What it means for us to actually 'see' a black hole

'We have seen what we thought was unseeable," the astronomer said, like someone who knows history's ear is pressed against the door. He stood in the hushed attention of the room in Washington as he called up the image on the screen behind him. You know it by now: a smoke ring, an orange doughnut, a blurry circlet of light closed around a profound darkness. By the end of the day, it would be familiar to millions of people as the first photo ever taken of a black hole.

The Event Horizon Telescope (EHT), a collaboration of eight radio telescopes around the world, pieced together this picture from observations made from Antarctica and Arizona. Because no single telescope is powerful enough to distinguish a detail as relatively small as a 55 million-light-year-distant black hole, scientists harnessed these eight observatories together to simulate a much greater instrument, one as large as the Earth itself. It took two years and more than 200 people sifting and refining the data gathered on four days in April 2017 to bring the final famous image into focus.

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Watching the story echo across the news on Wednesday, I was struck by the profound simplicity of this image, obtained through such complex means. Puzzled out through algorithms and vectors, the photo is not an artist's rendering or a model - our previous stabs at picturing a black hole - but it is still a construction. The radio waves collected by the telescopes and assembled into the picture are translated into color for our benefit. You can't pick up a telescope, even one the size of the Earth, and see it for yourself. And yet, over the course of the day, I heard one line repeated again and again: We are seeing a black hole. Not "detecting" it by its radio signature, not digging up more evidence of its existence - actually seeing it.

Far greater than its scientific value - besides its pure technological achievement, the experiment is perhaps most remarkable for not overturning the established rules of relativity - the picture seems to matter to us because it is a picture. Our certainty, rendered in undeniable orange and black, has a new heft.

We didn't lack proof that black holes existed. Ever since Albert Einstein reluctantly predicted them with his theory of relativity in 1916, we've been gathering evidence. In 1935, physicist Subrahmanyan Chandrasekhar hypothesized that a star might become so massive it would collapse under its own gravity. Then in 1969, Donald Lynden-Bell suggested that supermassive black holes at the center of galaxies could be responsible for the huge energy signatures detected there, far beyond what stars alone could generate. In 2015, near-definitive proof arrived with the cosmic "zing!" that was the sound of two black holes colliding, a billion light-years away. For most scientists, the detection of gravity waves cinched it: Black holes were out there, churning up spacetime, even if we never saw them.

Yet images of outer space have an uncanny power over us: These are sights so supernatural that they never quite fit our understanding. Naked, our eyes perceive the night sky as a cosmic swirl of dust and diamonds, hints of a greater complexity. In the early 1600s, Galileo and others pointed the first telescopes into the sky to describe the mountainous face of the moon, the dark-splotched sun turning slowly on its axis, the three gleaming moons of Jupiter wobbling in their distant orbits. Sight gave us irrefutable proof of the vastness of the universe, its chaotic, balletic physics. And, seeing all this for the first time, we believed in it.

I grew up under these cosmic visions. I remember napping beneath posters of the Pillars of Creation and the Cats Eye Galaxy at the Space Telescope Science Institute in Baltimore, while my mom sorted through data and proofread grant proposals at her desk. When I was a little older, I would entertain myself by launching a toy propeller in the hallway, to watch it drift along the currents shifting above my head. My mom told me that looking into space with a telescope was like looking back in time. Nothing ever disappeared from the universe, she said, it just got farther away. I understood this to mean that everything could be known, if we could just see far enough.

Our telescopes are bigger and more powerful than ever. They are mirrors tilting at the sky from mountaintops and deserts, or drifting in orbit like huge insects on their stiff solar-cell wings. They swallow light, not only the visible spectrum of blues and reds but also the X-rays and radio waves we are blind to, revealing the intricate structures of distant galaxies and misty Technicolor nebulas. These are landscapes we could never see for ourselves, but, in seeing their images, they become real to us.

Even so, black holes had eluded us. They are the definition of unseeable: mass and volume turning inside out, cosmic sinkholes from whose irresistible gravity nothing can escape - not a single lousy photon.

But light has always helped us understand what we couldn't see: T.S. Eliot wrote of the "visible reminder of invisible light," tracing the seen and the unseen both. Early astronomers inferred the invisible presence of gravity by the arcs of stars around their pivots. Modern ones use the oscillating brightness of stars to guess at the planets that may orbit them, tugging these distant suns off balance. And Hubble's observations of an expanding universe, rippling with distorted light, have revealed the presence of dark matter, an idea so strange and slippery it's difficult to get purchase on it.

At the Space Telescope Science Institute, my mom studied black holes, too, the ones that scientists were beginning to understand sat at the center of most galaxies, spinning their skirts of stars around them like dervishes. We couldn't see them, but we could see the radiant clouds of light and energy surrounding them. The paradox of the black hole is that, while no light can escape its limits, the regions just beyond the event horizon are some of the most energetic and bright places in the universe. From her, I saw how you could follow the trail of the visible, hunting for signs of the invisible.

Behind the image of the halo at the center of Messier 87, one of two galaxies in the EHT's crosshairs, is a reminder of the challenges we face when looking for these unseeable objects. The second target of the EHT, Sagittarius A*, is the black hole at the center of our own galaxy. Though much closer to us than M87, it is also smaller and therefore more prone to shifting out of focus, like a fidgety student on picture day.

But, unseen, it makes itself known. In the brightly lit hub of the Milky Way, stars slingshot in tight orbits around a massive and invisible object, accelerating with each brush past the engine spinning there, then slowing again at the zenith before turning back again. We can see them clearly. We can discern, by their dance, what holds them in thrall. We cannot see it yet. But we know it's there.

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