To understand black holes, physicists turn to a mathematical ‘Rosetta stone’

Physicists have found a new way to investigate a long-standing black hole enigma using the mathematics of particle physics.

Black holes aren’t entirely black. They emit a faint mist of particles called Hawking radiation, a concept central to major puzzles that swirl around black holes. But Hawking radiation is so faint that it’s not possible to observe directly.

Now, several teams of physicists have found a new angle on the phenomenon. They are taking advantage of a mathematical connection between two seemingly distinct types of physics — a link known as the double copy. 

Fundamental physics theories fall into two distinct camps: A theory called the standard model describes the physics of subatomic particles, while the general theory of relativity describes gravity. The double copy draws a mathematical connection between these two seemingly separate theories. That relationship can be used as a mathematical translation tool, switching a calculation from one “language” of physics to another. The swap can make calculations easier or reveal new insights.

According to the double copy, many phenomena of general relativity are mathematically equivalent to those of certain particles in the standard model, with one change: In general relativity, there are two copies of a particular part of the equation. Since this relationship was discovered in 2010 and developed in the years since, it has become a useful tool for understanding a variety of gravitational effects.

“It allows us to calculate things we’ve never been able to calculate before, just by recycling results in a clever way,” says theoretical physicist Chris White of Queen Mary University of London.

Until now, scientists didn’t have a standard model analog for Hawking radiation based on the double copy. Finding one “constitutes a major advance for these techniques,” says theoretical physicist Cynthia Keeler of Arizona State University in Tempe, who was not involved with the research. That’s because Hawking radiation connects the large and the small, with enormous black holes emitting tiny particles. The discovery shows that the double copy can bridge both scales.

In a paper accepted to the Journal of High Energy Physics, White and colleagues determined how Hawking radiation translates into the language of the standard model. In that language, the mathematical alter ego of Hawking radiation is a charged particle scattering off a spherical shell of charged matter collapsing in on itself. That’s mathematically equivalent to the emission of a particle of Hawking radiation.

Two other teams came to essentially the same conclusion, finding a mathematical analog of Hawking radiation. Reported in February in Physical Review Letters, the pair of papers shows that physics intrinsic to black holes is contained in the standard model of particle physics, says theoretical physicist Anton Ilderton of the University of Edinburgh, a coauthor of one of the studies. “These papers have started to show how to extract that information from the standard model.”

Scientists hope to explore even more inscrutable features of black holes in this way. For example, researchers would like to also find a standard model analog for a black hole’s event horizon, the boundary beyond which nothing that enters can escape. “That’s the big question that we would like to answer,” says theoretical physicist Uri Kol of Harvard University, who was not involved with the research. “These papers provide tools that can be used to address this question.”

Hawking radiation alone is intriguing enough for further study. After physicist Stephen Hawking conceived of the radiation in 1974, physicists realized it implied a puzzle. As a black hole spews particles, it shrinks and eventually obliterates itself. Physicists don’t understand what happens to the information it once swallowed. According to quantum physics, information can’t be destroyed. Studying features of Hawking radiation translated into the language of the standard model may help illuminate what’s going on.

Hawking radiation is a “Rosetta stone” problem, says theoretical physicist John Joseph Carrasco of Northwestern University in Evanston, Ill., coauthor of one of the Physical Review Letters papers. By studying it, physicists could become more fluent with gravity’s language.