Besides, "Maxwell's sensor controlled opening" sounds boring.
edit: To cite Theory of Heat ( 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.
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.
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/...
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.
(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.
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
ETA: nicklecompte says it perfectly below: https://news.ycombinator.com/item?id=26912158
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'.
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
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.
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
> act
This isn't magical: https://en.wikipedia.org/wiki/Conservative_force
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.
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.
Why?
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.