In the cosmic rulebook, black holes are known for merging, not splitting. But a new theoretical study suggests a loophole, finding that under exceptionally extreme conditions, a single black hole might just be able to break in two.
The Unbreakable Rule of Black Holes
For decades, our understanding
of black holes has been governed by a fundamental principle rooted in both Einstein's theory of general relativity and thermodynamics. This idea, known as Hawking's area theorem, states that the total surface area of a black hole's event horizon—the boundary beyond which nothing can escape—can never decrease. When two black holes collide and merge, the new, larger black hole has an event horizon with a surface area equal to or greater than the sum of the original two, satisfying this rule. However, if a single black hole were to split into two smaller ones, the combined surface area of the new event horizons would generally be less than the original. This would violate the area theorem, which is tied to the second law of thermodynamics. Splitting would cause a decrease in the system's total entropy, a measure of disorder, making the process thermodynamically unfavorable. This is why physicists have long believed that black holes can combine but never divide.
A New Theoretical Wrinkle
A recent study published in the journal 'Physical Review D' challenges this long-held assumption, at least in theory. An international team of physicists investigated if any circumstances could permit a black hole to fragment. They found a potential loophole that hinges on one crucial factor: rotation. The researchers' calculations suggest that for a non-rotating black hole, splitting is indeed thermodynamically forbidden, just as expected. But the situation changes for a black hole spinning at an exceptionally high rate. Extreme rotation could, in principle, allow a black hole to bypass the normal thermodynamic restrictions that prevent it from breaking apart. The paper, titled 'When Black Holes Admit Pair-of-pants Topologies', describes how the event horizon would evolve through time, with one 'tube' of spacetime branching into two 'legs', resembling a pair of trousers.
The Cosmic Fine Print
This theoretical possibility comes with a very large asterisk. The conditions required are described as 'exceptionally extreme'. For fragmentation to even be a possibility, the black hole must be rotating at or near its maximum possible speed. Even then, another major hurdle exists. The two newly formed black holes would have to separate quickly enough to avoid immediately merging back into one. According to co-author Francesco Sannino, this requirement almost completely closes the window of opportunity, leaving only a tiny possibility for black holes spinning at the absolute limit of their rotational speed. The researchers also explored more exotic scenarios, such as those involving extra spatial dimensions beyond the three we experience. In some of these theoretical models, black-hole-like objects can become unstable and fragment more easily, but this relies on physics beyond Einstein's established theory.
Why This Theory Matters
While we are unlikely to witness a black hole shattering anytime soon—no such event has ever been observed—this research is significant. It pushes at the boundaries of our understanding of gravity and the fundamental laws of the universe. Exploring these theoretical limits helps physicists probe the intersection of general relativity and quantum mechanics, two pillars of modern physics that have yet to be fully unified. The researchers note that the probability of a split depends on the behaviour of gravity beyond Einstein's theory. They suggest that small, primordial black holes, if they exist, might be more prone to splitting than the large astrophysical black holes we currently observe. Potentially, gamma-ray telescopes could search for signals from tiny fragments that might be produced during these events, offering an indirect way to test the theory. This work opens up a new, albeit highly speculative, avenue of cosmic phenomena to investigate.
