Plenary Colloquia SIFS
La SIFS organizza dei Plenary Colloquia online su temi vari connessi alla Fisica Statistica, come attività culturale e momento di condivisione per tutti i soci.
I seminari sono organizzati con cadenza mensile e sono poi resi disponibili online tramite il canale Youtube ufficiale della SIFS, per chi fosse interessato e non riuscisse a seguirli in diretta.
I colloquia sono tenuti online sulla piattaforma Microsoft Teams.
Istruzioni per partecipare ai Colloquia
prima dell'orario stabilito per il colloquium verrà pubblicato nella Home del sito web della SIFS e su questa pagina, il link per poter accedere al meeting su Teams;
chiunque può accedere al meeting semplicemente cliccando sul link, anche in assenza di un account Teams o Microsoft. Consigliamo fortemente l'uso di Google Chrome che è completamente supportato da Teams, mentre altri browsers potrebbero dare problemi;
una volta cliccato sul link è sufficiente seguire le indicazioni, inserendo un nome per essere individuabili nella riunione. Vi chiediamo di accedere alla riunione spegnendo videocamera e microfono prima di entrare o immediatamente, in modo da alleggerire la piattaforma;
il colloquium verrà registrato, per essere reso accessibile successivamente tramite il canale Youtube SIFS: partecipando alla riunione date il consenso per la registrazione;
il colloquium durerà circa 1 ora, divisa in circa 50 minuti di seminario e 10 minuti di domande. Le domande possono essere formulate tramite la chat della riunione e verranno poste allo speaker solo al termine della presentazione.
Optimal Transport in Plants and Forests
Amos Maritan, Department of Physics and Astronomy, University of Padova
Abstract: Forests represent one of the most complex systems with a high degree of structural and functional diversity.
Water transport through plants is a key driver of the carbon and other biogeochemical cycles and is a crucial link in plant adaptation to climate and vegetation response to climate change.
First, we discuss the shape of the xylems, the conduits responsible for transporting water to the leaves: wider conduits transport more water but are more vulnerable to conduction-blocking gas embolisms, and cost more for a plant to build, a tension necessarily shaping xylem conduit diameters along plant stems. A solution based on Pareto multi-optimization leads to the prediction of a universal shape of xylems which will be compared with data spanning terrestrial plant orders, life forms, habitats, and sizes.
In the second part we will present an optimization principle leading to the prediction of the total evapotranspiration of water in single plants and its consequences in the tree size distribution in natural plant communities, an issue of great importance for carbon storage. Predictions are tested in forests at various latitudes and deviations from the predictions are used to quantify degrees of disturbances.
Date: April 29, 2021 - 16.30 (Italian time)
Manipulating molecules one at a time: from energy to information
Felix Ritort, Small Biosystems Lab, Departament de Física de la Matèria Condensada, Facultat de Física, Universitat de Barcelona
Abstract: “Take a single DNA molecule and pull from its extremities while recording the force-extension curve until it gets fully straightened.” This thought experiment, which was just a dream a few decades ago, has become standard in many research labs worldwide. Force spectroscopy techniques (such as laser optical tweezers) are a fabulous tool for manipulating and monitoring biological molecules' direct action at the individual level. By measuring forces in the piconewton range and energies roughly equivalent to 1kT, we have the experimental accuracy in resolving the thermal energy unit. Recent advances in such technologies combined with theoretical developments in nonequilibrium physics offer the exciting prospect of experimentally testing fundamental physical principles and concepts in single-molecule experiments .
This talk will illustrate laser optical tweezers for single-molecule manipulation and some of the main applications in physics and biology. In particular, I will also present results on a Maxwell demon's experimental realization using single DNA molecules pulled under feedback protocols as a pioneering example of the thermodynamics of data processing and information .
1. F. Ritort, The noisy and marvelous molecular world of biology, Inventions, 4(2) (2019) 24
2. M. Ribezzi-Crivellari and F. Ritort, Large work extraction and the Landauer limit in the Continuous Maxwell Demon, Nature Physics 15 (2019) 660–664
Date: March 25, 2021 - 16.30 (Italian time)
Anomalous transport in classical one-dimensional systems
Antonio Politi, Institute for Complex Systems and Mathematical Biology, University of Aberdeen
Abstract: Transport properties of one-dimensional systems are reviewed, starting from the definition of heat flux, recently revisited to make it correct also over microscopic scales.
