Without a doubt, black holes are among the most mysterious types of object in the universe. Scientists grasped their existence in the early 20th century in the wake of Albert Einstein's revolutionary proposals about gravity. It took until 1964 for scientists to even propose that they had observed one, and it was only in 2019 that astronomers were able to release the now-famous photo of the supermassive black hole at the center of the galaxy M87, the first real image of a black hole.
With that said, Christopher Nolan's blockbuster film "Interstellar," which quite notably features a black hole, was released in 2014. So Nolan, even if he prefers avoiding CGI, had no other option but to use computers to create the movie's black hole. And he could do that only because there is a long tradition of generating images of a black hole, as far back as 1979.
The universe, as it turns out, loves following rules. The entire history of physics — actually just all of science — can be summed up in one phrase: the quest to figure out what the rules of the universe are. Scientists define those rules using math equations. So all it took to create an image of a black hole was just plugging those equations into a computer and letting it spit out a result. How hard could it be? Incredibly difficult, as it turns out, because the first problem was figuring out the equations in the first place.
For a long time, scientists understood gravity as it was formulated in 1687 by Isaac Newton: Every object in the universe applies a force on every other object in the universe, and that force is proportional to the product of the masses of the objects (how much stuff there is) and inversely proportional to the square of the distance between the centers of the objects. In simpler terms, larger objects and shorter distances produce more gravity.
And then Einstein came and made everything a lot more complicated. As it turns out, there were a few cases where Newton's theory of gravity didn't perfectly predict what was going on. While Newton was able to describe the force created by gravity, he was never able to determine what made that force happen — he gave us the "what" but not the "how" and "why." Einstein's theories built on the work of Newton and other physicists to fill in that gap, and in doing so, solved some of the cases where Newton's theory broke down.
In 1905, Einstein revealed his theory of special relativity, which was based on two ideas: first, that laws of physics never change as long as you move at a constant speed, and second, that the speed of light in a vacuum is always the same, no matter what. If you've ever heard of "spacetime" or the "space-time continuum," this is where it comes from. In order for the speed of light to always remain the same, time and distance both have to change, sometimes in unexpected ways (famously, time dilation). This weird, counterintuitive connection between physical space and the passage of time is a critical part of how our universe works.
For the next 10 years, Einstein worked on figuring out whether he could fit gravity into his theory of spacetime, and that's what general relativity did. To massively oversimplify, every object in the universe physically curves the field of spacetime, altering the trajectory of any object that passes near it. According to Einstein's theory, we perceive that change in trajectory as gravity, and nothing, not even light, is immune.
Not long after, some physicists, like Karl Schwarzschild, noticed a potential problem: What if an object had such a large mass that it could distort spacetime so much that not even light, the fastest object in the universe, could escape? Well, when light is absorbed into something, our eyes perceive that object as black. And this black object would have to be absurdly dense, so dense that at its center it might appear like it had created a tiny hole in the fabric of spacetime.
In other words — hold on, the exact same words actually — a black hole.
Scientists have built upon these theories ever since, confirming that Einstein, Schwarzschild and others were right time and time again. However, trying to explain the concept to the public is an entirely different matter. Generally, people like to know what something looks like, and the rules of a black hole make answering that question really difficult.
But in 1979, French cosmologist Jean-Pierre Luminet figured out a way to do it using the theories and equations that dictate how gravity pulls objects like cosmic space into a black hole (it's a rather messy process, as it turns out). Contrary to the public's impression of a black hole just being a giant galactic vacuum cleaner, a black hole simply exerts the force of gravity, no more and no less. And gravity, in case you haven't picked up on it already, is very complicated.
So when Luminet went about creating his image of a black hole, he used equations to predict how the light itself would be affected by passing near the black hole. His result showed a small ring of light around an otherwise completely black circle. Then, he figured out how the orbit of the space dust around the black hole might affect its image, what might happen if the black hole itself was rotating, and other complications. And then, he used a 1970s-era computer to print a simulated picture.
In his paper, he suggested that this image might accurately represent, among other black holes, "the supermassive black hole whose existence in the nucleus of M 87 has been suggested recently."
In April 2017, a massive project led by Harvard astronomer Shep Doeleman scanned the sky around M87 using telescopes all around the globe. They then spent two years analyzing the data and creating an image:
(Event Horizon Telescope)
All in all, it's safe to say Luminet did a pretty good job.