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Possessing only an undergrad level of physics, I've never understood why this is a remotely interesting question. It seems to effectively be equivalent to "Ima

Possessing only an undergrad level of physics, I've never understood why this is a remotely interesting question. It seems to effectively be equivalent to "Imagine we have a refrigerator with a massless, frictionless motor that also doesn't need to be plugged in. Isn't it crazy that this refrigerator would violate the second law of thermodynamics?"

bb88 5y ago
Entropy can't be reversed, even if you have a massless, frictionless, motor that doesn't need to be plugged in. That is effectively what Maxwell's demon is.
- deleted 5y ago
  [deleted]
- tsimionescu 5y ago
  Of course entropy can be reversed, I literally do it in my sleep! However, reversing entropy requires an amount of energy commensurate with the amount of entropy you are reducing. Maxwell's demon would appear as a violation of this. A massless, frictionless motor could trivially reverse all the entropy in the universe given enough time, but it is not clear that Maxwell's demon REQUIRES something equivalent.
  - bb88 5y ago
    Maxwell's demon requires erasure which would subsequently increase entropy. https://www.nature.com/news/the-unavoidable-cost-of-computation-revealed-1.10186#:~:text=Landauer's%20theory%20offered%20the%20first,entropy%20lost%20by%20the%20demon https://www.nature.com/news/the-unavoidable-cost-of-computat....
    - tsimionescu 5y ago
      As someone else was noting, this result is based on Landauer's limit, which is itself ultimately based on the second law of thermodynamics. If Maxwell's demon existed, the second law would be wrong, so Landauer's limit would be wrong, so there could be a way to erase information without increasing entropy. I'm also not sure why Maxwell's demon would require erasure.
      - bb88 5y ago
        Wikipedia summarizes this well. https://en.m.wikipedia.org/wiki/Maxwell%27s_demon https://en.m.wikipedia.org/wiki/Maxwell%27s_demon I think it's clear that by now the 2nd law always wins. Always.
canjobear 5y ago
The interesting thing is that this theoretically perfect refrigerator operates in a way that hints at deep connections between intelligence, information, and entropy.
- selestify 5y ago
  What’s the connection between intelligence and entropy?
  - ffhhj 5y ago
    information
  - a1369209993 5y ago
    "Intelligence" is a little misleading, it's more along the lines of "awareness of one's environment". Per Liouville's Theorem, information about your surroundings is a conserved quantity. A physical system (persistent along a time-like curve) that has more information about a particular part of it's environment (eg "the content of this movie is so-and-so") in one light cone of a event than the other, must have correspondingly more information about some other part of it's environment (eg "this heat sink that I am erasing scratch space with has particle velocities within bounds such-and-such") in the other light cone compared to the first one. If one light cone has much larger quantities of compressible ('cold', kind of) information than the other, such that interesting information about our surrondings always appears on the opposite side of events from it, we call that light cone "the past". I don't know that we have a better explanation than the anthropic principle for why one one of our light cones is like that, though.
    - thaumasiotes 5y ago
      > we call that light cone "the past" Hmmm. I had the impression that the flow of time was implicit in the fact that we talk about "light cones" instead of "light spheres". Is that not the case?
      - 5y ago
ineptech 5y ago
A motor that doesn't require energy violates the second law by definition; a selectively-enforced barrier that lets hot particles through but not cold particles didn't obviously provably cost energy. If that sounds like a cop out, remember that the Demon is a metaphor that's supposed to represent an as-yet-undiscovered technology. Like, instead of a literal massless door that opens and closes, something like, "What if we discovered a clever way of designing a little gap between two chambers and ionizing the particles such that only hot ones go through."
- thaumasiotes 5y ago
  > a selectively-enforced barrier that lets hot particles through but not cold particles didn't obviously provably cost energy. I think you're leaving something out of your description. We have passive selective barriers that will admit low-energy particles while reflecting high-energy particles. (Cotton, say, will reflect visible light while being transparent to infrared light.) So what do you get by hanging a sheet between two rooms? Nothing. I think Maxwell's Demon will only let hot particles pass through in one direction, but that's nothing new either; we have one-way mirrors.
  - Dylan16807 5y ago
    > I think Maxwell's Demon will only let hot particles pass through in one direction, but that's nothing new either; we have one-way mirrors. We don't, actually. We have semi-transparent mirrors that allow a small fraction of the light through, and they appear "one-way" only when one side is vastly brighter than the other.
  - jayd16 5y ago
    If one way mirrors worked like that we wouldn't need heat pumps. We do because they don't work like that.
- zackees 5y ago
  Replace “hot particles” with “moving electrons” on a sheet of graphemes and replace “door” with “diode” and the experiment has already been proven in reality to generate charge and extract work out of the environment.
hedora 5y ago
Imagine the door had some mass and some friction, but the particles were really heavy, so even a few correct openings could create a large energy imbalance. There’s no apparent relationship to the energy gain per separated particle and the energy used by the door. That makes it hard to prove it’s not generating energy. That’s what makes it an interesting problem.
- scrooched_moose 5y ago
  Interesting, thanks. I hadn't considered the disconnect between the particles and the door.
libeclipse 5y ago
I agree but for a different reason. It seems obvious that Maxwell's demon would itself use energy, thereby paying for the "decreased entropy" with this energy. This is essentially the same as: > The second vital piece of the puzzle was the principle of erasure. In 1961, the German American physicist Rolf Landauer showed that any logically irreversible computation, such as the erasing of information from a memory, would result in a minimal nonzero amount of work converted into heat dumped into the environment, and a corresponding rise in entropy.
- tomcam 5y ago
  I wish someone would address your point. This leapt to my mind immediately but I don’t understand enough about the physics to go any further with it.
- deleted 5y ago
  [deleted]
- exporectomy 5y ago
  Why is it obvious that Maxwell's demon would use energy? It could be a frictionless spring-loaded door with latches to hold it open or closed and it bounces between states when it's unlatched.
  - cambalache 5y ago
    How you will "tell" the door when to unlatch itself without expending energy?
- TchoBeer 5y ago
  iirc Laundauer assumes the second law, so it's not a good refutation here.
exporectomy 5y ago
That refrigerator would still use energy compressing the refrigerant. That's Work=F*x energy which won't go to zero with a massless frictionless motor. Or do you mean you also have a generator to recover the energy from the temperature differences it creates? So it's not really doing refrigerating because you're using all the cooling power to run the motor?
Sosh101 5y ago
It's interesting because it's one of the intersections between physics and information theory. Why it's not possible is not at all obvious.
lmilcin 5y ago
This approach is assuming thermodynamics as a unquestioningly true law. That could be fine for a student that is learning physics but not for a scientist. Treating something as unquestioningly true is a sign of belief and religion. Now, for most everyday situation it is practical to assume that a device can't be creating energy from nothing, can't be ordering atoms in two containers without using energy, because we know there exists second law of thermodynamics. But Maxwell's Demon is an experiment specifically aimed at testing thermodynamics. We can use the second law of thermodynamics exactly because we expend/expended effort to test it to a sufficiently high degree and we are revisiting as we learn new facts and push the boundary.
dw-im-here 5y ago
Because it's a way to investigate the properties of information and entropy

