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It took physicists 115 years to tame Maxwell\u2019s Demon. <\/p>\n The universe bets on disorder. Imagine, for example, dropping a thimbleful of red dye into a swimming pool. All of those dye molecules are going to slowly spread throughout the water. Physicists quantify this tendency to spread by counting the number of possible ways the dye molecules can be arranged. There\u2019s one possible state where the molecules are crowded into the thimble. There\u2019s another where, say, the molecules settle in a tidy clump at the pool\u2019s bottom. But there are uncountable billions of permutations where the molecules spread out in different ways throughout the water. If the universe chooses from all the possible states at random, you can bet that it\u2019s going to end up with one of the vast set of disordered possibilities.<\/p>\n Seen in this way, the inexorable rise in entropy, or disorder, as quantified by the second law of thermodynamics, takes on an almost mathematical certainty. So of course physicists are constantly trying to break it.<\/p>\n One almost did. A thought experiment devised by the Scottish physicist James Clerk Maxwell in 1867 stumped scientists for 115 years<\/a>. And even after a solution was found, physicists have continued to use \u201cMaxwell\u2019s demon\u201d to push the laws of the universe to their limits.<\/p>\n In the thought experiment, Maxwell imagined splitting a room full of gas into two compartments by erecting a wall with a small door. Like all gases, this one is made of individual particles. The average speed of the particles corresponds to the temperature of the gas \u2014 faster is hotter. But at any given time, some particles will be moving more slowly than others.<\/p>\n What if, suggested Maxwell, a tiny imaginary creature \u2014 a demon, as it was later called<\/a> \u2014 sat at the door. Every time it saw a fast-moving particle approaching from the left-hand side, it opened the door and let it into the right-hand compartment. And every time a slow-moving particle approached from the right, the demon let it into the left-hand compartment.<\/p>\n After a while, the left-hand compartment would be full of slow, cold particles, and the right-hand compartment would grow hot. This isolated system would seem to grow more orderly, not less, because two distinguishable compartments have more order than two identical compartments. Maxwell had created a system that appeared to defy the rise of entropy, and thus the laws of the universe.<\/p>\n \u201cHe tried to prove a system where the entropy would decrease,\u201d said Laia Delgado Callico<\/a>, a physicist at King\u2019s College London. \u201cIt\u2019s a paradox.\u201d<\/p>\n Two advances would be crucial to solving Maxwell\u2019s demon. The first was by the American mathematician Claude Shannon, regarded as the founder of information theory<\/a>. In 1948, Shannon showed that the information content of a message could be quantified with what he called the information entropy. \u201cIn the 19th century, no one knew about information,\u201d said Takahiro Sagawa<\/a>, a physicist at the University of Tokyo. \u201cThe modern understanding of Maxwell\u2019s demon was established by Shannon\u2019s work.\u201d<\/p>\n The second vital piece of the puzzle was the principle of erasure.\u00a0In 1961, the German American physicist Rolf Landauer showed that any logically irreversible computation, such as the erasing of information from a memory, would result in a minimal nonzero amount of work converted into heat dumped into the environment, and a corresponding rise in entropy. Landauer\u2019s erasure principle provided a tantalizing link between information and thermodynamics. \u201cInformation is physical,\u201d he later proclaimed<\/a>.<\/p>\n In 1982, the American physicist Charles Bennett put the pieces of the puzzle together<\/a>. He realized that Maxwell\u2019s demon was at core an information-processing machine: It needed to record and store information about individual particles in order to decide when to open and close the door. Periodically it would need to erase this information. According to Landauer\u2019s erasure principle, the rise in entropy from the erasure would more than compensate for the decrease in entropy caused by the sorting of the particles. \u201cYou need to pay,\u201d said Gonzalo Manzano<\/a>, a physicist at the Institute for Quantum Optics and Quantum Information in Vienna. The demon\u2019s need to make room for more information inexorably led to a net increase in disorder.<\/p>\n Then in the 21st century, with the thought experiment solved, the real experiments began. \u201cThe most important development is we can now realize Maxwell\u2019s demon in laboratories,\u201d said Sagawa.<\/p>\n In 2007 scientists used a light-powered gate<\/a> to demonstrate the idea of Maxwell\u2019s demon in action; in 2010, another team devised a way to use the energy produced by the demon\u2019s information to coax a bead uphill<\/a>; and in 2016 scientists applied the idea of Maxwell\u2019s demon to two compartments containing not gas, but light.<\/p>\n \u201cWe switched the roles of matter and light,\u201d said Vlatko Vedral<\/a>, a physicist at the University of Oxford and one of the study\u2019s co-authors. The researchers were ultimately able to charge a very small battery<\/a>.<\/p>\n Others wondered if there might be less demanding ways to use information to extract useful work from a similar system. And research published in February in Physical Review Letters<\/em><\/a> seems to have found a way to do so. The work makes the demon into a gambler.<\/p>\n The team, led by Manzano, wondered if there was a way to implement something like Maxwell\u2019s demon but without the information requirements. They imagined a two-compartment system with a door, as before. But in this case, the door would open and close on its own. Sometimes particles would randomly separate themselves into hotter and colder compartments. The demon could only watch this process and decide when to turn the system off. In theory this process could create a small temperature imbalance, and therefore a useful heat engine, if the demon was smart about when to end the experiment and lock any temperature imbalance in place, much as a smart gambler on a hot streak knows when to leave the table. \u201cYou can either play all night on the roulette table, or you can stop if you win $100,\u201d said \u00c9dgar Rold\u00e1n<\/a>, a physicist at the International Center for Theoretical Physics in Italy who was a co-author on the study. \u201cWe\u2019re saying we don\u2019t need such a complicated device as Maxwell\u2019s demon to extract work in the second law. We can be more relaxed.\u201d The researchers then implemented such a gambling demon in a nanoelectronic device, to show it was possible.<\/p>\n
Samuel Velasco\/Quanta Magazine<\/p><\/div>\nHow Maxwell\u2019s Demon Continues to Startle Scientists<\/h1>\n
The thorny thought experiment has been turned into a real experiment \u2014 one that physicists use to probe the physics of information.<\/em><\/h3>\n