events

3rd NEXT International Conference: News Expectations and Trends in Statistical Physics,13-18 August 2005, Kolimbari - Crete, Greece
Complexity and Nonextensivity: New Trends in Statistical Mechanics , 14-18 March 2005, Yukawa Institute for Theoretical Physics - Kyoto, Japan.
Complexity, Metastability and Nonextensivity 20-26 july 2004 Centre E. Majorana, Erice, Sicily, Italy

News and Expectations in Thermostatistics NEXT03 21-28 september 2003 Villasimius - Cagliari, Italy

MBS2003 in honour of Marcello Baldo for 60th birthday, 6-8 october 2003 Catania

Trends and perspectives in extensive and non-extensive statistical mechanics in honour of Constantino Tsallis for his 60th birthday, 19-21 november , 2003, Angra dos Reis, Brazil.

The standard central limit theorem mathematically grounds Boltzmann-Gibbs statistical mechanics. Recently proved central limit theorems constitute the corresponding mathematical grounding of nonextensive statistical mechanics. These theorems will be presented. In addition to this, recent experimental confirmations of this theory will be presented as well. Bibliography: (i) J.P. Boon and C. Tsallis, eds., Europhysics News 36 (6) (European Physical Society, Nov/Dec 2005); (ii) L.G. Moyano, C. Tsallis and M. Gell-Mann, Europhys. Lett. 73, 813 (2006); S. Umarov, C. Tsallis and S. Steinberg, cond-mat/0603593; (iii) S. Umarov, C. Tsallis, M. Gell-Mann and S. Steinberg, cond-mat/0606038 and cond-mat/0606040; (iv) P. Douglas, S. Bergamini and F. Renzoni, Phys. Rev. Lett. 96, 110601 (2006).

In nonextensive statistical mechanics, two kinds of definitions have been considered for expectation value of a physical quantity: one is the ordinary definition and the other is the so-called qexpectation value employing the escort distribution. In this lecture, it is shown that the correct one is the use of the q-expectation value. The proof is based on the Shore-Johnson theorem for consistency between the maximum entropy principle and the minimum cross-entropy (i.e., relative-entropy) principle.

The recently proposed time-average approach to thermodynamics is reexamined and generalized to nonextensive thermostatistics. It is shown that the Tsallis entropy, S_q, is always temporally extensive, that is, the production rate of S_q is constant, whenever physically relevant. The universal upper bound is also derived for such production rate.

Si discute la stabilita\u2019 del moto quantistico (fidelity) in relazione alle proprieta\u2019 dinamiche del sistema. Verra\u2019 poi esaminato il ruolo giocato dal caos dinamico nel processo di decoerenza quantistica in assenza di interazione con l\u2019ambiente esterno. Attraverso un semplice modello si mostra che il decadimento dell\u2019eco di Loschmidt quantistico (fidelity) e\u2019 determinato dal decadimento di una funzione di correlazione classica. Infine si presentano i risultati di una simulazione numerica dell\u2019esperimento delle due fenditure che mostrano come il caos interno distrugga le frange di interferenza.

Gravitational Structure Formation, the Cosmological Problem and Statistical Physics

Models of structure formation in the universe postulate that matter distributions observed today in galaxy catalogs arise, through a complex non-linear dynamics, by gravitational evolution from a very uniform initial state. Dark matter plays the central role of providing the primordial density seeds which will govern the dynamics of structure formation. We critically examine the role of cosmological dark matter by considering three different and related issues: Basic statistical properties of theoretical initial density fields, several elements of the gravitational many-body dynamics and key correlation features of the observed galaxy distributions are discussed, stressing some useful analogies with known systems in modern statistical physics. In these different contexts we focus on the role of dark matter in standard cosmological models of structure formation, stressing the crucial observational tests and several theoretical open problems

Lo studio dei fenomeni indotti dal rumore in sistemi fisici non lineari e lontani dall'equilibrio è uno dei recenti approcci usati per la comprensione dei sistemi complessi, di natura fisica e biologica. Verranno introdotti brevemente i seguenti fenomeni indotti dal rumore: (i) risonanza stocastica, (ii) attivazione risonante, e (iii) stabilità indotta dal rumore. Possibili applicazioni interdisciplinari di un tale approccio sono: a) la dinamica di una giunzione Joesphson, b) il modello di FitzHugh-Nagumo per la dinamica neuronale, e c) la dinamica di popolazioni (sistemi ecologici marini, crescita di cellule cancerogene, etc..).

