Boltzmann Lectures

Ludwig Eduard Boltzmann, born in Wien on 20 February 1844, is considered the founding father of statistical physics. He developed the probabilistic laws that explain the physical properties of matter and the concepts of entropy and irreversibility.

The Boltzmann lectures are organized by the group of Statistical Physics at SISSA. Each year a distinguished guest is invited to give a talk to commemorate and celebrate the Austrain physicist on the occasion of his birthday.


22 February 2023

Prof.Fabian Essler: Eigenstate Thermalization and Conservation Laws.

How do isolated many-particle quantum systems relax and ultimately give rise to the laws of Statistical Mechanics is a question that has exercised theoretical physicists since the early days of Quantum Theory. Much of our current understanding is based on a set of ideas known as the Eigenstate Thermalization Hypothesis (ETH). The latter provides statistical information of properties of energy eigenstates of “generic” systems, viewed through the lens of “few-body” physical observables. After reviewing the basics of eigenstates thermalization I turn to the question what takes the role of the ETH for integrable many-particle quantum systems. These are characterized by the existence of a macroscopic number of conservation laws, which lead to a much richer structure of energy eigenstates at finite energy densities.

Fabian Essler is a condensed matter theorist who focuses on low dimensional quantum many-body systems. He has a long-standing interest in quantum integrable models and classical driven diffusive systems. He co-authored a monograph on the exact solution of the one dimensional Hubbard model. Some of his current research interests include non-equilibrium evolution in quantum many-body systems, finite-temperature dynamical response in integrable models, and stochastic quantum dynamics.

21 February 2022

Prof. Subir Sachdev: Statistical mechanics of metals without quasiparticles, and of charged black holes

The very successful theory of metals is based upon a Boltzmann equation for electronic quasiparticles. But the “strange metals” found in high temperature superconductors do not have well-defined quasiparticle excitations, which raises the problem of a theory of electrical transport in such metals. In their quantum theory of charged black holes, Gibbons and Hawking applied the Boltzmann-Gibbs ensemble to the Einstein-Maxwell theory, and obtained results for black hole entropy which did not have an evident interpretation in terms of the eigenstates of a quantum Hamiltonian. I will describe progress in resolving these long-standing problems in very different fields of physics using insights from the solvable Sachdev-Ye-Kitaev model of fermions with random interactions.

Subir Sachdev was educated at the Indian Institute of Technology, Delhi, the Massachusetts Institute of Technology, and Harvard University. He has held professional positions at Bell Labs, Yale University, and Harvard where he is now the Herchel Smith Professor of Physics. During 2021-22 he is also the Maureen and John Hendricks Distinguished Visiting Professor at The Institute for Advanced Study, Princeton.
He has been elected to national academies of science in India and the U.S. and is a recipient of a number of awards and honors which include the Dirac Medal from the International Center for Theoretical Physics, and the Lars Onsager Prize from the American Physical Society.

22 February 2021

Prof. Leticia Cugliandolo: Thermodynamic concepts out of equilibrium: from classical to quantum.

Statistical Mechanics is the theoretical tool that describes the equilibrium collective behaviour of macroscopic classical and quantum systems. Notably, it has explained full branches of physics such as thermodynamics and phase transitions. In so doing, it gave a microscopic meaning to so far phenomenological variables such as temperature and pressure. At present, there is no equivalent framework for generic out-of-equilibrium macroscopic systems. One is forced to analyse their dynamics on a case-by-case basis and try to extract from these studies, for example, variables that could behave as the thermodynamic ones. In this talk, I will give two seemingly different but timely examples of out of equilibrium systems: classical active matter and a quantum isolated spin chain. Although we may ignore the details of their asymptotic behaviour and their full statistical description, I will show that an effective temperature can be identified and used as a guideline to grasp at least some of the macroscopic properties of these problems. I will then explain how these results fit within ideas developed in the context of a glassy system and speculate about how one could build upon these results to improve our understanding of the physics far from equilibrium.

Leticia Fernanda Cugliandolo is a professor at Sorbonne Université and Institut Universitaire de France, Paris. She is an Argentine condensed matter physicist known for her research on non-equilibrium thermodynamics, spin glass, and glassy systems. She won the Prix Paul Langevin in 2002, and in the same year won the Marie Curie Award of the European Commission. She won the Irène Joliot-Curie Prize for female scientist of the year in 2015.

20 February 2020

Herbert Spohn: Hydrodynamics of Integrable Many-Body Systems

Integrable many-body systems have an extensive number of conservation laws. This implies that conventional hydrodynamics, based on mass, momentum, and energy, has to rewritten. Surprisingly, on large space-time scales the two particle phase shift is the only microscopic information retained (except for Bose, Fermi, classical). For the purpose of the talk, prof. Spohn will mainly focus on the classical Toda lattice.

Herbert Spohn is Professor (Emeritus) at the Technical University Munich. Outstanding mathematician and mathematical physicist, he has worked on kinetic equations, stochastic dynamics of particle systems and growths processes, hydrodynamic limits, disordered systems, and functional integration and stochastic analysis. He has obtained several prizes, among which the D. Heineman Prize for Mathematical Physics (2011) and the IUPAP Boltzmann Medal (2019).

20 February 2019

Paul Fendley: Between Order and Disorder

Boltzmann’s development of statistical mechanics gives a precise way of understanding the phases of matter as competition between energy and entropy. At high temperatures, entropy wins and disorder is favoured. At low temperatures, energy typically wins, favouring ordering, e.g. lining up spins in a ferromagnet. However, theoretical and experimental work in recent decades has shown that some quantum systems do not order, even at zero temperature. Rather, they exhibit topological order, where ordering patterns can be understood only by the behaviour at long distances. Fendley will describe what topological order is, along with a few of its remarkable physical properties.

Paul Fendley is a theoretical physicist studying statistical mechanics and related mathematics. One particular focus of his research is on novel behaviour emerging in many-body systems. He is currently Senior Research Fellow at All Souls College and Professor of Physics at the University of Oxford.

20 February 2018

Immanuel Bloch: Using Ultracold Quantum Gases to Probe New and Old Frontiers of Statistical Physics

Immanuel Bloch will address one of the fundamental questions in statistical physics: how to conciliate the laws of quantum mechanics for a macroscopic system – which predict a memory of the initial state of the system – with the familiar irreversible phenomena that bring any extended system to a thermal equilibrium, where all memory of the initial state is lost. Bloch has led outstanding experiments on this crucial theme. During his talk, he will present a series of new results on cold atom quantum systems made of mixtures of fermions, which lead to a physical phenomenon known as Many Body Localization Transition. Moreover, he will discuss the possibility to realize quantum systems with negative temperature in the laboratory.

Immanuel Bloch is Scientific Director and Managing Director of the Max Planck Institute of Quantum Optics, Garching, Germany. For his outstanding work on quantum many-body system studied by means ultracold atomic quantum gases on optical lattices he received many prizes and awards, including the European Physical Society Prize in 2011 for Fundamental Aspects of Quantum Electronics and Optics, the 2013 Körber European Science Prize and the 2015 Harvey Prize from Israel Technion Institute.
Image: Statistical Physics group.

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