random thoughts on Maxwell's demon

This just came into my head. I don't think I really understand the significance of Maxwell's demon.

Please don't try to explain it to me. It's just a random thought.

Sorry, you can't make that statement and request at PF and expect cooperation.

Maxwell's demon is a famous thought experiment that seemed to suggest that it is possible to get free energy - to make heat flow from cold to hot.

The "entity" which is what Maxwell actually called it, just stands there and opens a door when more energetic molecules happen to be aimed at the door. He doesn't open it for less energetic particles. All he is doing is separating hotter from colder.

This is not impossible, and happens all the time in Nature: hot air rises by itself, no "demon' necessary. Separate the top half of any container of air from the bottom half, and you have what the demon did.

The container needs to be isolated. Without gravity, the top half would not get hotter than the bottom half.

What do you mean by "isolated"? And I'm sorry for not stipulating this wouldn't work in outer space. Didn't think anyone would go there.

Isolated means that energy is not allowed to pass through the walls of the container. The energy to lift the hot gas comes from outside the box.

Eh? The air has to have some non-zero temp to begin with. Put some air in a thermos and the hotter, more energetic particles automatically work their way to the top while the less energetic ones sink. The colder ones can be packed more closely together, the more energetic ones rise to the zone of least density.

You can most easily see what I am saying by trying to create a horizontal temperature gradient. Or by moving the container to a place where there is no gravity.

I know what it is. I was just thinking that maybe I don't understand it's significance.

“The second law of thermodynamics holds, I think, the supreme position among the laws of nature. If someone points out to you that your pet theory of the Universe is in disagreement with Maxwell’s equations - then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation - well, those experimentalists do bungle things up sometimes. but if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing to do but to collapse in deepest humiliation.”

I saw the original in a movie theater. Are you saying that there's a more complete version?

I know what it is. I was just thinking that maybe I don't understand it's significance.

It's not really about violating the second law. It's about having to view a huge number of individual things as a uniform thing.

It's not really about violating the second law.

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.

One of the best established facts in thermodynamics is that it is impossible in a system enclosed in an envelope which permits neither change of volume nor passage of heat, and in which both the temperature and the pressure are everywhere the same, to produce any inequality of temperature or of pressure without the expenditure of work.

Also, make note of this from the link:


For the purpose of this discussion, heat is the same thing as energy, i.e. the envelope should permit no passage of energy. The examples you have given us all depend on gravity to provide the energy for the heated gas to rise. This energy is in the form of a potential gradient which comes from outside the container.

Yes it is. I took this from your link.

Your statement makes no sense. Maybe you should stick to art.

This is only one of the instances in which conclusions which we have drawn from our experience of bodies consisting of an immense number of molecules may be found not to be applicable to the more delicate observations and experiments which we may suppose made by one who can perceive and handle the individual molecules which we deal with only in large masses.

In dealing with masses of matter, while we do not perceive the individual molecules, we are compelled to adopt what I have described as the statistical method of calculation, and to abandon the strict dynamical method, in which we follow every motion by the calculus.

It would be interesting to enquire how far those ideas about the nature and methods of science which have been derived from examples of scientific investigation in which the dynamical method is followed are applicable to our actual knowledge of concrete things, which, as we have seen, is of an essentially statistical nature, because no one has yet discovered any practical method of tracing the path of a molecule, or of identifying it at different times.

I do not think, however, that the perfect identity which we observe between different portions of the same kind of matter can be explained on the statistical principle of the stability of the averages of large numbers of quantities each of which may differ from the mean. For if of the molecules of some substance such as hydrogen, some were of slightly greater mass than others, we have the means of producing a separation between molecules of different masses, and in this way we should be able to produce two kinds of hydrogen, one of which would be somewhat denser than the other. As this cannot be done, we must admit that the equality which we assert to exist between the molecules of hydrogen applies to each individual molecule, and not merely to the average of groups of millions of molecules.

That's what I'm saying: gravity is the demon, and this happens all the time. The air near the ceiling is always at a higher temp than the air near the floor.