General relativity, Einstein's theory of gravity, is best tested at its most extreme—close to the event horizon of a black hole. This regime is accessible through observations of shadows of supermassive black holes and gravitational waves—ripples in the fabric of our Universe from colliding stellar-mass black holes. For the first time, scientists from the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav), the Event Horizon Telescope (EHT) and the LIGO Scientific Collaboration, have outlined a consistent approach to exploring deviations from Einstein's general theory of relativity in these two different observations. This research, published in Physical Review D, confirms that Einstein's theory accurately describes current observations of black holes, from the smallest to the largest.