Random thoughts on Maxwell's demon

[ThomasT]

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.

 

[Ivan Seeking]

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.

I was watching the original Andromeda Strain movie tonight. I've seen this a number of times but never realized I had only seen a highly edited version of the original movie. Suddenly there were scenes coming up I'd never seen before! That's always a nice susprise on a movie you thought you knew forwards and backwards. I only had it on because I was working and not really paying attention. Only by chance did I look up at just the right time.

 

[zoobyshoe]

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.

 

[Jimmy Snyder]

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.

 

[zoobyshoe]

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.

 

[Jimmy Snyder]

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.

 

[zoobyshoe]

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.

 

[Jimmy Snyder]

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.

 

[zoobyshoe]

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.

It doesn't work horizontally (without an "entity") and I already agree it wouldn't work vertically without gravity.

 

[Ivan Seeking]

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

Surely you understand what the significance of free energy would be? It would mean there would be no need for an energy supply, like oil. It would mean you can get something for nothing. Maxwell's Demon appeared to violate the second law of thermodynamics - loosely stated, there are no free rides. In it's simplest form the 2nd law says that without the addition of energy to the system, heat flows from hot to cold. I have a favorite quote for this issue.

“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.”

-------- Arthur S. Eddington (British Astrophysicist, 1882-1944) in The nature of the Physical World (1928)

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

I'd never seen the theater release; only the version edited for TV. That had never occurred to me before.

 

[zoobyshoe]

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

Here are Maxwell's original words (it's not long):

http://web.lemoyne.edu/~giunta/demon.html

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.

 

[Ivan Seeking]

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.

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

 

[Jimmy Snyder]

It's not really about violating the second law.

Yes it is. I took this from your link.

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.

 

[Jimmy Snyder]

Also, make note of this from the link:

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.

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.

 

[zoobyshoe]

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.

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.

 

[zoobyshoe]

Yes it is. I took this from your link.

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

The gedanken is prelude to this:

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.

 

[256bits]

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.

That's the 'gravity demom' /
Maxwell's demon would make the air hotter near the floor and cooler near the ceiling.

 

[Jimmy Snyder]

That's what I'm saying: gravity is the demon, and this happens all the time.

No, the demon is not allowed to work across the walls of the container. Maxwell's demon is inside the container.

 

[zoobyshoe]

That's the 'gravity demom' /
Maxwell's demon would make the air hotter near the floor and cooler near the ceiling.

Ah! I think you're right! Vertically speaking, this is what Maxwell would be describing.

 

[zoobyshoe]

No, the demon is not allowed to work across the walls of the container. Maxwell's demon is inside the container.

I know. The original remark to Ivan merely points out the automatic separation of hot from cold without any human effort. Meaning: if you really want to separate hot air from cold without any effort, let gravity do it; no impossible demon necessary. The main implication of that being; even if you miraculously violated the 2nd law, all you'd get is the same thing that happens all the time anyway by gravity in any room. This doesn't represent any amazing "free energy". People ignore it, take it for granted. When they imagine of it being done miraculously in violation of the 2nd law, suddenly it becomes psychologically important, despite the fact it's not enough of a temperature separation to do very much with.

 

[ThomasT]

Thanks for the replies.

 

[willem2]

I know. The original remark to Ivan merely points out the automatic separation of hot from cold without any human effort. Meaning: if you really want to separate hot air from cold without any effort, let gravity do it; no impossible demon necessary. The main implication of that being; even if you miraculously violated the 2nd law, all you'd get is the same thing that happens all the time anyway by gravity in any room. This doesn't represent any amazing "free energy". People ignore it, take it for granted. When they imagine of it being done miraculously in violation of the 2nd law, suddenly it becomes psychologically important, despite the fact it's not enough of a temperature separation to do very much with.

If you start with constant temperature air, you won't get any separation of hot and cold air. This would allow you to run a heat engine by putting an object in the hot zone and letting it radiate its heat to the cold zone, thus creating a temperature difference.

 

[AdrianMay]

We just had a detailed thread about this demon here: https://www.physicsforums.com/showthread.php?p=3784856#post3784856

The bottom line is that Maxwell's demon is still a convincing thought experiment for attacking the second law. There are false rumours all over the internet that information theory somehow helps by pointing out limits on the efficiency of the demon, but all that theory starts by assuming the 2nd law and deriving the limits from there. It would thus be circular to use the limits to rescue the 2nd law from the demon.

I don't think gravity is much of a demon though. It cashes in gravitational potential energy to do the sorting, which is perfectly legal under the 2nd law.

 

[zoobyshoe]

We just had a detailed thread about this demon here: https://www.physicsforums.com/showthread.php?p=3784856#post3784856

The bottom line is that Maxwell's demon is still a convincing thought experiment for attacking the second law. There are false rumours all over the internet that information theory somehow helps by pointing out limits on the efficiency of the demon, but all that theory starts by assuming the 2nd law and deriving the limits from there. It would thus be circular to use the limits to rescue the 2nd law from the demon.

I don't think gravity is much of a demon though. It cashes in gravitational potential energy to do the sorting, which is perfectly legal under the 2nd law.

I know. If your goal is to break the law, gravity won't help. If your goal is to separate air by temperature and harvest "free" energy thereby, gravity has been there all along, no demon necessary. That is all I was ever saying.

 

[AdrianMay]

But you appeared to be saying that the demon doesn't do anything that gravity can't. I'd say there is something special about the demon: he breaks the second law. Gravity never threatened to because it harvested GPE to do the separation. The demon manages the separation in the complete absence of gravity. So where's the energy coming from? That's a rather demonic party trick.

 

[zoobyshoe]

That's a... ...party trick.

This is what I'm trying to emphasize! The result achieved by extraordinary means is no more important than a party trick unless the result, itself, is extraordinary, in and of itself.

Suppose a magician actually used real magic to create a rabbit from nothing and pull it out of a hat. It would be conceptually astounding, he'd have broken all the laws of physics and completely upset the known paradigm. And all he'd have accomplished, though, is to create a new rabbit! We already have plenty of rabbits. A new rabbit is a trivial change to humanity. Rabbits already breed...like rabbits.

Why is the means whereby it's accomplished so important when the result is so mundane? I guess it depends on whether you're ultimately into concepts or practical realities.

The significance of the story, which is what ThomasT asked about, lies down a completely different path, one which just about everyone seems to miss: as I mentioned earlier, Maxwell wasn't proposing we should apply ourselves to figuring out how to get round the 2nd law, nor was he making a point about how the 2nd law was inviolate. He was making some completely different point about scale and statistical analysis:

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.

His exact point seems to be nothing more than to say that we have to give careful consideration to what conclusions we are drawing about the microscopic situation from macroscopic analyses, the specific illustrative example being that the temperature we ascribe to some container of gas actually says nothing about the energy of any actual given molecule of gas we might observe if we could observe an individual molecule, that if you could actually separate the more energetic ones from the less energetic, both portions would deviate a lot, one being higher, the other lower, from the measured temperature of the gas.

On the other hand, his concluding paragraph asserts that sometimes a macroscopic statistical average actually reflects a microscopic uniformity, the example given being the mass of a hydrogen molecule:

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.

His original goal was:

Before I conclude, I wish to direct attention to an aspect of the molecular theory which deserves consideration.

And that "aspect" of molecular theory he's directing our attention to, seems to be that, sometimes you are averaging a huge number of different things, at others you are averaging a huge number of uniform things. Careful consideration should be given to which is which least the microscopic reality be misunderstood.

The interjected question:

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.

Asks to what extent knowledge applies from one scale to the other. (In a sense, he's prematurely anticipating quantum physics.)