Single black holes could theoretically split into two smaller entities, but only if they spin near their maximum possible speed. A new study published in Physical Review D reveals that thermodynamics normally forbids this fragmentation, leaving an extreme, near-limit rotational loophole.
We are used to watching cosmic giants collide. When two black holes smash together, they merge to create a more massive black hole, setting space ringing with gravitational waves. But physics has long posed a stubborn reverse question: could the opposite happen? Could a single black hole tear itself apart to form two smaller ones?
A team of researchers has calculated that the answer is yes, under highly restrictive conditions. Their theoretical work was published on Oct. 22 in the journal Physical Review D.
Why Thermodynamics Normally Forbids a Split
The main obstacle stopping a black hole from dividing is not gravity alone, but entropy. In physics, entropy measures how many microscopic arrangements can correspond to the same large-scale state. The overall entropy would drop if a standard black hole without any rotation were to divide into a pair of smaller ones.
That outcome represents a drop in entropy, which runs afoul of the second law of thermodynamics.
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.
Francesco Sannino, professor of physics at University of Naples Federico II and Quantum Field Theory Center at University of Southern Denmark
The study’s authors are Giacomo Cacciapaglia, Manuel Del Piano, Vania Vellucci, and Francesco Sannino, who holds affiliations with the University of Naples Federico II in Italy as well as the Quantum Field Theory Center at the University of Southern Denmark.
Extreme Spin and the Pair-of-Pants Horizon Shape
Rotation changes the thermodynamic equation. A rapidly spinning black hole possesses rotational energy that alters the relationship between its mass, angular momentum, and entropy. Under specific theoretical conditions, that spin can provide a loophole, allowing the system’s overall entropy balance to work out during a division.
Yet solving the entropy hurdle is only half the battle. Any newly formed fragment must also muster enough momentum and energy to escape the primary remnant’s gravitational pull instead of immediately falling back together.
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.
Francesco Sannino, University of Naples Federico II and University of Southern Denmark
If such a rare split were to occur, the mathematical modeling shows the black hole’s outer boundary stretching into an unusual geometry. The event horizon would branch through time in a trouser-like shape, with one resulting fragment slipping down each distinct section. Researchers call this configuration a pair-of-pants topologies.
Alternative Dimensions and Primordial Black Holes
To test the boundaries of their models, the authors also explored more exotic theoretical scenarios, including theories with extra dimensions. In higher-dimensional models, some black holes can become unstable and begin to break apart, potentially making splitting thermodynamically favorable.
But the authors caution that these models remain theoretical constructs rather than direct observations of our universe.
No instance of a single black hole splitting apart has been observed. Quantifying the exact likelihood of such an event depends heavily on understanding how gravity behaves beyond Einstein’s general relativity.
Looking ahead, the researchers suggest that small primordial black holes would be more prone to splitting than massive astrophysical black holes. Indirect detection might be possible if gamma-ray telescopes such as Fermi-LAT pick up unique radiation signals from tiny fragments generated during cosmic encounters.