Black holes are, of course, awesome. But, for scientists, they are more awesome. If a rainbow is marvellous, then understanding how all the colors of the rainbow are present, unified, in ordinary white light—that’s more marvellous. (Though, famously, in his poem “Lamia,” John Keats disagreed, blaming “cold philosophy” for unweaving the rainbow.) In recent years, the amount of data that scientists have discovered about black holes has grown exponentially. In January, astronomers announced that the James Webb Space Telescope had observed the oldest black hole yet—one present when the universe was a mere four hundred million years old. (It’s estimated that it’s now 13.8 billion years old.) Recently, two supermassive black holes, with a combined mass of twenty-eight billion suns, were measured and shown to have been rotating tightly around each other, but not colliding, for the past three billion years. And those are just the examples that are easiest for the public to make some sense of. To me, a supermassive black hole sounds sublime; to a scientist, it can also be a test of wild hypotheses. “Astrophysics is an exercise in incredible experiments not runnable on Earth,” Avery Broderick, a theoretical physicist at the University of Waterloo and at the Perimeter Institute, told me. “And black holes are an ideal laboratory.”
Broderick says that he studies black holes because they are very simple, theoretically and mathematically. As he explained it, a black hole has a mass, an electrical charge, and an angular momentum (meaning that it can spin). “And that’s pretty much it,” he said. “Their behavior is extreme, but the apparatus is something we think we understand.” Another “simple” way to think of a black hole is as an extraordinary amount of mass in a relatively small space. It exerts a gravitational pull so strong that not even light can escape it. Imagine the mass of Earth condensed to the volume of a marble; imagine a million suns condensed to the volume of a single sun—that’ll give you an idea of a black hole. Some black holes are formed by stars that have collapsed in on themselves. Other black holes are thought to have been formed by the inward collapse of enormous clouds of gas. (There are other theories, too.) To look “into” a black hole—from which no photon or wave or ray ever returns—requires considerable creativity. The interior of a black hole can only be deduced from changes exterior to it. Active black holes are encircled by intense brightness and billion-degree heat, given off by matter falling toward them—think of the fire of an incoming asteroid—while the black hole itself is unthinkably cold, a tiny fraction of a degree above absolute zero.
It’s in these simple, outlandish objects, Broderick explained, “that twentieth-century physics breaks down.” But what is the twentieth-century physics that is said to break down? Basically, there’s Albert Einstein’s theory of general relativity (which made a tiny but far-reaching correction to Isaac Newton’s concept of gravity), and there’s quantum mechanics. “General relativity is thought of as the theory of the very large and massive, and quantum mechanics is the theory of the very small or very cold,” Broderick said. Black holes are massive (general relativity), and cold (quantum mechanics). But, when scientists attempt to use these theories to describe what happens in the interior of a black hole, the implications are, as another astrophysicist put it, “a disaster.” Or, as Broderick put it, the theories “give very different answers.”
Some scientific terms have more charisma than others. Schrödinger’s cat, dark matter, the “plum pudding” model of the atom—these are more evocative than “eigenstates,” “neutron stars,” or “ribosomes.” “I had been searching for just the right term for months, mulling it over in bed, in the bathtub, in my car, wherever I had quiet moments,” the late physicist John Wheeler once said, about trying to find better language for what was then called a “gravitationally completely collapsed object.” Wheeler, who also read poetry, loved dreaming up vivid language for new concepts in physics. It is to Wheeler that we owe “wormholes,” the “participatory universe,” and “quantum foam”—names that carry something of the spirit of what they describe.
In 1967, Wheeler attended a discussion of the astrophysicist Jocelyn Bell’s recent discovery of pulsars—celestial objects that flash out radiation. The discussion centered on what caused this curious phenomenon. A “completely collapsed object” was one explanation. When Wheeler spoke, he used that cumbersome term several times before finding himself simply saying “black hole.” The name stuck. (That, anyhow, is one of the origin stories—the term had been used in articles a few years earlier, and it is also said to have been shouted out as a suggestion to Wheeler by a conference attendee.)
