Entropy and Translation Length in the Ray Graph

Abstract

Let $Γ$ be the mapping class group of the plane minus a Cantor set, acting on the ray graph $\mathcal{R}$, and for $γ\in Γ$ let $τ(γ)$ denote the translation length of $γ$ on $\mathcal{R}$. We prove that $τ(γ) \le h(f)/\log(2)$ for every $C^\infty$ diffeomorphism $f$ of $S^2$ representing $γ$, where $h$ denotes topological entropy. The proof falls into two parts: a combinatorial argument bounding distance between two rays in $\mathcal{R}$ by the logarithm of the geometric intersection number; and a geometric argument that promotes geometric control (coarse length of iterates of a fixed ray) to combinatorial control (geometric intersection number). We conjecture that the $C^\infty$ hypothesis on $f$ can be removed.