Nonadditive entropy: The concept and its use

The thermodynamical concept of entropy was introduced by Clausius in 1865 in order to construct the exact differential dS = Q / T , where Q is the heat transfer and the absolute temperature T its integrating factor. A few years later, in the period 1872-1877, it was shown by Boltzmann that this quantity can be expressed in terms of the probabilities associated with the microscopic configurations of the system. We refer to this fundamental connection as the Boltzmann-Gibbs (BG) entropy, namely (in its discrete form) , where k is the Boltzmann constant, and { p i } the probabilities corresponding to the W microscopic configurations (hence ∑ W i=1 p i = 1 . This entropic form, further discussed by Gibbs, von Neumann and Shannon, and constituting the basis of the celebrated BG statistical mechanics, is additive . Indeed, if we consider a system composed by any two probabilistically independent subsystems A and B ( i.e. , , we verify that . If a system is constituted by N equal elements which are either independent or quasi-independent ( i.e. , not too strongly correlated, in some specific nonlocal sense), this additivity guarantees S BG to be extensive in the thermodynamical sense, i.e. , that in the N ≫ 1 limit. If, on the contrary, the correlations between the N elements are strong enough, then the extensivity of S BG is lost, being therefore incompatible with classical thermodynamics. In such a case, the many and precious relations described in textbooks of thermodynamics become invalid. Along a line which will be shown to overcome this difficulty, and which consistently enables the generalization of BG statistical mechanics, it was proposed in 1988 the entropy . In the context of cybernetics and information theory, this and similar forms have in fact been repeatedly introduced before 1988. The entropic form S q is, for any q 1 , nonadditive . Indeed, for two probabilistically independent subsystems, it satisfies . This form will turn out to be extensive for an important class of nonlocal correlations, if q is set equal to a special value different from unity, noted q ent (where ent stands for entropy . In other words, for such systems, we verify that , thus legitimating the use of the classical thermodynamical relations. Standard systems, for which S BG is extensive, obviously correspond to q ent = 1 . Quite complex systems exist in the sense that, for them, no value of q exists such that S q is extensive. Such systems are out of the present scope: they mi