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Maxwell's demon

Maxwell's Demon: A thought experiment that seems to defy the 2nd Law of Thermodynamics. Created by Sal Khan.

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• What if there is no "demon", ie. no outside interference? What if the reason that only the slower of the fast particles make it to the colder side (and vice-versa) is solely due to probability? (Continue to imagine a small passageway if it helps.) Obviously the chance is essentially zero, but (as with Sal's example of a ball suddenly flying up into the air after lying motionless, from the previous video) it should be theoretically possible, right?
• That single probability represents 1 microstate. Because entropy is a macroscopic value and measures change in entropy regarding all microstates, that probability doesn't violate the second law of thermodynamics.
• What if the membrane separating the two sides was composed of a material that allowed this to happen naturally without computation? Where fast particles could penetrate through it from the cold side and repelled on the hot side side and slow particles could pass through from the hot side and be repelled from the cold side?

This video does not disprove "Maxwell's Demon" just by saying that humans' way of figuring out which particles can pass would generate too much heat from computation.
• A membrane would just be a surface with lots of little trap doors, each manned by its own demon. You can just think of what Sal (and Maxwell) described as being an individual "pore" in the membrane.

Regardless of the form the demon takes, there are certain things it must do: It must first measure the speed of each particle that approaches, then use this information to decide whether to open or close the door. It simply won't work to operate the door without first measuring the speed of the particle.

So the demon (in any form) MUST be an information processing system!

The proof comes from information theory. Since the demon has to store the measurements of each particle long enough to decide whether to open or close the door, it must have some sort of memory. At some point the demon must erase its memory. Landauer's principle shows that erasing information necessarily creates a tiny amount of waste heat. It's this waste heat from erasing the demon's memory that saves the second law.
• Dear Sal, I don't get the point of this thought experiment. If it is not even possible to create such a system why consider and debate about it? I can't see the significance of it. Maybe you could explain the usefulness of this thought experiment?
• The original thought experiment was to disprove the second law. The importance of the entire story, is as a reminder; if you believe you may have violated the second law, you probably missed something.
• Isn't evaporation a kind of Maxwell's Demon? When some particles of water are fast enough to escape the body of water and go into the atmosphere, you are basically taking energy away from that body, cooling it down, which is probably why evaporation is a cooling phenomenon. Am I missing something here?
• The liquid gets cooler but the atmosphere gets hotter. Entropy is at least conserved.
• Sal said that all the slow particles would be on one side and all the fast would be on another, but wouldn't some particles' kinetic energy be transferred to others as they bumped into each other, therefore making it impossible for all of the fast molecules to be on one side and all of the slow molecules to be on the other?
• collisions between two particles change their individual KE's, but not the total (average) KE
(1 vote)
• What if you just randomly open the door, and by some chance keep reducing the entropy randomly? (If it can happen, it will happen)
• This possibility represents just one microstate, and entropy is a macrostate variable, so this still wouldn't violate the second law.
• that was confusing can someone help me?
• Basically, this 'thought experiment' is saying that we can violate the 2nd Law of Thermodynamics in this way: we start with two containers filled with a gas at the same temperature and then visualize a creature (the demon) who helps the gas atoms to 'sort' themselves by opening a trap door between the containers when he sees an atom come whizzing towards him with a particularly high kinetic energy. Similarly, when he sees an atom with a low kinetic energy come moseying along, he opens the door and lets it pass into the other container. Remember, temperature is simply average kinetic energy, so after the demon has done this a great number of times the high kinetic energy atoms and the low kinetic energy atoms will be neatly separated, and the two containers will be at different temperatures. Well, now it certainly seems that our rule that entropy (the dispersal of energy) will always increase has been violated by showing that the heat from high energy atoms is not dispersing into the lower temperature container. Actually though, physicists say that we have to include the entropy of the demon himself as he does the computational work of separating the atoms, and therefore the overall entropy of the system has in fact increased.
• I agree that this video can not get rid of the problem by saying that the mechanism needed would offset the loss in entropy. So instead of something hypothetical like a mysterious membrane let's use something simple, like chance. Say a study was conducted into this problem where his setup is replicated an extremely large number of times with the key difference being that the door opened randomly with no regard for the particles passing through. In nearly all of the tests this would not produce the end we are looking for, but this allows for the possibility of it happening even once. This means the principle stands and we still have what seems to be a problem.
• The entropy created by that one particle would be offset by the work that it took to open the door, or the friction that was created by the door when it was opened or closed. Because it is impossible in this universe to create a frictionless or weightless door (which would create a reversible process) this would cause positive entropy, which would offset all the possible entropy losses from a single particle going through the door. So, the change in entropy will still be positive.
(1 vote)
• Hey Sal. I'm assuming you know what the Heisenberg Uncertainty Principle is. Basically, it says you can't know both the momentum and position of a particle at the same time. Therefore, how would the demon know when to open and close the door if he does not know about the particles' characteristics? I might be wrong, just wanted to clarify.