Stephen Hawking says his group has solved a black hole puzzle

Stephen Hawking, the world’s most famous living physicist, thinks he has solved a mystery. It’s one that has puzzled scientists for more than 40 years: What happens to information about matter as it falls into a black hole?

Black holes are regions in space that contain huge amounts of matter. All that mass is packed together very tightly. The result is that a black hole’s gravity is so strong that not even light can escape. So if you fell into a black hole, you’d die. (Don’t worry, though. A person would never actually come anywhere near a black hole!)

But black holes don’t last forever. In the 1970s, Hawking showed that the energy in black holes slowly leaks away into space. It “evaporates” until nothing is left. It does this through a process now known as Hawking radiation.

The same thing should be true of the information about matter inside a black hole, such as its shape or its electrical charge. If the matter inside a black hole disappeared, so should any record of what had been inside it.

But that would defy a basic law of how the universe works. That law says that information is never lost. So the idea that information in a black hole could simply evaporate posed a major problem. Physicists called it the information paradox. (A paradox is an idea or a statement that is true, but seems logically impossible.)

Now, Hawking claims that he and two colleagues have solved that information paradox.

This trio proposes that the information about matter that falls into a black hole is actually stored in a boundary area that surrounds the black hole. This boundary is called an event horizon. A layer of light called a hologram slides along the event horizon. It’s stuck there, as if it were rowing upstream and getting nowhere, says physicist Andrew Strominger. He works at Harvard University in Cambridge, Mass., and is one of Hawking’s collaborators.

The researchers say the hologram acts like a very observant border guard. It stores a detailed record of every bit of matter that drifts into the black hole.

Here’s how the team thinks the process works: Every proton, atom or other bit of matter that gets pulled into the black hole causes some of the light in the hologram to shift along the event horizon. And each such disturbance is unique. Such shifts are called supertranslations. Each creates a unique record for each particle that enters the black hole. When Hawking radiation leaks out beyond the event horizon, it also carries the hologram’s information away too, bit by bit.

Strominger says the challenge is proving that supertranslations can really store the huge amount of information about a black hole’s entire contents. “This might not be the only kind of storage device that the hologram uses,” notes. But the idea of supertranslations preserving the information in black holes is an important step forward, he says.

Other physicists have proposed a similar idea. But Hawking and his colleagues say their proposal describes the specific process that allows each black hole in the universe to record and hold information about what is inside it. “This resolves the information paradox,” Hawking said on August 25. He presented the idea at the Hawking Radiation conference. It was held at the KTH Royal Institute of Technology in Stockholm, Sweden.

Strominger is more cautious. He notes that their research on this is not yet complete.

Other physicists say the idea sounds interesting. Still, they find it is only as convincing as the mathematical “evidence” to support it. And that is what’s not yet complete. But Strominger is confident has his team’s research will completely change how physicists think about black holes. Researchers hope that resolving the information paradox will help them understand how gravity works at the scale of tiny particles, such as atoms, and the even smaller scale of particles that make up those atoms. At this scale, the behavior of matter is ruled by a special set of laws known as quantum mechanics.

Several physicists say it’s hard to make judgments about the new announcement without reviewing a scientific paper. “Stephen whet our appetite but didn’t really flesh out the ideas,” says Michael Duff, who attended Hawking’s talk. Duff works for Imperial College London. As a theoretical physicist, he specializes in using mathematical models to understand the nature of matter and energy. The suspense will continue until at least the end of this month. That’s when Hawking says his team plans to post a paper online.

Ummm, maybe not. Strominger says to expect a longer wait.

Power Words

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black hole  A region of space having a gravitational field so intense that no matter nor radiation (including light) can escape.

event horizon An imaginary sphere that surrounds a black hole. The more massive the black hole, the bigger the sphere. Anything that happens inside the event horizon is invisible, because gravity is so strong that under normal circumstances even light can’t escape. But according to some theories of physics, in certain situations small amounts of radiation can escape.

gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

Hawking radiation The particles emitted from the event horizon on the outer edges of a black hole. Energy can be converted into a pair of particles. If that happens very close to outer edge of a black hole, one of those particles can tunnel out and become detected — providing the only direct physical clue to the black hole’s presence. These emissions are called Hawking radiation for Stephen Hawking, the famous British physicist who came up with the idea that black holes can emit particles.

hologram  An image made of light and projected onto a surface, depicting the contents of a space.

information paradox  (in physics) A problem created by two conflicting ideas about how black holes work and how the universe works. Black holes eventually disappear, and presumably, the information they contain about what’s in them also disappears. But  this disappearance breaks a law of quantum mechanics, which says that information is never “lost” to the universe.

matter Something which occupies space and has mass. Anything with matter will weigh something on Earth.

mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from. For objects on Earth, we know the mass as “weight.”

model  A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.

paradox  An idea or a statement that is true, but that seems logically impossible.

physics  The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. It’s an alternative to quantum physics in explaining the motions and behavior of matter. A scientist who works in that field is known as a physicist.

proton A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

quantum mechanics  A branch of physics dealing with the behavior of matter on the scale of atoms or subatomic particles.

supertranslation  (in physics) A rearrangement of light in a black hole’s event horizon (the boundary surrounding the black hole) that, according to some physicists, occurs when a particle of matter enters a black hole.

theoretical physics  A branch of physics that uses mathematical models to understand the nature and properties of matter and energy. A scientist who works in that field is known as a theoretical physicist.