Maxwell's demon or how to get around the second law of thermodynamics

Hello dear guests and subscribers of my channel. Today I want to talk to you about the so-called Maxwell's Demon, which was generated during the discussion of the second law of thermodynamics. So let's get started.

Not only in science fiction novels you can find some unusual creature, but even in a seemingly extremely far from fiction, science like physics turns out to have a place for unusual and even fantastic creatures, such as demons.

Perhaps the most famous such creature was the so-called Maxwell Demon, which was created by James Clerk Maxwell himself, the creator of the Maxwell system of equations. And he (the demon) was invented during the active debate around the second law of thermodynamics.

What does the second law of thermodynamics say?

So, according to the second law of dynamics, it has enough formulations, but at the same time the physical meaning is the same: the system, in isolation, unable to independently transition from a less ordered state to a more ordered one state.

For example, let's imagine a certain volume of gas where molecules move at different speeds. According to the second law, the gas is not able to independently split into two halves, where one will contain a gas with low-velocity molecules, and the other will contain a gas with high-velocity molecules.

Also, a large number of processes are categorized as reversible. So, for example, ordinary water can be frozen, and after defrosting, you can get liquid water again.

Metal can be both magnetized and then demagnetized. There are also irreversible processes, such as burning something. But all these processes, according to the second law of thermodynamics, will lead either to the preservation or to a decrease in the degree of ordering of the system.

This situation haunted and disturbed the scientific minds of the 19th century. And it was at this time that Maxwell formulated his non-trivial solution, which, as it seemed at that time, made it possible to gracefully bypass the second law of thermodynamics and prevent the inevitable increase in chaos in a closed system.

He set up such a thought experiment.

Thought experiment or how Maxwell's Demon appeared

The essence of Maxwell's experiment:

Imagine a container divided into two equal halves. Moreover, in the partition (this divides the container into two identical parts and is still absolutely impermeable to gas) there is a microscopic opening-door that can only let one gas atom pass at a time.

And at the same time, one half of the container is completely filled with gas, and the second is filled with pure vacuum.

Now let's mentally imagine that a micro-watchman is presented to this checkpoint-door, observing the gas molecules in the most intent manner.

And at the same time, he (the watchman) opens the passage for fast molecules and lets them into the second half of the container with a vacuum, and leaves the insufficiently high-speed ones where they were.

It is logical that if the work of the checkpoint with the watchman lasts a significant time, then the gas will be divided into two parts. One will have a cooled gas with slow molecules, and the other will have a hot gas with hot gas molecules.

Thus, the system will be ordered in relation to the initial state, and thus the second law of thermodynamics is violated.

Moreover, the obtained temperature imbalance is quite acceptable to use to obtain work (According to the Cycle and Carnot's theorem).

And this means if the notorious watchman is left at the checkpoint for an unlimited time, then we will get nothing more than a perpetual motion machine.

It was this watchman-controller that other scientists called the Demon of Maxwell. And it seemed in this experiment everything was taken into account, and oh, how it does not hurt a perpetual motion machine. But there is one significant catch.

What is Demon Maxwell's problem

Almost from the beginning, the thought experiment was questioned, and here's why:

For the endless work of the demon watchman, energy supply in the form of a stream of photons is required, which is needed to illuminate and sift the arriving molecules.

Also, sorting molecules, the demon cannot but interact with the molecules, which means they themselves will acquire thermal energy from the gas. This means that entropy will inevitably increase.

And the total entropy of such a system will not decrease in any way. This means that there is no violation of the second law.

A significant counterargument against the existence of Maxwell's Demon came after the birth of quantum mechanics.

So, in order to correctly sort the oncoming gas molecules, the watchman needs to accurately measure their speed, which in principle is impossible according to Heisenberg's uncertainty. Also, according to the same principle, the exact position of the molecule cannot be determined.

And this means that inevitably some of the molecules, in front of which the door will be opened, will miss it.

That is, Maxwell's demon is essentially a macroscopic elephant in the china shop of the microworld, living by its own rules.

And if he (the demon watchman) is presented in accordance with the laws of quantum mechanics, then he will be unable to sort the molecules. Therefore, it is not dangerous to the second law of thermodynamics.

This is how the mythical creature of physics, the Demon of Maxwell, was destroyed.

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