A magnet with only one pole (north, without a south) is not a fragment but a particle in its own right. Physicists call it a magnetic monopole (a carrier of a single magnetic charge). Such visitors could have been born in the early universe, when the forces were just separating. And alongside them, there might have existed a field of axions — invisible, nearly massless particles that likely make up dark matter.
If a monopole finds itself in an axion-rich environment, a film stretches around it — a closed wall, much like a soap bubble. It traps the monopole inside, like a nut in its shell. The axion field changes shape, flowing around the central object, and the result is a stable structure — a 'monopole bag.' The fate of such a bubble depends on how much it collapses under its own gravity.
In one scenario, the collapse stops. What remains is an object without an event horizon — not a black hole, but a dense clump of fields. In another, gravity wins, and a black hole is born. But not an ordinary one: inside it, there is no point of infinite density, i.e., no singularity. Matter does not vanish into nothingness; instead, it is distributed in a clever way. Such a black hole is called regular. It resembles a dyon — simultaneously magnetic and electrically charged.
The secret lies in a special term in the equations. Physicists call it the Chern–Simons term. It couples the axion field to electromagnetism and endows the resulting black hole with unusual electric and magnetic properties. Even after collapse, the axion profile does not disappear but continues to influence the surrounding space. Thus, the monopole and its axion shell leave behind an object that challenges our usual notions of gravitational collapse.
Magnetic monopoles have not been found yet, but their existence is predicted by many theories that unify fundamental interactions.
In Frederik Pohl's novel Gateway, the protagonists find a ship that uses monopoles to travel through black holes.