Stephen Hawking always had something to say. He shook up the world of cosmology with more than 150 papers, dozens of which became renowned. He was told he had only a brief time on Earth, but spent half a century captivating audiences in lecture halls, on TV and in the pages of his books. For newspaper editors, almost any utterance of his could make a headline, and he knew it. Hawking warned about the threats of nuclear war, genetically modified viruses, artificial intelligence and marauding aliens. He pronounced on the human condition and once dismissed the role of God in creating the universe. The statement caused a fuss, as the denial of invisible superbeings still can in the 21st century.

It is an unwritten law of nature that when a personality steps into the foreground, their work must take a step back. In Hawking’s case, being the most famous scientist of our time had a mysterious ability to eclipse his actual achievements. At his best Hawking was spectacular: he made intuitive leaps that will keep scientists busy for decades.



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It began with Albert Einstein. Where Isaac Newton had thought gravity was an attraction borne by the fields of massive objects, Einstein said mass curved space itself. By his reckoning, the planets of the solar system circled the sun not because of some unseen force, but simply because they followed the curvature of space. The late US physicist John Wheeler once summarised the theory with characteristic simplicity: “Matter tells space how to curve; space tells matter how to move.”



Einstein’s formulation of gravity, set forth a century ago in the general theory of relativity, raised an exotic and somewhat unsettling possibility: that a truly massive object, such as an enormous star, could collapse under its own gravity, and would then become a speck of infinite density called a singularity. The gravitational pull of these weird cosmic dots would be so intense that not even light could escape them.



The idea that singularities were real and lurked in the darkness of space was not taken terribly seriously at first. But that changed in the 1950s and 60s, when a clutch of papers found that singularities – now known as black holes, a term coined by Wheeler – were not only plausible but inevitable in the universe.

This led to a surge in fascination with the objects that coincided with Hawking’s arrival as a PhD student at Cambridge University.



Hawking was never one to think small. His goal was a complete understanding of the universe. So while others pondered the creation of black holes in space, Hawking applied the same thinking to the cosmos itself. He joined forces with Roger Penrose, the Oxford mathematician, and showed that if you played time backwards and rewound the story of the universe, the opening scene was a singularity. It meant that the universe, with all of its warming stars and turning planets, including Earth with all its lives, loves and heartbreaks, came from a dot far smaller than this full stop.



Even before they worked together, Penrose got a flavour of Hawking’s sharp mind. Penrose had delivered a lecture on the big bang and Hawking, nearly a decade his junior, was in the audience. “I remember him asking some very awkward questions at the end,” Penrose said. “He obviously knew the weak points in what I was saying. It was clear he was someone to contend with.”

Hawking went back to black holes for his next act. Although the matter at the heart of a black hole is compacted into an infinitesimal point, black holes spin and have a “size” that depends on the amount of mass that falls into them. The greater the mass, the larger they are, and the farther out the so-called event horizon, the point where light falling into the black hole cannot come out. A supermassive black hole such as the one at the centre of the Milky Way captures light from as far away as 12.5m kilometres. If the Earth, at a mere six billion trillion tonnes, were compressed into a singularity, the resulting black hole would measure less than 2cm wide.



In the late 1970s, Hawking declared that a black hole could only ever get bigger. The maths behind the claim was strikingly similar to the equation that underpins one of the fundamental laws of nature – that entropy, a measure of disorder, can also only increase. When one physicist, Jacob Bekenstein, declared that the similarity was no coincidence, and that the area of a black hole was actually a measure of its entropy, Hawking and many other physicists balked. For a black hole to have entropy, it must be hot and radiate heat. But as everyone knows, nothing can escape a black hole, not even radiation. Or can it?



When Hawking set out to prove Bekenstein wrong, he made the most spectacular discovery of his career. Black holes did have a temperature, they did radiate heat – later known as Hawking radiation – and they could therefore shrink with time. As he remarked some time later: “Black holes ain’t so black.” It meant that, given enough time, a black hole would simply evaporate out of existence. For a typical black hole, that time is longer than the age of the universe. However, mini black holes, which are smaller than atoms, would be more dynamic, releasing heat with ferocious intensity until they finally explode with the energy of a million one megaton hydrogen bombs.



Hawking’s revelation shocked cosmologists, and the claim threw up a fresh and thorny problem that became known as the black hole information paradox. As Hawking himself realised, if black holes simply evaporated, then all of the information they held from infalling stars, planets and clouds of cosmic dust could be lost forever. It might not make for sleepless nights for most people, but most people are not theoretical physicists. The loss of information from the universe would contradict a basic rule of quantum mechanics. Hawking argued, nevertheless, that black holes destroyed information, while other physicists vehemently disagreed. In 1997, one of them, John Preskill at the California Institute of Technology, accepted a bet on the subject from Hawking. To the winner was promised an encyclopaedia of his choosing.



Marika Taylor, a former student of Hawking’s and now professor of theoretical physics at Southampton University, says that while the information paradox remains a paradox today, most physicists now believe that information is not destroyed in black holes. The answer may lie in the principles of holography, the process of capturing a 3D image on a two-dimensional sheet. When applied to black holes, the holographic principle shows that the event horizon can keep an audit of whatever falls inside. How it does so is unclear, but according to the theory, it retains a kind of imprint of the information. “Many people think that effectively, the black hole event horizon itself behaves like a giant computer hard disk,” Taylor said. “When the black hole evaporates into radiation, the information will be carefully encoded in the radiation that comes out.”



Hawking conceded his bet in 2004 and handed Preskill a copy of Total Baseball: The Ultimate Baseball Encyclopaedia. But even as he admitted defeat, Hawking was convinced the information released by a black hole would be mangled and impossible to read. To make the point, Hawking quipped that he should have burned the encyclopaedia and given Preskill the ashes.



To settle the matter once and for all, scientists need to detect Hawking radiation as it streams from a black hole and read the information it carries. But that is a fanciful idea. “We’d have to sit for millions or even billions of years to see this,” said Taylor. A more realistic hope is that subtle features of black holes may leave their mark on the gravitational waves that physicists can now detect with instruments such as Ligo, the US laser interferometer gravitational-wave observatory.



Hawking was, of course, far more than just a physicist. The stratospheric success of A Brief History of Time was driven by a blend of charisma, good writing, a profound theme and an excellent title. It put hard physics in the hands of millions, and even if millions did not finish the book, it changed the world. “If you look at the popular science press in physics, it looks totally different from 30 years ago,” said Sabine Hossenfelder, a research fellow at the Frankfurt Institute for Advanced Studies. “Everybody wants to know about black holes. People talk about the big bang over dinner. And Hawking has played a large role in this.” Hossenfelder read A Brief History of Time before she became a teenager. “I hated it because I didn’t understand anything,” she said. “And it’s the reason I’m a physicist today, because I thought I have to understand it.”



For Max Tegmark, a physics professor at MIT, Hawking was one of the most influential scientists of all time. The two worked together to raise publicity over the threats of nuclear war and the potential pitfalls of artificial intelligence. He was a person who wasn’t afraid to think about the big questions, Tegmark said. Having been told he would die young, Hawking pushed for actions that would ensure humanity did not. He thought we should “stop rolling the dice,” Tegmark said, and “plan ahead, to take advantage of this incredible cosmic opportunity we have.”



Hawking took opportunities whenever they arose, and his legacy will be richer for it. “When you think of the impact that Albert Einstein, Isaac Newton and others have had, it’s mainly in the past,” Tegmark said. “But when you think of the impact of Stephen Hawking, it’s clearly mostly in the future still. Stephen is going to be guiding our research for years to come.”

