Black Hole Imaging and the Event Horizon Telescope: From Theory to Observation—Shadows, Photon Rings, and Tests of General Relativity

On April 10, 2019, the Event Horizon Telescope (EHT) collaboration released humanity’s first direct image of a supermassive black hole: M87*, located at the center of the galaxy Messier 87, approximately 53 million light-years from Earth. Three years later, on May 12, 2022, the collaboration revealed the first image of Sagittarius A* (Sgr A*), the four-million-solar-mass black hole at the center of the Milky Way. These landmark achievements represent the culmination of decades of theoretical developments, technological advances in Very Long Baseline Interferometry (VLBI), and unprecedented international scientific cooperation involving more than 300 researchers from 80 institutions worldwide. This paper provides a comprehensive examination of the theoretical foundations and observational techniques that enable black hole imaging. We emphasize key relativistic phenomena such as gravitational light bending, photon spheres, event horizons, and the formation of black hole shadows. In particular, we explain why the observed shadow radius is approximately 2.6 times the Schwarzschild radius rather than a direct image of the event horizon itself, discuss the infinite replication of images due to multiple photon orbits, and analyze relativistic effects including Doppler beaming and gravitational lensing of emission from accretion flows. We review the VLBI methodology that effectively transforms Earth into a planet-sized telescope with angular resolution approaching 25 microarcseconds, address the formidable computational challenges involved in correlating and imaging petabytes of data, and clarify why imaging Sgr A* proved substantially more difficult than imaging M87*, despite its much closer distance. The paper presents and compares results from both M87* and Sgr A* observations, highlighting their implications for testing general relativity in the strong-field regime, constraining black hole masses and spins, and probing alternative theories of gravity. The striking similarity between the images of M87* and Sgr A*, despite their vastly different mass scales—6.5 billion solar masses versus 4 million—provides compelling evidence for the universality of general relativity in governing black hole physics. Finally, we discuss future prospects, including the next-generation Event Horizon Telescope (ngEHT), expanded global telescope coverage, shorter-wavelength observations, and the possibility of time-resolved “black hole movies,” along with their anticipated impact on our understanding of accretion physics, jet formation, and the fundamental geometry of spacetime.