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Physics

Hawking’s legacy

The British physicist showed black holes can emit radiation and shrink in size

Hawking and a representation of a black hole, one of his chief research interests

Imagem: Andrew Cowie /AFP / Getty Images | NASA

British physicist Stephen Hawking, who died on March 14 at the age of 76, after living for more than half a century with amyotrophic lateral sclerosis (ALS), was a theorist who, though often overshadowed by his media celebrity, made important contributions to our understanding of the origin of the Universe and the physics of black holes. In 1963, he learned he had a motor neuron disease that would progressively impair his motor function and confine him to a wheelchair for decades. He is considered to have been most productive in science during a period of 10 or 15 years following his diagnosis, before acquiring world fame. “Until the mid-1970s, Hawking did very solid work,” says physicist George Matsas of the Institute for Theoretical Physics at São Paulo State University (IFT-UNESP). “Thereafter his ideas became more speculative.”

Hawking was born in Oxford and studied physics at the city’s famed university. But at the age of 20, after earning his degree, he moved to archrival Cambridge to pursue a PhD in physics, and more specifically in cosmology. He completed his PhD in 1966 with a thesis titled “Properties of Expanding Universes.” His first major breakthrough came in 1970, working alongside British mathematician Roger Penrose, then a professor at Birkbeck College at the University of London and 10 years his senior.

In an article published on January 27 that year in the Proceedings of the Royal Society A, Hawking and Penrose demonstrated an implication of Einstein’s general theory of relativity, which treats gravity as a geometric property of space-time. The duo showed that the German physicist’s ideas led inexorably to the conclusion that the Universe had to have begun in the distant past through a gravitational singularity—a region of infinite curvature in space-time. “They demonstrated mathematically that reversing the arrow of time would lead to a classical singularity,” explains Daniel Vanzella of the São Carlos Institute of Physics at the University of São Paulo (IFSC-USP). That initial singularity, in which all matter collapsed into a point of infinite density, was the Big Bang—the dawn of the Universe. While the notion of a ​​Big Bang predates Hawking and Penrose’s paper, they showed that general relativity made this initial singularity inevitable within a classical context (i.e. without taking quantum ingredients into account).

In the early 1970s, while still working on general relativity and the concept of the singularity, Hawking turned his attention to one of the most mysterious objects in the Cosmos: black holes. The physicist’s early studies agreed with the classical framework. First defined as extremely compact regions in space-time where gravity is so strong that nothing—not even light—can escape it, black holes emerge from Hawking’s early work as indestructible entities. Any matter or light that brushes the event horizon, a boundary marking the point of no return around a black hole, is sucked inside. According to this interpretation, black holes can only increase and never decrease in mass.

The question became much more complex and took an unexpected turn when in addition to general relativity, Hawking factored the ideas of quantum mechanics into his analyses and calculations on black-hole dynamics. In a paper published on March 1, 1974, in Nature, he first posited that black holes should emit subatomic particles, a theoretical proposition that would later become known as Hawking radiation. “As a black hole emits this thermal radiation one would expect it to lose mass,” the Cambridge physicist wrote in his paper. This radiation was a quantum effect that caused black holes to gradually lose energy (and mass). It was evidence that, contrary to what general relativity predicts, black holes can in fact shrink and evaporate, meaning they can be destroyed. “The smaller the black hole, the hotter its radiation,” says theoretical physicist André Landulfo of the Federal University of ABC (UFABC). As Hawking himself would later say, black holes are not so black after all.

Virtual particles
The mechanism behind Hawking radiation involves what physicists call virtual particles. The vacuum is populated by particles and antiparticles that are quickly created and annihilated in pairs. This occurs so rapidly that, according to Heisenberg’s uncertainty principle—one of the fundamental laws of quantum mechanics—these particles cannot be directly observed. As a result, they are called “virtual particles.” However, near the event horizon of a black hole, something special will sometimes occur: one of the particles of the pair will fall into the black hole with negative energy while its counterpart will appear outside with positive energy. If this happens, the particles will become real and no longer virtual. The particle outside the event horizon would be observed at infinity as thermal radiation, now famously known as Hawking radiation. The particle inside would cause the black hole to lose energy, making it shrink. “At the end of this process, it is as if the black hole has evaporated by emitting radiation,” explains Matsas.

Hawking showed that the Universe according to general relativity could only have begun with a Big Bang

The concept of Hawking radiation, though never experimentally confirmed, has established itself as one of the most important ideas about how black holes work. It is generally regarded as Hawking’s greatest contribution. “For any black hole that is expected to arise in normal astrophysical processes, however, the Hawking radiation would be exceedingly tiny, and certainly unobservable directly by any techniques known today,” said Roger Penrose, now professor emeritus at Oxford University, in an obituary published in The Guardian shortly after the death of his colleague. Matsas notes that effects analogous to Hawking radiation have been observed in other areas of physics, such as sound waves being emitted from acoustic black holes—clouds of ultracold atoms—from which sound should not be expected to escape.

Throughout Hawking’s life, subjects such as the origin of the Universe, time, and space, and the nature of black holes, would continue to be present in his writing for both scientific and wider audiences. But the celebrity physicist’s mind also gravitated toward new questions, some more speculative in nature—such as the extra dimensions predicted by superstring theory and the existence of other universes (besides ours).

After Albert Einstein (1879–1955), the best-known scientist of the twentieth century, few physicists have become as popular among lay audiences as Hawking did. He wrote popular physics books, such as the international bestseller A Brief History of Time (1988), which sold 10 million copies and has been translated into 40 languages, and was never embarrassed to appear in public. He made guest appearances on television programs such as Star Trek: The next generation and The Big Bang Theory. He was portrayed as a cartoon character on The Simpsons. His synthesized voice featured on two tracks by British rock band Pink Floyd. And his life story was played out in documentaries and even in a movie, The Theory of Everything (2014), which earned an Oscar for the British actor who portrayed him, Eddie Redmayne.