Rolling spheres and the Willmore energy

The Willmore energy plays a central role in the conformal geometry of surfaces in the conformal 3-sphere $S^3$. It also arises as the leading term in variational problems ranging from black holes, to elasticity, and cell biology. In the computational setting a discrete version of the Willmore energy is desired. Ideally it should have the same symmetries as the smooth formulation. Such a Möbius invariant discrete Willmore energy for simplicial surfaces was introduced by Bobenko. In the present paper we provide a new geometric interpretation of the discrete energy as the curvature of a rolling spheres connection in analogy to the smooth setting where the curvature of a connection induced by the mean curvature sphere congruence gives the Willmore integrand. We also show that the use of a particular projective quaternionic representation of all relevant quantities gives clear geometric interpretations which are manifestly Möbius invariant.