Differential geometry of rigid bodies collisions and non-standard billiards

The configuration manifold M of a mechanical system consisting of two unconstrained rigid bodies in ℝⁿ is a manifold with boundary (typically with singularities.) A full description of the system requires boundary conditions specifying how orbits should be continued after collisions, that is, the assignment of a collision map at each tangent space on the boundary of M giving the post-collision state of the system for each pre-collision state. We give a complete description of the space of linear collision maps satisfying energy and (linear and angular) momentum conservation, time reversibility, and the natural requirement that impulse forces only act at the point of contact of the colliding bodies. These assumptions are stated geometrically in terms of a family of vector subbundles of the tangent bundle to ∂M: the diagonal, non-slipping, and impulse subbundles. Collision maps are shown to be the isometric involutions that restrict to the identity on the non-slipping subspace. We then make a few observations of a dynamical nature about non-standard billiard systems, among which is a sufficient condition for the billiard map on the space of boundary states to preserve the canonical measure on constant energy hypersurfaces.