bb88 5y ago

Entropy can't be reversed, even if you have a massless, frictionless, motor that doesn't need to be plugged in. That is effectively what Maxwell's demon is.

deleted 5y ago
[deleted]
tsimionescu 5y ago
Of course entropy can be reversed, I literally do it in my sleep! However, reversing entropy requires an amount of energy commensurate with the amount of entropy you are reducing. Maxwell's demon would appear as a violation of this. A massless, frictionless motor could trivially reverse all the entropy in the universe given enough time, but it is not clear that Maxwell's demon REQUIRES something equivalent.
- bb88 5y ago
  Maxwell's demon requires erasure which would subsequently increase entropy. https://www.nature.com/news/the-unavoidable-cost-of-computation-revealed-1.10186#:~:text=Landauer's%20theory%20offered%20the%20first,entropy%20lost%20by%20the%20demon https://www.nature.com/news/the-unavoidable-cost-of-computat....
  - tsimionescu 5y ago
    As someone else was noting, this result is based on Landauer's limit, which is itself ultimately based on the second law of thermodynamics. If Maxwell's demon existed, the second law would be wrong, so Landauer's limit would be wrong, so there could be a way to erase information without increasing entropy. I'm also not sure why Maxwell's demon would require erasure.
    - bb88 5y ago
      Wikipedia summarizes this well. https://en.m.wikipedia.org/wiki/Maxwell%27s_demon https://en.m.wikipedia.org/wiki/Maxwell%27s_demon I think it's clear that by now the 2nd law always wins. Always.

canjobear 5y ago

The interesting thing is that this theoretically perfect refrigerator operates in a way that hints at deep connections between intelligence, information, and entropy.

