History of Science: Discovery of Carbon-14 Opens Window on Past Civilizations — February 27, 1940

In 1974, a brilliant 32-year-old physicist published a paper in Nature that was less than two pages long. And it overturned one of our fundamental assumptions about black holes.

The author was Stephen Hawking, whose paper “Are Black Holes Exploding?” became his most lasting legacy.

According to Einstein’s theory of relativity, black holes are so massive that not even light can escape their grasp. According to that logic, black holes should grow only as the universe ages, eating up nearby matter or merging with other black holes, eventually reaching supermassive scales.

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But for several years before his seminal paper, Hawking had been studying how quantum mechanics – the strange laws governing elementary particles – influence the growth and evolution of black holes. Building on the work of theoretical physicist Jacob Beckenstein, he combined general relativity, the laws of thermodynamics, and relatively simple quantum physics to deduce that black holes radiate small amounts of heat.

In his popular 1988 book, The History of Time, Hawking argued that the reason is that pairs of “virtual” particles pop up throughout the universe and disappear when they come into contact. But occasionally, one member of the pair appears just outside the black hole’s event horizon, and the other just inside its boundary. One falls inside and the other escapes with a slight fever. Over time, this loss of heat or radiation causes the black hole to shrink and its surface gravity to increase. As a result, the black hole would accelerate its radiation, possibly resulting in the black hole’s eventual evaporation via an explosion.

(In fact, later research has greatly simplified the particle-antiparticle explanation and showed that Hawking radiation actually appears as a result of the acceleration of observers close to the black hole’s event horizon.)

For black holes larger than the mass of the sun, evaporation by what is now known as “Hawking radiation” would take longer than the age of the universe, the study concluded. But Hawking also wondered whether small primordial black holes could have formed from ancient “quantum fluctuations.” These tiny black holes, weighing less than about 1 trillion kilograms, would have exploded long ago, he concluded.

“This is a fairly small explosion by astronomical standards, but it is equivalent to one million 1M tonne hydrogen bombs,” Hawking pointed out in his paper.

Hawking radiation quickly became firmly entrenched in physical theory. But it also revealed a major contradiction in black hole physics. Evaporation meant that the “information” that fell into the black hole was lost forever. This would violate a central principle of quantum mechanics: information cannot be created or destroyed. Over the next 40 years, until his death in 2018, Hawking gradually unraveled the black hole information paradox.

In a public lecture in Sweden in 2015, Hawking reiterated his suggestion that information can indeed escape from black holes, perhaps via wormholes.

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“Black holes are not as black as they are portrayed. They are not eternal prisons as once thought,” Hawking said. “Things can come out from outside the black hole, and maybe even into another universe.”

After his death, some of his collaborators published a series of papers that seemed to resolve this contradiction. They hypothesized that information is not lost when entering a black hole, but instead flows backwards.

And in 2024, physicists proposed a way to find it. Information swallowed by black holes will leave traces as subtle ripples in the space-time surrounding these cosmic monsters. These ripples will appear in gravitational waves, which we already detect using large observatories.

Scientists have yet to find direct evidence of exploding black holes or primordial black holes. However, the James Webb Space Telescope recently detected an ancient galaxy that can be explained by a primordial black hole.