6 ms·

Isn't this like the Maxwell's demon? Deciding what to do on micro level can create macro level changes that break physics, but it's not actually possible, so th

by ajuc 4y ago

Isn't this like the Maxwell's demon? Deciding what to do on micro level can create macro level changes that break physics, but it's not actually possible, so there's no paradox.

bee_rider 4y ago
There's nothing really "not actually possible" about Maxwell's Demon. It is maybe impractical from an engineering point of view to create the demon, but this doesn't resolve the problem, which is based in the more fundamental physics. I think a more accurate description of the resolution is that this "deciding what to do on the micro level" must itself have an energy cost -- making it a great example of the link between information and thermodynamic entropy.
- ajuc 4y ago
  Yes, that's what I meant. This is similar - because you have to put a mirror at "just the right time" and you ignore the energetic cost of that.
  - MockObject 4y ago
    What if we have a mirror moving randomly around the box? That would lead to the same effect, whereas a randomized Maxwell's Demon would not.
    - ajuc 4y ago
      I'm not sure. Wouldn't the introduced waves cancel out then?
      - MockObject 4y ago
        Only if it lead to abnormally cool photons as much as anomalously hot ones.
- kgwgk 4y ago
  If there is nothing "not actually possible" what's the problem that needs to be solved?
  - unholiness 4y ago
    At the time it was proposed, it was thought that a thermal reservoir on its own could never do useful work. This is always what's observed in practice and laws of heat were based around it e.g. U = Q - W. In order to derive those heat equations from the second law, we now know it requires an assumption: that you (or any entropy-containing component not modeled in the system) cannot have specific knowledge of the microstate of the system, only its macroscopic properties. For a long time it seemed absurd that such an assumption would necessary at all, for these seemingly universal laws, and there was no clear way to thermodynamically model the knowledge of the actor inside this system such that you wouldn't need such an assumption.
    - kgwgk 4y ago
      > For a long time it seemed absurd that such an assumption would necessary at all, for these seemingly universal laws, Not so long. The second law dates from 1850 and the requirement of such an assumption is what Maxwell’s demon illustrated less than forty years later. > and there was no clear way to thermodynamically model the knowledge of the actor inside this system such that you wouldn't need such an assumption. There is still no way to model an actor inside a thermodynamical system in equilibrium - by definition.
  - bee_rider 4y ago
    There isn't a problem. The initially apparent problem is that the demon appears to be generating a temperature gradient "for free" by just swinging a gate open (for fast molecules) or closed (for slow ones) (because there's no fundamental physical cost for gate-swinging). It is resolved by taking into account the fact that the demon must use at least a bit of memory, while acquiring the 'status' of the molecule (let through or don't). So, we can at least say that the demon, no matter how slow and lazy (efficient) it wants to be, must pay the information-theory based cost of erasing that bit.
- shaded-enmity 4y ago
  How do you build a mechanism that has a perfect knowledge about a system while being part of said system?
  - bee_rider 4y ago
    I'm not sure I see the link here (although it is definitely possible that I'm jsut missing something, I'm no physicist). I don't think Maxwell's demon needs perfect knowledge of the whole system -- it is just locally deciding to let through "fast" molecules and block "slow" ones.
    - shaded-enmity 4y ago