selestify 5y ago
What’s the connection between intelligence and entropy?
- ffhhj 5y ago
  information
- a1369209993 5y ago
  "Intelligence" is a little misleading, it's more along the lines of "awareness of one's environment". Per Liouville's Theorem, information about your surroundings is a conserved quantity. A physical system (persistent along a time-like curve) that has more information about a particular part of it's environment (eg "the content of this movie is so-and-so") in one light cone of a event than the other, must have correspondingly more information about some other part of it's environment (eg "this heat sink that I am erasing scratch space with has particle velocities within bounds such-and-such") in the other light cone compared to the first one. If one light cone has much larger quantities of compressible ('cold', kind of) information than the other, such that interesting information about our surrondings always appears on the opposite side of events from it, we call that light cone "the past". I don't know that we have a better explanation than the anthropic principle for why one one of our light cones is like that, though.
  - thaumasiotes 5y ago
    > we call that light cone "the past" Hmmm. I had the impression that the flow of time was implicit in the fact that we talk about "light cones" instead of "light spheres". Is that not the case?
    - 5y ago

ineptech 5y ago

A motor that doesn't require energy violates the second law by definition; a selectively-enforced barrier that lets hot particles through but not cold particles didn't obviously provably cost energy. If that sounds like a cop out, remember that the Demon is a metaphor that's supposed to represent an as-yet-undiscovered technology. Like, instead of a literal massless door that opens and closes, something like, "What if we discovered a clever way of designing a little gap between two chambers and ionizing the particles such that only hot ones go through."

thaumasiotes 5y ago
> a selectively-enforced barrier that lets hot particles through but not cold particles didn't obviously provably cost energy. I think you're leaving something out of your description. We have passive selective barriers that will admit low-energy particles while reflecting high-energy particles. (Cotton, say, will reflect visible light while being transparent to infrared light.) So what do you get by hanging a sheet between two rooms? Nothing. I think Maxwell's Demon will only let hot particles pass through in one direction, but that's nothing new either; we have one-way mirrors.
- Dylan16807 5y ago
  > I think Maxwell's Demon will only let hot particles pass through in one direction, but that's nothing new either; we have one-way mirrors. We don't, actually. We have semi-transparent mirrors that allow a small fraction of the light through, and they appear "one-way" only when one side is vastly brighter than the other.
- jayd16 5y ago
  If one way mirrors worked like that we wouldn't need heat pumps. We do because they don't work like that.
zackees 5y ago
Replace “hot particles” with “moving electrons” on a sheet of graphemes and replace “door” with “diode” and the experiment has already been proven in reality to generate charge and extract work out of the environment.

hedora 5y ago

Imagine the door had some mass and some friction, but the particles were really heavy, so even a few correct openings could create a large energy imbalance. There’s no apparent relationship to the energy gain per separated particle and the energy used by the door. That makes it hard to prove it’s not generating energy. That’s what makes it an interesting problem.

scrooched_moose 5y ago
Interesting, thanks. I hadn't considered the disconnect between the particles and the door.

libeclipse 5y ago

I agree but for a different reason. It seems obvious that Maxwell's demon would itself use energy, thereby paying for the "decreased entropy" with this energy. This is essentially the same as: > The second vital piece of the puzzle was the principle of erasure. In 1961, the German American physicist Rolf Landauer showed that any logically irreversible computation, such as the erasing of information from a memory, would result in a minimal nonzero amount of work converted into heat dumped into the environment, and a corresponding rise in entropy.

tomcam 5y ago
I wish someone would address your point. This leapt to my mind immediately but I don’t understand enough about the physics to go any further with it.
deleted 5y ago
[deleted]
exporectomy 5y ago
Why is it obvious that Maxwell's demon would use energy? It could be a frictionless spring-loaded door with latches to hold it open or closed and it bounces between states when it's unlatched.
- cambalache 5y ago
  How you will "tell" the door when to unlatch itself without expending energy?
TchoBeer 5y ago
iirc Laundauer assumes the second law, so it's not a good refutation here.

exporectomy 5y ago

That refrigerator would still use energy compressing the refrigerant. That's Work=F*x energy which won't go to zero with a massless frictionless motor. Or do you mean you also have a generator to recover the energy from the temperature differences it creates? So it's not really doing refrigerating because you're using all the cooling power to run the motor?

Sosh101 5y ago

It's interesting because it's one of the intersections between physics and information theory. Why it's not possible is not at all obvious.

lmilcin 5y ago

This approach is assuming thermodynamics as a unquestioningly true law. That could be fine for a student that is learning physics but not for a scientist. Treating something as unquestioningly true is a sign of belief and religion. Now, for most everyday situation it is practical to assume that a device can't be creating energy from nothing, can't be ordering atoms in two containers without using energy, because we know there exists second law of thermodynamics. But Maxwell's Demon is an experiment specifically aimed at testing thermodynamics. We can use the second law of thermodynamics exactly because we expend/expended effort to test it to a sufficiently high degree and we are revisiting as we learn new facts and push the boundary.

dw-im-here 5y ago

Because it's a way to investigate the properties of information and entropy