The Unbreakable Rule of Black Holes
In the universe, some rules feel absolute. One of the most fundamental involves black holes and the laws of thermodynamics. For a long time, physicists believed that black holes could only merge and grow larger. This idea is partly tied to a concept called
entropy, which is a measure of disorder in a system. According to the laws of physics, the total entropy of the universe can't decrease. The entropy of a black hole is related to the surface area of its event horizon—the point of no return. If one black hole were to split into two smaller ones, their combined surface area would be less than the original. This would mean a decrease in entropy, making the split 'thermodynamically unfavorable' and essentially forbidden. This principle has been a cornerstone of black hole physics for decades.
A Crack in the Cosmic Law
A recent theoretical study, published in the journal Physical Review D, challenges this long-held assumption. Researchers have identified a highly specific, extreme scenario where a black hole might just be able to fragment. The key, according to their calculations, lies in rotation. The team found that the entropy hurdle could potentially be overcome if a black hole is spinning at an exceptionally high speed. A rapidly spinning black hole has lower entropy compared to a non-spinning one of the same mass, which creates a potential opening for fragmentation to occur. However, even this is not a simple process. Just because a split is possible doesn't mean it will happen easily. The conditions must be almost perfect.
The Pair-of-Pants Problem
For a black hole to split, its event horizon would need to contort into a shape that researchers have whimsically dubbed a 'pair-of-pants topology'. Imagine the event horizon, the boundary of the black hole, starting as a single sphere. If it were to split, this sphere would have to stretch and then divide into two separate 'legs', each one forming the event horizon of a new, smaller black hole. The issue is that even if the spin is high enough to allow for this shape to form, there's another major hurdle: the two new black holes would be incredibly close to each other. Their immense gravity would almost certainly cause them to immediately fall back together and merge again. The researchers found that this window for a successful split and separation 'largely closes again', unless the black hole is spinning at or very near its absolute maximum possible rate.
Beyond Einstein and Into Other Dimensions
The researchers also explored more exotic scenarios to see if fragmentation could be more likely. This involves looking beyond Einstein's theory of general relativity and considering theories that include extra spatial dimensions. In some of these higher-dimensional models, certain objects that resemble black holes, like 'black strings', can become unstable and break apart. Exploring these alternative theories of gravity might reveal that splitting is more common than current physics suggests. However, these ideas are even more theoretical and push the boundaries of our current understanding of the cosmos. As co-author of the study Francesco Sannino noted, the probability of a split depends on how gravity behaves beyond Einstein's theory, which is a very difficult problem on its own.
Searching for the Impossible
It is crucial to remember that this entire concept remains purely theoretical. No such event has ever been observed. So, how would scientists even begin to look for evidence? The researchers suggest that the best candidates for this kind of fragmentation would be smaller, primordial black holes, which are thought to have formed in the early universe. Detecting a split directly would be incredibly difficult, but it might be possible to see the aftermath. One possibility is to search for specific signals in high-energy light, such as gamma rays, which might be produced by tiny black hole fragments. Telescopes like the Fermi LAT could potentially be used to hunt for these indirect signs.
