# The Maxwell's demon

What is the "the Maxwell's demon"? Why did he excites the minds of great scientists? It is easy to explain. Scientists are looking for processes that would allow heat to pass from the body less heated to a more heated bodies. But we know that heat can only move from hot to cold bodies. This is called the second law of thermodynamics. Against this law decided to fight "the Maxwell's demon".

In 1871 the great English scientist James Maxwell tried to solve this problem. For this, he came up with a fantastic creature called "the Maxwell's demon", which has the function of such a mechanism. It can track the movement of each individual molecule and know its speed. For example, take a vessel divided by a partition into two parts. In this vessel, "the Maxwell's demon" would sit at the door in the wall. We can get him to open the door only to fast or just before the slow molecules. "the Maxwell's demon" will skip the fast molecules in one part of the vessel, and slow in another part. Then one of the vessel will have a higher temperature and pressure than the other. Thus we get an unlimited supply of energy without the cost of any work. The entropy of the system is greater in the initial state than in the end. This is contrary to the principle of the thermodynamic entropy change in closed systems.

The paradox is solved if we consider a closed system, including "the Maxwell's demon" and the vessel. For the operation of "the Maxwell's demon" need the energy transfer to it from an external source. Due to this energy is produced separation of hot and cold molecules in the vessel, i.e. the transition to a state with lower entropy.

With the development of information theory it was found that the measurement process may not result in an increase of entropy under the condition that it is thermodynamically reversible. However, in this case, the daemon must remember velocity measurement results (erase them from the memory of the demon makes the process irreversible). Because the memory is finite, at some point, "the Maxwell's demon" forced to erase the old results. This leads to an increase in entropy of the whole system.

Many times scientists argued that "the Maxwell's demon" is a joke of the great physicist. Indeed, "the Maxwell's demon," is not effective in a vessel with two molecules. In half the cases, they could be in one part of the vessel. In the case of multiple molecules of the probability of this is very small.

But, "the Maxwell's demon" is trying to find new arguments in his defense. In one scientific journal says that "the Maxwell's demon" exists only in the form of a laser, which can separate the molecules with high energy from unexcited molecules.

Until now, there is no rigorous evidence "the Maxwell's demon". "The Maxwell's demon" stirs interest in the further search.