More than a century ago, Albert Einstein stunned the world when he explained the universe through his theory of general relativity. The theory not only described the relationship between space, time, gravity and matter, it opened the door to the theoretical possibility of a particularly mind-boggling phenomenon that would eventually be called black holes.

The concept that explains black holes was so radical, in fact, that Einstein, himself, had strong misgivings. He concluded in a 1939 paper in the Annals of Mathematics that the idea was “not convincing” and the phenomena did not exist “in the real world.”

The unveiling of the first-ever picture of a black hole by the Event Horizon Telescope in April 2019, however, not only confirmed Einstein’s original theory, but also provided indisputable proof that the gravitational monsters are, in fact, real.

## The Space-Time Theory

As described by American physicist John A. Wheeler, general relativity governs the nature of space-time, particularly how it reacts in the presence of matter: “matter tells space-time how to curve, and space-time tells matter how to move.”

Picture a flat rubber sheet (space-time) suspended above the ground. Place a bowling ball in the middle of the sheet (matter) and the sheet will distort around the mass, bending half way to the floor— this is matter telling space-time how to curve. Now roll a marble (matter) around the rubber sheet (space-time) and the marble’s trajectory will change, being deflected by the warped sheet— this is space-time telling matter how to move. Matter and space-time are inextricably linked, with gravity mediating their interaction.

Now, place a singularity—a theoretical point of infinite density—onto the sheet, what would happen to space-time? It was German theoretical physicist Karl Schwarzschild, not Einstein, who used general relativity to describe this hypothetical situation, a situation that would become the most extreme test of general relativity.

At a certain threshold, Schwarzschild found that the hypothetical singularity would literally punch through space-time. In mathematics, singularities are interesting numerical solutions, but astrophysical singularities were, at the time, thought to be an abomination— there was no known mechanism that could produce them.

Schwarzschild, however, persisted until his death in 1916, realizing that an astrophysical singularity would warp space-time so severely that even light would not be fast enough to get out of the space-time hole that the singularity would create. The point of no return, a spherical region surrounding the singularity, would become known as the “event horizon.”

Known physics breaks down beyond the event horizon and, as no information can escape, we can have no experience as to what lies inside. Though this was an interesting concept, there was no known mechanism that could create a singularity in nature, so the idea was largely overlooked.

## Concept of Black Holes Are Born

That was until 1935, when Indian astrophysicist Subrahmanyan Chandrasekhar realized that, should a massive star run out of fuel, the sheer gravitational pressure of that mass would be concentrated to a point, causing space-time to collapse in on itself. Chandrasekhar had bridged the gap between mathematical curiosity and a scientific possibility, seeding the theory behind the formation of a real singularity with extreme consequences for the fabric of space-time.

Even with Chandrasekhar’s contributions toward the modern understanding of the nature of black holes, astrophysical singularities were assumed to be, at best, extremely rare. It stayed that way until the 1960s when British theoretical physicists Stephen Hawking and Roger Penrose proved that, far from being rare, singularities were a part of the cosmic ecosystem, and are a part of the natural evolution of massive stars after they run out of fuel and die.

And it wasn’t until 1967, 12 years after Einstein’s death in 1955, that these astrophysical singularities became known as “black holes”—a term coined by American physicist John A. Wheeler during a conference in New York to describe the grim fate of a massive star after it runs out of fuel and collapses in on itself.

The black hole “teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but,” Wheeler wrote in his 1999 autobiography.

Thanks to astronomers and computer scientists working with the Event Horizon Telescope (EHT), a network of eight linked telescopes, humanity was finally able to visualize these "infinitesimal dots." Although Einstein wasn’t alive to see evidence of black holes—the result of real singularities about which he remained doubtful—his theory of relativity made their discovery possible.

And, no doubt he also would have marveled at the ghostly crescent surrounding a near-perfect dark disk: proof that even the most outrageous theories can turn out to be true.

Ian O'Neill is an astrophysicist and science writer