Maxwell's Demon

Maxwell's demon is the name of a thought experiment that has been intensely discussed by physicists and philosophers investigating the relation between energy, information, and time. In this experiment, an imaginary but not supranatural being is using his extraordinary sensorial and intellectual powers to violate the second law of thermodynamics and, therefore, to falsify the fundamental principle by which modern physics explains the irreversible direction of time. The demon is named after one of the greatest physicists of all time, James Clerk Maxwell (1831–18 79), who developed the theory of the electromagnetic field and conceived, in 1867, the experiment with the fictitious being.

The problem Maxwell wanted to solve by his thought experiment is whether a physical process might be conceived that, though being consistent with all known laws of physics, breaks the second law of thermodynamics. This law states that the entropy of a thermodynamically closed system never decreases so that it is possible to objectively measure the irreversible flow of time by comparing the entropy of different states of such a system. How did Maxwell suppose his demon to perform the feat of violating the second law? The demon, who has superhuman abilities but must act according to the laws of physics, is sitting in a closed container that is filled with gas and divided, by an impermeable partition, into two halves. In the partition there is a door that can be opened and closed at will. The demon observes the gas molecules approaching the door in both halves and sorts them by deciding which molecules are allowed to move through the door so that, for example, the [Page 829]faster ones will be gathered in the left half and the slower ones in the right half. Because the left half gradually becomes warmer and the right one cooler, the resultant temperature difference can be used to do work. Suppose that Maxwell's demon and the door could operate in a reversible manner, giving up some amount of energy when opening the door without friction and taking in the same amount of energy when closing it without friction. Then the container would constitute, for all practical purposes, a perpetuum mobile that, thanks to the demon, could completely transform heat flowing from a reservoir at lower temperature to a reservoir at higher temperature into work.

Even a short description of Maxwell's demon shows that information plays a crucial role in reasoning about the thought experiment. The demon must know the positions and the velocities of the molecules that are approaching the door to decide reasonably whether it is to be opened or closed. To bring order into a disordered system (thermody-namically speaking, to lower its entropy), information about the components of the system is needed. Therefore the physical nature of information must be taken into account if we want to find the weak spot of Maxwell's demon and to show that the thought experiment is not a counterargument against the universal validity of the second law of thermodynamics. The history of the discussion about Maxwell's demon mirrors the difficulties encountered in the development of a physics of information. Arguments for the possibility of Maxwell's demon, and against it, revolved around the information-gathering activity of the imaginary being: How does the demon come to know the location and speed of single molecules, and is the ther-modynamic cost of gathering this information high enough to rescue the second law of thermodynamics because it produces at least the same amount of entropy that it makes possible to consume?

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