Black holes are one of the most critical predictions of the theory of general relativity, as black holes cast dark shadows of light due to strong curvature in space and time, which is not predicted in Newtonian gravity. From an astrophysical point of view, black holes also play unique roles in the evolution of galaxies and the universe since they could be responsible for launching relativistic jets which may strongly interact with the surroundings. Thus, directly imaging the black hole shadow region can provide a unique way to explore the theories of gravity and sharpen our understanding of relativistic astrophysics.
On the other hand, direct imaging of the black holes has been nearly impossible with existing observing facilities. While black holes are believed to be abundantly present in the local and distant universe, they are at vastly far distances compared to the tiny spatial scales over which the aforementioned physical processes occur. This combination results in enormously small, sub-nanoradian scale angular sizes of event-horizon-scale structures in typical nearby massive black holes.
Over the last decades, therefore, concerted global efforts have been made to image the black hole by radio observations at short millimeter wavelengths, adopting the technique of very-long-baseline interferometry, which synthesizes an Earth-sized virtual telescope.
The Event Horizon Telescope (EHT) is an outcome of such efforts. Now EHT began delivering crucial results, such as the first-ever image of a supermassive black hole at the heart of the giant galaxy M87 and its view in linearly polarized light.
In these lectures, I will cover several topics relevant to the EHT: (1) Black holes, general relativity, and background astrophysical context. (2) Fundamental principles and basics of radio interferometric observations and the EHT. (3) Recent results from the EHT 2017 observations and key related studies. (4) Future perspectives in this field, from both the theory and instrumentation sides.
