I have a problem with Maxwell's demon

[bland]

The problem I have is that I fail to see how it caused so much consternation. The way I understand it the demon could passively allow or not allow particles of gas to pass or not pass, and thus gradually create a more ordered state free of charge.

I have learned that the problem with the demon was 'solved' because it had to keep a tally of what it did and eventually this information needed to be erased in order to store more information and the erasure caused heat and there fore entropy to be increased.

The problem I have is that even before the above, would not the demon be generating heat anyway merely by opening and closing a door or somehow directing particles. Why is it not sufficient to say that the demon was using energy in order to create more order? It seems obvious to me but I must obviously not be understanding something because I'm this would also be obvious to the people in the 19 century who were worried about the demon.

 

[Nugatory]

The problem I have is that even before the above, would not the demon be generating heat anyway merely by opening and closing a door or somehow directing particles. Why is it not sufficient to say that the demon was using energy in order to create more order? It seems obvious to me but I must obviously not be understanding something because I'm this would also be obvious to the people in the 19 century who were worried about the demon.

It's not enough to say that the demon must be using energy. You have to demonstrate that the demon must use more energy than the usable energy that we can extract from the system after it's been sorted into hot and cold.

 

[bland]

@Nugatory

Ok thanks for that. That sort of answers a bit of my questions but it raises other questions now that are perhaps even more strange. As I'd never seen any reference to the demon as any sort of physical system only a concept responsible for passively directing particles. Now that you've cleared that up, what is the form that physicists view this so called 'demon'. So then I come back to the original puzzlement for me which is why it caused so much consternation.

Surely it would be up to someone to show that a system could be constructed that would use less energy than the energy that it could build up. But that's like asking someone to show how their perpetual motion machine works. It's not going to work. Therefore why all the worry about Maxwell's demon. What's the difference between this demon and perpetual motion machine?

 

[Lord Jestocost]

Regarding Maxwell’s demon and the confusion persisting around this topic, I recommend John D. Norton’s article "The Worst Thought Experiment" ( http://philsci-archive.pitt.edu/12337 ).

 

[bland]

Regarding Maxwell’s demon and the confusion persisting around this topic, I recommend John D. Norton’s article "The Worst Thought Experiment" ( http://philsci-archive.pitt.edu/12337 ).

Thanks for that link, it certainly sort of clears up the madness for me, even if it still doesn't account for the collective madness which is how could these very clever people postulate such obvious nonsense and not have it immediately debunked.

The article though seems to concentrate on the very extra particular egregiousness of Szilard's explanation. The most strangest part for me is the following...

It is a gas of a single molecule whose density fluctuates wildly as the molecule bounces rapidly to and fro inside the confining chamber.

How is a box containing a single molecule said to be 'fluctuating in density' just because it's possible to place an imaginary wall in the box and then claim it's now in a smaller space? That's like building a wall around the universe by postulating an imaginary wall around myself and defining the outside of the wall as the inside!

Also...

The common assumption was that all the other processes could, in principle, be carried out without creating thermodynamic entropy and thus could be idealized as dissipationless processes.

This assumption is the most egregious of all assumptions in this literature. It is fatally and disastrously wrong.

It turns out that a simple description of what a Maxwell’s demon is required to do is incompatible with the Liouville theorem of Hamiltonian dynamics.

I don't know what the Liouville theorem but it seems that Maxwell's demon is certainly incompatible with physics in general.

Still as the writer points out at the end, even before Szilard the assumptions made are against the general principles of what we may allow to be true and/or irrelevant in thought experiments.

 

[Nugatory]

Thanks for that link, it certainly sort of clears up the madness for me, even if it still doesn't account for the collective madness which is how could these very clever people postulate such obvious nonsense and not have it immediately debunked.

Who are these people of whom you speak and exactly what obvious nonsense were they postulating? That's a serious question, and the answer would be a simple declarative sentence in the form "The postulate that ... made by ...is obvious nonsense"?

If you're working with 19th-century classical physics, as Maxwell and his contemporaries were, there's no obvious reason why the energy to move the barrier cannot, even in principle, be less than the energy difference between the fast-moving and slow-moving particles impinging on the barrier; and therefore no obvious justification for the claim that Maxwell's demon is impossible even in principle. (We can't use violation of the second law to prove the impossibility of the demon the way we use violation of energy conservation to prove the impossibility of perpetual motion machines, because the second law depends on statistical methods that apply only to systems that don't contain a demon).

 

[bland]

Who are these people of whom you speak and exactly what obvious nonsense were they postulating? That's a serious question, and the answer would be a simple declarative sentence in the form "The postulate that ... made by ...is obvious nonsense"?

As the article linked to suggests the flaw was more about misusing the concept of a thought experiment and it is this that I intuitively felt to be wrong, although I was of course ready to doubt my intuition and defer to those who know better. As is mentioned in the article, Einstein could postulate a magical being if he wanted who was pulling the rope connected to a room through space while accelerating to simulate gravity, because it was irrelevant to the point being made. However the magical demon with supernatural powers was not separate from the thought experiment. The demon for example was postulated (by everyone who accepted it) to be intelligent which would by necessity mean considerable entropic cost, I suppose, I'm not stating this as a fact because I really don't know. Never mind about how it could detect the molecules. How could it be possible to work out if there was a net decrease in entropy because the demon would have to be described in detail and the detail would become huge, unlike the demon who was dragging Einstein's chamber.

EDIT:
@Nugatory. My original bewilderment is precisely why 19thC physicist were themselves so nonplussed by Maxwell's demon. If as you say there's no reason in principle why the energy to move the barrier cannot be less than the energy build up in the difference on both sides of the dividing line, then... why didn't someone do the calculation? I guess it's because they'd have to bring turn the demon into a physical system, not to mention supplying it will energy. It would certainly take energy to achieve the separation of the fast and slow moving molecules and the difference becomes itself a source of energy so why isn't that a perpetual motion machine being postulated?

 

[f95toli]

@Nugatory. .. why didn't someone do the calculation? I guess it's because they'd have to bring turn the demon into a physical system, not to mention supplying it will energy. It would certainly take energy to achieve the separation of the fast and slow moving molecules and the difference becomes itself a source of energy so why isn't that a perpetual motion machine being postulated?

Because the calculation is far from trivial...
Note that this is still a very active field of research (just try typing Maxwell's demon into Google Scholar); especially for nanoscale devices where the energies involved are are tiny and the number of particles so small that it is not at all obvious that you can NOT create a demon.

See e.g.
https://www.nature.com/articles/srep21126

Moreover, the fact that the solution to the problem in a sense involves thinking about information as something "physical" was completely new at the time.

Also, don't take the "demon" part too seriously; all it means (and meant) was some sort of fairly simple feedback system (which is also how it is implemented in real experiments). It does not refer to some sort of intelligent creature.

 

[bland]

@f95toli

OK, thanks for that clarification and link.