A black hole, once a mathematical curiosity, is brought to light

Blue Marble, meet Black Shadow.

In what Carlos Moedas, the European commissioner of research, called “a huge breakthrough for humanity,” the Event Horizon Telescope team revealed the first-ever image of the “shadow” of a black hole.

The color-shifted image, an ethereal swirling glow encircling a stark black disc, shows millimeter-wave radiation surrounding a black hole at the center of M87, a galaxy more than 50 million light-years away in the constellation Virgo.

Created using data from eight radio telescopes around the earth run by a global consortium of scientific institutions, the Event Horizon Telescope’s image could prove as much of an iconic image of a black hole as the 1972 photo taken from Apollo 17 is of our planet. A milestone in black hole astronomy, it is the first direct visual evidence of an object once thought to be an mathematical artifact, a glitch in Albert Einstein’s geometric model of gravity, the general theory of relativity.

“We have transformed a mathematical concept, the event horizon” said Goethe University Frankfurt physicist Luciano Rezzolla during the press conference in Brussels, “into a physical object.”

The technique used to image the black hole’s shadow will allow for many more observations of this black hole and of Sagittarius A*, the supermassive black hole at the center of our own galaxy – observations that could reveal new findings about the workings of gravity itself.

“This is going to open up a whole set of tests of general relativity that have never been possible up until now,” says astrophysicist Christopher Impey, the author of the 2018 book “Einstein’s Monsters: The Life and Times of Black Holes. “This is the first time you get to test general relativity and the ultimate regime of superstrong gravity, where space and time are heavily, hugely distorted.”

Even though black holes are the most extreme known objects in the universe, humans first thought of them not by actually detecting them but by imagining them. The first to consider such an object was the English polymath John Michell in a 1784 essay. He wrote in passing that, according to Newtonian physics, which depicts gravity as a force and light as a particle with mass, there could exist a “dark star,” a body so massive that any light particles, if they got close enough, would fall into it.

Working independently 12 years later, the French scholar Pierre-Simon Laplace gave the idea a more thorough mathematical treatment. He correctly calculated the relationship between the mass of a body and the point of no return for light that surrounds it, which today is called the event horizon. Even though Laplace employed two ideas now understood to be false – Newton’s theories of light and gravity – he was pretty close.

“He actually got the right answer for that,” says Cornell University astrophysicist Dong Lai, an expert on black holes. “This sometimes happens in physics. If you make two mistakes, you can get the right answer.”

Michell’s and Laplace’s speculations were abandoned in the 19th century, following Thomas Young’s 1801 discovery that light travels as a wave, leaving no known way for the force of gravity to interact with it. And in any case, even if such “dark stars” existed, there was no known way at the time for astronomers to detect them.

But the idea resurfaced in physics in 1916, when Albert Einstein revolutionized our understanding of the cosmos with his general theory of relativity. He conceived of gravity not as a force, as Newton did, but as a consequence of the way massive objects curve space and time.

“One really has to use the theory of general relativity to really appreciate the true meaning of black holes,” says Dr. Lai.

Einstein himself didn’t initially notice the possibility for black holes in his equations. And when the German physicist Karl Schwarzschild wrote to him that year to raise the possibility of black holes and calculating the radius of their event horizons, Einstein initially dismissed black holes as a mathematical anomalies.

Lia Medeiros, a theoretical astrophysicist at the University of California, Santa Barbara and the University of Arizona’s Steward Observatory, likens the math of black holes to dividing by zero. “That thing your math teacher told you not to do in high school? We do it all the time,” she says.

The output of this illicit mathematical operation is an extremely distorted pocket of space-time such that even light, the fastest thing there is, gets trapped.

That’s what creates the black hole’s shadow, says Dr. Medeiros, who worked on the University of Arizona’s Event Horizon Telescope team. The black hole at the center of M87 is surrounded by a glowing disc of hot gas and other material that heats up as it plummets into the black hole. Some of the light from the disc, following the curvature of space created by the black hole, bends around it, creating an outline of the event horizon. The rest of the light falls in, leaving a dark, circular void.

“You essentially sequester off a little piece of the universe,”  says Dr. Impey. “You pinch off a piece of space-time so that it’s invisible and hidden from view.”

Mainstream scientists began taking the idea of black holes seriously again in the 1960s, using X-ray detectors on a sounding rocket to detect emissions from the first known black hole, Cygnus X-1, in 1964. So far, only “40 or 50 very good cases of black holes” have been detected, says Dr. Impey, even though they are thought to be very common and lie at the center of every galaxy.

“It’s been very hard work to find black holes,” says Dr. Impey.

But this latest image confirms the power of theoretical astrophysics. The shape of the black hole’s shadow matches theoretical predictions made 40 years ago by astrophysicists Jim Bardeen and Jean-Pierre Luminet.

“The fact that this was calculated so many years ago,” says Dr. Medeiros, “and the fact that we actually observed it, and it was exactly what we had predicted, is incredible. It really makes me believe in humans as a species.”

The image, say observers, heralds a new era in the study of general relativity. Will Einstein’s model, having held up spectacularly against every test imaginable in Earth’s gravitational field, hold up to tests performed in a gravitational field more than two trillion times the mass of our sun?

“That we’re looking at black holes, for me, is super exciting, says Dr. Medeiros. “We’re probing extreme gravity in a way that’s never been probed before.”