By living an extraordinarily rich, productive life despite his profound disability, Stephen Hawking inspired millions around the world. But Hawking, who died on Wednesday at the age of 76, made his greatest contributions as a theoretical physicist.
Over the years, his books, papers, and lectures turned generations into armchair cosmologists and transformed our understanding of the universe — especially with regard to the strange celestial objects known as black holes.
The first black hole was discovered in 1971, and we now believe that 100 million or so are sprinkled across the universe. Most astronomers now believe that black holes lie at the center of most, if not all, galaxies, including our own Milky Way.
But at the time of Hawking's birth in 1942, black holes were little more than a mathematical quirk — a prediction of Albert Einstein's 1916 theory of general relativity. The term black hole itself wasn't coined until the 1960s, when scientists began to realize that Einstein's math actually described real objects — gaping abysses of raw gravitational force so powerful that they suck in dust, gas, and stars and stop light itself from escaping.
In the 1960s, Hawking and fellow British physicist Roger Penrose built on Einstein's theories to describe the physical characteristics of black holes and showed that when a star collapses it forms an infinitely dense point called a singularity — the birth of a black hole.
Hawking also helped confirm the Big Bang theory. Drawing once again from Einstein's equations, he and Penrose showed that 13.8 billion years ago the universe emerged violently from a single compressed point no bigger than an atom.
"We didn't know these things until they proved them," Sean Carroll, a professor of physics at Caltech and one of the world's leading cosmologists, says of Hawking and Penrose. "So through that alone I think it's safe to say that Hawking taught us more about gravity and space-time than any person since Einstein."
Hawking's biggest breakthrough was yet to come.
Black holes not so black
In 1974, Hawking published a paper that contained one of the strangest ideas yet about black holes.
At the time, the consensus among physicists was that once something gets sucked into a black hole it can never escape. They believed that as black holes kept swallowing up all matter around them, they grew inexorably.
Hawking showed that black holes can actually shrink. The reason, he said, was that black holes shed particles and radiate energy — a phenomenon that came to be known as "Hawking radiation." And this conclusion meant that rather than being voids producing nothing at all — as physicists had long thought — black holes actually glow.
"As Hawking himself put it, 'black holes ain't so black,'" Carroll says. "This was a stunning finding that surprised everybody, and we're still trying to understand its implications."
Hawking went on to show that while large black holes emit radiation as a slow dribble, small ones glow, emitting lots of radiation quickly. He showed that all black holes eventually evaporate or boil themselves away, expiring in a brilliant burst of energy equivalent to a million 1-megaton hydrogen bombs.
As Hawking's mind kept coming up with new ideas, his body was slowly withering — a result of the neurodegenerative disease amyotrophic lateral sclerosis, or Lou Gehrig's disease, with which he had been diagnosed with at age 21. Ironically, his growing paralysis may have enabled him to think about black holes in ways other physicists never could.
"Most physicists work with equations using pen and paper, but because of his disability, Hawking found it easier to visualize things in his mind," says Alan Lightman, a professor of theoretical physics at MIT and one of Hawking's long-time friends. "So he developed some new graphical methods for visualizing the trajectory of light rays through the warped space geometry created by the immense gravity of a black hole. And he applied these methods to making these major breakthroughs."
The information paradox
For all Hawking's remarkable insights, he was never able to solve one of the greatest puzzles involving black holes.
Every star is associated with lots of information — ranging from its shape and chemical composition to how fast it spins. But when a star collapses to become a black hole, it seems that all that information is lost forever.
But this appears to violate a bedrock concept of quantum physics, the rules that describe how the universe works at the atomic and subatomic levels. Quantum physics holds that information can never truly be lost.
"The reason why this is upsetting to physicists is that all our current theories of the universe assume that the amount of information in the world stays constant," Lightman says. "You can move information from one place to another, but you cannot destroy it."
Exactly what happens to the lost information contained within stars when they collapse remains a mystery, although Hawking — who spent 40 years on the problem — came up with a number of possible solutions.
In the early 2000s, he proposed that the information isn't actually lost but is somehow encoded in the radiation black holes emit. But neither Hawking nor his colleagues came up with anything resembling a concrete proof. Thus the so-called "information paradox" remains unsolved — and may stay unsolved for the foreseeable future.
Or it just might be that the Whole Earth Telescope — a project that links 30 observatories in 12 countries to create an Earth-sized telescope — will detect a tiny black hole and actually observe Hawking radiation. If that were to happen, physicists could probe for clues as to whether this radiation does indeed encode the missing information.
"The black holes we know about are so large that their Hawking radiation is predicted to be much fainter than even the background light in the cosmos, too faint to pick up," Carroll says. "If we could identify a really tiny black hole, the radiation emitted would be much larger."
Hawking himself might not have been surprised should the real answer to the information paradox be far stranger. In one of his final published works, Hawking argued that black holes might serve as gateways to parallel universes and that missing information might actually be transferred to those universes.
Whatever the truth, Hawking remained convinced that by attempting to understand the universe's inner workings, humans can learn valuable lessons about themselves. As he explained in a 2015 lecture, black holes offer can their own form of inspiration.
"Black holes are not the eternal prisons they were once thought," he said. "Things can get out of a black hole both on the outside and possibly come out in another universe. If you feel you are trapped in a black hole, don't give up. There is a way out."