Why is it important to study black holes? What is there to be learned from the EHT observations?

As a scientific collaboration, the aim of the EHT is not only to prove the existence of black holes, but also to understand the physics of black holes and their surrounding environments. There is ample indirect evidence from various astronomical studies indicating that black holes exist, including the investigation of nearby objects that are subjected to the gravitational pull of a black hole. These are circumstances well explained by the General Theory of Relativity (GR). It was the direct observation of the immediate environment surrounding a black hole—the event horizon—that had never been achieved, until now. With the release of the first results in April 2019, the EHT has filled a significant gap in our empirical knowledge.

Black holes are laboratories for testing fundamental theories that explain how the Universe works on the largest and the smallest scales (e.g., GR and Quantum Physics). While each of these theories works well in its respective regime, physicists currently do not understand how to create a single physical theory that would be universal and, hence, explain the physics of black holes in detail. With the EHT results, scientists are able to directly resolve the conditions of spacetime at the black hole boundary.

In addition to the fundamental physical theories, there are many details of plasma physics that are not completely understood. Properties of the hot gas surrounding and being pulled into the black hole beyond the event horizon are not fully known. But, understanding these properties is crucial for interpreting black hole images because it is this glowing plasma that produces radiation captured radio telescope arrays. Effectively, this glowing hot gas illuminates the shape of spacetime around the black hole and EHT observations will help to better understand the properties and behavior of these extreme environments.