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Maxwell's Demon
Posted on Saturday, August 30, 2003 @ 18:25:20 GMT by vlad

Science Robert Lerwill writes: "This is brief history of how Maxwell's Demon has been viewed by the scientific establishment. My intention in giving this summary is to show that it is not a settled matter, but is in fact the subject of an ongoing debate. Over the 132 years since the idea was first published, many reasons have been put forward to show that Maxwell's Demon cannot exist. Most of them have been accepted at the time. Most of them have later been shown to be wrong.

Maxwell's demon is a molecule size demon that sits over a hole in a barrier between two gas-filled chambers and has the power to bar or let pass molecules that try to go through the hole. He takes it upon himself to only allow "hot" molecules through from right to left and "cold" ones from left to right. Soon he will create a spontaneous temperature difference between the two halves in violation of the 2nd law of thermodynamics.

This is the original description of the demon. However the idea and the use of the term has been extended. The "intelligence" behind the demons action can be mechanical. The demon can separate pressure instead of temperature. Any scheme that acts on individual molecules to sort them in some way and so spontaneously reduce entropy has become to be referred to as an example of Maxwell's demon.

In a letter Maxwell wrote in 1868, he said he come up with the notion of the demon "To show that the 2nd law of thermodynamics is only a statistical certainty." In other words, he used them to illustrate that the law only applied for macroscopic processes, where statistics came into play. He thought the sustained and concerted unidirectional action of a microscopic process, like the demon, could violate the law.

Later physicists sought to show that Maxwell's demon and its analogues were impossible. The first notable attempt was by Leo Szilard in 1929. He proposed a version of the engine that created a pressure difference rather than a temperature difference. However he showed that the energy needed to "see" the molecules approaching the hole would exceed the energy separated.

This idea was accepted for over a century. However Bennet (Scientific American, Nov 1987) eventually showed that Szilard was wrong and that it was possible to detect the position of an individual molecule without expending energy. His alternate theory was that it was the storing of information about the molecules position that took up the energy.

Landauer showed in 1989 that Bennet was wrong to claim energy was required for storage. He claimed instead that it was the decision making process itself that took up the excess energy.

Following on from this, Caves claimed in 1990 that Landauer was wrong and that if an array of cells was used, the decision making process could be made to consume less than the excess energy (Physical review letters, vol. 64 pg.2111).

Last episode I heard was Zurek et al had found a flaw in Caves reasoning, and Caves conceded. They were preparing a paper for Physical Review Letters in June 1990. But the story does not end there. Zurek thought that it still might be possible to build an analogue of Maxwell's demon using overlapping processes. All the analysis so far had all been carried out on discrete processes (Science News, Vol. 137).

Any advances on Zurek?
(from Yahoo free_energy list)



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"Maxwell's Demon" | Login/Create an Account | 3 comments | Search Discussion
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Re: Maxwell's Demon (Score: 0)
by Anonymous on Sunday, August 31, 2003 @ 05:01:33 GMT
What about the Uncertainty Principle? If you know exactly where the molecule is (in a very small hole) you cannot know whether it is "hot" or "cold".



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