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I never understood what was so important about Maxwell's Demon. If an external, supernatural force acts upon a system to reduce entropy how is this in any way c

by c3534l 6y ago

I never understood what was so important about Maxwell's Demon. If an external, supernatural force acts upon a system to reduce entropy how is this in any way contradictory or surprising? Articles, like this one, explain the thought experiment, then are just content to assume that this is obviously a puzzle of some kind and I just think "so what?"

FeepingCreature 6y ago
I think "demon" just acts as a placeholder for "component whose behavior we can describe in intentional terms." As I understand it, the demon is not supposed to be a supernatural force, it's supposed to be a physical component in the system. We know now that building such a component is theoretically impossible, which is why the demon would need to appeal to supernatural means, but this was not clear for a long time. Besides, "Maxwell's sensor controlled opening" sounds boring. edit: To cite Theory of Heat ( https://upload.wikimedia.org/wikipedia/commons/b/b8/J._Clerk_Maxwell._Theory_of_Heat_%281871%29.pdf https://upload.wikimedia.org/wikipedia/commons/b/b8/J._Clerk... ): "But if we conceive a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are still as essentially finite as our own (emphasis mine), would be able to do what is at present impossible to us. ... 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." Which IMO makes it clear Maxwell thought of his demon as a fundamentally natural system.
- kryogen1c 6y ago
  >As I understand it, the demon is not supposed to be a supernatural force, it's supposed to be a physical component in the system. if you did that, it would be obvious what the problem is with the thought experiment: physical components have entropy. moving pieces need motive force which make heat, and movement also makes heat. the thought experiment only makes sense with a nonsense demon, because the though experiment isnt possible.
  - TeMPOraL 6y ago
    Which, I believe, was literally the point of the experiment - proof by contradiction.
  - codeulike 6y ago
    Its obvious to us 150 years later because we are very familiar with the idea that processing data needs energy, its as natural an idea to us as plugging in a phone. It wasn't so obvious in 1867.
    - engineer_22 6y ago
      I don't think you give Maxwell enough credit.
      - codeulike 6y ago
        Maxwell was a giant obviously but with his thought experiment of the Demon (which he quite casually came up with in a letter to a friend) he suspected that he had successfully poked holes in the second law - to "show that the 2nd Law of Thermodynamics has only statistical certainty" as he put it. So he thought the law was a statistical guide that could be gotten around in special circumstances. It took the thinkers of the time a long time to realise that tracking all the molecules would need energy. Whereas if you re-state the thought experiment using todays technology - "imagine a device that tracks all the molecules and opens a hatch to only let the fast ones through" its obvious to us - with our modern conception of processing - that such a device would need a fair bit of juice. Source: https://archive.org/details/lifescientificwo00knotuoft/page/214/mode/2up?view=theater https://archive.org/details/lifescientificwo00knotuoft/page/...
        chriswarbo 6y ago
        > its obvious to us - with our modern conception of processing - that such a device would need a fair bit of juice It wouldn't. The hatch itself could operate with no net energy lost/gained, e.g. using a counterweight to cancel out the momentum. Any required calculations could be carried out with a reversible computer, e.g. a billiard ball computer. This way, once our computation is finished, we just can just run it in reverse to get back any energy that was spent (e.g. anything that needed charging during the computation, will get discharged on the way back; and likewise for any lifting, pushing, etc.). The only thing which requires energy is resetting the device. This cannot be done reversibly, since it has to 'forget' whether the hatch got opened or not; i.e. reset(opened) -> init and reset(closed) -> init, so we can't "undo" the 'reset' operation to recover any energy it used.
- lmkg 6y ago
  In modern terms, we would use the word "oracle" instead of "demon."
  - dreamcompiler 6y ago
    Not necessarily. Our computers have dozens of things we call "daemons" running in the background and nobody considers them supernatural. The word "daemon" for a computer program was inspired by Maxwell's demon, because it works tirelessly in the background. http://www.takeourword.com/TOW146/page4.html http://www.takeourword.com/TOW146/page4.html
