Some thoughts on theoretical physics

Some thoughts are presented on the inter-relation between beauty and truth in science in general and theoretical physics in particular. Some conjectural procedures that can be used to create new ideas, concepts and results are illustrated in both Boltzmann-Gibbs and nonextensive statistical mechanics. The sociological components of scientific progress and its unavoidable and benefic controversies are, mainly through existing literary texts, briefly addressed as well.Comment: Short essay based on the plenary talk given at the International Workshop on Trends and Perspectives in Extensive and Non-Extensive Statistical Mechanics, held in November 19-21, 2003, in Angra dos Reis, Brazil. To appear in a Physica A special volume (2004) edited by E.M.F. Curado, H.J. Herrmann and M. Barbosa. 23 pages, including 3 figures. The new version has 25 pages and the same figures. The texts by Saramago and by Bersanelli are now translated into English. A few typos and minor improvements are included as wel

Negative specific heat in a Lennard-Jones-like gas with long-range interactions

We study, through molecular dynamics, a conservative two-dimensional Lennard-Jones-like gas (with attractive potential $\propto r^{-\alpha}$). We consider the effect of the range index $\alpha$ of interactions, number of particles, total energy and particle density. We detect negative specific heat when the interactions become long-ranged ($0\le \alpha/d<1$).Comment: LaTeX, 8 pages, 4 eps figures, contributed paper to the Proceedings of the International School and Workshop on Nonextensive Thermodynamics and physical applications, NEXT 2001, 23-30 May 2001, Cagliari (Italy) (Physica A) (New Title, new Fig. 4

Dynamical scenario for nonextensive statistical mechanics

Statistical mechanics can only be ultimately justified in terms of microscopic dynamics (classical, quantum, relativistic, or any other). It is known that Boltzmann-Gibbs statistics is based on the hypothesis of exponential sensitivity to the initial conditions, mixing and ergodicity in Gibbs $\Gamma$-space. What are the corresponding hypothesis for nonextensive statistical mechanics? A scenario for answering such question is advanced, which naturally includes the {\it a priori} determination of the entropic index $q$, as well as its cause and manifestations, for say many-body Hamiltonian systems, in (i) sensitivity to the initial conditions in Gibbs $\Gamma$-space, (ii) relaxation of macroscopic quantities towards their values in anomalous stationary states that differ from the usual thermal equilibrium (e.g., in some classes of metastable or quasi-stationary states), and (iii) energy distribution in the $\Gamma$-space for the same anomalous stationary states.Comment: Invited paper at the Second Sardinian International Conference on "News and Expectations in Thermostatistics" held in Villasimius (Cagliari)- Italy in 21-28 September 2003. 12 pages including 2 figure

Equipartition and Virial theorems in a nonextensive optimal Lagrange multipliers scenario

We revisit some topics of classical thermostatistics from the perspective of the nonextensive optimal Lagrange multipliers (OLM), a recently introduced technique for dealing with the maximization of Tsallis' information measure. It is shown that Equipartition and Virial theorems can be reproduced by Tsallis' nonextensive formalism independently of the value of the nonextensivity index.Comment: 13 pages, no figure

Generalized thermostatistics based on deformed exponential and logarithmic functions

The equipartition theorem states that inverse temperature equals the log-derivative of the density of states. This relation can be generalized by introducing a proportionality factor involving an increasing positive function phi(x). It is shown that this assumption leads to an equilibrium distribution of the Boltzmann-Gibbs form with the exponential function replaced by a deformed exponential function. In this way one obtains a formalism of generalized thermostatistics introduced previously by the author. It is shown that Tsallis' thermostatistics, with a slight modification, is the most obvious example of this formalism and corresponds with the choice phi(x)=x^q.Comment: Invited talk at Next2003, uses Elsevier LaTeX macro

Dynamical anomalies and the role of initial conditions in the HMF model

We discuss the role of the initial conditions for the dynamical anomalies observed in the quasi-stationary states of the Hamiltonian Mean Field (HMF) model.Comment: 8 pages, 5 figures, submitted to Physica A for the proceedings of the conference Frontier Science 2003 Pavia, Italy, 8-12 September 200

