What the Sight of a Black Hole Means to a Black Hole Physicist
At this historic moment, the world has paused to take in the sight of humanity’s first image of the strangest phenomenon in the known universe, a remarkable legacy of the general theory of relativity: a black hole. I am moved not just by the image; overwhelmingly I am moved by the significance of sharing this experience with strangers around the globe. I am moved by the image of a species looking at an image of a curious empty hole looming in space.
I am at the National Press Club, in Washington, D.C., a hive of excitement. Scientists with the Event Horizon Telescope aspired for years to take the first-ever picture of a supermassive black hole, so when they gathered journalists and scientists together today for a press conference, there wasn’t much doubt as to what we were here to see. But still, there are surprises. At the podium is Sheperd Doeleman, the director of the Event Horizon Telescope. He welcomes us, “black hole enthusiasts.” I have the strongest memory of standing at the chalkboard in an otherwise empty classroom at the Massachusetts Institute of Technology with Shep, my funny friend with his funny, unmistakable, burnt-mahogany hair. Covered in chalk dust, we acquired the hard-earned mathematics of Albert Einstein’s theory of relativity. Video: Sheperd Doeleman, director of the Event Horizon Telescope, unveiled the first-ever photo of a black hole at a press conference at the National Press Club in Washington, D.C. National Science Foundation We knew the words already, the standard lore: All forms of matter and energy bend space and time, and light and matter follow those curves. The words have to be taken on trust. But the mathematics we could acquire. It would belong to us. When Einstein conceived of relativity, he gave us a gift that has been passed from person to person around the world. Relativity, defying its name, is true for all of us. Maybe my memory of that particular board is so crisp precisely because that moment defines the cusp between before and after acquiring relativity. Now I cannot imagine my own mind without it. Relativity permeates my thoughts so that I think in relativity the way writers think in their natural language. Since that time at MIT, Shep and I have both found our way via relativity to the most remarkable of its predictions, black holes. Black holes were conceived of as a thought experiment, a fantastical imagining. Imagine matter crushed to a point. Don’t ask how. Just imagine that. While enlisted in the German army during World War I, Karl Schwarzschild discovered this possible solution to Einstein’s newly published theory of relativity, apocryphally between calculating ballistic trajectories from the trenches on the Russian front. Schwarzschild inferred that space-time effectively spills toward the crushed center. Racing at its absolute speed, even light gets dragged down the hole, casting a shadow on the sky. That shadow is the event horizon, the stark demarcation between the outside and anything with the misfortune to have fallen inside. Einstein thought nature would protect us from the formation of black holes. To the contrary, nature makes them in abundance. When a dying star is heavy enough, gravity overcomes matter’s intrinsic resistance and the star collapses catastrophically. The event horizon is left behind as an archaeological record while the stellar material continues to fall inward to an unknown fate. In our own Milky Way galaxy there could be billions of black holes. Supermassive black holes, millions or even billions of times the mass of the sun, anchor the centers of nearly all galaxies, though nobody yet knows how they formed or got so heavy. Maybe they formed from dead stars that merged and escalated in size, or maybe they directly collapsed out of more primordial material in a younger universe. However they formed, there are as many supermassive black holes as there are galaxies — hundreds of billions in the observable universe. We had never seen a black hole before today. No telescope had ever taken a picture of one. We have indirectly inferred the presence of black holes when they’ve cannibalized companion stars, powered energetic jets in twisted magnetic fields, and captured stars in their orbit. We have even heard black holes collide and merge, ringing space-time like mallets on a drum. We had never taken a direct picture of a black hole before because black holes are tiny, despite their dramatic reputation as weapons of mayhem and destruction (yes, the Nova film I hosted was called “Black Hole Apocalypse”). A black hole the mass of the sun would have an event horizon a mere 6 kilometers across. Compare that to the 1.4-million-kilometer breadth of the sun itself. The supermassive black hole at the center of the Milky Way, dubbed Sagittarius A*, is 4 million times the mass of the sun but only about 17 times wider. Consider the challenge of capturing a portrait of an entirely dark object only 17 times the width of an ordinary star at a distance of 26,000 light-years. Resolving an image of Sagittarius A* is comparable to resolving the