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Maxwell's demon is a thought experiment that appears to disprove the second law of thermodynamics.

Lord Kelvin would later call it a "demon". In the thought experiment, a demon controls a door between two chambers containing gas. As individual gas molecules (or atoms) approach the door, the demon quickly opens and closes the door to allow only fast-moving molecules to pass through in one direction, and only slow-moving molecules to pass through in the other. Because the kinetic temperature of a gas depends on the velocities of its constituent molecules, the demon's actions cause one chamber to warm up and the other to cool down. This would decrease the total entropy of the system, seemingly without applying any work, thereby violating the second law of thermodynamics. The concept of Maxwell's demon has provoked substantial debate in the philosophy of science and theoretical physics, which continues to the present day. It stimulated work on the relationship between thermodynamics and information theory. Other researchers have implemented forms of Maxwell's demon in experiments, though they all differ from the thought experiment to some extent and none have been shown to violate the second law.

The demon must let molecules pass in both directions in order to produce only a temperature difference.

Szilárd pointed out that a real-life Maxwell's demon would need to have some means of measuring molecular speed, and that the act of acquiring information would require an expenditure of energy. Since the demon and the gas are interacting, the total entropy of the gas and the demon combined must be considered. The expenditure of energy by the demon causes an increase in the entropy of the demon that is larger than the lowering of the entropy of the gas. Norton have argued that Szilárd and Landauer's explanations of Maxwell's demon begin by assuming that the second law of thermodynamics cannot be violated by the demon, and derive further properties of the demon from this assumption, including the necessity of consuming energy when erasing information, etc.

From this viewpoint, the measurement process is regarded as a process where the correlation (mutual information) between the engine and the demon increases, decreasing the entropy of the system in an amount given by the mutual information. More recently, Kastner has argued that the uncertainty principle forces an entropy increase when the molecule is localized to one side or the other in the Szilard engine, and that is what prevents the demon from violating the second law. For the case of the original Demon who is sorting molecules by speeds, Kastner and Schlatter argue that the uncertainty principle prevents the Demon from sorting the molecules due to their delocalization upon measurement of momentum.

Single-atom traps used by particle physicists allow an experimenter to control the state of individual quanta in a way similar to Maxwell's demon.

This technique is widely described as a "Maxwell's demon" because it realizes Maxwell's process of creating a temperature difference by sorting high and low energy atoms into different containers. Only a year later and based on an earlier theoretical proposal, the same group presented the first experimental realization of an autonomous Maxwell's demon, which extracts microscopic information from a system and reduces its entropy by applying feedback. The operation of the demon is directly observed as a temperature drop in the system, with a simultaneous temperature rise in the demon arising from the thermodynamic cost of generating the mutual information.

Historian Henry Brooks Adams, in his manuscript The Rule of Phase Applied to History, attempted to use Maxwell's demon as a historical metaphor, though he misunderstood and misapplied the original principle. Adams interpreted history as a process moving towards "equilibrium", but he saw militaristic nations (he felt Germany pre-eminent in this class) as tending to reverse this process, a Maxwell's demon of history.

"Eaters of the lotus: Landauer's principle and the return of Maxwell's demon" (PDF).

Quick Facts

• As individual gas molecules (or atoms) approach the door, the demon quickly opens and closes the door to allow only fast-moving molecules to pass through in one direction, and only slow-moving molecules to pass through in the other.
• The concept of Maxwell's demon has provoked substantial debate in the philosophy of science and theoretical physics, which continues to the present day.
• This technique is widely described as a "Maxwell's demon" because it realizes Maxwell's process of creating a temperature difference by sorting high and low energy atoms into different containers.
• Other researchers have implemented forms of Maxwell's demon in experiments, though they all differ from the thought experiment to some extent and none have been shown to violate the second law.
• In the thought experiment, a demon controls a door between two chambers containing gas.

Source material: Wikipedia - "Maxwell's demon". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.