Search engines have become key media for our scientific, economic, and social activities by enabling people to access information on the Web in spite of its size and complexity. On the down side, search engines bias the traffic of users according to their page-ranking strategies, and some have argued that they create a vicious cycle that amplifies the dominance of established and already popular sites. We show that, contrary to these prior claims and our own intuition, the use of search engines actually has an egalitarian effect,redirecting more traffic toward less popular sites, even in comparison to what would be expected from users randomly surfing the Web.

Dr. Andrea Carati
Dipartimento di Matematica, Università di Milano, Italy

Abstract

In meccanica statistica si mostra come sia possibile, utilizzando il concetto di medie di insieme, recuperare i principi fondamentali della termodinamica di equilibrio. In questo seminario si vuole mostrare come sia possibile derivare il primo ed il secondo principio usando direttamente il concetto di media temporale, senza passare attraverso il concetto di media di insieme, quindi senza basarsi su nessuna proprietà ergodiche della dinamica. In particolare si mostra come si possa ritrovare l'usuale espressione di Boltzmann per l'entropia sotto opportune ipotesi sulla dinamica. Si mostra anche come dinamiche diverse diano origine ad espressioni diverse per l'entropia.

Prof. Luigi Galgani
Dipartimento di Matematica, Università di Milano, Italy

Abstract

The original FPU numerical result consisted in exhibiting numerical results in which a classical dynamical system (a model of a one-dimensional crystal) appeared to relax to a state different from the one expected according to equilibrium statistical mechanics (energy equipartition among the normal modes). Such a result was generally considered to be a kind of paradox, and many attempts were thus made with the aim of proving that the phenomenon of nonequipartition disappears in situations of physical interest (namely, in the thermodynamic limit). Here it is discussed how the FPU phenomenon is interpreted within a metastability scenario, which was proposed already in the year 1982 by a group of people around Parisi (Fucito et al), and was recently supported by a group of people in Milano, with results holding in the thermodynamic limit (here, a reinterpretation of the works of Zabusky and Kruskal on the KdV equation does play a relevant role). In particular it is shown how such a scenario allows one to understand a relevant physical phenomenology concerning the times that are required when concrete measurement of the specific heats are performed, in the region where they start decreasing toward zero. A short discussion is also given of the FPU problem in dimensions higher than one.

I will discuss in this talk a simple spin model made up of 'up' and 'down' spins to show that the microcanonical entropy can be a nonconcave function



of the mean energy. This will provide a counterexample to what is claimed in most textbooks of statistical mechanics, namely that entropy is always



concave. As a result of the nonconcavity of the entropy, the system has equilibrium properties within the microcanonical ensemble that have no



equivalent in the canonical ensemble (phenomenon of nonequivalent ensembles). I will show that these nonequivalent microcanonical properties



can be calculated by resorting to a generalized version of the canonical ensemble which restores equivalence with the microcanonical ensemble.

Co-expression of statistically over-represented peptides in proteomes: a
key to phylogeny ?

It is proposed that the co-expression of statistically significant motifs among the sequences of a proteome is a phylogenetic trait. From the co-expression matrix of such motifs in a group of prokaryotic proteomes a suitable definition of a phylogenetic distance is introduced and the corresponding distance matrix between proteomes is constructed. From the distance matrix a phylogenetic tree is inferred, following a standard procedure. It compares well with a reference tree, deduced from a distance matrix obtained from the alignment of ribosomal RNA sequences. Our results are consistent with the hypothesis that biological evolution manifestsitself with a modulation of basic correlations between shared peptides of short length, present in protein sequences. Moreover, the simple procedure we propose reconfirms that it is possible, sampling entire proteomes, to average the effects of lateral gene transfer and infer reasonable phylogenies.