For most people, the name evokes what the Nobel Prize-winning astrophysicist Saul Perlmutter termed “the Darth Vader aspect” of black holes: the objects are “deadly, silent, all-powerful, and looming.” The popular understanding of black holes, though not precise, has the accuracy that we associate with the poetry that Wheeler loved to read. “Not only does it swallow anything that comes too near it but no one lives to tell the tale . . . there are footprints leading in, and no footprints leading out,” Perlmutter said, describing how nonspecialists might think of them. “If black holes weren’t real, I think the science-fiction writers would have wanted to invent them.”
A hundred years ago, almost no one believed that black holes were real, not even Einstein, who wrote the equations that predicted them. He said that they were impossible, a quirk of mathematics. In 1935, at a meeting of the Royal Astronomical Society, the young astrophysicist Subrahmanyan Chandrasekhar presented his work, which suggested, instead, that what was impossible was essentially that black holes didn’t exist. Sir Arthur Eddington, one of the most respected elders of astronomy, had arranged Chandrasekhar’s talk and knew what he was going to speak about—and he scheduled himself to speak directly afterward. Eddington, who was polite, open-minded, and encouraging to younger scientists, dismissed Chandrasekhar’s idea as ludicrous, saying, “There should be a law of nature to prevent a star from behaving in this absurd way!” Eddington’s viewpoint won; talking about stellar collapse became a great way not to be taken seriously.
Decades passed. Then a few weird things were noticed. Astronomers began seeing something at the center of galaxies so bright that it outshone all the other stars in the galaxy put together. (This was the energy from things falling into a black hole.) Elsewhere, cosmic rays were detected from a part of the sky termed Cygnus X-1, in a pattern that seemed inexplicable. Ultimately, the concept of black holes came to account for and unify these baffling and seemingly disparate observations. Denying the existence of black holes became more awkward than accepting it. In the nineteen-seventies, Stephen Hawking had made a bet with Kip Thorne, a fellow-physicist, about whether the cosmic X rays coming from Cygnus X-1 could really be the result of a black hole. Hawking bet No, though he made the wager to counterbalance the outcome that he wanted to be true; he had spent years studying black holes. Only in 1990 had enough evidence accumulated to settle the bet in Thorne’s favor. (The payout was a subscription to Penthouse; if Hawking had won, he would have received the satirical magazine Private Eye.)
From there, knowledge accelerated. Not only were black holes real but a giant one was at the center of our galaxy—actually, at the center of every galaxy—actually, not only at the center of galaxies but all over them, with our Milky Way alone housing many millions. And not only are there unthinkable numbers of black holes but sometimes they collide, sending tiny ripples of gravitational waves across the universe. In 2015, scientists at LIGO, an observatory whose construction had taken decades of dreaming and designing and revising, ran an experiment that within days detected those gravitational waves. The researchers described what they observed, evocatively, as ripples in the fabric of space-time.
The gravitational waves sent out by colliding black holes make a sound—or translate into a sound—something like a bell being rung. “You can tell if I’m pounding on this desk, or if I’m pounding on the floor,” Will Farr, an astrophysicist, said to me in his bright office at the Flatiron Institute, in Manhattan. “If I had musical instruments, it would sound prettier, but also you could tell if it was a trumpet or a trombone or drums or a clarinet—even when they’re making the same note.”
The sound made when you strike an object—or pull a bow across it, or blow air into a reed connected to it—carries information about the object’s shape, material makeup, and temperature. “You can identify an instrument by listening to what we call the spectrum—the different frequencies of the modes that are excited when you hit it,” Farr said. Black holes have modes, too, though they radiate in gravitational waves, not sound waves. So if you, say, pounded on a black hole, in theory, you could tell quite a bit about it. Two black holes colliding in effect do that pounding which scientists cannot themselves do—an experiment out in the cosmos.
