  In Haller and Beron-Vera (2013) we developed a variational principle for the
detection of coherent Lagrangian vortex boundaries. The solutions of this
variational principle turn out to be closed null-geodesics of the Lorentzian
metric associated with a generalized Green-Lagrange strain tensor family. This
metric interpretation implies a mathematical analogy between coherent
Lagrangian vortex boundaries and photon spheres in general relativity. Here we
give an improved discussion on this analogy.