nicklecompte 6y ago
It is indeed obvious that an external force can apply energy to a system to reduce entropy. The trick is that, if you ignore the thermodynamics of information processing, Maxwell's demon can work with arbitrarily little energy (the only physical work it is doing is opening/closing a door with arbitrarily little mass). In fact Maxwell's demon can perform any amount of thermodynamic work with essentially zero physical work. It really does seem to be a violation of energy conservation. The only way to solve this is by considering the thermodynamics of Maxwell's demon's brain (creating and destroying state as it determines whether or not to let a given molecule pass the door) - and in particular understanding that there is a fundamental thermodynamic cost to storing or deleting information, it is not just a consequence of imperfect circuit/neuron design.
Traubenfuchs 6y ago
Reducing it to something absurd, is it anything beyond "If we just ignore this one rule of physics, this other rule of physics also stops working. Gotcha!"?
- thatcherc 6y ago
  I think the real trick with the thought experiment is: which law of physics is the Demon ignoring? The answer (spoilers) is that the demon is ignoring the entropy of the information it must learns about the particles that pass through the gate - either the Demon stores info about which particles are let through, which has entropy of its own, or the Demon erases that data, which comes with an energy cost that must subtly heat something up and again increases entropy elsewhere. Either way, the entropy of the total system still increases, even though the entropy of the gas decreases.
junippor 6y ago
I think the point is that it's not "supernatural". Conceptually you could implement it. The opening and closing involves zero work. In reality the work won't be zero due to friction, but you can in principle at least approximate it really well.
- raducu 6y ago
  The work will never be zero, and I have a hunch it could well be greater than your overall temperature gain. If the opening works on non-charged particles, it has to have mass and opening and closing it is not possible without work. The work might be tiny, but your temperature gain is probably event tinier. If the particle is charged, you don't need a physical gate, but you do work on the particle by exerting a force on it. You also somehow must detect the particle, and that also takes energy.
  - deleted 6y ago
    [deleted]
  - FabHK 6y ago
    > If the opening works on non-charged particles, it has to have mass and opening and closing it is not possible without work. You could open the gate by pulling it up, which would require work, but you could recover the entire work by letting it sink down again. It has been shown that the solution was not the physical work of opening/closing the gate, but the work required in information processing, in particular deleting information.
- peteretep 6y ago
  > The opening and closing involves zero work Why?
  - chriswarbo 6y ago
    It can be done conservatively https://en.wikipedia.org/wiki/Conservative_force https://en.wikipedia.org/wiki/Conservative_force
  - deleted 6y ago
Strilanc 6y ago
The demon is introduced as a magical effect, but the obvious followup question is why can't you actually implement it. Why can't you make a machine that lowers entropy by opening and closing a door to let atoms go one way but not the other? Why can't you make a one-way valve? What specifically is magical about the demon? There is an important insight hiding inside that demon.
- jakobnissen 6y ago
  The demon is not really supposed to be magical, but instead "some physical process or machine", e.g. a future nanomachine or something. It's like a "perpetual motion machine" - a hypothetical object that can be used in thought experiments. If it could be shown that creating a Maxwell's demon that creates less entropy than it reduces is impossible, then the paradox is solved. A [brownian ratchet](https://en.wikipedia.org/wiki/Brownian_ratchet https://en.wikipedia.org/wiki/Brownian_ratchet) is an example of a (failed) implementation.
- ctdonath 6y ago
  What’s magical: it can observe, decide, and act with zero energy (or at least less energy than real-world thermodynamics requires).
  - chriswarbo 6y ago
    > observe This isn't magical: it's just entanglement/causality. If "observing" required energy, then atoms would either blow apart (due to their electrons not "observing" their nuclei), or collapse (due to their electrons losing energy by "observing" their nuclei). > decide This isn't magical: https://en.wikipedia.org/wiki/Billiard-ball_computer https://en.wikipedia.org/wiki/Billiard-ball_computer > act This isn't magical: https://en.wikipedia.org/wiki/Conservative_force https://en.wikipedia.org/wiki/Conservative_force

