When a Black Hole Can Split Apart express
A black hole is like a spinning drop of viscous liquid. Normally, such a drop tends to stay whole: splitting would reduce its surface area, which contradicts the law of non-decrease of entropy (a measure of disorder, like a broken cup doesn't glue itself back together). For non-rotating black holes, discovered by Karl Schwarzschild, splitting is impossible. However, with furious rotation, centrifugal forces stretch the black hole, and a microscopic fragment can pinch off. This process resembles nuclear fission in the liquid-drop model. Physicists have also considered worlds with extra dimensions, where spacetime curvature behaves differently, and black holes can become unstable, like long liquid threads ready to snap into pieces. Such decay could alter the population of primordial black holes, born in the young Universe.
Original abstract
as written by the authors arXiv:2606.24642
We investigate the conditions under which black hole fragmentation, the splitting of a black hole horizon into multiple smaller ones, may occur. The simplest realization is that of a single black hole horizon splitting into two, giving rise to the eponymous pants topology. In classical general relativity, the Bekenstein-Hawking area law forbids such processes for Schwarzschild black holes. For spinning Kerr black holes, purely kinematic analyses impose constraints that prevent fragmentation, even in regimes where entropy considerations might allow it, except possibly in near-extremal cases. We then hunt for scenarios where black holes can wear pants: from the well-known Gregory-Laflamme instability in higher dimensions, to the potential effect of superradiant instabilities in non-axisymmetric radiation trapping, to finally gravitational models that modify the relations between entropy and/or horizon radius and the black hole mass in four dimensions. In all such cases, emission of small fragments can be entropically favored, however its occurrence still depends on the kinematic configuration of the initial state. Our analysis clarifies the theoretical landscape where black holes may fragment, which is particularly relevant for primordial black holes and catastrophic events such as black hole mergers.
Merging black holes, on the contrary, always increases the total area: this process resembles the merging of mercury droplets and is accompanied by powerful gravitational waves.