5 ms·

>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

>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.
- philipswood 6y ago
  That's the thing: it doesn't! (i.e. Processing data doesn't need energy - more correctly "work") In the limit, you can process information adiabatically.(*) i.e. you can perform a computation without heat exchange, and you can perform computations reversibly. Maxwell's demon as a thought experiment is actually quite a deep and subtle probe into the boundary between information and thermodynamics. What you CAN'T do adiabatically or reversibly is clear or set bits and throw away their initial state. In theory you could make a classical mechanical turing machine that could perform a computation for an arbitrarily small amount of work, but to prepare a blank tape for it (or to configure a tape into a known configuration) will take a certain minimal expenditure of work. * Ignoring Quantum Physics for now The Wikipedia page for Reversible Computing might be a place to start reading if you're interested in following up.
FabHK 6y ago
No. The paradox makes perfect sense with a technical device. It is conceivable to have fine mechanisms that can be operated with minimal loss of energy and entropy (not bounded away from zero). Thus, the thought experiment poses a challenge. As the article alluded to, the paradox was solved by the fact that information processing is required, which is also in theory reversible, except for the process of erasing information, which interestingly requires some minimum energy. That then would be just enough to compensate for the energy won from the demon, thus not violating the laws of thermodynamics. The "demon" is nomenclature in parallel with Laplace's demon (a thought experiment with a machine that knows position and velocity of all particles, and thus can compute the entire past and future of the universe, in Newtonian physics). https://en.wikipedia.org/wiki/Laplace's_demon https://en.wikipedia.org/wiki/Laplace's_demon ETA: nicklecompte says it perfectly below: https://news.ycombinator.com/item?id=26912158 https://news.ycombinator.com/item?id=26912158
chriswarbo 6y ago
> 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. This is such a bad answer, on so many levels. > physical components have entropy But do they? It's a bit of a hand-wavey statement on its own, but even if we made it more precise it would be putting the cart before the horse. If we want to understand or explain entropy, then we can't make assertions like this; it would just lead to circular reasoning (fundamentally, our "explanations" would distill down to "physical components have entropy because physical components have entropy"). > moving pieces need motive force which make heat This is simply incorrect. "Heat" is a large-scale, statistical phenomenon: the kinetic energy of uncorrelated internal movement of components (e.g. the molecules of a gas moving and spinning). Moving pieces don't "make heat"; pieces moving in uncorrelated ways is heat. Hence we have to focus on the correlation, which is exactly what we mean by entropy. Also "motive force" sounds a bit hand-wavey, and possibly anthropocentric (e.g. it conjures images of bolting things to a steam engine). A better term might be energy gradient, or path of stationary action. > movement also makes heat No it does not. First of all, movement is relative. When cosmic rays hit the Earth, they measure Earth's movement as being close to the speed of light. A ballistic missile at apogee measure's the Earth's movement as zero. This is consistent, since movement is relative. Yet heat is not relative: there is no "heat dilation", since heat has net-zero movement in all directions (net movement is correlated and hence, by definition, not heat). Both the cosmic ray and the missile must measure the Earth's heat as the same; hence heat cannot depend on movement. Secondly 'conservative forces' like gravity and electromagnetism do not lose energy: they can cause perpetual motion; especially at the atomic scale and below, where non-conservative pseudo-forces like friction don't play a role. This is the scale of Maxwell's Demon. Your faulty reasoning has also arrived at the wrong conclusion. Maxwell's Demon (or a physical analogue, like a nanomachine) can separate gas into hot and cold compartments, without causing any heat, without the need of any external 'motive force', in a conservative/reversible way. If we follow the correct line of reasoning (which took 115 years to discover, as the article mentions!) we find the catch: it would require unlimited memory. That's something that's not at all "obvious", and certainly not clear from a treatment of 'motive forces' and 'heat'.

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.
philipswood 6y ago
That's the thing: it doesn't! (i.e. Processing data doesn't need energy - more correctly "work") In the limit, you can process information adiabatically.(*) i.e. you can perform a computation without heat exchange, and you can perform computations reversibly. Maxwell's demon as a thought experiment is actually quite a deep and subtle probe into the boundary between information and thermodynamics. What you CAN'T do adiabatically or reversibly is clear or set bits and throw away their initial state. In theory you could make a classical mechanical turing machine that could perform a computation for an arbitrarily small amount of work, but to prepare a blank tape for it (or to configure a tape into a known configuration) will take a certain minimal expenditure of work. * Ignoring Quantum Physics for now The Wikipedia page for Reversible Computing might be a place to start reading if you're interested in following up.

FabHK 6y ago

No. The paradox makes perfect sense with a technical device. It is conceivable to have fine mechanisms that can be operated with minimal loss of energy and entropy (not bounded away from zero). Thus, the thought experiment poses a challenge. As the article alluded to, the paradox was solved by the fact that information processing is required, which is also in theory reversible, except for the process of erasing information, which interestingly requires some minimum energy. That then would be just enough to compensate for the energy won from the demon, thus not violating the laws of thermodynamics. The "demon" is nomenclature in parallel with Laplace's demon (a thought experiment with a machine that knows position and velocity of all particles, and thus can compute the entire past and future of the universe, in Newtonian physics). https://en.wikipedia.org/wiki/Laplace's_demon https://en.wikipedia.org/wiki/Laplace's_demon ETA: nicklecompte says it perfectly below: https://news.ycombinator.com/item?id=26912158 https://news.ycombinator.com/item?id=26912158

chriswarbo 6y ago

> 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. This is such a bad answer, on so many levels. > physical components have entropy But do they? It's a bit of a hand-wavey statement on its own, but even if we made it more precise it would be putting the cart before the horse. If we want to understand or explain entropy, then we can't make assertions like this; it would just lead to circular reasoning (fundamentally, our "explanations" would distill down to "physical components have entropy because physical components have entropy"). > moving pieces need motive force which make heat This is simply incorrect. "Heat" is a large-scale, statistical phenomenon: the kinetic energy of uncorrelated internal movement of components (e.g. the molecules of a gas moving and spinning). Moving pieces don't "make heat"; pieces moving in uncorrelated ways is heat. Hence we have to focus on the correlation, which is exactly what we mean by entropy. Also "motive force" sounds a bit hand-wavey, and possibly anthropocentric (e.g. it conjures images of bolting things to a steam engine). A better term might be energy gradient, or path of stationary action. > movement also makes heat No it does not. First of all, movement is relative. When cosmic rays hit the Earth, they measure Earth's movement as being close to the speed of light. A ballistic missile at apogee measure's the Earth's movement as zero. This is consistent, since movement is relative. Yet heat is not relative: there is no "heat dilation", since heat has net-zero movement in all directions (net movement is correlated and hence, by definition, not heat). Both the cosmic ray and the missile must measure the Earth's heat as the same; hence heat cannot depend on movement. Secondly 'conservative forces' like gravity and electromagnetism do not lose energy: they can cause perpetual motion; especially at the atomic scale and below, where non-conservative pseudo-forces like friction don't play a role. This is the scale of Maxwell's Demon. Your faulty reasoning has also arrived at the wrong conclusion. Maxwell's Demon (or a physical analogue, like a nanomachine) can separate gas into hot and cold compartments, without causing any heat, without the need of any external 'motive force', in a conservative/reversible way. If we follow the correct line of reasoning (which took 115 years to discover, as the article mentions!) we find the catch: it would require unlimited memory. That's something that's not at all "obvious", and certainly not clear from a treatment of 'motive forces' and 'heat'.