by c3534l 6y ago

FeepingCreature 6y ago

I think "demon" just acts as a placeholder for "component whose behavior we can describe in intentional terms." As I understand it, the demon is not supposed to be a supernatural force, it's supposed to be a physical component in the system. We know now that building such a component is theoretically impossible, which is why the demon would need to appeal to supernatural means, but this was not clear for a long time. Besides, "Maxwell's sensor controlled opening" sounds boring. edit: To cite Theory of Heat ( https://upload.wikimedia.org/wikipedia/commons/b/b8/J._Clerk_Maxwell._Theory_of_Heat_%281871%29.pdf https://upload.wikimedia.org/wikipedia/commons/b/b8/J._Clerk... ): "But if we conceive a being whose faculties are so sharpened that he can follow every molecule in its course, such a being, whose attributes are still as essentially finite as our own (emphasis mine), would be able to do what is at present impossible to us. ... 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." Which IMO makes it clear Maxwell thought of his demon as a fundamentally natural system.

kryogen1c 6y ago
>As I understand it, the demon is not supposed to be a supernatural force, it's supposed to be a physical component in the system. if you did that, it would be obvious what the problem is with the thought experiment: physical components have entropy. moving pieces need motive force which make heat, and movement also makes heat. the thought experiment only makes sense with a nonsense demon, because the though experiment isnt possible.
- TeMPOraL 6y ago
  Which, I believe, was literally the point of the experiment - proof by contradiction.
- codeulike 6y ago
  Its obvious to us 150 years later because we are very familiar with the idea that processing data needs energy, its as natural an idea to us as plugging in a phone. It wasn't so obvious in 1867.
  - engineer_22 6y ago
    I don't think you give Maxwell enough credit.
    - codeulike 6y ago
      Maxwell was a giant obviously but with his thought experiment of the Demon (which he quite casually came up with in a letter to a friend) he suspected that he had successfully poked holes in the second law - to "show that the 2nd Law of Thermodynamics has only statistical certainty" as he put it. So he thought the law was a statistical guide that could be gotten around in special circumstances. It took the thinkers of the time a long time to realise that tracking all the molecules would need energy. Whereas if you re-state the thought experiment using todays technology - "imagine a device that tracks all the molecules and opens a hatch to only let the fast ones through" its obvious to us - with our modern conception of processing - that such a device would need a fair bit of juice. Source: https://archive.org/details/lifescientificwo00knotuoft/page/214/mode/2up?view=theater https://archive.org/details/lifescientificwo00knotuoft/page/...
      - chriswarbo 6y ago
        > its obvious to us - with our modern conception of processing - that such a device would need a fair bit of juice It wouldn't. The hatch itself could operate with no net energy lost/gained, e.g. using a counterweight to cancel out the momentum. Any required calculations could be carried out with a reversible computer, e.g. a billiard ball computer. This way, once our computation is finished, we just can just run it in reverse to get back any energy that was spent (e.g. anything that needed charging during the computation, will get discharged on the way back; and likewise for any lifting, pushing, etc.). The only thing which requires energy is resetting the device. This cannot be done reversibly, since it has to 'forget' whether the hatch got opened or not; i.e. reset(opened) -> init and reset(closed) -> init, so we can't "undo" the 'reset' operation to recover any energy it used.
lmkg 6y ago
In modern terms, we would use the word "oracle" instead of "demon."
- dreamcompiler 6y ago
  Not necessarily. Our computers have dozens of things we call "daemons" running in the background and nobody considers them supernatural. The word "daemon" for a computer program was inspired by Maxwell's demon, because it works tirelessly in the background. http://www.takeourword.com/TOW146/page4.html http://www.takeourword.com/TOW146/page4.html

