The Shadow's Photograph: A Narrative Review of Black Hole Imaging from Bardeen's Photon Rings to the Event Horizon Telescope

Abstract

Black hole imaging---the pursuit of the darkness's silhouette against the sky's light---moved from Bardeen's photon rings and Luminet's disk image through Falcke's shadow prediction and Doeleman's horizon-scale detections to the Event Horizon Telescope's M87 image, the Sagittarius A* shadow, and the metric's tests. This article presents a narrative review of that arc's canonical line: Bardeen's 1973 Kerr geodesics, Luminet's 1979 disk image, Falcke, Melia, and Agol's 2000 shadow prediction, Doeleman and colleagues's 2008 horizon-scale structure, Fish and colleagues's 2011 time-variable emission, Issaoun and colleagues's 2019 Sgr A* size, the 2019 M87 shadow results, the 2019 imaging and mass results, Psaltis and colleagues's 2020 shadow test, the 2022 Sagittarius A* shadow, and James and colleagues's 2015 Interstellar lensing. The review is organized around three themes: the shadow's theory, in which the photon orbits's capture cross-section, the disk's image, and the shadow's prediction defined what a black hole looks like; the horizon's resolution, in which the millimeter VLBI's detections and the size's and the scattering's measurements assembled the telescope; and the images, in which the M87's ring, the Sagittarius A*'s shadow, and the metric's tests turned the prediction into the photograph. It is concluded that black hole imaging is the general relativity's portrait---and that its arc is the shadow's journey from the equation's prediction to the telescope's image.