**Of Planets, Stars and Eternity**

After Newton’s great achievements, it seemed to scientists and philosophers of the Enlightenment that we should be able to predict everything about the future of the physical world from a knowledge of its present state. One of the areas where that should be easiest is to predict the future of the solar system, and indeed our ability to predict the motions of the planets in the short term is extremely good. But what about the long term? Can we even say whether our planets will someday be thrown far from the sun by the cumulative forces of gravitational perturbations?

This problem has occupied mathematicians since the 19th century, and has led to great advances in our understanding of dynamical systems — but the original question remains open. Prof. Villani spoke to the Forum about the study of the long time behavior of such systems from, from the Solar system itself to galaxies and related questions from fluid mechanics.

### Speaker Profile

**Dr. Cédric Villani** is a French mathematician working primarily on partial differential equations, Riemannian geometry and mathematical physics. He was awarded the Fields Medal in 2010 for his work on Landau damping and the Boltzmann equation. He described the development of his theorem in his autobiographical book *Théorème vivant* (2012), published in English translation as *Birth of a Theorem: A Mathematical Adventure* (2015).

After attending the Lycée Louis-le-Grand, Villani was admitted at the École normale supérieure in Paris and studied there from 1992 to 1996. He was later appointed as an assistant professor in the same school. He received his doctorate at Paris Dauphine University in 1998, under the supervision of Pierre-Louis Lions, and became professor at the École normale supérieure de Lyon in 2000. He is now professor at Lyon University. He has been the director of Institut Henri Poincaré in Paris since 2009.