For about 40 years, physicists have puzzled over a strange problem:
What happens when an electrically charged particle encounters a hypothetical magnetic monopole?
Some calculations suggested that the outgoing particle simply disappears from the theory.
Now, researchers from Ghent University, the University of Cambridge and the University of Oxford may have found an answer.
Their study, published in Nature Physics, shows that the particle may not disappear at all. Instead, it can change into a very different kind of quantum object.
The researchers studied what happens when a quantum wave packet, representing a particle, meets a special boundary called a “duality defect.”
This boundary can be thought of as an interface between two different descriptions of the same underlying quantum system—almost like crossing into a mirror version of the quantum world.
Computer simulations produced a remarkable result. Every time the particle reached the defect, it passed straight through. There was no reflection. In other words, transmission occurred with 100% probability.
But the object emerging on the other side was no longer an ordinary particle.
Instead, it became a nonlocal object connected to the defect by an invisible quantum “string.” Rather than being concentrated in one place like a conventional particle, its properties were spread across the system, making it resemble a faint quantum wisp.
This finding could help explain a famous puzzle involving magnetic monopoles. A magnetic monopole is a hypothetical particle with only one magnetic pole—north or south—rather than the paired poles found in ordinary magnets.
In the 1980s, physicists Curtis Callan and Valery Rubakov studied what should happen when charged particles scatter from magnetic monopoles. Their calculations produced a disturbing result: The expected outgoing particle appeared to be missing.
More recent theories suggested that the particle had not truly vanished. Instead, it might have entered an unusual “twisted” state that conventional descriptions failed to recognize.
The new study demonstrates a similar process in a much simpler system: a chain of interacting quantum spins that researchers can simulate on computers.
The defect used in the experiment relies on a famous concept called Kramers–Wannier duality, which connects two apparently different descriptions of a physical system. When the simulated particle crossed this boundary, it emerged in exactly the kind of unusual string-like state predicted by newer theories.
The researchers also found that quantum entanglement—the deep connection between different parts of a quantum system—is central to the process. The defect contains its own hidden quantum state space, which determines how particles pass through it and what they become afterward.
Importantly, the idea may eventually be tested experimentally. Because the researchers used a relatively simple quantum spin model, similar behavior could potentially be recreated using cold atoms, trapped ions or superconducting quantum processors.
Such an experiment could provide the first direct observation of a particle effectively changing its identity as it crosses a topological quantum boundary.
What once looked like a disappearing particle may therefore be something much stranger: a particle transformed into a spread-out quantum object, still present but hiding in a form physicists previously struggled to see.
