There is a certain comfort in the finality of a black hole, the sense that once something crosses the event horizon, the story is over. Black holes merge; they do not divide. They swallow; they do not split. But physics has a way of softening the edges of our certainties, and a new theoretical study published in Physical Review D has found a narrow loophole in the laws that normally forbid a black hole from splitting in two. The loophole is not wide. It requires a black hole spinning at nearly the maximum rate the laws of physics allow. But it exists, and that is enough to unsettle a comfortable assumption.
The research, led by Francesco Sannino of the University of Southern Denmark and the University of Naples Federico II, with colleagues Giacomo Cacciapaglia, Manuel Del Piano, and Vania Vellucci, began with a simple question: which physical laws prevent a black hole from dividing, and could any of them be bypassed ? The primary obstacle is thermodynamics. A black hole has entropy, a measure of its internal disorder, and splitting into two smaller black holes would reduce the total entropy of the system. The process is thermodynamically unfavorable, like a shattered vase reassembling itself .
But things change when a black hole rotates. For a rapidly spinning black hole, the calculations show, the entropy arithmetic becomes more complicated. A window opens—a narrow one, but a window nonetheless—in which a split could occur without violating the laws of thermodynamics . The resulting black holes would have to separate rather than immediately fall back together, and Sannino acknowledged that when this requirement is included, "the window largely closes again, except possibly for black holes rotating close to their maximum possible rate" .
If such a split did occur, the black hole's event horizon—the boundary beyond which nothing can escape—would take on a shape that the researchers describe with a term that is both precise and oddly domestic: "pair-of-pants topology." One section of the horizon would divide into two "legs," each becoming a new black hole, connected briefly by a bridge that dissolves as they separate . The image is strange, but it captures the mathematics: a single surface becoming two, connected by a passage that does not last.
The study also explores more exotic possibilities involving additional dimensions of space, where black-hole-like objects are already known to become unstable and fragment. Under some of these conditions, splitting can become thermodynamically favorable, particularly when one resulting black hole is much smaller than the other . But the authors are careful not to overstate what they have found. They cannot yet assign a probability to a black hole splitting. That depends on how gravity behaves beyond Einstein's theory, and the work of quantifying it continues.
What they can say is that small primordial black holes—remnants of the early universe—are more likely candidates for splitting than the massive astrophysical black holes formed by stellar collapse. The reason is simple: smaller objects can rotate faster, and the loophole depends on rotation near the maximum . Detection, if it ever comes, would be indirect. Gamma-ray telescopes like Fermi-LAT could search for signals from tiny black-hole fragments that might form during mergers, a whisper of a split rather than a direct observation .
For now, the study remains a theoretical exploration, a careful probing of the boundaries of what the laws of physics permit. It does not claim that black holes split. It claims that the prohibition is not absolute, that under conditions extreme enough and rare enough, the universe might allow a black hole to become two. Whether any black hole has ever taken that path is a question that observation, not calculation, will have to answer.
AI Image Disclaimer: The images in this article are generated using AI tools and are illustrative only; they do not depict the actual research, the researchers, or any observed black hole.
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