Farr described the moment of collision between two holes as forming a shape like a “black peanut,” which eventually settles into a single, merged black hole. “Ringdown” is the term used to describe both that final merging and the “sound”—the waves—emitted by it. The ripples from a ringdown, if recorded precisely enough, could tell the story of a black hole: its spin, its mass, and its charge, and possibly information about the million-degree hot-gas ring external to it. “Basically, there’s currently no other way to probe this region of space-time except with gravitational waves,” Farr said. Light waves, radio waves, X rays—none of these ways of reading the universe let scientists see as far down toward the horizon of a black hole. “That’s one reason why it’s been revolutionary to have this tool.”
Ringdown may be used by researchers to test if general relativity accurately describes black holes. If it does, then the ringdown tones will come out one way; if not, then they won’t. “By examining the deviations in detail, tone by tone, you can try to understand where the deviation from general relativity is coming,” Farr said.
It will be a while before the public has a “feel” for gravitational waves, or for how black holes do or don’t unify quantum mechanics and general relativity. But those ideas still filter into the public imagination, however fuzzily. In 2020, the astrophysicist Andrea Ghez, of the University of California, Los Angeles, and a colleague received a Nobel for having tracked the path of objects near the center of our galaxy in sufficient detail to indicate that there must be a supermassive black hole there. In 2014, through an ingenious and sparsely funded passion project conducted by hundreds of scientists, the Event Horizon Telescope (E.H.T.) was assembled, with the goal of “photographing” a black hole. The E.H.T. is not one telescope but a network of initially eight and now eleven, spread across the planet. Together, the telescopes function as if they were one telescope with a lens the size of Earth. The E.H.T. gathered data on a distant black hole and then on a nearer one at the heart of the Milky Way. Images came through of the two black holes—or, rather, of the ring of stuff falling into the black holes.
To share the work with the general public, the scientists translated the data into astonishing images that were published all around the world. Because what the telescopes “saw” is at wavelengths outside the spectrum visible to the human eye, E.H.T. scientists made decisions about, for example, what color to use to depict the extreme heat of the plasma around the event horizon. (Even though blue flames are hotter than orange-red ones, they chose an orange-red color.) A number of scientists I spoke with about the images were gently indifferent, saying that they didn’t add to their fundamental understanding of black holes. Most were more impressed with the creative means of collecting the data than with the images themselves. (In contrast, for a certain crowd, the waveform that appeared when LIGO detected gravitational waves is a popular tattoo.) For other earthlings, that darkness, surrounded by its burning ring of fire, had the intimate quality of a Polaroid—but one of the alien that we call our universe.
Broderick, who was one of the first theoretical physicists to join the E.H.T., told me that, for scientists, the adage “seeing is believing” doesn’t always hold true; scientists often have to decide if they can trust what they see. He framed the E.H.T. as contributing both societal impact and scientific impact. It was “pleasingly novel to work on something I can tell my in-laws and mother about,” he said. He pointed out that it was estimated that half the world’s population had seen the first black-hole image: “I’m actually kind of curious; when was the last time so many newspapers had the same thing on their front page? We all shared this experience together, and there are precious few such things, and most of them are negative.” He saw the E.H.T. as a chance to make part of the story of physics real and visible.
As a kid, Broderick watched “Star Trek,” “Doctor Who,” and “anything adventurous,” he said, with his father, whom he would visit in the summer and on holidays. “He would record these movies and shows in anticipation of me coming, so we would binge-watch before it was a thing,” he said. His father was quadriplegic, and enjoyed photography and movies. Broderick told me that his viewing of “Star Trek” is not unrelated to his work as a theoretical physicist. In every episode, he said, the Starfleet travelled to incredible new places, and saw something different, and that was the adventure. “A part of what drew me to astrophysics was that Starfleet doesn’t exist. But I get to travel the universe with telescopes, large computers, blackboards, whiteboards, chalk—with whatever.” ♦
