Black Holes Swallow Everything, Even the Truth
The most mysterious objects in the universe are fundamentally unknowable.
In 1967, the physicist John Wheeler was giving a lecture about a mysterious and startling phenomenon in deep space that the field was just beginning to understand. But it didn’t have a great name to match. Wheeler and his audience were equally tired of hearing “gravitationally completely collapsed object” over and over, so someone threw out an idea for a different name. A few weeks later, at another conference, Wheeler debuted the suggestion: black hole. And it’s perfect, isn’t it? What else would you call a dark abyss that swallows light and matter and doesn’t let go?
Decades later, black holes—invisible, impenetrable, and many light-years away—are more familiar to us than ever before. We know that supermassive versions sit at the center of most galaxies, including our own Milky Way. In 2019, we even got pictures that show a black hole as an imposing shadow against the glow of cosmic material. Scientists have detected the gravitational ripples that result when black holes smash into each other; the entire cosmos, we recently learned, might be humming with the force of such collisions. The list goes on.
But the big mystery still remains: We don’t know what lies at the center of a black hole, beyond the boundary where matter winks out of view forever. “The question is definitely answerable in that someone could fall inside the black hole and find out the answer,” Eliot Quataert, a theoretical astrophysicist at Princeton, told me in an email. “The problem is they couldn’t convey that answer to someone outside the black hole—because nothing can get out.”
Because such hands-on observation is impossible, scientists must approach the subject theoretically. The effort involves mind-bending physics calculations, endless thought experiments, and seriously entertaining the possibility that the universe is delightfully weirder than we can imagine. It also requires accepting that we’ll never truly know for sure what’s inside a black hole.
For us earthlings, a bottomless pit more massive than the sun might be difficult to fathom. “Black holes are not solid objects, like planets or asteroids,” Shane Larson, a physics professor at Northwestern University, told me. They’re more like regions in space, seemingly empty spots made noticeable by the stars orbiting wildly around them. “It’s kind of like an open window,” Larson said: an invisible line that separates exterior from interior.
To understand why black holes scramble our understanding of cosmic forces, we need to think about their structure. If a black hole were a Ferrero Rocher chocolate ball, the first layer of chocolate and crushed hazelnuts would be the region just outside the event horizon, where gravity is still weak enough that a nearby star could safely whiz by and not fall in. Next up is a layer of crispy wafer; this is the event horizon, the point of no return. Under this layer lies smooth chocolate filling, through which trapped cosmic material is sucked toward the center. And then there’s the heart of the Ferrero candy, the whole, roasted hazelnut. This is the singularity, a tiny, concentrated point of infinite density.
Physicists believe they understand what the space beyond the event horizon—nearly all the way through the chocolate filling—should look like, based on Einstein’s theory of general relativity. If an astronaut were to fall into a black hole, she would descend, and as she went deeper, she would experience the very fabric of space-time warping all around her. The 1915 theory describes well what must occur in such an extreme environment, where gravity overwhelms all other forces in the universe. The trouble starts farther in, closer to the singularity, where “the laws of physics as we currently understand them break down,” Larson said.
Deep inside a black hole, general relativity isn’t enough to explain what’s happening; you also need a different kind of physics, quantum mechanics, which deals with the tiniest particles of the universe, atoms and their even-smaller components. “When collapsing all matter down to a point,” as a black hole does, “the size gets small enough that quantum effects become important,” James Miller-Jones, an astronomer at Curtin University, in Australia, told me in an email. Unfortunately, general relativity and quantum mechanics do not get along.
According to the principles of general relativity, once stuff goes into a black hole, it’s lost for good. You can determine some fundamental properties of the black hole, such as its mass, but not its constituent parts. In the 1970s, Stephen Hawking showed that black holes actually evaporate very slowly, emitting radiation from just outside the event horizon. This development should have been thrilling for quantum mechanics, which dictates that information can’t be destroyed. If someone took apart a completed puzzle and scattered it around your garden, you could collect the pieces and put them together again, Nicholas Warner, a physics and astronomy professor at the University of Southern California, told me. It would take some time and effort without a picture to guide you, but quantum theory says you could do it. But the particles wafting off black holes seem entirely devoid of information about the contents of their interior—a clear-cut violation of that principle. It’s as if someone had run all those puzzle pieces through a washing machine, and “they all become this gloopy, gray mess,” Warner said.
Theoretical physicists around the world are trying to reconcile the mismatch between general relativity and quantum mechanics. Warner is a member of the camp that believes that Einstein’s theory—the very principles that predicted the existence of black holes before astronomers found evidence of them—is incomplete. Other experts feel the same but say Einstein isn’t the only one to blame. “Maybe you have to change quantum mechanics too,” Daniel Harlow, a physicist at MIT, told me. Harlow and his colleagues have posited that the radiation that Hawking discovered is indeed encoded with information from the depths, and our understanding of quantum mechanics isn’t good enough yet to unscramble it.
Everyone is searching for a theory of quantum gravity that avoids any contradiction. “We don’t have one of those,” Charles Hailey, an astrophysicist at Columbia University, told me. “Not even close.” But some physicists I spoke with said the field could crack it within decades, certainly in this century. Scientists would finally know—but only in the theoretical sense, of course.
That’s the thing about black holes. We can get only so close to the truth, only experience certain kinds of knowing. Even the powerful telescopes that have shown us sparkling galaxies nearly all the way back to the Big Bang can’t help us here. “Observationally, we’re almost certainly not going to learn anything about the inside of black holes in this century,” Carl Rodriguez, a physics professor at the University of North Carolina at Chapel Hill, told me in an email. The best we can do is to study some effects of black holes. Maya Fishbach, an astrophysicist at the University of Toronto, thinks we’ll learn more by studying the invisible gravitational waves that fan out when two black holes collide and merge into one. Those waves carry with them information about the newly formed black hole, which vibrates in the aftermath of its creation like a bell. “Just like listening to a ringing bell can tell us what the bell is made of, listening to the black hole [ringing] can tell us what the black hole is made of,” Fishbach told me.
But the purest form of discovery will always remain out of reach. “If ‘know’ means a student could take a field trip, observe directly with their own senses, then come back and write a class report about what they observed, then we will never know what is inside,” Larson said. Perhaps that’s not the worst thing. “If it were easy to encounter situations like that”—environments with extreme, drag-you-into-the-abyss gravity—”it would probably be bad for us,” Harlow said. Safer to study black holes from afar, in our quiet cosmic neighborhood, where gravity is far weaker and we overcome it each day, simply by pulling back the covers and getting out of bed in the morning.