In 1974, Stephen Hawking put forward an interesting idea: Using the principles of quantum physics, he predicted that even though nothing can escape a black hole’s event horizon, these cosmic beasts can actually emit particles. And by expelling these particles, black holes will shrink over very long periods of time, eventually evaporating and possibly exploding.
Hawking sparked a debate that has continued for more than 50 years. The idea that black holes evaporate puts two fundamental theories of physics — general relativity and quantum mechanics — into conflict.
But, assuming Hawking and others are right, how would a black hole evaporate, leaving nothing behind? Before we explore this mind-bending idea, it’s important to know that theoretical physicists are still debating how this is possible. But over the past few decades, several possible explanations have emerged for how black holes manage to vanish and defy the rules of our universe.
Albert Einstein predicted the existence of black holes in 1915 with his theory of general relativity, which describes how gravity is a property of the curvature of space-time. Based on this theory, black holes are objects with very high masses compressed into a singular region, where gravity is so strong that not even light can escape its pull.
“According to general relativity, everything can only go inside [a black hole] and nothing can ever come out,” said astrophysicist Heino Falcke of Radboud University in the Netherlands, who was involved in capturing the first image of a black hole in 2019. “Everything thrown inside is completely crushed into a point.”
But nearly 60 years later, Hawking’s calculations showed that maybe not everything is crushed by black holes. In quantum mechanics, pairs of particles — particles and antiparticles — come into existence and go away. These particles usually cancel each other out.
But Hawking argued that field fluctuations at the event horizon, a black hole’s “point of no return” beyond which nothing can escape, mean that these particles don’t always cancel out: One of the particles might get sucked into the black hole, while the other escapes into space, leaving behind a cloud called Hawking radiation. According to this theory, as more and more particles get ejected, black holes begin to lose energy and mass, and eventually disappear.
This process would be very slow. A black hole with the mass of the sun might take 10^67 years to completely evaporate — longer than the current age of the universe. And scientists haven’t found proof of this yet; black holes don’t emit thermal radiation, which means Hawking radiation can’t be detected. Some scientists are trying to get a look at this elusive radiation in laboratories and at smaller black holes, which are set to evaporate more quickly than the black holes in our galaxy.
The Black Hole Paradox
However, Hawking’s idea has some caveats that lead to confusing questions. Evaporation gives rise to a puzzle called the black hole information paradox. If a black hole evaporates and disappears, the particles left behind do not contain information about the original state of matter. This violates a basic concept in physics — that one system at one time should determine, or reflect, its state in another — also known as predictability.
Scientists are still debating how to solve this paradox. “The amazing thing about Hawking’s paradox is that any solution to it requires you to give up some sacred principle of physics,” Harlow said. Hawking’s solution was to give up predictability, as he lamented in a 1976 paper.
Some physicists are investigating the rules surrounding thermodynamics to resolve this discrepancy, and how entropy affects quantum information. Another group of physicists is investigating locality, the principle that objects are only directly affected by their surroundings. They believe the information paradox could be resolved through something called quantum nonlocality — the idea that particles inside a black hole share their quantum state with correlated particles outside it.
Despite progress in understanding black hole evaporation, the mysteries continue to grow. In a 2023 study in the journal Physical Review Letters, Falcke and his colleagues argued that the information paradox may not be limited to black holes. By reworking Hawking’s calculations, the team proposed that all objects may have the same problem. All things can evaporate, making the puzzle even deeper.