 by a hole closed by a small door. A demon is able to see single molecules. Opening the door at the right moment traps the higher velocity molecules in one section, say the left one, and the slower velocity ones in the other. In this way the demon is able to increase the temperature difference between the two sections, violating the second thermodynamic principle. Variants of Maxwell's demon were later discussed by Szilard, Brillouin, Rothstein, and other authors [10, 81]. In Szilard's model, M. D. Migliore the selection of molecules in thermal equilibrium requires that the demon sees the individual molecules, that is, a sort of detection is required, which on the one hand increases the state of knowledge of the demon with respect to the environment; on the other hand, it causes an increase in the entropy of statistical mechanics, which is "transformed" into information by the demon. Overall, the entropy of the system does not decrease, and therefore there is no violation of the second thermodynamic principle. Generally speaking, the many examples reported in literature converge on the conclusion that even for information there is no "free meal" and considering information completely independent of physics can cause the violation of first principles. Information and physics are intimately related, and any method that allows us to transmit an unlimited amount of information with finite energy simply violates the laws of our universe. What are the implications of this conclusion in communication systems? Com- munication systems basically encode information in different states of a physical observable. Accordingly, for any E approximation level, the set of the states must admit a finite number of e-distinguishable balls. This requires that the set is totally bounded4 [19, 46]. In a physical approach to the communication system, the total boundness of the observed quantity (i.e., the electromagnetic field in our case) on the observation surface is automatically guaranteed by the properties of the radiation operator [