The centuries-old history of understanding black holes

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Very little about black holes, which are among the strangest objects in the universe, is simple. By studying the massive ripples that black holes create in spacetime and learning how they are formed, scientists have a more complete idea of ​​these mind-altering objects than ever before. But the short history of mankind’s understanding of black holes was shattered along the way by major twists and turns.

Although the existence of black holes is all but certain, experts weren’t so sure half a century ago. Robert Mann, a physicist at the University of Waterloo who studies black holes and quantum information, says that when he was a graduate student in the 1970s, “professors really had doubts about it.”

The first inklings that black holes exist predate the American Constitution. As early as 1783, the Rev. John Michell, a British scientist, envisioned black holes as “dark stars.” Michell asked what a star would look like if it were so massive that the speed it would take to escape its gravitational pull was “faster than light,” says Mann.

Michell’s question was good. But a few years later, in the 1790s, renowned French mathematician Pierre-Simon Laplace and other pioneering thinkers convinced the scientific community that light behaves like a wave and is therefore unaffected by gravity, says Mann. This new conception of light made Michell’s theory seem irrelevant.

But the idea was revived after 1915 when Albert Einstein proposed his General Theory of Relativity. The theory states that any object with mass warps spacetime in proportion to its gravity, allowing a certain amount of matter to become so dense that it collapses into an infinitely dense point called a singularity — the heart of a black hole .

People often say Einstein predicted black holes, but that’s not entirely true, says Javier Garcia, an astrophysicist at Caltech who uses X-rays to study the fundamental properties of black holes. “Einstein developed the theory” necessary for their existence, says Garcia, but he didn’t predict the objects themselves.

In 1915, Einstein used general relativity to explain Mercury’s motion around the sun. These and other successful applications of Einstein’s theory encouraged scientists to explore its deeper implications.

[Related: Black holes have a reputation as devourers. But they can help spawn stars, too.]

Within a year, Karl Schwarzschild, who was “a conscripted lieutenant in the German army but a theoretical astronomer by trade,” as Mann puts it, heard of Einstein’s theory. He was the first to work out a solution to Einstein’s equations that showed that a singularity can form — and nothing, if it gets too close, can move fast enough to escape a singularity’s gravitational pull.

Then, in 1939, physicists Rober Oppenheimer (famous or disgraceful) and Hartland Snyder were trying to figure out if a star could create Schwarzschild’s impossible-sounding object. They reasoned that with a large enough dust ball, gravity would cause the mass to collapse and form a singularity, which they showed with their calculations. But when World War II broke out, progress in this area stalled until the late 1950s, when people began retesting Einstein’s theories.

Physicist John Wheeler, pondering the effects of a black hole, asked one of his graduate students, Jacob Bekenstein, a question that baffled scientists in the late 1950s. As Mann paraphrased, “What happens when you pour hot tea into a black hole?”

The answer, of course, is that the black hole drinks it up. But the hot tea creates a paradox. Anything with a certain temperature gives off heat. And mixing hot and cool objects creates an exchange – for example, if you put ice cubes in a hot bath, 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 warmer “contradicts everything we know about thermodynamics,” says Mann.

In 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 in but never escape, and they would appear completely obscure to any observer.

Wheeler’s student Bekenstein then worked with Stephen Hawking to discover that black holes do in fact emit energy. This radiation, caused by quantum fluctuations in space, releases little energy. But their research proved that black holes have heat — which definitely answered the question Wheeler asked a decade and a half earlier.

Their introduction of quantum physics to black holes solved one paradox but created another, Mann says. Quantum mechanics requires that information cannot be destroyed. And currently, scientists have no way of telling about the material that went into a black hole based on the small amount of radiation it emits — that information is being lost.

“There is still no agreement on how to solve this problem,” Mann says, although some researchers believe they are close to the solution.

[Related: What we can learn from baby black holes]

Hawking helped solve another mystery that has been inherent in black holes since their inception. The Schwarzschild solution, which Schwarzschild developed in the early 20th century, not only prevented light from escaping. It also contained a hole in space-time at the core of the black hole – the singularity. But at the time, scientists weren’t sure if this was a general property 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 produced a singularity, was not just a one-time solution for impossibly round stars – any sufficiently large mass would do so.

X-ray observations of potential black holes had accumulated over the decades, but it wasn’t until the first LIGO detections, announced in 2016, that astronomers had direct evidence that black holes exist. Not only that, they crash together to form larger black holes and emit gravitational waves, Mann says.

Scientists still don’t know what to do with the information paradox or singularities. “Nevertheless, we see these objects. And we took a picture of one, too,” says Mann, referring to the first image of luminous matter around a black hole taken by the Event Horizon Telescope in 2019.

New telescopes continue to illuminate the darkest objects in the universe. And when major upgrades are complete, the Event Horizon Telescope team hopes to capture the first video of a black hole.

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