For as long as we have studied black holes, their story has run in one direction: two merge into one, and the ripples of that union roll outward through space. A new theoretical study dares to imagine the reverse, a single black hole dividing into two, and finds that nature may allow it only under the most extreme conditions, when a black hole spins almost as fast as physics permits. The old law that disorder must not spontaneously fall has kept such splitting out of reach, yet at the edge of rotation a narrow opening appears, and whether the universe has ever taken it remains a question for t
Black Holes Might Split in Two, but Only If They Spin Near Their Maximum Speed
A black hole splitting would be a drop in entropy, strongly disfavored by thermodynamics
Mark
So the headline idea is that a black hole can split in two. Why is that surprising?
Mimi
Because everything we've observed goes the other way. Black holes merge, and the collisions produce gravitational waves. A split would be the reverse.
Mark
And what normally forbids it?
Mimi
Entropy. A split would lower it, and thermodynamics strongly disfavors that. The article compares it to scattered crayons, which don't sort themselves.
Luke
But that only applies to non-rotating black holes, right? The spinning case is where the loophole lives.
Mimi
Yes. And even then the pieces have to separate rather than fall straight back together. Sannino says that requirement largely closes the window again, except near maximum spin.
Mark
What's the trouser shape about?
Mimi
It's how the event horizon would divide, with one new black hole in each leg. They call it pair-of-pants topologies.
Luke
Let's be clear about what this is. It's a theoretical calculation. Nobody has observed a split.
Mark
Do we know how likely it is?
Luke
No. Sannino says they can't give a single probability, because it depends on how gravity behaves beyond Einstein's theory. That's unresolved.
Mimi
He does expect small primordial black holes to split more readily than big astrophysical ones, and suggests Fermi-LAT could look for gamma-ray signals from tiny fragments.
Luke
Which is an expectation and a proposed search, not a detection.
Il Polso
- A long-held rule of thermodynamics seems to forbid black holes from splitting, and that tension drives the whole inquiry.
- Splitting would lower entropy, a move nature strongly resists, so the theory must hunt for a loophole in the rules.
- The loophole appears only when a black hole spins near its maximum rate, and even then the two fragments must separate rather than re-merge.
- Researchers are mapping the strange pair-of-pants geometry of the split and working out how gravity beyond Einstein might change the odds.
- Gamma-ray telescopes such as Fermi-LAT may soon search for the faint high-energy traces of tiny black hole fragments, though the result remains theoretical for now.
For as long as we have studied black holes, their story has run in one direction: two merge into one, and the ripples of that union roll outward through space. A new theoretical study dares to imagine the reverse, a single black hole dividing into two, and finds that nature may allow it only under the most extreme conditions, when a black hole spins almost as fast as physics permits. The old law that disorder must not spontaneously fall has kept such splitting out of reach, yet at the edge of rotation a narrow opening appears, and whether the universe has ever taken it remains a question for the patient search of future telescopes.
For as long as physicists have studied black holes, their history has moved in one direction. Two collide, merge into a heavier one, and the violence sends gravitational waves rippling outward. This new study asks whether the film might run backward, whether one black hole could divide into two.
The researchers answer with a qualified yes, and the qualification is severe. A black hole could split only while spinning close to the fastest rate physics allows. Ordinarily, thermodynamics stands in the way, since black holes carry entropy and splitting would lower it, something nature strongly disfavors. For a non-rotating black hole that objection holds firmly. For a rapidly spinning one, the team found a loophole.
Even that opening is narrow. Francesco Sannino of the University of Naples Federico II explained that the two resulting black holes must also be able to separate rather than fall back together. With that requirement included, the window nearly closes again, except possibly for black holes spinning close to their maximum.
The geometry of the split is striking. The event horizon would divide in a shape resembling a pair of trousers, with one new black hole slipping down each leg. A split is more likely when one fragment is much smaller than the other, and small primordial black holes should be the most prone of all.
The work yields no single probability yet, because the answer depends on how gravity behaves beyond Einstein's theory. A possible way to test it lies in gamma rays. Telescopes such as Fermi-LAT could search for high-energy signals from tiny fragments that might form during mergers. For now the result is a calculation, and whether any black hole in nature meets these demanding conditions is the question that future searches may begin to answer.
For as long as physicists have studied black holes, the direction of travel has been one-way. Two of them collide, merge into a single, heavier black hole, and the violence of the event sends gravitational waves, ripples in the fabric of space, rolling outward. A new theoretical study asks whether the film can run backward: whether one black hole might, under the right conditions, divide into two.
The answer from the research team is a qualified yes, and the qualification is severe. A black hole could split only if it is spinning at close to the maximum speed physics allows. Ordinarily, the laws of nature stand in the way, and the researchers set out to find whether there are circumstances in which those laws can be temporarily sidestepped.
The main obstacle comes from thermodynamics. Black holes carry entropy, a measure of disorder in a system. A tidy box of crayons sorted by color has low entropy; scatter them and entropy rises. Putting them back in order takes energy from outside the system, a parent's effort, because entropy does not fall on its own. A black hole splitting in two would amount to a drop in entropy, which thermodynamics strongly disfavors. That objection holds, though, only if the black hole is not rotating. For a rapidly spinning one, the situation could differ, and the team found that a loophole does exist.
That is not the end of the problem. Francesco Sannino of the University of Naples Federico II in Italy, a member of the team, said in a statement that the two resulting black holes must also be able to separate rather than immediately fall back together. Once that requirement is included, he said, the window largely closes again, except possibly for black holes rotating close to their maximum possible rate.
The team also worked out the geometry of such a split. The event horizon, the outer boundary of a black hole, would divide in a shape resembling a pair of trousers, with one of the new black holes slipping down each leg. The researchers call these pair-of-pants topologies. They also determined that a split is more likely when one of the two resulting black holes is considerably smaller than the other.
The work does not yield a number. Sannino said the team cannot yet give a single probability for a black hole splitting, because it depends on how gravity behaves beyond Einstein's theory, and the researchers are working to quantify that. What he does expect is a pattern by size: small primordial black holes should be more prone to splitting than much larger astrophysical black holes.
There may also be a way to look for it. Sannino said a split might be detected indirectly, through high-energy light in the form of gamma rays. Telescopes such as Fermi-LAT can search for signals from tiny black hole fragments that might form during mergers. For now the result is a theoretical calculation, and whether any black hole in nature meets the demanding conditions it describes is the open question that gamma-ray searches could begin to address.
Citazioni salienti
We cannot yet give a single probability for a black hole to split. It depends on how gravity behaves beyond Einstein's theory, and we are working to quantify this.— Francesco Sannino, University of Naples Federico II
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