Imaging Horizons: Probing the Nature of Black Holes

For the first time it has become possible to image a black hole at sub-horizon scales. The extraordinary resolution required to do this has being achieved by pushing Very Long Baseline Interferometry (VLBI) to millimeter and sub-millimeter wavelengths. Already, mm-VLBI & infrared observations of the supermassive black hole at the center of the Milky Way has placed an important constraint upon fundamental gravitational physics: compact horizons almost certainly exist, even if General Relativity fails to describe black holes! Future measurements promise to test whether or not the Kerr metric is an adequate description of strong gravity in the vicinity of supermassive black holes. In addition, mm-VLBI observations are being brought to bear upon the many unresolved astrophysical questions regarding black hole accretion and outflows, providing crucial empirical insight into these processes at the smallest relevant scales. I will discuss how these observations are being done, what we expect to see (and have seen!), and their implications for fundamental questions in black hole science.