Low temperature specific heat of glasses: a non-extensive approach

Specific heat is calculated using Tsallis statistics. It is observed that it is possible to explain some low temperature specific heat properties of glasses using non-extensive approach. A similarity between temperature dependence of non-extensive specific heat and fractal specific heat is also discussed.Comment: 5 pages, 4 figure

Generalized Simulated Annealing

We propose a new stochastic algorithm (generalized simulated annealing) for computationally finding the global minimum of a given (not necessarily convex) energy/cost function defined in a continuous D-dimensional space. This algorithm recovers, as particular cases, the so called classical ("Boltzmann machine") and fast ("Cauchy machine") simulated annealings, and can be quicker than both. Key-words: simulated annealing; nonconvex optimization; gradient descent; generalized statistical mechanics.Comment: 13 pages, latex, 4 figures available upon request with the authors

Is the entropy Sq extensive or nonextensive?

The cornerstones of Boltzmann-Gibbs and nonextensive statistical mechanics respectively are the entropies $S_{BG} \equiv -k \sum_{i=1}^W p_i \ln p_i$ and $S_{q}\equiv k (1-\sum_{i=1}^Wp_i^{q})/(q-1) (q\in{\mathbb R} ; S_1=S_{BG})$. Through them we revisit the concept of additivity, and illustrate the (not always clearly perceived) fact that (thermodynamical) extensivity has a well defined sense {\it only} if we specify the composition law that is being assumed for the subsystems (say $A$ and $B$). If the composition law is {\it not} explicitly indicated, it is {\it tacitly} assumed that $A$ and $B$ are {\it statistically independent}. In this case, it immediately follows that $S_{BG}(A+B)= S_{BG}(A)+S_{BG}(B)$, hence extensive, whereas $S_q(A+B)/k=[S_q(A)/k]+[S_q(B)/k]+(1-q)[S_q(A)/k][S_q(B)/k]$, hence nonextensive for $q \ne 1$. In the present paper we illustrate the remarkable changes that occur when $A$ and $B$ are {\it specially correlated}. Indeed, we show that, in such case, $S_q(A+B)=S_q(A)+S_q(B)$ for the appropriate value of $q$ (hence extensive), whereas $S_{BG}(A+B) \ne S_{BG}(A)+S_{BG}(B)$ (hence nonextensive).Comment: To appear in the Proceedings of the 31st Workshop of the International School of Solid State Physics ``Complexity, Metastability and Nonextensivity", held at the Ettore Majorana Foundation and Centre for Scientific Culture, Erice (Sicily) in 20-26 July 2004, eds. C. Beck, A. Rapisarda and C. Tsallis (World Scientific, Singapore, 2005). 10 pages including 1 figur

On the extensivity of the entropy Sq, the q-generalized central limit theorem and the q-triplet

First, we briefly review the conditions under which the entropy $S_q$ can be extensive (Tsallis, Gell-Mann and Sato, Proc. Natl. Acad. Sc. USA (2005), in press; cond-mat/0502274), as well as the possible $q$-generalization of the central limit theorem (Moyano, Tsallis and Gell-Mann, cond-mat/0509229). Then, we address the $q$-triplet recently determined in the solar wind (Burlaga and Vinas, Physica A {\bf 356}, 375 (2005)) and its possible relation with the space-dimension $d$ and with the range of the interactions (characterized by $\alpha$, the attractive potential energy being assumed to decay as $r^{-\alpha}$ at long distances $r$).Comment: 9 pages including 3 figures. Invited lecture at the International Conference on Complexity and Nonextensivity - New Trends in Statistical Mechanics (Yukawa Institute for Theoretical Physics, Kyoto, 14-18 March 2005). To appear in Prog. Theor. Phys. Supplement, eds. M. Sakagami, N. Suzuki and S. Ab