In this review Marc Barthelemy describes how statistical physics helps understand some of the key aspects of cities. Modelling the structure and evolution of cities became critical because policy makers need robust theories and new paradigms for mitigating problems arising with their growth. Fortunately, the increased data available about urban systems opens the possibility of constructing a quantitative ‘science of cities’ with the aim of identifying and modelling essential phenomena. Statistical physics plays a major role in this effort by bringing tools and concepts able to bridge theory and empirical results.

Organizing commitee: Brando Bellazzini, John Joseph Carrasco and Filippo Vernizzi

Single (top) vs double copy (bottom) theory This past year a team at IPhT around John Joseph Carrasco — in collaboration with physicists at UCLA and Penn State University in the United States, and Uppsala University in Sweden — completed a calculation long thought to be impossible: the direct understanding of the dimensions of space-time in which the most symmetric particle-based field theory of gravity (maximal supergravity) would cease to be predictive when quantum effects are relevant. One can isolate order by order the quantum effects in graviton scattering. For two gravitons scattering off of each other, this calculation required consideration of the fifth-order quantum correction. This puts strong constraints on the behavior of fundamental symmetries in controlling the high-energy behavior of this theory.