## Between Order and Disorder

On Wednesday 20 February SISSA celebrates Ludwig Eduard Boltzmann’s birthday with a lecture by physicist Paul Fendley entitled “Between Order and Disorder”. The event will take place in room 128-129 at 2.30 pm.

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.

Ludwig Eduard Boltzmann, born in Wien on 20 February 1844, is considered the founding father of statistical physics. The *Boltzmann Lecture* is a project promoted by the group of Statistical Physics at SISSA and, in particular, by the coordinator Pasquale Calabrese and Giuseppe Mussardo, author of the documentary film “Boltzmann. The genius of disorder” (2006).

Image: Giuseppe Mussardo