Black holes occur in the middle of galaxies where mass collapses to form gravity-dense regions. Anything that falls into a black hole cannot escape — not even light, making it impossible to see black holes directly.
Black holes are massive entities and the Earth is tiny in comparison. Therefore, we need giant telescopes to capture the entirety of the event horizon. As such telescopes are impractical, astronomers have integrated eight telescope arrays spread across continents to work as one large earth-sized telescope, known as the Event Horizon Telescope (EHT).
Telescopes with short wavelength and large diameters yield sharp images. EHT uses a powerful radio astronomy tool called the Very Large Baseline Interferometry (VLBI) technique. In VLBI, angular resolution is a crucial factor to obtain high-resolution images from a telescope. It depends on two parameters – the diameter of the antennae and the wavelength of the radiation signal reaching it.
One or more pairs of antennae separated physically are virtually linked, expanding the antenna’s diameter. Owing to the vast distances, the signals reaching Earth are almost linear. Hence, each telescope can observe only a part of the event horizon. When collated (albeit virtually), several baselines or diameters of the antennae behave as one large earth-sized antenna. The more the number of antennae, the higher is the resolution of the image. Astronomical, statistical, and advanced imaging algorithms are used with atomic clocks to time-code the measurements made at each facility. A weeklong observation in 2017 generated petabytes of data which took the EHT team two years to analyse revealing the first-ever picture of a black hole.