entropy of the gas system without any apparent energy input, thereby creating order from disorder.
Modern interpretations recognize that the demon itself must expend energy to measure and sort the molecules, which compensates for the decrease in entropy of the gas. This energy expenditure ensures that the second law of thermodynamics is not violated. Information theory and statistical mechanics have provided deeper insights into the demon's actions, showing that the processing and erasing of information by the demon require energy.
While the original thought experiment remains theoretical, modern experiments have demonstrated similar principles using nanoscale systems. Researchers have created devices that mimic the demon's sorting mechanism by using external energy sources to control the movement of particles, thereby validating the theoretical insights about energy and information processing.