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. This article illustrates this point by focusing on fundamental objects in cities: the distribution of urban population, segregation phenomena and spin like models, the polycentric transition of the activity organization, energy considerations about mobility and models inspired by gravity and radiation concepts, CO2 emitted by transport, and finally scaling that describes how various socio-economical and infrastructures evolve when cities grow. ...

Effective field theory is one of the deepest and most useful guiding principles in physics. Its tools and methods allow one to study the universal aspects of entire classes of unknown microscopic models, with their main features being captured by symmetries and few relevant parameters of the effective degrees of freedom. Because of its universality, it finds  applications across all scale in physics: from super-Hubble scales all the way to the Planck length. It is successfully applied to cosmology to describe the early cosmic inflation, the current cosmic acceleration, the dynamics of the large scale structure and the dark matter. Its methods have found recent applications in the theory of gravitational wave emission by binary inspirals, and in the new multimessanger astrophysics: new and conceptually compelling ways to perform calculations and predictions are being developed by making contact with the methods of scattering amplitudes of particle physicists. ...

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.
 
To give an idea of the size of the problem, consider a word count of everything ever written by human beings (from cuneiform tablets and stone carvings to books, magazines, emails, etc). This was estimated at one time to be some 40 quadrillion words. ...