Sly, calculating and unreal: who is Maxwell's demon

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Sly, calculating and unreal: who is Maxwell's demon
Sly, calculating and unreal: who is Maxwell's demon

In the late 19th century, British physicist James Maxwell proposed a thought experiment that seemingly violates the laws of thermodynamics. As a result, the central character of this experiment was named Maxwell's demon. Let's try to figure out what is remarkable about this fictional entity.

Maxwell's demon

Maxwell's Demon is a hypothetical entity proposed by James Clerk Maxwell in one of his thought experiments, presumably in 1871.

What does the demon and Maxwell have to do with it? In general, the essence proposed by Maxwell is a kind of contradictory god from the machine, one might say, who discovered a way to circumvent one of the most fundamental and indisputable laws of the Universe - the second law of thermodynamics. Initially, the scientist's colleagues did not take the thought experiment seriously and were even confused, because this “essence” could mean that you can finally forget about wasting coal and just get a job endlessly, in fact, out of nothing.

However, in reality, this is not entirely true. You can read about the second law of thermodynamics and entropy in our other material. And now we will try to figure out why Maxwell's demon caused bewilderment among the luminaries of science at the end of the 19th century.

Maxwell's demon - a loophole in entropy

Maxwell's thought experiment was originally mentioned in the scientist's correspondence with Peter Tate around 1867. It was later introduced to the public in Maxwell's book on thermodynamics called Theory of Heat, published in 1872.


Despite the fact that Maxwell himself never used the word "demon" when describing the experiment, his agent opened the door (in the partition in our gas box) between the chambers as a "limited being." This entity was first named "demon" by William Thomson, known as Lord Kelvin, to describe Maxwell's agent in Nature in 1874. As a justification, he argued that he wanted to designate the mediating nature of the essence in this way and in no case was going to emphasize the negative connotation of the word itself.

So, back to the experiment. This is primarily a closed system. The proposed apparatus consists of a simple cuboid, which contains some kind of arbitrary gas. The cuboid is divided into two sections of equal size with the same, uniform temperature. On the wall dividing the section, a demon sits, carefully selecting randomly scattered particles so that all particles with high kinetic energy are collected in one section, while the rest - with low kinetic energy - remain in another.

We can say that this demon is a metaphor for a device or machine capable of carefully analyzing the speed or kinetic energy of each particle in any container. Based on its analysis, the adaptation can determine exactly which particles it should, roughly speaking, keep for itself, and which - get rid of.


Meanwhile, this contradicts the generally accepted opinion that gas particles at a constant temperature move with the same speed. Nevertheless, this same speed is their average speed, which means that there are particles moving at a higher speed, and there are particles moving at a lower speed, reducing everything to an average value.

Through this process - the actions of the demon Maxwell - all high-energy particles are subsequently driven into one section. The demon raised the temperature of one part of the box compared to the other. This excess temperature or pressure can be used to power the turbine or piston. Yes, it follows that we get energy from literally nothing. In other words, the demon has reduced entropy without spending any effort.

It is necessary to understand, however, that the cunning demon used his tricks and, as a result, was able to contradict the law of entropy, but he did not violate the law of conservation of energy. He simply redistributed random kinetic energy to create a differential pressure enough to draw energy from an initially balanced system. The demon's cunning deceived nature itself!

Can such an apparatus exist?

Be that as it may, such an apparatus cannot be created in reality. Nature is not easily deceived. Of course, the cunning and clever demon was able to avoid the oppressive sanctions of the second law of thermodynamics, but he cannot get away from the all-seeing eye of the first law of thermodynamics.

According to the first law of thermodynamics, no machine is capable of functioning without a source of heat, and in the process of work it can also partially absorb it. Or, the process performance will never reach 100 percent. Machines not only need the stimulus in the form of heat, they also need to absorb it, thereby increasing their own temperature.

The conversion of thermal energy into mechanical energy in steam engines is not absolute. Some of the heat is absorbed by the engine itself, reducing overall performance and increasing the entropy surrounding it.

If the demon is a high-tech machine that selectively tracks certain particles, the question arises: where does it get its energy from to do its job? Even if he somehow manages to do this, the expansion in relation to the thermal performance of the machine still denies the possibility of a decrease in entropy.


A demon or machine would have to get information about the particles. Take photons, for example. In the process of interacting with them, a complex apparatus like Maxwell's demon will inevitably spend energy and absorb some of the heat itself, increasing the total entropy and bringing it to its original value.

The point of the argument is that, according to calculations, any demon will inevitably "generate" more entropy by separating molecules than it can ever "destroy" it - this is in line with the principles on which it is based. In other words, it would take much more thermodynamic work to determine the velocity of molecules and select them to pass through the door between sections than the amount of energy gained from the temperature difference that arose after the work done.

Be that as it may, it should be noted that Maxwell was very cunning. However, if not for the first law of thermodynamics, nothing would have saved the second law from public shame.

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