What is Maxwell’s Demon thought experiment? Maxwell’s Demon’s thought experiment is a theoretical way that the Second Law of Thermodynamics could be challenged.
The Second Law of Thermodynamics is the law that explains entropy. It says that any system always heads from order towards disorder and the only way to get it back to order is to apply energy. It says that heat energy always moves spontaneously from an area of high heat energy to an area of low heat energy. You can see this for yourself by just leaving your hot coffee on the kitchen table in a cold room. Very quickly, the heat will have moved from the hot coffee to the cold room. It never goes the other way. The cold room still has energy, but you never see the heat energy from the cold room moving into the hot coffee and making it even hotter. The only way to make the coffee hotter, or to reheat it, is to add energy. Just like the only way to reorder a system that has gone to disorder is to apply energy. For example, if you put a sugar cube in a glass of water, the sugar cube will dissolve. The cube breaks into smaller pieces and the sugar particles mix rapidly throughout the water. It is far easier for them to be in a mixed state than an unmixed state. There is only one possible way the sugar molecules can be arranged into a sugar cube, but there are countless ways that those molecules can be arranged throughout the glass of water. The only way to get the sugar back into cube form is by applying energy. You have to heat the water to evaporate it to get the sugar back. Then you apply force to press the sugar back into cube shape. Entropy always goes only one way unless energy is added back. The entropy of the universe will only increase.
Maxwell’s Demon thought experiment was a theoretical way of challenging this law. The Maxwell in the experiment was James Clerk Maxwell, a Scottish physicist and mathematician. He accomplished many things in his life. He came up with the theory of electromagnetic radiation, he made the first durable color photograph, amongst numerous other things. He is often ranked as one of the most influential scientists of all time, even by Einstein, who said Maxwell “did work that was the most profound and the most fruitful physics has experienced since the time of Newton.” He was working in the mid 19th century, around about the same time that the first two Laws of Thermodynamics were created by Rudolf Clausius and William Thomson. He came with his thought experiment just after. So, what was it?
The experiment imagines two chambers that are full of gas. A gas is a higher-energy state of matter than liquids or solids, and the molecules move constantly because of all the energy they have. Between the two chambers is a door. In each chamber there are fast moving molecules and slow moving molecules because they all have different amounts of energy, as is normal. In the regular universe, under the Second Law of Thermodynamics, they temperature between the molecules would slowly equalize until they were all the same. Maxwell proposed that there is a creature between the two chambers that can open and close the door very quickly. It would need to be a supernatural being to be able to do it fast enough. The gas molecules bounce all over the place and when fast moving particles in the left chamber are heading towards the door, the demon would quickly open the door so that they move into the right chamber and when slow moving particles are moving towards the door in the right chamber, the demon would open and close the door so that they move into the left changer. By doing this, all of the slow moving particles would be in one chamber and the fast moving particles would be in the other chamber, Order would have increased, and entropy would have decreased. The amount of energy in the one chamber would have increased significantly without any work having to be done. This would go against the Second Law of Thermodynamics.
There are many arguments over how this experiment doesn’t break the law. The main one is that the chamber, the door, and the demon are all part of the system. The gas molecules may be defying entropy, but the demon needs to apply energy to open and close the door, which adds to the entropy of the system. The demon also has to measure the atoms and that information cannot stay in its head forever, When it erases the information, that is adding to entropy. There is also the point that in any chamber the gas molecules would slowly lose heat to the air or to the surface of the chamber because nothing can be insulated to 100%. Still, it is a very interesting thought experiment.
Sources
https://en.wikipedia.org/wiki/Maxwell%27s_demon
https://en.wikipedia.org/wiki/James_Clerk_Maxwell
https://openstax.org/books/physics/pages/12-3-second-law-of-thermodynamics-entropy
https://physics.stackexchange.com/questions/614630/why-only-heat-energy-is-linked-with-entropy
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