  The Event Horizon Telescope image of the supermassive black hole in the
galaxy M87 is dominated by a bright, unresolved ring. General relativity
predicts that embedded within this image lies a thin "photon ring," which is
composed of an infinite sequence of self-similar subrings that are indexed by
the number of photon orbits around the black hole. The subrings approach the
edge of the black hole "shadow," becoming exponentially narrower but weaker
with increasing orbit number, with seemingly negligible contributions from high
order subrings. Here, we show that these subrings produce strong and universal
signatures on long interferometric baselines. These signatures offer the
possibility of precise measurements of black hole mass and spin, as well as
tests of general relativity, using only a sparse interferometric array.