Date: February 25, 2021 - 16.30 (Italian time)
Spontaneous vs. stimulated brain activity: A statistical physics approach
Lucilla De Arcangelis, University of Campania "Luigi Vanvitelli"
Abstract: The understanding of the fundamental relation between brain resting activity and the response to stimuli is a long-standing fascinating question. Recent experiments have shown that the spontaneous brain activity is characterized by avalanches showing absence of characteristic size, result successfully interpreted in the context of criticality. However, in order to support the idea that the brain acts close to a critical point it is crucial to evidence the existence of long-range correlations. The temporal organization of neuronal avalanches in the rat cortex in vitro is characterized by a complex organization, leading to the characteristic brain oscillations and a dynamic balance between excitation and inhibition. The question concerning the relation between spontaneous and evoked activity is addressed by means of the coarse-grained Wilson Cowan model. An approach inspired in non-equilibrium statistical physics allows to derive a fluctuation-dissipation relation, suggesting that measurements of the correlations in spontaneous fluctuations in the brain activity alone could provide a prediction for the system response to a stimulus. Theoretical predictions are in good agreement with MEG data for healthy patients performing visual tasks.
Date: February 4, 2021 - 16.30 (Italian time)
2020 Plenary Colloquia
Extreme Values Statistics: An overview and perspectives
Satya Majumdar, Laboratoire de Physique Theorique et Modeles Statistique (LPTMS), Université de Paris-Sud (Orsay)
Abstract: Extreme value statistics (EVS) concerns the study of the statistics of the maximum or the minimum of a set of random variables. This is an important problem for any time-series and has applications in climate, finance, sports, all the way to physics of disordered systems where one is interested in the statistics of the ground state energy. While the EVS of 'uncorrelated' variables is well understood, little is known for strongly correlated random variables. Only recently this subject has gained much importance both in statistical physics and in probability theory. In this talk, I will give an overview and perspectives on this interdisciplinary and rapidly evolving area of research.
Date: December 3, 2020 - 16.30 (Italian time)
Absolute Negative Mobility: can it take place in thermal equilibrium?
David Mukamel, Weizmann Institute of Science
Abstract: Absolute Negative Mobility (ANM) is a phenomenon whereby current in a stationary system is in a direction opposite to the driving field. Naïve argument suggests that ANM cannot take place in systems in thermal equilibrium as this could lead to a violation of the second law of thermodynamics. Thus numerous previous theoretical and experimental studies of ANM have dealt with the response to a driving field in nonequilibrium steady states. In this talk a simple lattice model of a driven tracer is introduced and demonstrated to exhibit ANM in equilibrium, with no violation of the basic laws of thermodynamics. The limits of validity of the naïve argument are elucidated and the entropy production which accompany the motion of the tracer is calculated.
Date: November 19, 2020 - 16.30 (Italian time)
How amorphous materials yield under stress: a new kind of out of equilibrium phase transition
Giulio Biroli, Ecole Normale Supérieure
Abstract: I will show that a new kind of out of equilibrium phase transition is at the core of how amorphous solids yield in response to external deformations—a phenomenon that is crucial both for practical applications and for theoretical reasons. Such phase transition has strong connections with phenomena studied in the theory of disordered systems such as the zero-temperature spinodal of the Random Field Ising Model, the Depinning transition, and rare events.
Our results unveil that despite large differences in the materials’ microscopic interactions and typical scales (from colloids to molecular glasses) a large degree of universality emerges as there are only two ways in which amorphous solids respond to a deformation: One, typical of well-annealed materials, is characterized by an abrupt failure with a macroscopic stress drop and the sudden emergence of sharp shear bands; the other, typical of poorly annealed materials, shows merely a smooth crossover. By varying the preparation protocol, one can change the response of a given material from one to the other, and this change is controlled by a random critical point, akin to the one of the zero temperature Random Field Ising Model.
Date: October 29, 2020 - 16.30 (Italia time)
Rough landscapes and glass dynamics: from inference to machine learning
Chiara Cammarota, Università di Roma "La Sapienza", King's College London
Abstract: The realm of statistical mechanics has been enlarged to describe systems, such as glass forming materials, where structural disorder plays the predominant role. Interestingly the spectrum of applications of this new physics goes much beyond the scope of condensed matter and extends to the currently booming field of data science. In this colloquium I will focus on the challenge of signal-reconstruction from noisy collections of data, omnipresent in machine learning applications and in classical inference problems. By leveraging tools and ideas from glass physics, I will show how we can describe, predict, and enhance the performances of algorithms introduced to tackle these reconstruction problems.
