Information theory and the second law of thermodynamics
In this chapter, we discuss a relation between the concept of the amount of information and that of physical entropy. As is well known, the latter allows us to express quantitatively the second law of thermodynamics, which forbids, in an isolated system, the existence of processes accompanied by an increase of entropy. If there exists an influx of information dI about the system, i.e. if the physical system is isolated only thermally, but not informationally, then the above law should be generalized by substituting inequality dH ≥ 0 with inequality dH + dI ≥ 0. Therefore, if there is an influx of information, then the thermal energy of the system can be converted (without the help of a refrigerator) into mechanical energy. In other words, the existence of perpetual motion of the second kind powered by information becomes possible.