Maxwell's demon is a character in an 1867 thought experiment by the Scottish physicist James Clerk Maxwell, meant to illustrate the second law of thermodynamics. This law "forbids" (among other things) two bodies of equal temperature, brought in contact with each other and isolated from the rest of the Universe, from evolving to a state in which one of the two has a significantly higher temperature than the other. The second law is also expressed as the claim that entropy never decreases.
In Maxwell's thought experiment, two containers, A and B, filled with the same gas at equal temperatures, are placed next to each other. A little 'demon' guards a trapdoor between the two containers, observing the molecules on both sides. Whenever a faster-than-average molecule from A flies towards the trapdoor, he opens it, and the molecule will move from A to B. Then he waits until a slower-than-average molecule from B comes flying towards the trapdoor, which he opens again, letting the molecule through to A. Thus, the average speed of the molecules in B increases and that in A decreases. But since average molecular speed corresponds to temperature, this means that the temperature in B increases and that in A decreases, which is a violation of the second law of thermodynamics.
So, why would a setup like Maxwell's demon not work? The question was first answered in 1929 by Leó Szilárd. Any real "demon" that does this would not be a disembodied spirit receiving its information telepathically; acquiring information about the world requires physical interaction with it. In determining what side of the gate a molecule must be on, the demon must store information about the state of the molecule. Eventually, the demon will run out of information storage space and must begin to erase the information that has been previously gathered. Erasing information is a thermodynamically irreversible process that increases the entropy of a system. Maxwell's demon therefore reveals a deep connection between thermodynamics and information theory.
Real-life versions of Maxwellian demons (with their entropy lowering effects of course duly balanced by increase of entropy elsewhere) actually occur in living systems, such as the ion channels and pumps that make our nervous systems work, including the human brain. Molecular-sized mechanisms are no longer found only in biology; they are also the subject of the emerging field of nanotechnology.
External links and references
- Harvey S. Leff, Andrew F. Rex (editors), Maxwell's Demon 2: Entropy, Classical and Quantum Information, Computing, Institute of Physics, 2003 — an anthology and comprehensive bibliography of academic papers pertaining to Maxwell's demon and related topics. Chapter 1 provides a historical overview of the demon's origin and solutions to the paradox. The 1st edition from 1990 (out of print) contained several additional relevant papers.
- Charles H. Bennet, "Demons, Engines and the Second Law", Scientific American, pp.108–116 (November, 1987).