Imaging black holes: a VLBI success story

Advances in Very Long Baseline Interferometry (VLBI) have enabled, for the first time, horizon-scale images of the two most accessible supermassive black hole candidates: M87* at the centre of the elliptical galaxy Messier 87 and Sgr A* at the centre of the Milky Way. These images reveal ring-like emission surrounding central brightness depressions whose angular sizes are consistent with the black-hole signatures predicted by general relativity. The theoretical framework and early predictions of observable black hole signatures, developed from the 1970s onward, established the scientific motivation and defined the instrumental requirements for this endeavour. Achieving the required angular resolution and sensitivity demanded transformational enhancements to existing VLBI arrays, pursued along two complementary paths: space VLBI, extending baselines beyond the Earth, and mm-VLBI, observing at the shortest accessible radio wavelengths from the ground. The latter strategy, realised through the addition of the Atacama Large Millimeter Array to a global network of mm-wavelength telescopes, advances in receiver technology, data processing, and image reconstruction methods, and the formation of an international collaboration, yielded the first images of the predicted black hole shadows. This established the observational foundation for a broad programme of follow-up studies.