We’re still in the dark about a key black hole paradox
Until recently, the existence of black holes was far from a given.
Very little about black holes, among the strangest objects in the universe, is straightforward. Scientists have a more complete conception of these mind-bending objects than ever before, by studying the massive ripples black holes create in spacetime and learning about how they form. But the brief history of humanity’s understanding of black holes was rocked with major twists and turns along the way.
Although the existence of black holes is all but certain, just a half-century ago experts weren’t so sure. Robert Mann, a physicist at the University of Waterloo who studies black holes and quantum information, says when he was a grad student in the 1970s, “professors really doubted it.”
The first inklings that black holes exist are older than the American Constitution. Way back in 1783, the Reverend John Michell, a British scientist, conceived of black holes as “dark stars.” Michell asked what a star would look like if it was so heavy that the velocity needed to escape its gravitational pull was “faster than light,” Mann says.
Michell’s question was a good one. But a few years later, in the 1790s, the renowned French mathematician Pierre-Simon Laplace and other pioneering thinkers convinced the scientific community that light behaved like a wave and, therefore, was not affected by gravity, Mann says. This new conception of light made Michell’s theory look irrelevant.
But the idea was revived after 1915, when Albert Einstein proposed his theory of general relativity. The theory says that any object with mass curves spacetime in proportion to how heavy it is, and allows for a certain amount of matter to become so dense it collapses into an infinitely dense point called a singularity–the heart of a black hole.
People often say Einstein predicted black holes, but this isn’t quite right, says Javier Garcia, an astrophysicist at Caltech who uses X-rays to study the fundamental properties of black holes. “Einstein developed the theory” that’s necessary for their existence, Garcia says, but didn’t predict the objects themselves.
In 1915, Einstein used general relativity to explain the motion of Mercury around the sun. This and other successful applications of Einstein’s theory encouraged scientists to explore its deeper implications.
Within a year, Karl Schwarzschild, who was “a lieutenant in the German army, by conscription, but a theoretical astronomer by profession,” as Mann puts it, heard of Einstein’s theory. He was the first person to work out a solution to Einstein’s equations, which showed that a singularity could form–and nothing, once it got too close, could move fast enough to escape a singularity’s pull.
Then, in 1939, physicists Rober Oppenheimer (of Manhattan Project fame, or infamy) and Hartland Snyder tried to find out whether a star could create Schwarzschild’s impossible-sounding object. They reasoned that given a big enough sphere of dust, gravity would cause the mass to collapse and form a singularity, which they showed with their calculations. But once World War II broke out, progress in this field stalled until the late 1950s, when people started trying to test Einstein’s theories again.
Physicist John Wheeler, thinking about the implications of a black hole, asked one of his grad students, Jacob Bekenstein, a question that stumped scientists in the late 1950s. As Mann paraphrased it: “What happens if you pour hot tea into a black hole?”
The answer is the black hole drinks it up, of course. But the hot tea causes a paradox. Anything with some temperature emits heat. And mixing hot and cool objects causes an exchange–when you put ice cubes in a hot bath, for instance, the ice cubes warm up and the bath cools down.
If a black hole swallows everything and emits nothing, that means it doesn’t emit heat, and must have zero temperature. A black hole that sucks in hot matter and never gets any warmer “contradicts everything we know about thermodynamics,” Mann says.
By the 1960s, these objects had a catchy name, “black hole.” The term explained two features: They were holes, in the sense that things could fall into them but never escape, and they would appear totally dark to any observers.
Wheeler’s student, Bekenstein, went on to work with Stephen Hawking to find that black holes do in fact give off energy. This radiation, caused by quantum fluctuations in space, releases only a tiny bit of energy. But their research proved black holes have heat–definitively answering the question Wheeler asked a decade-and-a-half prior.
Their introduction of quantum physics to black holes solved one paradox, but created another, Mann says. Quantum mechanics requires that information can’t be destroyed. And currently, scientists don’t have a way to tell anything about the stuff that went into a black hole from the little radiation it gives off–that information is lost.
“There is still no agreement on how to solve this problem,” Mann says, though some researchers believe they’re close to solving it.
[Related: What we can learn from baby black holes]
Hawking helped solve another mystery that had clung to black holes since the beginning. The black hole solution that Schwarzschild came up with in the early 20th century didn’t just prevent light from escaping. It also included a hole in spacetime at the core of the black hole–the singularity. But at the time, scientists weren’t sure whether this was a general quality of black holes, or just a quirk of the specific systems Schwarzschild, and later Oppenheimer and Snyder, chose to calculate.
Hawking and Roger Penrose showed that Schwarzschild’s solution, which created a singularity, wasn’t just a one-off for impossibly round stars—any sufficiently large mass would do so.
X-ray observations of potential black holes had stacked up over the decades, but it wasn’t until the first LIGO detections, announced in 2016, that astronomers had direct evidence that black holes exist. And not only that, they crash together to make bigger black holes and radiate gravity waves, Mann says.
Scientists still don’t know what to make of the information paradox or the singularities. “Yet we see these objects. And we’ve also taken a picture of one,” Mann says, referring to the first image of the glowing matter around a black hole taken by the Event Horizon Telescope in 2019.
New telescopes continue to shed light on the universe’s darkest objects. And when major upgrades are ready, the Event Horizon Telescope team hopes to take the first video of a black hole.