Lying at the intersection of physics and astronomy, black holes provide an ideal laboratory to study some of the most fundamental questions regarding the physical laws of the universe. One test of Einstein’s General Relativity is gravitational redshift. AGN broad emission lines arise from a region close to the black hole where gas is photo-ionized. The proximity of the broad line region to the black hole causes significant special- and general-relativistic effects on the emission lines. For more info, see paper.