Combined correlation integral and spatio-temporal correlations among earthquakes

Scale invariant properties of seismicity in time, space and energyargue for the presence of complex triggering mechanisms where, like a cascade process,each event produces aftershocks. We propose a novelmethod of data analysis that, based on the space-time combinedgeneralization of the correlation integral leads to a self-consistent visualization and analysis of both spatial andtemporal correlations. When analyzing global seismicity we discovered a universal relationlinking the spatial Influence Length of a given earthquake to the time elapsed from theevent itself. Following an event, time correlations (i.e. causality effects) exist in a region that shrinks over time. A different process is acting in the short-rangewhere events are randomly set in space, evidencing a sub-diffusive growth of the seismogenic zone.

Dipartimento di Fisica e Astronomia - Venerdi 2 luglio - Aula I ore 11.00

A topological perspective of the glass transition

Centro SMC - INFM
Dipartimento di Fisica, Universita' di Roma La Sapienza
irene.giardina@roma1.infn.it

The energy landscape approach to supercooled liquids has gainedmuch attention in the recent years. Its main aim is to connect thetopological features of the potential energy surface to the dynamicalbehaviour of the system. While at low temperature the equilibriumlandscape is dominated by minima, it turns out that at higher temperaturesthe saddles (i.e. stationary points with unstable modes) are the relevant topological objects. In this context the dynamical slowing down occurringat the Mode Coupling temperature can be associated to a change in the topology of the equilibrium landscape.In this seminar I will revise our main results in this field, I will show how and to what extent a topological analysis of glassy systemscan be pursued, and what are the main open questions to be addressed in the future.

The main technical and computationa aspects of DNA microchips will be discussed.

Department of Physics, University of Firenze,
and Istituto Nazionale di Ottica Applicata, Italy
e-mail arecchi@ino.it

The central issue of cognitive science is how a large collection of coupled neurons combines external data with internal memories into new coherent patterns of meaning. Such a process is called "feature binding", insofar as the coherent patterns combine together features which are extracted separately by specialized cells, but which do not make sense as isolated items. A powerful conjecture ,with experimental confirmation, is that feature binding in perceptual tasks implies the mutual synchronization of axonal spike trains in neurons.
Based on recent laboratory investigations of homoclinic chaotic systems, and how they mutually synchronize by weak coupling, a novel conjecture on the dynamics of the single neuron is formulated. Homoclinic chaos implies the recurrent return of the dynamical trajectory to a saddle focus, in whose neighbourhood the response to an external perturbation is very high and hence it is very easy to lock to an external stimulus. Thus homoclinic chaos appears as the easiest way to encode information in time by a code consisting of trains of equal spikes.
A time code based on a synchronized sequence of spikes requires a decision time sufficiently longer than the minimal interspike separation t1 , so that the total number of different set elements is related in some way to the size / t1. In the brain while .
In a sensory layer of the brain cortex an external stimulus localized at some input spreads over a large assembly of coupled neurons building up a collective state univocally corresponding to the stimulus, thus synchronization of trains of different individual neurons is the basis of a coherent perception.
The percept space can be given a metric structure by introducing a distance measure, so that, at least in principle, the set of different perception can be classified.
The distance in percept space is conjugate of the duration time in the sense that an uncertainty relation in percept space is associated with time limited perceptions.
This coding of different percepts by synchronized trains of spikes entails a fundamental quantum limitation. If the synchronized train is truncated at a time , then the corresponding identification of a percept P is not fully accomplished, but it carries an uncertainty cloud around P . As the two uncertainty clouds around two distinct percepts overlap, interference occurs; this is a quantum behavior. Thus the quantum formalism is not exclusively limited to microscopic phenomena, but here it is conjectured to be the appropriate description of truncated perceptions.
The quantum feature here explored is uniquely tied up to the coding and reading operations which take place in passing from external signals on the sensor transducers to complete perceptions driving an action; it is not related to Planck's h-bar but to the details of the perceptual chain.