The atomic nucleus is not just a ball of protons and neutrons. Inside it, like a tiny magnet, there is its own distribution of magnetization. Until now, it has been difficult to see: nuclei live for tiny fractions of a second, and their magnetic field is weak. Now physicists at CERN have found a way to look inside.

They took the short-lived isotope potassium-47 and placed it in a strong magnetic field. The nucleus, like a compass needle, began to rotate around the direction of the field: this is called Larmor precession. The frequency of this rotation was measured to one part per million. From tiny deviations of the frequency from the calculated value, they were able to understand how magnetization is distributed inside the nucleus: how much is created by spins (the intrinsic rotation of particles) and how much by their orbital motion.

Comparison with theory brought a surprise: calculations overestimate the spin contribution. Even taking into account complex particle interaction effects, theory does not agree with experiment. But when scientists took the spatial shape of magnetization from another theory, everything matched. So the issue is not the total amount of magnetism, but how it is 'smeared' across the nucleus.

The new method is a window into the heart of the atom. It will help test theories of nuclear structure and search for traces of new particles that could affect the magnetic properties of atoms. The tiny potassium nucleus has become a touchstone for big physics.