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MAXWELL'S THOUGHT EXPERIMENT 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." In other words, Maxwell imagines two containers, ''A'' and ''B'', filled with the same Gas at equal temperatures, placed next to each other. A little "demon" guards a trapdoor between the two containers, observing the Molecules on both sides. When a faster-than-average molecule from ''A'' flies towards the trapdoor, the demon opens it, and the molecule will fly from ''A'' to ''B''. Thus, the average Speed of the molecules in ''B'' will have increased, while the molecules in ''A'' will have slowed down on average. However, since average molecular speed corresponds to temperature, the temperature in ''A'' will have decreased and in ''B'' will have increased; this is contrary to the second law of thermodynamics. CRITICISM, DEVELOPMENT, APPLICATION ''Is Maxwell correct?'' ''Could such a demon, as he describes it, actually violate the second law?'' One of the most famous responses to this question was suggested in 1929 by Leó Szilárd and later by Léon Brillouin . 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. The Second Law states that the total entropy of an isolated system must increase. Since the demon and the gas are interacting, we must consider the total entropy of the gas and the demon combined. The expenditure of energy by the demon will cause an increase in the entropy of the demon, which will be larger than the lowering of the entropy of the gas. For example, if the demon is checking molecular positions using a flashlight, the flashlight battery is a low-entropy device, a chemical reaction waiting to happen. As its energy is used up emitting photons (whose entropy must now be counted as well!), the battery's chemical reaction will proceed and its entropy will increase, more than offsetting the decrease in the entropy of the gas. Szilárd's insight was expanded upon in 1982 by Charles H. Bennett . In 1960, Rolf Landauer realized that certain measurements need not increase thermodynamic entropy as long as they were thermodynamically reversible. Due to the connection between thermodynamic entropy and information entropy, this also meant that the recorded measurement must not be erased. In other words, 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. {Link without Title} . Real-life versions of Maxwellian demons (with their entropy-lowering effects, of course, duly balanced by increase of entropy elsewhere) occur in living systems, such as the Ion Channel s and Pumps that make our Nervous System s work, including the human Brain . Single atom traps allow an experimenter to control the state of individual quanta in the same way as Maxwell's demon. Molecular-sized mechanisms are no longer found only in biology; they are also the subject of the emerging field of Nanotechnology . A mechanical implementation exists as a commercially-available device, called a Ranque-Hilsch Vortex Tube . Due to conservation of angular momentum, hotter molecules are spun to the outside of a tube while cooler molecules spin in a tighter whirl within the tube, allowing venting of each from opposite ends of the tube. ADAMS AND THE DEMON AS HISTORICAL METAPHOR Historian Henry Adams in his manuscript ''The Rule of Phase Applied to History'' attempted to use Maxwell's demon as an historical Metaphor though he seems to have misunderstood and misapplied the principle. Adams interpreted History as a process moving towards Equilibrium but saw Militaristic nations, and Adams felt Germany pre-eminent in this class, as tending to reverse this process, a Maxwell's Demon of history. Adams made many attempts to respond to the criticism of his formulation from his scientific colleagues but the work remained incomplete at Adams' death in 1912 . It was only published posthumously. MAXWELL'S DEMON IN POPULAR CULTURE
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