Seminarium Teorii Względności i Grawitacji

The optical properties of spacetimes are often studied using null congruences, i.e. 2- or 3-parameter families of null geodesics. I will present in my talk a different, more general approach, based on the bi-local geodesic operators (or Jacobi propagators). I will focus on a number of interesting results you can obtain in a fairly simple way in the bi-local formalism, including an inequality between the parallax distance and angular diameter distance, valid in any spacetime satisfying the null energy condition.