Article

# Nonadditive entropy: the concept and its use. European Phys. J. A 40, 257-266

Santa Fe Institute 1399 Hyde Park Road 87501 Santa Fe USA
(Impact Factor: 2.74). 12/2008; 40(3):257-266. DOI: 10.1140/epja/i2009-10799-0
Source: arXiv

ABSTRACT

The thermodynamical concept of entropy was introduced by Clausius in 1865 in order to construct the exact differential dS = d \delta
Q/T , where d \delta
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) SBG=-kåi=1W pi lnpi\ensuremath S_{BG}=-k\sum_{i=1}^W p_i \ln p_i , 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., pijA+B=piA pjB, "(i,j)\ensuremath p_{ij}^{A+B}=p_i^A p_j^B, \forall(i,j) , we verify that SBG(A+B)=SBG(A)+SBG(B)\ensuremath S_{BG}(A+B)=S_{BG}(A)+S_{BG}(B) . 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 SBG to be extensive in the thermodynamical sense, i.e., that SBG(N) µ N\ensuremath S_{BG}(N) \propto N in the N ≫ 1 limit. If, on the contrary, the correlations between the N elements are strong enough, then the extensivity of SBG 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 Sq=k [1-åi=1W piq]/(q-1) (q Î R; S1=SBG)\ensuremath S_q=k [1-\sum_{i=1}^W p_i^q]/(q-1) (q\in{R}; S_1=S_{BG}) . In the context of cybernetics and information theory, this and similar forms have in fact been repeatedly introduced before
1988. The entropic form Sq is, for any q
¹ \neq 1 , nonadditive. Indeed, for two probabilistically independent subsystems, it satisfies Sq(A+B)/k=[Sq(A)/k]+ [Sq(B)/k]+(1-q)[Sq(A)/k][Sq(B)/k] ¹ Sq(A)/k+Sq(B)/k\ensuremath S_q(A+B)/k=[S_q(A)/k]+ [S_q(B)/k]+(1-q)[S_q(A)/k][S_q(B)/k] \neq S_q(A)/k+S_q(B)/k . 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 qent (where ent stands for entropy . In other words, for such systems, we verify that Sqent(N) µ N (N >> 1)\ensuremath S_{q_{ent}}(N) \propto N (N \gg 1) , thus legitimating the use of the classical thermodynamical relations. Standard systems, for which SBG 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 Sq is extensive. Such systems are out of the present scope: they might need forms of entropy different from Sq, or perhaps --more plainly-- they are just not susceptible at all for some sort of thermostatistical approach. Consistently
with the results associated with Sq, the q -generalizations of the Central Limit Theorem and of its extended Lévy-Gnedenko form have been achieved. These recent theorems
could of course be the cause of the ubiquity of q -exponentials, q -Gaussians and related mathematical forms in natural, artificial and social systems. All of the above, as well as presently
available experimental, observational and computational confirmations --in high-energy physics and elsewhere-- are briefly
reviewed. Finally, we address a confusion which is quite common in the literature, namely referring to distinct physical mechanisms
versus distinct regimes of a single physical mechanism.

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• "black holes – along the same lines of reasoning. We believe that this can either be achieved by relaxing the requirement of full thermodynamic equilibrium or by using different statistics, such as Rényi or Tsallis' statistics [39] [40] "
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• "Indeed as Bouchaud and Potter point out, the equation (7) is linearly related to the Tsallis [8] [9] entropy function and the entire process of obtaining the weights, p i is equivalent to minimizing a free -utility function: "
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• "During the last years some papers have been published in which again Gibbs Paradox (non-additive entropy), the concept of entropy and the mixing process of particle systems were considered, see for example [1] [2] [3] [4] [5] [6]. From the definition of additivity (or non-additivity) follows that this notion is related to a system composed of several components, S=∑ i S i i≥2. "
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ABSTRACT: In this work the non-additive entropy is examined. It appears in isolated particle systems composed of few components. Therefore, the mixing of isolated particle systems S=S1+S2 has been studied. Two cases are considered T1=T2 and T1\leqT2, where T1,T2 are the initial temperatures of the system S1 and S2 respectively. The concept of similar systems containing interacting particles is introduced. These systems are defined by a common temperature and an identical time evolution process, i.e. the approach to the same thermodynamic equilibrium. The main results are: 1) The properties of the similar particle systems yield the non-additive entropy and free energy. The Gibbs Paradox is not a paradox. 2) The relation between the initial temperatures T1 and T2 governs the mixing process. 3) In the two cases T1=T2, T1\leqT2 mixing of the systems S1, S2 results in a uniform union system S=S1+S2. The systems S, S1, S2 are similar one to the other. 4) The mixing process is independent of the extensive quantities (volume, particle number, energy) and of the particle type. Only the mean energy plays an important role in the mixing of the systems S1, S2. 5) Mixing in the case T1\leqT2 is in essence a thermalization process, but mixing in the case T1=T2 is not a thermodynamic process. 6)Mixing is an irreversible process. Keywords: Entropy; Similar systems of interacting particles; Mixing of systems; Thermal equilibrium