What Is Hawking's Black Hole Paradox? 50-year Mystery Cracked, Experts Say

Scientists believe they have cracked one of the most pressing mysteries surrounding black holes that has boggled experts for decades.

Black holes are among the most extreme and strange objects we know of, birthed from the collapse of gigantic stars. They are objects with such an intense gravitational pull that light itself cannot escape once it falls in.

One particular characteristic of black holes has puzzled scientists for decades. In the 1970s, world-renowned physicist Stephen Hawking and colleagues found that black holes emit something called Hawking radiation—a faint glow of light that actually drains the energy of a black hole. Over time, a black hole will emit so much Hawking Radiation that it will disappear completely.

Crucially, scientists' calculations showed that Hawking radiation cannot reveal anything about what is inside the black hole. In other words, when a black hole eventually dies, all of the information about the things it gobbled up in its lifetime disappears forever.

The problem is that this should not be possible according to quantum mechanics—the rules of physics that govern how the universe works at its most fundamental level. Quantum mechanics states that information can never be destroyed. According to the Massachusetts Institute of Technology, if an observer knows the current state of any system, then they can know everything about its past and future.

"Information thrown into the black hole will vanish as radiation is thermal and does not contain any information. This would be a huge problem for quantum mechanics: a fundamental principle of quantum mechanics is that any process in physics can be mathematically reversed," Professor Xavier Calmet, professor of physics at the University of Sussex in the U.K. who led a new study into the issue, told Newsweek. "It should be possible to rewind the movie and watch it backwards! Is general relativity wrong? Is quantum mechanics wrong?"

This problem is known as the black hole information paradox. On Thursday, Calmet and other scientists from Italy and the U.S. published what they say is a solution.

In essence, the study shows that black holes have external features—the researchers call these features "hair"—that allows outside observers to tell what's going on inside the black hole.

This is in contrast to what's called the "no-hair theorem," which states that only three things can be known about a black hole by looking at it from the outside: its mass, its electric charge, and its angular momentum.

However, Calmet and colleagues think that, actually, black holes do give away more information than previously thought.

According to their calculations, objects falling into a black hole leave imprints on the black hole's gravitational field that, in theory, preserve at least some of those objects' information.

"We are showing that Hawking radiation and space-time geometry are entangled," Calmet said. "This is a huge quantum system that we call the quantum state of the graviton field and this state contains all the information that ever went into the black hole."

Calmet told Newsweek that, in theory, it is possible for an outside observer to piece together information about what has fallen into a black hole by studying these black hole 'hairs'—though practically this would be "as hopeless as trying to reconstruct a piece of coal after it has been burnt."

The significance of this finding, though, is that the rules of quantum mechanics don't need to be rewritten to solve the black hole information paradox.

"Our solution doesn't require any speculative idea; instead our research demonstrates that the two theories can be used to make consistent calculations for black holes and explain how information is stored without the need for radical new physics," Calmet told The Guardian newspaper.

Calmet conceded that it is a "big claim" that would take the scientific community some time to accept.

Indeed, Professor Toby Wiseman, a theoretical physicist at Imperial College London, told The Guardian that the study was "a good bit of work" but did not show that the entirety of the information that falls into a black hole can be recovered.

He believes that the two universal theories of quantum mechanics and gravity—which have never been reconciled—will have to "come together" in order to resolve the paradox.

Tom Hartman, associate professor of physics at Cornell University with expertise in quantum gravity and black holes, told Newsweek that in his view, the new study "does not make significant progress on the black hole information paradox."

Update, 3/21/22 8:37 a.m. ET: This article has been updated to include comment from Hartman and Calmet.

Stephen Hawking
Renowned scientist Stephen Hawking seen at Cambridge University in the U.K. in June, 2008. In the 1970s, Hawking and colleagues theorized that back holes emitted a glow called Hawking radiation. Shaun Curry/AFP/Getty