Level 1 — Absolute Beginner

A black hole is a place in space with very strong gravity. Nothing can get out of it, not even light.

Two black holes can join and make one bigger black hole. Scientists have seen this happen. But can one black hole break into two?

A new study says maybe. It can happen only if the black hole spins very, very fast. Then the spin can help the two pieces fly apart.

This is only maths on paper. Nobody has ever seen a black hole break in two. Scientists want to look for signs of it in space.

space

The huge empty area outside the Earth where the stars are.

gravity

The force that pulls things towards each other.

light

What lets us see; it comes from the sun and from lamps.

join

To come together and become one.

break

To come apart into pieces.

spin

To turn round and round quickly.

study

Careful work by scientists to learn something new.

piece

One part of something bigger.

Level 2 — Elementary

Black holes are regions of space where gravity is so strong that nothing can escape. Astronomers regularly detect two black holes merging into one, and the waves that collision sends through space are now measured on Earth. The opposite event has never been seen.

A new calculation, published in the journal Physical Review D, asks exactly why. The answer involves a quantity called entropy, which is linked to the surface area of the black hole. For a black hole that is not spinning, the entropy grows with the square of its mass. Splitting such an object into two smaller ones would lower the total, and physics does not allow that.

Rotation changes the sums. A spinning black hole has a smaller surface and less entropy than a still one of the same mass. If that stored spin is turned into the motion of two fragments rushing away from each other, the two new black holes can end up with as much entropy as the original, or even more.

There is a second problem. The two pieces must have enough energy to keep moving apart instead of falling back together. When the team added that condition, the window almost closed again. It seems to stay open only for black holes turning at close to the fastest rate physics permits. One of the authors, Francesco Sannino of the University of Naples Federico II, says the team cannot yet say how likely such an event would be.

region

An area or part of something larger.

escape

To get away from a place.

merge

To join together into one.

entropy

A measure of disorder in a system, which in nature does not decrease.

surface area

The size of the outer boundary of an object.

mass

How much matter something contains.

fragment

A piece broken off from something bigger.

permit

To allow.

Level 3 — Intermediate

Black holes merge. That much is now routine astronomy, confirmed by a decade of gravitational wave detections. Whether the reverse can ever happen, a single black hole dividing into two, has remained an open question, and a new paper in Physical Review D sets out both the reason it should be impossible and the narrow circumstances in which it might not be.

The barrier is thermodynamic. In 1971 Stephen Hawking showed that the area of a black hole's event horizon cannot shrink, a result known as the area theorem, and horizon area is tied directly to entropy. Because the area of a non-rotating black hole scales with the square of its mass, dividing one into two smaller holes of the same combined mass would leave less total area, and therefore less entropy, than before. Nature does not run that way, and a recent precision analysis of the merger event GW250114 tested the area law more stringently than ever.

Rotation is where the authors find room. A spinning black hole has a smaller horizon, and so less entropy, than a stationary one of equal mass. If that rotational energy is paid out as the orbital motion of two fragments separating from each other, the entropy budget can balance, or even improve. The geometry of such an event, traced through time, resembles a pair of trousers: a single trunk branching into two legs, which is why theorists speak of pair of pants topologies.

Then the conditions tighten. The fragments must also carry enough energy and momentum to escape each other rather than immediately recombine, and once that is imposed the opening narrows to black holes rotating very close to their maximum possible rate. No numerical threshold is quoted. Francesco Sannino of the University of Naples Federico II, who also works at the Quantum Field Theory Center at the University of Southern Denmark, says a single probability cannot yet be given, because the answer depends on how gravity behaves beyond Einstein's theory. The team expects small primordial black holes to be the likeliest candidates, and suggests that gamma ray instruments such as Fermi-LAT might one day pick up an indirect signature.

gravitational wave

A ripple in space and time produced by violent events such as merging black holes.

thermodynamic

Relating to heat, energy and disorder in physical systems.

event horizon

The boundary around a black hole beyond which nothing can return.

theorem

A statement proved to be true by mathematical argument.

stringently

Strictly and demandingly.

topology

The branch of mathematics concerned with shape and connection rather than exact size.

momentum

The quantity of motion an object has, from its mass and its speed.

primordial

Existing from the earliest times, here from the first moments of the universe.

Level 4 — Advanced

Theoretical physics often advances by asking why something forbidden is forbidden, and then examining the proof for soft edges. Mergers of black holes are now observational routine; fission is not, and has never been seen. A paper in Physical Review D interrogates the prohibition itself, and reports that the door is not quite bolted, though the gap is extremely narrow and the argument remains entirely on paper.

The prohibition is thermodynamic rather than mechanical. Hawking's 1971 area theorem forbids the event horizon from shrinking, and horizon area is proportional to entropy, which supplies black hole physics with its own second law. Since the horizon area of a Schwarzschild hole scales as the square of its mass, cleaving one into two of equal combined mass yields less total area and so less entropy, which is not merely unlikely but disallowed. The constraint has lately been tested with unusual rigour: analysis of the merger GW250114 confirmed the area law to high precision.

Rotation supplies the slack. A Kerr black hole of given mass has a smaller horizon, hence lower entropy, than its non-rotating counterpart, so a rapidly spinning object sits further from the entropy ceiling than it appears. Convert that rotational reservoir into the orbital energy of two receding fragments and the entropy ledger can be balanced or improved. Traced as a spacetime diagram, the horizon of such a process forms a single trunk that bifurcates into two limbs, the pair of pants topology familiar from string worldsheets. A second requirement then bites hard: the daughter holes must possess sufficient energy and momentum to separate rather than promptly recoalesce, and imposing it collapses the parameter space to objects spinning very near extremality, with no numerical bound offered.

The authors are careful about what this does and does not establish. Francesco Sannino, of the University of Naples Federico II and the Quantum Field Theory Center at the University of Southern Denmark, notes that no single probability can yet be assigned, since the result is sensitive to how gravity behaves beyond general relativity; one illustrative calculation in a modified gravity setting finds that a ten solar mass hole shedding a fragment of around twenty billion kilograms, a negligible fraction of the whole, could raise total entropy. Small primordial black holes are considered the most plausible venue, and a Hawking radiation signature detectable by gamma ray instruments such as Fermi-LAT is floated as an indirect test. A mathematical loophole, as one of the reports covering the work observes, is not evidence that nature uses it.

fission

The splitting of one object into two or more parts.

prohibition

A rule that forbids something.

proportional

Changing at the same rate as something else.

cleave

To split or divide something.

rigour

Strictness and exactness in method or reasoning.