Black holes were first postulated in 1915 as part of Einstein’s Theory of General Relativity. They are so mysterious that it was long believed impossible to observe one directly. The very first image of a black hole, therefore, is a remarkable scientific achievement. Some have complained that the image itself is a letdown, as it resembles every drawing of black hole. But nothing should detract from the incredible effort required to produce it.
Ten years ago, Avery E. Broderick and Abraham Loeb, two of the scientists on the huge team that eventually captured the image, laid out in Scientific American some of the challenges of the imaging project. One problem they faced was that, despite what science fiction tells us, most black holes actually aren’t that large as astronomical phenomena go. According to Broderick and Loeb, a “typical” black hole is only about 90 km across, a mere speck in the grand scheme of things. Even a really large black hole fits within the area defined by Neptune’s orbit. The imaging project was possible, in part, because the black hole they chose was exceptionally enormous: 6.5 billon times our sun’s mass and almost 29,000 times it diameter. Still, from 55 million light years away, it looks small.
Another problem was that the gas circling the event horizon (i.e., the black part) is spinning exceptionally quickly. It takes very high resolution instruments to pick up such rapid motion. Furthermore, very few black holes actually have a spinning halo of visible gas; most are just dark. Prospective targets for imaging are limited. One takeaway is that there are many more black holes in existence than have been discovered.
But the biggest obstacle in getting the image was diffraction. No matter what kind of electromagnetic wave is being detected—be it light, radio, or x-rays—the waves will scatter into a series of indistinguishable blurred rings when an object is small and faint. A larger telescope increases the observable distance before diffraction sets in, but there are limits to practical telescope size. Shorter wavelengths like infrared can be detected with a smaller telescope, but it’s relative. A suitable telescope dish needs to to be thousands of kilometers wide to capture longer wavelengths.
Which is, of course, impossible, unless you use multiple telescopes. There are various telescopes in the world that consist of arrays of smaller telescopes, networked to act as one. Imaging the black hole required networking several arrays to all receive the same image, but none of these telescopes was initially designed for such work. It took years for researchers to figure out how to get a group of telescope arrays networked in order to be able to observe Saggitarius A, the supermassive black hole at the center of the Milky Way.
Windows of available telescope time and proper weather conditions at multiple sites all had to come together. New techniques had to be developed just to process the image. The rest, as they say, is history.