Stephen Hawking, one of the most influential physicists of the twentieth century and perhaps the most celebrated icon of contemporary science, has died at the age of 76.
The University of Cambridge confirmed that the physicist died in the early hours of 14 March at his home in Cambridge, England.
Since his early twenties, Hawking had lived with amyotrophic lateral sclerosis (ALS), a disease in which motor neurons die, leaving the brain incapable of controlling muscles. Hawking’s health had been reportedly deteriorating; just over a year ago, he was hospitalized during a trip to Rome.
His death was marked by statements from scientists around the world. Astrophysicist Neil deGrasse Tyson, director of the Hayden Planetarium in New York City, wrote on Twitter: “His passing has left an intellectual vacuum in his wake. But it’s not empty. Think of it as a kind of vacuum energy permeating the fabric of spacetime that defies measure.”
“The reaction among physicists is just profound shock and sadness,” says Malcolm Perry, a Cambridge theoretical physicist who was a student of Hawking’s in the early 1970s. “He was a truly extraordinary man,” says Roger Penrose, a theoretical physicist at the University of Oxford, UK, who in 1970 co-authored a seminal paper with Hawking on the nature of black holes.
Another of Hawking’s former students at Cambridge, theoretical physicist Raphael Bousso, told Nature that his teacher was a brilliant physicist who also excelled at communicating science to the public. “These are two distinct skills. Stephen excelled at both.”
Bousso, now at the University of California, Berkeley, recalls how he had to learn to shake off his awe and relax around Hawking. “Stephen was a joyful and light-hearted person, not to be burdened by excessively respectful and convoluted interactions,” he says.
The British physicist was born in Oxford in 1942. He was diagnosed with ALS when he was 21, while a doctoral student in cosmology at the University of Cambridge. Hawking first realized that something was wrong when he went ice skating with his mother one day, he recalled in a speech at his 75th birthday celebration last year. “I fell over and had great difficulty getting up,” he told the audience. “At first I became depressed. I seemed to be getting worse very rapidly.”
Although physicians initially gave him just a few years to live, his disease advanced more slowly than expected. He went on to have an active career for decades, both as a theoretical physicist and as a popularizer of science. Still, Hawking progressively lost use of most of his muscles, and for the last three decades of his life was communicating almost exclusively through a voice synthesizer.
Over the years, Hawking became one of the most recognized names in contemporary science. His books, particularly A Brief History of Time (1988), became blockbuster successes. He relished making cameo appearances on television shows such as Star Trek: The Next Generation, The Simpsons and The Big Bang Theory.
But scientifically, his name is most closely associated with the physics of black holes, which he began to study when they were still considered mere mathematical curiosities in Albert Einstein’s general theory of relativity. In the early 1970s, he began to investigate what quantum physics could reveal about the event horizon, a black hole’s surface of no return. Hawking shocked the physics world when he calculated that this surface should slowly emit radiation (soon to become known as Hawking radiation). Black holes were not truly black.
This emission, he reasoned, should ultimately lead the black hole to shrink and disappear1. Even more shocking to researchers was Hawking’s realization in 1976 that Hawking radiation should erase information from the Universe, in apparent contradiction to some of the basic tenets of quantum theory2.
“The importance of this work was not so much the effect itself, but that he was able to provide the one clear-cut physical implication that we know of which brings together the two great revolutions of twentieth-century physics, namely general relativity and quantum mechanics, and showing that this necessarily brought in profound notions from thermodynamics,” says Penrose.
Two years ago, together with Perry and Andrew Strominger at Harvard University in Cambridge, Massachusetts, Hawking began to sketch a possible way out of the black-hole information paradox. The three of them, along with Strominger’s student Sasha Haco, had been working on a follow-up paper. “We believe we are just in the final stages of doing the [calculations] required,” Perry says. “The paper will have his name on it.”
Perhaps because most of his work was of speculative nature and difficult to test experimentally, Hawking was never awarded a Nobel prize. In 2016, some wondered whether he finally might win one, when Jeff Steinhauer, a physicist at the Technion–Israel Institute of Technology in Haifa, announced that he had found convincing evidence of Hawking radiation — not in an actual black hole, but in a laboratory analogue made of extremely cold atoms. However, some experts still consider those results inconclusive, and many say that their relevance to true black holes is uncertain.
A more direct test of some of Hawking’s findings might yet come from the study of astrophysical black holes through gravitational waves, initiated by the US-based Laser Interferometer Gravitational-wave Observatory (LIGO). Hawking and others have linked the surface area of a black hole’s event horizon to its entropy, a measure of disorder. When interviewed by Nature’s news team in 2016 about LIGO’s first detection of gravitational waves from merging black holes, Hawking said that he hoped future detections would be sensitive enough to confirm a prediction he had made in the 1970s: that the surface area of a post-merger black hole should be larger than the combined surface area of the original objects that formed it. This is consistent with the thermodynamic law that governs entropy in the Universe, which states that over time, entropy can never decrease. “I would like them to test my area theorem,” he said.
“His work on the behaviour of black holes provides some of the key principles and ideas that help us to interpret the data from our gravitational-wave observations of black-hole mergers,” says Bruce Allen, who studied under Hawking during his PhD in the early 1980s and is director of the Max Planck Institute for Gravitational Physics, Hanover, Germany.
With cosmologist Thomas Hertog, another former student, Hawking explored the concept of cosmic inflation — a brief period of extremely fast expansion in the first moments of the Big Bang — and how it could spawn several universes, a ‘multiverse’. “We set out to develop methods to transform the idea of a multiverse into a coherent testable scientific framework,” says Thomas Hertog, a cosmologist at KU Leuven in Belgium and former student of Hawking who co-authored studies with him. “This was Hawking: to boldly go where Star Trek fears to tread.”
Nature 555, 423-424 (2018)