Milestone: Black hole radiation theorized
Date: March 1, 1974
Where: Cambridge, England
WHO: Stephen Hawking
In 1974, a brilliant 32-year-old physicist published a not-quite-two-page paper in the journal Nature – blowing up one of our fundamental assumptions about black holes.
According to Einstein’s theory of relativity, black holes are so massive that nothing, not even light, can escape their clutches. By that logic, black holes should only grow as the universe ages, gobbling up nearby matter or merging with other black holes to eventually reach supermassive scales.
But for a few years before his groundbreaking paper, Hawking had been investigating how quantum mechanics – the strange laws that govern subatomic particles – would affect black hole growth and evolution. Building on the work of theoretical physicists Jacob Beckensteinhe combined general relativity, the laws of thermodynamics and relatively simple quantum physics to deduce that black holes radiate minimal amounts of heat.
In his popular 1988 book “A Brief History of Time,” Hawking argued that it was because pairs of “virtual” particles pop in and out of existence throughout the universe, annihilating on contact. Occasionally, however, one member of the pair would appear just outside a black hole’s event horizon, while the other would be just inside that boundary. One would fall in, while the other would escape, carrying with it a little bit of warmth. Over time, this loss of heat, or radiation, will shrink the black hole, causing its surface gravity to increase. This in turn will cause the black hole to accelerate its radiation, leading to the black hole’s eventual vaporization, possibly via explosion.
(Actually, later research showed that the particle-antiparticle explanation is grossly oversimplified, and Hawking radiation actually appears as a result of the acceleration of an observer near a black hole’s event horizon.)
For black holes with the mass of the Sun or larger, evaporation of what is now known as “Hawking radiation” would take longer than the age of the universe, the study concluded. But Hawking also wondered whether tiny primordial black holes formed from “quantum fluctuations” at the dawn of time. These tiny black holes, smaller 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 about 1 million 1 Mton hydrogen bombs,” Hawking said dryly in his paper.
Hawking radiation soon became firmly entrenched in physics theory. But it also revealed a huge paradox in black hole physics: Evaporation meant that “information” that fell into a black hole was lost forever. This, in turn, would violate a central tenet of quantum mechanics: that information cannot be created or destroyed. In the next four decades, until his death in 2018Hawking would chip away at the black hole information paradox.
In a public lecture in Sweden in 2015, Hawking repeated his suggestion that information can indeed escape a black hole, possibly via a wormhole.
“Black holes are not as black as they are painted. They are not the eternal prisons they once thought.” Hawking said. “Things can come out of a black hole both on the outside and possibly come out in another universe.”
After his death, some of his collaborators published a series of articles which seemed to resolve the paradox; information is not lost when it enters a black hole, they put, but arose.
And in 2024, physicists proposed a way to find it: The information swallowed by a black hole would leave traces in subtle ripples in space-time around these cosmic monsters. These ripples would reveal themselves in gravitational waves we already detect using massive observatories.
Scientists have yet to find direct evidence of black hole explosions or primordial black holes. But the James Webb space telescope recently discovered an ancient galaxy that could be explained by primordial black holes.