      I'm no physicist either, I just like to ask questions :) How does the demon attain the knowledge of what is "fast" and "slow" without continuous observation (and thus interaction) with the particles. Velocity is just function of position over time, so the demon needs at least 2 samples to make the most basic approximation. Where is the entropy for doing that coming from? How does the interference of the measuring apparatus factor into the whole process - what if the sole act of measurement changes the state of the particle from "fast" to "slow" or vice versa? Do we need to measure twice? But what if the second measurement causes the transition it was meant to detect?
- lamontcg 4y ago
  Yeah these though experiments all seem to depend on some kind of very tiny perfect subatomic process which has no energy input. Flip it around and the violation of conservation of energy should place a minimum requirement on the energy required to run a real Maxwell's Demon, or the mirror in this experiment (and real mirrors aren't infinitely thin mathematical abstractions).
  - bee_rider 4y ago
    I think this is exactly the point. A good way to ask "what does our model mean, exactly" is to imagine the perfect processes with no unnecessary energy losses. Real mirrors might not be infinitely thin mathematical abstractions, but if there isn't an actual, physically defined fundamental limit to how thin a mirror can be, then it would be weird if we could violate fundamental laws of the universe, but for want of such a mirror. Maxwell's Demon is neat because we can whittle things down and eventually get to needing to account for the physical cost of the bits in the thing's 'brain.' An interesting example of the fact that information is actually a physical quantity.
- jp57 4y ago
  Is it merely impractical though? In the thought experiement the demon needs to be able to know the state of the impending particle before it arrives, so it can decide whether to open the door. It can't do that without interacting with them and changing their state.
  - nobodyandproud 4y ago
    Here’s a more mechanical variation of Maxwells Demon: https://en.m.wikipedia.org/wiki/Brownian_ratchet https://en.m.wikipedia.org/wiki/Brownian_ratchet
  - bee_rider 4y ago
    I think it depends on how you define the demon. I first heard of it in the final, resolved version, so I see it as just impractical (because the solution is baked in). The interaction between the demon and the particles is the key. The final version, at least as far as I'm concerned, comes from Landauer and Bennett [https://en.wikipedia.org/wiki/Maxwell%27s_demon#Criticism_and_development https://en.wikipedia.org/wiki/Maxwell%27s_demon#Criticism_an...] -- the demon must accumulate information (which it can only do finitely) or erase it. Erasing information has a real physical cost, resolving the issue. When the demon was first invented, information theory hadn't been developed yet. So the mystery to Maxwell was that it looked like it was violating energy conservation, but that's just because there's a sneaky place we can store entropy temporarily or finitely. So, I think I was imprecise (or... wrong). A demon that does what we really want (generates free energy) is impossible. But it is impossible for weird reasons that Maxwell wouldn't have been aware of, and I don't think he explicitly explored them.
- deleted 4y ago
  [deleted]
- davesque 4y ago
  Yes, computation "costs" energy and increases entropy. The demon must compute its decisions before it can carry them out.
throwawaymaths 4y ago
This was my first gut instinct, except even simpler than Maxwell's demon: by partitioning the apparatus with the mirror, you're altering the statistical ensemble of possible wave states (increasing the relative information entropy) and that's where your energy comes from. That's just my gut though, I'm not a professional physicist.