nicklecompte 6y ago

It is indeed obvious that an external force can apply energy to a system to reduce entropy. The trick is that, if you ignore the thermodynamics of information processing, Maxwell's demon can work with arbitrarily little energy (the only physical work it is doing is opening/closing a door with arbitrarily little mass). In fact Maxwell's demon can perform any amount of thermodynamic work with essentially zero physical work. It really does seem to be a violation of energy conservation. The only way to solve this is by considering the thermodynamics of Maxwell's demon's brain (creating and destroying state as it determines whether or not to let a given molecule pass the door) - and in particular understanding that there is a fundamental thermodynamic cost to storing or deleting information, it is not just a consequence of imperfect circuit/neuron design.

Traubenfuchs 6y ago

Reducing it to something absurd, is it anything beyond "If we just ignore this one rule of physics, this other rule of physics also stops working. Gotcha!"?

thatcherc 6y ago
I think the real trick with the thought experiment is: which law of physics is the Demon ignoring? The answer (spoilers) is that the demon is ignoring the entropy of the information it must learns about the particles that pass through the gate - either the Demon stores info about which particles are let through, which has entropy of its own, or the Demon erases that data, which comes with an energy cost that must subtly heat something up and again increases entropy elsewhere. Either way, the entropy of the total system still increases, even though the entropy of the gas decreases.

junippor 6y ago

I think the point is that it's not "supernatural". Conceptually you could implement it. The opening and closing involves zero work. In reality the work won't be zero due to friction, but you can in principle at least approximate it really well.

raducu 6y ago
The work will never be zero, and I have a hunch it could well be greater than your overall temperature gain. If the opening works on non-charged particles, it has to have mass and opening and closing it is not possible without work. The work might be tiny, but your temperature gain is probably event tinier. If the particle is charged, you don't need a physical gate, but you do work on the particle by exerting a force on it. You also somehow must detect the particle, and that also takes energy.
- deleted 6y ago
  [deleted]
- FabHK 6y ago
  > If the opening works on non-charged particles, it has to have mass and opening and closing it is not possible without work. You could open the gate by pulling it up, which would require work, but you could recover the entire work by letting it sink down again. It has been shown that the solution was not the physical work of opening/closing the gate, but the work required in information processing, in particular deleting information.
peteretep 6y ago
> The opening and closing involves zero work Why?
- chriswarbo 6y ago
  It can be done conservatively https://en.wikipedia.org/wiki/Conservative_force https://en.wikipedia.org/wiki/Conservative_force
- deleted 6y ago

Strilanc 6y ago

The demon is introduced as a magical effect, but the obvious followup question is why can't you actually implement it. Why can't you make a machine that lowers entropy by opening and closing a door to let atoms go one way but not the other? Why can't you make a one-way valve? What specifically is magical about the demon? There is an important insight hiding inside that demon.

jakobnissen 6y ago
The demon is not really supposed to be magical, but instead "some physical process or machine", e.g. a future nanomachine or something. It's like a "perpetual motion machine" - a hypothetical object that can be used in thought experiments. If it could be shown that creating a Maxwell's demon that creates less entropy than it reduces is impossible, then the paradox is solved. A [brownian ratchet](https://en.wikipedia.org/wiki/Brownian_ratchet https://en.wikipedia.org/wiki/Brownian_ratchet) is an example of a (failed) implementation.
ctdonath 6y ago
What’s magical: it can observe, decide, and act with zero energy (or at least less energy than real-world thermodynamics requires).
- chriswarbo 6y ago
  > observe This isn't magical: it's just entanglement/causality. If "observing" required energy, then atoms would either blow apart (due to their electrons not "observing" their nuclei), or collapse (due to their electrons losing energy by "observing" their nuclei). > decide This isn't magical: https://en.wikipedia.org/wiki/Billiard-ball_computer https://en.wikipedia.org/wiki/Billiard-ball_computer > act This isn't magical: https://en.wikipedia.org/wiki/Conservative_force https://en.wikipedia.org/wiki/Conservative_force