Kac’s walk on $n$-sphere mixes in $n\log n$ steps

Abstract

Determining the mixing time of Kac’s random walk on the sphere $\mathrm{S}^{n-1}$ is a long-standing open problem. We show that the total variation mixing time of Kac’s walk on $\mathrm{S}^{n-1}$ is between $\frac{1}{2}n\log(n)$ and $200n\log(n)$ for all $n$ sufficiently large. Our bound is thus optimal up to a constant factor, improving on the best-known upper bound of $O(n^{5}\log(n)^{2})$ due to Jiang [Ann. Appl. Probab. 22 (2012) 1712–1727]. Our main tool is a “non-Markovian” coupling recently introduced by the second author in [Ann. Appl. Probab. 24 (2014) 114–130] for obtaining the convergence rates of certain high dimensional Gibbs samplers in continuous state spaces.