by ajuc 4y ago

Isn't this like the Maxwell's demon? Deciding what to do on micro level can create macro level changes that break physics, but it's not actually possible, so there's no paradox.

bee_rider 4y ago

There's nothing really "not actually possible" about Maxwell's Demon. It is maybe impractical from an engineering point of view to create the demon, but this doesn't resolve the problem, which is based in the more fundamental physics. I think a more accurate description of the resolution is that this "deciding what to do on the micro level" must itself have an energy cost -- making it a great example of the link between information and thermodynamic entropy.

ajuc 4y ago
Yes, that's what I meant. This is similar - because you have to put a mirror at "just the right time" and you ignore the energetic cost of that.
- MockObject 4y ago
  What if we have a mirror moving randomly around the box? That would lead to the same effect, whereas a randomized Maxwell's Demon would not.
  - ajuc 4y ago
    I'm not sure. Wouldn't the introduced waves cancel out then?
    - MockObject 4y ago
      Only if it lead to abnormally cool photons as much as anomalously hot ones.
kgwgk 4y ago
If there is nothing "not actually possible" what's the problem that needs to be solved?
- unholiness 4y ago
  At the time it was proposed, it was thought that a thermal reservoir on its own could never do useful work. This is always what's observed in practice and laws of heat were based around it e.g. U = Q - W. In order to derive those heat equations from the second law, we now know it requires an assumption: that you (or any entropy-containing component not modeled in the system) cannot have specific knowledge of the microstate of the system, only its macroscopic properties. For a long time it seemed absurd that such an assumption would necessary at all, for these seemingly universal laws, and there was no clear way to thermodynamically model the knowledge of the actor inside this system such that you wouldn't need such an assumption.
  - kgwgk 4y ago
    > For a long time it seemed absurd that such an assumption would necessary at all, for these seemingly universal laws, Not so long. The second law dates from 1850 and the requirement of such an assumption is what Maxwell’s demon illustrated less than forty years later. > and there was no clear way to thermodynamically model the knowledge of the actor inside this system such that you wouldn't need such an assumption. There is still no way to model an actor inside a thermodynamical system in equilibrium - by definition.
- bee_rider 4y ago
  There isn't a problem. The initially apparent problem is that the demon appears to be generating a temperature gradient "for free" by just swinging a gate open (for fast molecules) or closed (for slow ones) (because there's no fundamental physical cost for gate-swinging). It is resolved by taking into account the fact that the demon must use at least a bit of memory, while acquiring the 'status' of the molecule (let through or don't). So, we can at least say that the demon, no matter how slow and lazy (efficient) it wants to be, must pay the information-theory based cost of erasing that bit.
shaded-enmity 4y ago
How do you build a mechanism that has a perfect knowledge about a system while being part of said system?
- bee_rider 4y ago
  I'm not sure I see the link here (although it is definitely possible that I'm jsut missing something, I'm no physicist). I don't think Maxwell's demon needs perfect knowledge of the whole system -- it is just locally deciding to let through "fast" molecules and block "slow" ones.
  - shaded-enmity 4y ago
    I'm no physicist either, I just like to ask questions :) How does the demon attain the knowledge of what is "fast" and "slow" without continuous observation (and thus interaction) with the particles. Velocity is just function of position over time, so the demon needs at least 2 samples to make the most basic approximation. Where is the entropy for doing that coming from? How does the interference of the measuring apparatus factor into the whole process - what if the sole act of measurement changes the state of the particle from "fast" to "slow" or vice versa? Do we need to measure twice? But what if the second measurement causes the transition it was meant to detect?
lamontcg 4y ago
Yeah these though experiments all seem to depend on some kind of very tiny perfect subatomic process which has no energy input. Flip it around and the violation of conservation of energy should place a minimum requirement on the energy required to run a real Maxwell's Demon, or the mirror in this experiment (and real mirrors aren't infinitely thin mathematical abstractions).
- bee_rider 4y ago
  I think this is exactly the point. A good way to ask "what does our model mean, exactly" is to imagine the perfect processes with no unnecessary energy losses. Real mirrors might not be infinitely thin mathematical abstractions, but if there isn't an actual, physically defined fundamental limit to how thin a mirror can be, then it would be weird if we could violate fundamental laws of the universe, but for want of such a mirror. Maxwell's Demon is neat because we can whittle things down and eventually get to needing to account for the physical cost of the bits in the thing's 'brain.' An interesting example of the fact that information is actually a physical quantity.
jp57 4y ago
Is it merely impractical though? In the thought experiement the demon needs to be able to know the state of the impending particle before it arrives, so it can decide whether to open the door. It can't do that without interacting with them and changing their state.
- nobodyandproud 4y ago
  Here’s a more mechanical variation of Maxwells Demon: https://en.m.wikipedia.org/wiki/Brownian_ratchet https://en.m.wikipedia.org/wiki/Brownian_ratchet
- bee_rider 4y ago
  I think it depends on how you define the demon. I first heard of it in the final, resolved version, so I see it as just impractical (because the solution is baked in). The interaction between the demon and the particles is the key. The final version, at least as far as I'm concerned, comes from Landauer and Bennett [https://en.wikipedia.org/wiki/Maxwell%27s_demon#Criticism_and_development https://en.wikipedia.org/wiki/Maxwell%27s_demon#Criticism_an...] -- the demon must accumulate information (which it can only do finitely) or erase it. Erasing information has a real physical cost, resolving the issue. When the demon was first invented, information theory hadn't been developed yet. So the mystery to Maxwell was that it looked like it was violating energy conservation, but that's just because there's a sneaky place we can store entropy temporarily or finitely. So, I think I was imprecise (or... wrong). A demon that does what we really want (generates free energy) is impossible. But it is impossible for weird reasons that Maxwell wouldn't have been aware of, and I don't think he explicitly explored them.
deleted 4y ago
[deleted]
davesque 4y ago
Yes, computation "costs" energy and increases entropy. The demon must compute its decisions before it can carry them out.

throwawaymaths 4y ago

This was my first gut instinct, except even simpler than Maxwell's demon: by partitioning the apparatus with the mirror, you're altering the statistical ensemble of possible wave states (increasing the relative information entropy) and that's where your energy comes from. That's just my gut though, I'm not a professional physicist.