image of a piece of fruit on the moon. To resolve such a miniscule image requires a telescope the size of the entire Earth. Since those days in that chalk-dusted classroom at MIT, my funny, utterly unconventional friend has been determined to capture the image of a supermassive black hole all the same. During our years in graduate school, Shep’s hair was an allegory for his mind — wild and spirited. I admired the freedom I sensed in the way he thought, always forging unexpected connections, sometimes at the expense of the required lesson. His shocked eyes would warn me that a crazy idea had struck him just at that precise moment, as though he was as surprised as I was by the thought. The Event Horizon Telescope is a testament to bold ideas, as well as scientific ingenuity and collaboration. Exploiting large radio telescopes around the globe — relying on the newest, most sophisticated observatories and reviving some that were nearly defunct — EHT became a composite telescope the size of the Earth. As the planet spins and orbits, the target black holes rise into the field of view of component telescopes around the planet. To render a precise image, the telescopes need to operate as one, which involves sensitive time corrections so that one global eye looks toward the black hole. The radio telescope observatories involved in Event Horizon Telescope’s observations were (clockwise from top left): Atacama Large Millimeter/submillimeter Array (ALMA) in Chile; SubMillimeter Array (SMA) in Hawaii; South Pole Telescope (SPT) in Antarctica; Submillimeter Telescope (SMT) in Arizona; Atacama Pathfinder Experiment (APEX) in Chile; Large Millimeter Telescope (LMT) in Mexico; James Clerk Maxwell Telescope (JCMT) in Hawaii; and Institut de Radioastronomie Millimétrique (IRAM 30m) in Spain. Iztok Bončina/ESO; Steven H. Keys; University of Arizona/Junhan Kim; Dave Harvey; Juan de Dios Santander Vela; A. Woodcraft; Luyten; ESO/B. Tafreshi/TWAN Combining telescopes for better resolution was the basis of Shep’s doctoral thesis in the ’90s. By 2008, he led a small team that imaged structures comparable in size to nearby supermassive black holes. That proof of concept drove the EHT project, whose team was now confident that the required resolution was in reach. In the decade since, EHT had to address challenges the data posed and advance technologically, and Shep is quick to credit the international team for their stamina and for the cleverness of their collective contributions. Our supermassive black hole, Sagittarius A*, became the obvious target to pursue. Despite the abundance of supermassive black holes in galaxies, all others are too far away to resolve even with a telescope the size of the Earth. There is one exception. Messier 87, or M87, is an enormous elliptical galaxy 55 million light-years away that is known to harbor a staggering supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. At the small end of that range, M87 would be an impossible target for EHT. At the high end, it is possibly suitable. So M87 became a secondary target in the heated pursuit of Sagittarius A*. A black hole against the dark backdrop of empty space would be truly invisible. Sagittarius A* and M87 are helpfully illuminated by debris caught in hot disks orbiting very near their event horizons. The path of the light from the luminous orbiting material is bent along the curved space so that even light behind a black hole gets redirected our way. The disk appears to surround the black hole, allowing for a bright contrast against which its shadow is visible. EHT actually sees an area slightly outside the event horizon itself — a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the “last photon orbit.” (Were you to float there, you could see light reflected off the back of your head after completing a round trip. Or, if you turned around quickly enough, you might see your own face.) Closer than that, all the light falls in. We are gathered here, black hole theorists and observers, journalists and friends, in this room together to share an image we could already pretty well imagine and were excited to celebrate. But this was the surprise on hearing the announcement: It’s not Sagittarius A* they saw. It’s not our black hole. It’s M87! The image is unmistakable — a dark shadow the size of our solar system, enveloped by a bright, beautiful blob. While the scientific implications will take time to unpack, some of the anthropological impact feels immediate. The light EHT collected from M87 headed our way 55 million years ago. Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs. Looking at M87, I am reminded that scientific discoveries transcend those differences. We are all under the same sky, all of us bound to this pale blue dot, floating in the sparse local territory of our solar system’s celestial bodies, under the warmth of our yellow sun, in a sparse sea of stars, in orbit around a supermassive black hole at the center of our luminous galaxy. When asked his thoughts at the moment he first saw the image of the black hole in M87, Shep replied, “We saw something so true.” And it’s true for all of us.