Shred a document, and you can piece it back together. Burn a book, and you could theoretically do the same. But send information into a black hole, and it’s lost forever.
Well, maybe not.
For years, physicists have argued that black holes are the ultimate vaults, entities that suck in information and then evaporate without leaving behind any clues as to what they once contained.
WHAT LIES WITHIN
“According to our work, information isn’t lost once it enters a black hole,” says Dejan Stojkovic, associate professor of physics at University at Buffalo. “It doesn’t just disappear.”
A new paper in Physical Review Letters outlines how interactions between particles emitted by a black hole can reveal information about what lies within, such as characteristics of the object that formed the black hole to begin with, and characteristics of the matter and energy drawn inside.
This is an important discovery, Stojkovic says, because even physicists who believed information was not lost in black holes have struggled to show, mathematically, how this happens. The new paper offers explicit calculations demonstrating how information is preserved.
STEPHEN HAWKING’S THEORY
The research marks a significant step toward solving the “information loss paradox,” a problem that has plagued physics for almost 40 years, since Stephen Hawking first proposed that black holes could radiate energy and evaporate over time.
The theory posed a huge problem because it meant that information inside a black hole could be permanently lost when the black hole disappeared—a violation of quantum mechanics, which states that information must be conserved.
In the 1970s, Hawking proposed that black holes were capable of radiating particles, and that the energy lost through this process would cause the black holes to shrink and eventually disappear. Hawking further concluded that the particles emitted by a black hole would provide no clues about what lay inside, meaning that any information held within a black hole would be completely lost once the entity evaporated.
Though Hawking later said he was wrong and that information could escape from black holes, the subject of whether and how it’s possible to recover information from a black hole has remained a topic of debate.
PARTICLE INTERACTIONS
Instead of looking only at the particles a black hole emits, the new study also takes into account the subtle interactions between the particles. This shows it’s possible for an observer standing outside of a black hole to recover information about what lies within.
Interactions between particles can range from gravitational attraction to the exchange of mediators like photons between particles. Such “correlations” have long been known to exist, but many scientists discounted them as unimportant in the past.
“These correlations were often ignored in related calculations since they were thought to be small and not capable of making a significant difference,” Stojkovic says.
“Our explicit calculations show that though the correlations start off very small, they grow in time and become large enough to change the outcome.”
