## Comments about "Maxwell's demon" in Wikipedia

This document contains comments about the article Maxwell's demon in Wikipedia
• The text in italics is copied from that url
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.
Maxwell's demon is a thought experiment that would hypothetically violate the second law of thermodynamics.
This sentence is difficult to understand because it requires a clear definition of the following three concepts:
"thought experiment", "hypothetical" and "the second law of thermodynamics".
The problem is with a thought experiment is, that at least two people, try to perform a physical experiment completely, in their mind, without looking to all the physical problems involved. Thought experiments should not be used to perform science.
In the thought experiment, a demon controls a small massless door between two chambers of gas. As individual gas molecules (or atoms) approach the door, the demon quickly opens and closes the door to allow only fast-moving molecules to pass through in one direction, and only slow-moving molecules to pass through in the other.
This description is too complex. Here is a simpler one:
In the thought experiment, a demon controls a small massless DOOR between two chambers of gas. As one individual gas molecule (or atom) approach the DOOR, the demon quickly opens and closes the door to allow one fast-moving molecule to pass through in one direction, and only one slow-moving molecule to pass through in the other. one
This is a physical completely unrealistic experiment. The task to perform by the demon is impossible. Anyway to perform this task costs energy.
Because the kinetic temperature of a gas depends on the velocities of its constituent molecules, the demon's actions cause one chamber to warm up and the other to cool down.
That is correct. The question is what happens if the demon, after finishing his task, just keeps the door open.
My guess is that the two chambers will again reach equilibrium, but the final temperature will be lower. There is loss of energy.
This would decrease the total entropy of the two gases, without applying any work, thereby violating the second law of thermodynamics.
This increases the entropy in the hot chamber and decreases the entropy in the cool chamber. The total stays the same. The Second law of thermodynamics should confirm this.
The concept of Maxwell's demon has provoked substantial debate in the philosophy of science and theoretical physics, which continues to the present day.
The scientist should come to a conclusion: what is right and what is wrong. A clear definition of both points of view are paramount.
It stimulated work on the relationship between thermodynamics and information theory.
Thermodynamics and Information theory have 'nothing' in common. Both require a clear definition.
Other researchers have implemented forms of Maxwell's demon in experiments, though they all differ from the thought experiment to some extent and none have been shown to violate the second law.
This sentence is tricky because it is a waste of time to compare real experiments with thought experiments.
Science should not be based on "Maxwell's demon" as a thought experiment.

### 2 Original thought experiment

The second law of thermodynamics ensures (through statistical probability) that two bodies of different temperature, when brought into contact with each other and isolated from the rest of the Universe, will evolve to a thermodynamic equilibrium in which both bodies have approximately the same temperature.
A better description is the following:
Each and every experiment shows that two bodies of different temperature, when brought into contact with each other and isolated from the rest of the Universe, will evolve to a thermodynamic equilibrium in which both bodies have approximately the same temperature.
Brought into contact implies that there should be an ample transport of energy possible between the two bodies. Such an example demonstrates the second law of thermodynamics.
The second law is also expressed as the assertion that in an isolated system, entropy never decreases.
This sentence is not clear, because the concept entropy is not clear.
This sounds more like the assertion, that in an isolated system the total amount of energy is constant.
Now let us suppose that such a vessel is divided into two portions, A and B, by a division in which there is a small hole, and that a being, who can see the individual molecules, opens and closes this hole, so as to allow only the swifter molecules to pass from A to B, and only the slower molecules to pass from B to A. 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.
The problem is there exist no being which can decide that there is on side A a certain molecule mA which moves swifter than a certain molecule mB on side B (which moves slower) and takes care that mA moves through a hole from A to B and mB moves through a hole from B to A.
The problem is how can this being decide that the speed of mA is higher than the speed of mB
A second problem is that the more molecules are transported the more difficult it becomes to make a selection.
The more difficult implies the more energy it costs.
When a faster-than-average molecule from A flies towards the trapdoor, the demon opens it, and the molecule will fly from A to B.
This should be: When a molecule from A flies faster than the average molecules from B, it flies towards the trapdoor from A to B. (action 1) As a result of 'Action 1' the average speed of the molecules in B will have increased.
Likewise, when a slower-than-average molecule from B flies towards the trapdoor, the demon will let it pass from B to A.
This should be: When a molecule from B flies slower than the average molecules from A, it flies towards the trapdoor from B to A. (action 2) As a result of 'Action 2' the average speed of the molecules in A will will have slowed down
The average speed of the molecules in B will have increased while in A they will have slowed down on average.
It should be mentioned:
• That both actions should be done simultaneous.
• That to select a molecule from A which flies faster than the average molecules from B costs energy.
• That to select a molecule from B which flies slower than the average molecules from A costs energy
• The longer this process continuous the more difficult this selection becomes, implying the more energy it costs.

### 3. Criticism and development

Several physicists have presented calculations that show that the second law of thermodynamics will not actually be violated, if a more complete analysis is made of the whole system including the demon
First of all you must make a complete analysis of a process under investigation. Separate you have to give a short description of what the second law is. Next you have to combine the two. Only there after you can perform the calculations as indicated by the second law.
One of the most famous responses to this question was suggested in 1929 by Leó Szilárd, and later by Léon Brillouin. Szilárd pointed out that a real-life Maxwell's demon would need to have some means of measuring molecular speed, and that the act of acquiring information would require an expenditure of energy.
That is correct. Remember at molecule level.
Since the demon and the gas are interacting, we must consider the total entropy of the gas and the demon combined.

