Black holes might split in two under extreme conditions, theoretical calculations suggest, offering physicists a way to explore the limits of our understanding of gravity.
Two black holes can collide and merge into one. But could a single black hole divide into two smaller ones that go their separate ways? Reversing a merger might sound straightforward, but the rules governing black holes make it difficult.
The entropy barrier to splitting
One obstacle is entropy, a measure of how many different microscopic arrangements can give a physical system the same overall properties. Think of rolling two dice. Several combinations produce a total of seven, but only two ones produce a total of two. Entropy captures a similar distinction between an overall result and the different underlying arrangements that can produce it. More possible arrangements mean higher entropy.
For an ordinary black hole that is not rotating, two smaller black holes would have less combined entropy than the original. That makes splitting thermodynamically unfavorable. The usual requirement of thermodynamics is that the total entropy of an isolated system should stay the same or increase, rather than decrease.
Whether black holes can split is the subject of “When Black Holes Admit Pair-of-pants Topologies,” a theoretical study published in Physical Review D. The unusual title describes what a split would look like through time. A black hole’s outer boundary, called its horizon, would trace a trouser-like shape, with one section dividing into two “legs.”
“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, a co-author and professor of physics at the University of Naples Federico II in Italy and the Quantum Field Theory Center at the University of Southern Denmark.
Sannino’s co-authors are Giacomo Cacciapaglia, a Centre National de la Recherche Scientifique (CNRS) researcher at the Laboratoire de Physique Théorique et Hautes Énergies, a joint laboratory of CNRS and Sorbonne Université in Paris, and Manuel Del Piano and Vania Vellucci, both from the Quantum Field Theory Center at the University of Southern Denmark.
Rapid rotation offers a possible loophole
Their calculations suggest that rapid rotation could offer a loophole in the thermodynamic barrier. A spinning black hole might be able to divide under conditions that would make splitting unfavorable for a nonrotating one.
“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.
Uncertain odds beyond Einstein’s theory
The researchers also explore theories with additional dimensions of space. Within these theories, physicists already know of black-hole-like objects that can become unstable and begin to break apart. Under some of these conditions, splitting can be thermodynamically favorable, particularly when one resulting black hole is much smaller than the other. Even then, showing that entropy permits a split does not establish that the process will actually happen.
The possibilities also extend to primordial black holes, hypothetical objects that could have formed in the early universe.
“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.
Reference: “When black holes admit pair-of-pants topologies” by Giacomo Cacciapaglia, Manuel Del Piano, Francesco Sannino and Vania Vellucci, 22 October 2026, Physical Review D.
DOI: 10.1103/cx69-t466
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