On Wednesday, April 10, 2019, black holes were transformed from an abstraction to reality when the Event Horizon Telescope presented the first-ever image of a black hole. The image seen around the world was of the supermassive black hole at the center of a supergiant elliptical galaxy in the Virgo constellation. There were strong scientific reasons to think that almost every galaxy, including our own Milky Way galaxy, has a supermassive black hole at its center, yet, until then, there was no way to see the black hole or measure its diameter to provide a check on Einstein’s theory of gravity.
Black holes are a consequence of general relativity and have been under discussion for one hundred years. The diameter of a black hole is proportional to its mass: for 1 solar mass, that's 6 kilometers – the size of San Francisco. This location is called "the event horizon" because no events taking place inside that diameter can affect the outside world. These small San Francisco-size black holes could form when stars collapse in supernova explosions. A supermassive black hole, like the one we saw on April 10, could be 10,000,000 solar masses, with an event horizon of 60,000,000 kilometers, which is a few times the size of the Earth's orbit around the Sun. These black holes grow over cosmic time as galaxies evolve.
In the past decade, radio astronomers understood that imaging the central black hole in our galaxy and nearby galaxies was just out of reach, and technical improvements would make it feasible to achieve this dream. In 2012, the foundation gave a series of grants to the Smithsonian Astrophysical Observatory, a leader of the world-wide collaboration later dubbed the Event Horizon Telescope. The principal investigator of these grants was Sheperd "Shep" S. Doeleman who is a senior research fellow at the Harvard–Smithsonian Center for Astrophysics and the director of the Event Horizon Telescope project. These grants supported building the equipment to record the signals from each radio telescope in the array and the people to construct an image from those signals. These grants made the billion-dollar investments in radio telescopes around the world much more effective for the quest to create the first-ever image of a black hole.
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