### 6 Experimental work

In 2006, Raizen, Muga, and Ruschhaupt showed in a theoretical paper that as each atom crosses the one-way wall, it scatters one photon, and information is provided about the turning point and hence the energy of that particle. The entropy increase of the radiation field scattered from a directional laser into a random direction is exactly balanced by the entropy reduction of the atoms as they are trapped by the one-way wall.
The question is what means the entropy of the radiation field.
From a physical point of view the energy of the atom is decreased by the release of a photon or the energy equivalent.
This technique is widely described as a "Maxwell's demon" because it realizes Maxwell's process of creating a temperature difference by sorting high and low energy atoms into different containers.
However, scientists have pointed out that it is not a true Maxwell's demon in the sense that it does not violate the second law of thermodynamics; it does not result in a net decrease in entropy and cannot be used to produce useful energy.
a true demonstrate

### 8. See also

Following is a list with "Comments in Wikipedia" about related subjects
See also:

### Reflection 1 - A new natural experiment with red and yellow balls.

The purpose of this experiment to try to repeat the experiment with the demon, but not as a thought experiment, and to establish if there are energy issues.
Perform the following natural experiment:
• First take a container and place a divider in the container. Next place 50 yellow balls in the left part and 50 red balls in the right half.
• Remove the divider. Wait. Nothing will happen. The system is in a equilibrium state.
• Shake the container such that the balls are equally divided inside the container. Place again a divider inside the container such that the container is divided into two parts, which each part containing almost the same number of balls.
• Ask someone to select a red ball from the left part and place it in the right half. Ask someone to select a yellow ball from the right part and place it in the left half.
• Repeat these two actions until there are no more red balls in the left half and no more yellow balls in the right part.
What that means that you are back to the initial situation.
What this experiment shows:
• That both processes going from an ordered state to a random state, and back from a random state to an ordered state costs energy.
• The first process is a shaking process and the second one a selection processes.
Both of these processes are irreversible because energy in each case is required.

### Reflection 2 - A natural experiment with cold and hot water.

This experiment is identical as demonstrated in refelection 1, but now hot and cold water is used.
• First take a container and place a divider in the container. Next put cold water of 20 degrees in the left part and hot water of 40 degrees in the right part.
• Remove the divider. Wait. There will be some sort of hot water flow to the left and some sort of cold water flow to the right. You could claim the the cold water molecules will be heated up and the hot water molecules will be cooled down. Finally the system will become in equilibrium at a temperature of 30 degrees.
• Place again a divider inside the container such that the container is divided into two parts, which each part containing almost the same number of balls.
• The second phase of this experiment is to try to bring the state of the container, or better the state of each of the two parts, back to its intitial situation.
• In order to decrease the temperature at the left the demon has to select the molecule with the lowest temperature (or slowest movement) at the right and move this to the left. This will increase the temperature at the right and decrease the temperature at the left
• In order to increase the temperature at the right the demon has to select the molecule with the highest temperature (or fastest movement) at the left and move this to the right. This will decrease the temperature at the left and increase the temperature at the right.
Both of these actions have to be performed simultaneous
• That means to remove the extra heat from each of the water molecules at the left and transport that to a water molecule at the right side.
What this experiment shows:
• That a process which starts in an equilibrium situation has to go to state where two seperate parts have a different temperature. That means from an average temperature of 30 degrees to a state where one part has 20 degrees and the other part has 40 degrees.
The assumption exists that this does not require any energy
• The question is if that is true because than you have a perpetum mobile.

### Reflection 3 - The original thought experiment

In the original thought experiment start position is an equilibrium situation.
The idea behind the thought experiment is that a demon makes the selections.
The purpose is to increase the temperature in part A and to decrease the temperture in part B
In order to increase the temperature of part A the demon has to select the molecule with the highest temperature (or fastest movement) in part B and move this to part A. This will decrease the temperature in part B and increase the temperature in part A
In order to decrease the temperature of part B the demon has to select the molecule with the lowest temperature (or slowest movement) in part A and move this to part B. This will increase the temperature in part A and decrease the temperature in part B
Both of these actions have to be performed simultaneous
The general physical consideration is that to perform these tasks by the demon no energy is involved. In the original configuration there is no temperature difference between part A and part B but as a result of the actions by the Demon this changes and the temperature difference grows without any energy. This sounds like a perpetum mobile.
I don't think in reality this works.

### Reflection 4 - Maxwell's demon

The definition of a Maxwell's demon is a device, for example, which can devide a mixture of hot and warm molecules into two seperate collections of hot molecules and warm molecules.
What is important that the heat to warm the molecules must come from the cold molecules, which become cooler.

A Maxwell's demon is not a photon which is allowed to slowdown the speed of an atom, therby cooling the atom. The reason is that the infalling photon is considered as an external source of energy, which is in conflict with the closed system in which the demon is supposed to function.

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Created: 12 Februari 2019
Updated: 26 October 2022

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