Scientists just found a loophole that could let black holes split in two

The research, published in Physical Review D under the title "When Black Holes Admit Pair-of-pants Topologies," examines the physical laws that keep black holes intact and whether certain circumstances could allow them to break apart.

"In our study, we investigate which physical laws normally prevent a black hole from splitting into two. We also investigate if certain circumstances may allow a split to happen," says Francesco Sannino, co-author of the study and a professor of physics at University of Naples Federico II in Italy and Quantum Field Theory Center at University of Southern Denmark.

The research team also included Giacomo Cacciapaglia, a researcher with the French National Centre for Scientific Research (CNRS) at the Laboratoire de Physique Théorique et Hautes Énergies, a joint CNRS and Sorbonne Université laboratory in Paris. Additional contributors were Manuel Del Piano and Vania Vellucci, both from the Quantum Field Theory Center at the University of Southern Denmark.

Why Black Holes Normally Cannot Split Apart

One of the biggest obstacles to a black hole splitting comes from thermodynamics, the branch of physics that governs how energy behaves and how physical systems change.

A central concept is entropy, which describes how many different microscopic arrangements a system can have while appearing unchanged from the outside. Black holes have entropy, and this property places important limits on what can happen to them.

For a black hole that is not rotating, dividing into two smaller black holes would reduce the total entropy. Such a decrease makes the process thermodynamically unfavorable, helping explain why black holes do not ordinarily break apart.

However, the researchers found that the situation could change dramatically when a black hole spins at an exceptionally high speed.

Extreme Rotation Could Allow Black Holes to Split

The team's calculations suggest that rapid rotation may create a narrow opening in the physical rules that normally prevent black holes from dividing. In principle, a sufficiently fast spinning black hole could overcome the usual thermodynamic restriction.

But there is another major obstacle. Even if a black hole could divide, the two resulting objects would need to move far enough apart to avoid immediately merging again.

"But the two resulting black holes must also be able to separate rather than immediately fall back together. When we include this requirement, the window largely closes again, except possibly for black holes rotating close to their maximum possible rate," says Francesco Sannino.

The study's unusual title refers to the shape that a black hole's boundary would trace through space and time if such a split occurred. This boundary, called the horizon, would begin as a single structure before separating into two distinct branches.

Viewed across time, the resulting shape would resemble a pair of trousers, with one upper section dividing into two "legs." This is the idea behind the term "pair-of-pants topologies."

Extra Dimensions Offer Another Possibility

The researchers also investigated more unusual scenarios involving theories in which space has additional dimensions beyond the three we experience in everyday life.

Physicists have already identified objects resembling black holes in some of these theoretical models that can become unstable and start breaking apart. These possibilities suggest that the familiar rules governing black holes may not apply in the same way under more exotic conditions.

In certain cases, splitting could even become thermodynamically favorable, especially when one of the two resulting black holes is much smaller than the other.

However, a process being allowed or favored by thermodynamics does not necessarily mean it will happen. Researchers still need to determine whether such fragmentation can physically occur and what conditions would make it possible.

Could Astronomers Detect a Black Hole Splitting?

The scientists are now working to better understand how likely these events might be. The answer may depend on aspects of gravity that go beyond Einstein's theory, making the problem especially challenging.

"We cannot yet give a single probability for a black hole to split. It depends on how gravity behaves beyond Einstein's theory, and we are working to quantify this. We expect small primordial black holes to be more prone to splitting than much larger astrophysical black holes. We may detect a split indirectly: gamma-ray telescopes such as Fermi-LAT can search for signals from tiny black-hole fragments that might form during mergers," says Francesco Sannino.

Although there is currently no confirmed observation of a black hole splitting, the possibility of detecting signals from tiny fragments offers a potential way to investigate the idea. Such evidence could provide valuable clues about the behavior of gravity under some of the most extreme conditions in the universe.

Sannino is the director of the Quantum Field Theory Center (ℏQTC), which is affiliated with the Department of Mathematics and Computer Science (IMADA) at the University of Southern Denmark. He is also affiliated with the Danish Institute for Advanced Study and the Department of Physics "Ettore Pancini" at the University of Naples Federico II. In addition, he serves as a visiting professor at Durham University.