The flow of two falling balls mixes rapidly

In this paper we study the system of two falling balls in continuous time. We model the system by a suspension flow over a two dimensional, hyperbolic base map. By detailed analysis of the geometry of the system we identify special periodic points and show that the ratio of certain periods in continuous time is Diophantine for almost every value of the mass parameter in an interval. Using results of Melbourne (2007 Trans. Am. Math. Soc. 359 2421–41) and our previous achievements (Balint et al 2012 Chaos 22 026104) we conclude that for these values of the parameter the flow mixes faster than any polynomial. Even though the calculations are presented for the specific physical system, the method is quite general and can be applied to other suspension flows, too.