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.
|A MEETING OF MINDS At an August conference in Sweden, Stephen Hawking proposed a solution to the decades-old puzzle. It had to do with what happens to information about matter that has fallen into black holes. KTH Royal Institute of Technology|
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.