Date: October 15, 2020 - 16.30 (Italian time)
(Non equilibrium) thermodynamics of classical Integrable models
Leticia F. Cugliandolo, Sorbonne Université, Laboratoire de Physique Théorique et Hautes Energies, Institut Universitaire de France
Abstract: Motivated by recent experimental developments in atomic physics, a large theoretical effort has been devoted to the analysis of the quench dynamics of quantum isolated systems. In this talk I will describe the evolution of a family of classical many-body integrable models after instantaneous quenches of the same kind. My aim will be to distinguish the effects of the quantum/classical nature of the systems from those of the interactions, be them external and due to the environment, or internal (integrability vs. non-integrability). I will also show that the asymptotic dynamics of these models can be fully elucidated, and the stationary properties compared to the ones obtained using a Generalised Gibbs Ensemble. The latter can not only be built but also used to evaluate analytically all relevant observables, a quite remarkable fact for an interacting integrable system.
Date: October 1, 2020 - 16.30 (Italian time)
Simplicial complexes and dynamics
Ginestra Bianconi, School of Mathematical Sciences, Queen Mary University of London
Abstract: Networks are everywhere and they describe a large variety of complex systems such as social networks and the brain. In the last few decades, unveiling the underlying architecture of complex systems using the network approach has been key to reveal how the statistical properties of network structure affect dynamics, including most notably epidemic spreading and network robustness. Recently is has been realized that many complex systems such as brain networks and social networks include interactions among two or more nodes. These complex systems cannot be captured by networks including exclusively pairwise interactions, rather these systems should be represented by higher-order networks such as simplicial complexes.
In this talk I will show that taking into account higher-order interactions and combining network theory with topology can greatly enhance the ability to predict the function of complex systems starting from their structure.
I will overview recent results on the interplay between network topology and dynamics focusing on percolation and on synchronization phenomena.
A new topological approach  to synchronization on simplicial complexes will be presented. Here the theory of synchronization is combined with topology (specifically Hodge theory) for formulating the higher-order Kuramoto model that uses the higher-order Laplacians and provides the main synchronization route for topological signals. I will show that the dynamics defined on links can be projected to a dynamics defined on nodes and triangles that undergo a synchronization transition.
This model can be applied to study synchronization of topological signals in the brain and in biological transport networks as it proposes a new set of topological transformations that can reveal collective synchronization phenomena that could go unnoticed otherwise.
 Millán AP, Torres JJ, Bianconi G. Explosive higher-order Kuramoto dynamics on simplicial complexes. Physical Review Letters. 2020 May 27;124(21):218301
Date: July 16, 2020 - 16.30 (Italian time)
The thermodynamic uncertainty relation
Udo Seifert, Institute for Theoretical Physics, University of Stuttgart
Abstract: The thermodynamic uncertainty relation discovered in 2015 is arguably one of the most promising insights arising from stochastic thermodynamics. It relates the mean and fluctuations of any current to the overall entropy production in a non-equilibrium steady state. It provides a lower bound on the inevitable cost of temporal precision of processes, leading, e.g. to the minimal cost for measuring time in a finite temperature environment. As a tool for thermodynamic inference, it gives a model-free universal upper bound on the efficiency of molecular motors in terms of experimentally accessible observables. Current challenges to the theory include the still missing proof for underdamped dynamics and extensions to periodically and time-dependently driven systems.
Date: June 25, 2020 - 16.30 (Italian time)
A perspective view on fully developed turbulence
Roberto Benzi, Università Roma "Tor Vergata"
Abstract: In this talk I discuss some basic physical properties of three dimensional fully developed turbulence. In particular, I will focus on the case of homogeneous and isotropic turbulence (H.I.T). This is a relatively narrow view of turbulence when compared to the enormous number of problems where turbulence plays a significant role. In fact, there exist many different turbulent "problems". Nevertheless, in the case of H.I.T there has been a remarkable scientific effort over the last few decades which, probably for the first time, provided a well defined theoretical framework with a significant agreement against laboratory and/or numerical data. The aim of this talk is to review this effort and to illustrate our present knowledge on the problem.
Date: May 22, 2020 - 16.30 (Italian time)