Maxwell's demon

From Academic Kids

Maxwell's demon is a character in an 1867 thought experiment by the Scottish physicist James Clerk Maxwell, meant to raise questions about 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.

Maxwell described his thought experiment in this way:

"... if we conceive of a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are as essentially finite as our own, would be able to do what is impossible to us. For we have seen that molecules in a vessel full of air at uniform temperature are moving with velocities by no means uniform, though the mean velocity of any great number of them, arbitrarily selected, is almost exactly uniform. Now let us suppose that such a vessel is divided into two portions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower molecules to pass from B to A. He will thus, without expenditure of work, raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics."[1] (

In other words, Maxwell imagines 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 is Maxwell correct -- could a demon such as he describes actually violate the second law? One of the most famous responses to this question was suggested in 1929 by Le Szilrd. Szilrd 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. Szilrd's insight was expanded upon in 1982 by C.H. Bennett, who argued that to determine 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. According to Bennett, therefore, Maxwell's demon therefore reveals a deep connection between thermodynamics and information theory. [2] (

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.

Maxwell's demon in culture

Maxwell's demon plays a significant role in the book The Crying of Lot 49 by Thomas Pynchon, and also makes appearances in Her Majesty's Wizard by Christopher Stasheff, the 'Captain Baseball Bat Boy' cartoons in the videogame Max Payne 2, and in the manga Oh My Goddess!, by Kosuke Fujishima. Maxwell Demon is also a fictional charecter from the Movie Velvet Goldmine.

The term Maxwell's Demon is mentioned in the Square Super Famicom title, Radical Dreamers, when the player attempts to access the third ending.

Maxwell's Demon is mentioned as well in the Novel Homo Faber by swiss author Max Frisch.

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).

de:Maxwellscher Dmon fr:Dmon de Maxwell it:Diavoletto di Maxwell ja:マクスウェルの悪魔 zh-cn:麦克斯韦妖 tr:Maxwell'in Cini


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