Broken causal lens rigidity and sky shadow rigidity of Lorentzian manifolds
We prove that the topology, smooth structure, and metric of a compact Lorentzian manifold with boundary is uniquely determined by data at the boundary. The data consists of the lengths and directions of future-directed once-broken geodesics connecting points on the boundary, which are first timelike and then lightlike. This requires the strong causality condition and a weak convexity assumption, but it holds without any assumptions about conjugate points. With an additional convexity assumption we prove the analogous statement for future-directed once-broken timelike geodesics. If there are no conjugate points and lightlike geodesics never refocus, the analogous data using lightlike geodesics and once-broken lightlike geodesics may be used to reconstruct the manifold up to a conformal factor. This is a corollary of a result which shows that the conformal class is determined by the collection of sets of future-directed lightlike vectors at the boundary which give geodesics which all intersect in a single point.