In 1974, Stephen Hawking put forward an intriguing idea: Using the principles of quantum physics, he predicted that even though nothing is supposed to escape a black hole’s event horizon, these cosmic beasts can actually emit particles. And by ejecting these particles, black holes will shrink over a very long time, eventually evaporate and possibly explode.
Hawking sparked a debate that has continued for more than 50 years. The idea that black holes evaporate puts two fundamental tenets of physics — general relativity and quantum mechanics — into conflict.
But, assuming that Hawking and others are correct, 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 it’s possible. But in the past few decades, a number of potential explanations have emerged for how black holes seem to defy the rules of our universe by disappearing.
“We spent the last 50 years fighting about it,” Daniel Harlow, a physicist at MIT, told Live Science. “I would say now we understand it a lot better than Hawking did.”
Related: Could a black hole devour the universe?
Albert Einstein predicted the existence of black holes in 1915 with his theory of general relativity, which explains how gravity is a property of space-time’s curvature. Based on this theory, black holes are objects with a lot of mass compressed into a singular area, where gravity is so strong that even light can’t escape its pull.
“According to general relativity, everything can only go inside [a black hole] and nothing can ever come out,” said Heino Falcke, an astrophysicist at Radboud University in the Netherlands who was involved in capturing the first image of a black hole in 2019. “Everything thrown in is completely crushed into a point.”
But around 60 years later, Hawking’s calculations showed that perhaps not everything is crushed by black holes. In quantum mechanics, pairs of particles — particles and antiparticles — blink in and out of existence. These particles usually cancel each other out.
But Hawking argued that fluctuations of fields at the event horizon, a black hole’s “point of no return” beyond which nothing can escape, means that these particles don’t always cancel out: One of those particles can get sucked into the black hole, while the other gets ejected into space, leaving behind a cloud called…
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