Very long baseline interferometry as a precision strong gravity instrument

We are entering the era of precision black hole astronomy. Thanks to gravitational wave interferometers like LIGO-Virgo-KAGRA and potential first-generation space-based detectors like LISA, the next few decades will provide direct gravitational probes of supermassive black holes at horizon scales. However, gravitational wave detectors are not alone. Thanks to the unprecedented resolution of very-long-baseline interferometry (VLBI) instruments like the Event Horizon Telescope and potential space extensions like the Black Hole Explorer, black hole horizon-scale features (e.g., the photon sphere and inner shadow) are expected to come into focus within the next decade. In this letter, we will highlight the potential for precision measurements of black holes using VLBI and how it can provide an independent probe of these objects.