Gravitational wave detectors have shown that black holes frequently collide and merge, producing larger black holes. Physicists have now examined whether the process could run in reverse, allowing one black hole to split into two, and they concluded that the possibility cannot be ruled out.

The hypothetical process produces diagrams resembling trousers, inspiring the researchers to describe it as “wearing pants.” The idea may sound playful, but it confronts some of the most formidable properties associated with black holes, including immense gravity and the constraints imposed by thermodynamics.

Black holes exert gravity so powerful that not even light can escape. That makes the prospect of one portion breaking away from the rest seem unlikely, but Professor Francesco Sannino of the University of Southern Denmark and colleagues decided to investigate whether such a division could ever occur.

Gravity is not the only obstacle. The authors also point to thermodynamics and Stephen Hawking’s prediction that two smaller black holes possess less entropy, meaning less disorder within a system, than one large black hole. Gravitational wave observations have supported that prediction.

Because the entropy of the universe is always growing, a system whose entropy shrinks without external input would appear to violate the usual order of nature. On that basis, splitting one black hole into two smaller ones faces a major thermodynamic barrier.

The researchers identified a possible exception involving an original black hole rotating close to the maximum possible speed. Black holes spin because of the angular momentum of the stars that collapsed to form them, along with material that subsequently fell into them.

For a black hole, faster rotation corresponds to lower entropy when other factors remain constant. If a rapidly spinning black hole divided into two black holes that rotated more slowly, the arrangement could theoretically allow the proposed split under the conditions explored by the researchers.

The angular momentum of the original black hole would still need somewhere to go. The authors suggest that if the two newly created black holes orbited one another, their orbit could consume that angular momentum and help make the unusual configuration theoretically possible.

Even then, the conditions would be extreme. The researchers found that a split could theoretically occur only when the original black hole’s spin was close to its maximum possible value, which implies that such events would be rare.

A graph showing the black hole’s boundary over time would resemble a pair of trousers. The original black hole occupies the waist, while the two black holes produced by the proposed division extend into the legs, leading the authors to call the models “pair-of-pants topologies”.

Yet the trousers contain a conspicuous gap in the explanation. In the authors’ diagram, the crotch of the “pants” carries the label “unknown dynamics,” reflecting the unresolved question of what physical process could actually initiate the split.

One possible mechanism proposed by the researchers involves “superradiance.” In that scenario, waves become trapped in a black hole and are amplified until they generate an instability powerful enough that it might shake the black hole apart.

The possibility may become more plausible if dimensions beyond the familiar four are permitted. In those multidimensional examples, the most plausible splits involve a much smaller black hole breaking away from the original rather than the black hole dividing into two equal parts.

Extra dimensions do not eliminate the central mystery, however, because the researchers still lack an explanation for what would trigger the division. Sannino said the likelihood also depends on how gravity behaves beyond Einstein’s theory and that the team is working to quantify it.

“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,” Sannino said in a statement.

That possible indirect search could focus on signals from tiny fragments created during mergers. The research does not establish that black holes split, but it lays out conditions under which the process might be possible and identifies the missing dynamics that physicists must still explain.