SO Colloquium: Eyes on the Invisible: Charting New Horizons with the Event Horizon Telescope

The Event Horizon Telescope (EHT) collaboration has captured the first-ever image of a black hole's event horizon in the galaxy M87, and more recently, in the supermassive black hole at the center of our Milky Way, SgrA*. These images, consistent with Kerr black holes as described by General Relativity, provide the strongest evidence to date for the existence of supermassive black holes in galaxy nuclei. Notably, the first polarization images of M87* at event horizon scales have revealed a dominant poloidal magnetic field, confirming predictions from general relativistic magnetohydrodynamic simulations and highlighting magnetically arrested disks (MADs) as crucial in black hole accretion and relativistic jet formation processes. These achievements, stemming from EHT’s 2017 inaugural observing campaign, have set the stage for an array of new, exhilarating findings from subsequent campaigns. These include finer details of the emission structure near black holes and pioneering real-time movies of black hole accretion and jet launching, made possible by additional telescopes, enhanced data rates, and improved algorithms. Beyond these remarkable accomplishments, the EHT's journey is just beginning. The next-generation EHT (ngEHT) aims to significantly augment the current array with more stations, multi-frequency simultaneous observations, and extended time coverage. This expansion will dramatically enhance angular resolution, dynamic range, and temporal coverage, solidifying the EHT as a paramount astronomical facility over the next decade, and the only one capable of directly imaging black holes.