What Earth Owes to Black Holes

The cosmic objects have a reputation as monstrous maws, but without them the universe would make less sense.

A composite rendering of the center of the Milky Way, with its supermassive black hole shrouded in clouds of gas
A composite rendering of the center of the Milky Way, with its supermassive black hole shrouded in clouds of gas (NASA / CXC / UMass / D. Wang et al / NRF / SARAO / MeerKAT)

The first picture ever captured of a black hole, one situated in the center of another galaxy, was pretty blurry. Seen in silhouette, it appeared fuzzy, as did the ring of hot gas surrounding it. The reaction of the public did not necessarily match the unalloyed joy of astronomers accustomed to extracting cosmic wonders from lines in a graph. To anyone more familiar with black holes from epic space films, this one mostly looked like a flame-glazed donut.

But that portrait is one of the most extraordinary achievements in modern science, a display of humankind’s capacity to reach across light-years. Not so long ago, scientists couldn’t say with much confidence that black holes existed, nor did they know that a giant one sits at the center of our own galaxy.

Yesterday, the Nobel Committee recognized decades of black-hole research by awarding its physics prize to three scientists. Half the prize went to Roger Penrose, of the University of Oxford, who showed that black holes could exist, and half went to Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics and UC Berkeley, and Andrea Ghez, of UCLA, who provided the most convincing evidence that a particular black hole—the supermassive one at the center of our Milky Way—did. (Ghez, it’s worth noting, is only the fourth woman to receive the honor in nearly 120 years of Nobel history.)

Black holes are among the most mysterious phenomena in the universe. Forged from the cores of dead stars, they are so dense that nothing can escape their gravitational pull, not even light, which renders them invisible. Entire stars, once luminous, can be extinguished if they cross a black hole’s boundary, and pass the point of no return.

Albert Einstein predicted more than a century ago, based on his theories untangling the nature of gravity, that such strange objects could exist, but he thought the idea was too far-fetched. In 1965, after Einstein’s death, Penrose, the Oxford professor, published a paper showing, mathematically, that the forces of the universe could indeed produce black holes, and that inside their impenetrable depths resides something called a singularity, an inscrutable point which no known laws of physics can describe.

Such a thing might still seem too incredible to exist, but without black holes, the movements of faraway stars in our galaxy don’t always make sense. Genzel and Ghez spent many years poking into the cosmic cloud of interstellar gas and dust at the very center of the galaxy, with the world’s largest telescopes. They discovered stars orbiting a seemingly empty spot at startling speeds, a chaotic environment that could make sense only in the presence of a supermassive black hole. This region in our galaxy, known as Sagittarius A* (pronounced ay-star), has a mass 4 million times that of our sun, squeezed into a space smaller than our solar system.

Astronomers have found other black holes, too, by watching for the dizzying orbits of the unlucky stars around them. They have seen black holes in the glow coming from matter as it plunges into the invisible depths, a process so intense that particles light up beautifully. And they have felt them in the ripples in the fabric of space-time, the gravitational waves that fan out across the universe when two black holes collide. Black holes, it turns out, are everywhere, in the center of most galaxies and spread throughout them, and they come in different sizes. (Some even appear to be so big that, theoretically, they shouldn’t exist.) Earlier this year, astronomers found the closest known black hole to Earth 1,000 light-years away, almost on our doorstep by cosmic measures, in a constellation that can be seen with the naked eye. The variety in discoveries is quite impressive for an object best known for its nothingness.

That nearby black hole is no threat to Earth. No known black hole is. If anything, we benefit from their existence. The stellar explosions that produce black holes also spew elements such as carbon, nitrogen, and oxygen into space. The collisions of black holes and neutron stars help spread heavier elements, such as gold and platinum. These elements make up our Earth, and our own selves.

Supermassive black holes, in particular, might play an important role in star formation within galaxies, dictating when production slows or ceases altogether. “This fertile corner of the cosmos has been governed by all that has gone on around it, including the behavior of the black hole at our galactic center,” explains Caleb Scharf, the director of the Columbia Astrobiology Center, in his book Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos. “The entire pathway leading to you and me would be different or even nonexistent without the coevolution of galaxies with supermassive black holes and the extraordinary regulation they perform.”

Since black holes first captivated the public imagination decades ago, they have garnered a certain reputation. They are labeled monstrous, destructive, bent on devouring anything that dares approach their cosmic maw. But without them, the cosmos, and our own planet, would be less dense with wonder.