# Stephen Hawking Thinks We Can Solve a Major Black Hole Mystery With Hair

No toupees needed, though. The “hairs” in question are minute changes in spacetime at the fringes of black holes

The most famous living physicist has proposed a new way to solve one of most perplexing puzzles in modern physics: Can a black hole erase information from the universe? According to Stephen Hawking and his collaborators, the answer might be no—if you give black holes a lush head of hair.

“Whether or not this is the final answer is totally unclear to us,” says co-author Malcolm Perry, a theoretical physicist at the University of Cambridge. “We’re saying that it’s a step on the way.”

The most basic description of a black hole states that what goes in never comes out. Based on Einstein’s general theory of relativity, which describes how gravity affects the cosmos, a black hole is so massive that anything crossing its edge, or event horizon, is pulled in and lost forever, even light.

Then in the 1970s, Hawking discovered that black holes themselves can disappear. Thanks to the predictions of quantum mechanics, physicists think space is not a vacuum but is filled with so-called virtual particles that wink in and out of existence. These particles appear in pairs that are doomed to annihilate as soon as they come in contact with each other. But if a pair appears on either side of a black hole's event horizon, one gets swallowed and the other radiates away into space.

The escaping radiation steals energy from the black hole as it departs, so that the black hole loses mass over time. It eventually evaporates out of existence. According to Hawking’s calculations, the lingering radiation—the only trace of a vanished black hole—contains no useful information about how the black hole formed and what it ate.

That’s a problem for physicists hoping to unite relativity with quantum mechanics, which most experts say prohibits information loss. Shred an encyclopedia, burn it or lose it to the stupendously strong gravity of a black hole, and its remnants must always contain clues that, in theory, allow the tape to be rewound and its original contents to be known.

In search of this reversibility, Hawking revisited his original calculations. At the time he proposed black hole radiation, mathematical theorems had painted black holes as fairly featureless entities when viewed from the outside. All that could be measured was a black hole’s total mass and charge, as well as how fast it spun.

The new work, published online this week to the pre-print site arxiv.org, grants the cosmic monster additional distinguishing features known as “hairs,” in this case, minute changes in the vacuum just outside the event horizon.

“We show that if you throw something at a black hole it in the right way, you can implant hair,” says theoretical physicist Andrew Strominger of Harvard University. He and his colleagues illustrate this with charged particles.

Like energy, electric charge can neither be created nor destroyed. Because charge must be conserved in the universe, Hawking and his co-authors suggest that charged particles crossing the event horizon could leave traces of themselves in space in the form of strange particles of light, called soft photons, that have no energy. These bizarre photons change the vacuum of spacetime and allow it to preserve information about the original particles.

Hawking and his colleagues propose that the altered vacuum might store information about three-dimensional objects on the black hole's surface, in the same way that a hologram acts as a 3D representation of a flat image.

“Whether the mathematics of general relativity allows that is quite subtle,” says Lee Smolin, a theoretical physicist at the Perimeter Institute for Theoretical Physics. Smolin instead favors quantum explanations of black holes based on their centers instead of their edges, adding that he will be looking to other experts to check the math on the new paper.

No one has yet reported a mistake in the calculations, but concerns have begun to pop up that the theory is at best incomplete. Sabine Hossenfelder of the Nordic Institute for Theoretical Physics questions how much information the proposed soft hair could encode. She also points out that the paper does not explain how the hairs, which would disappear with the black hole once it evaporated, would transfer their information to the radiation that remains.

“I am not at all convinced that the new idea proposed by Hawking, Perry, and Strominger solves the information loss problem,” she writes on the blog Backreaction. “But it seems an interesting avenue that is worth further exploration. And I am sure we will see further exploration.”