Proton magnetic moment

A Dirac point particle of the proton's charge and mass would have exactly one nuclear magneton of moment. The proton has 2.7928, a discrepancy far too large to be a radiative correction, and the explanation is that the proton is not a point: it is three valence quarks in a boiling sea of gluons, and their orbital and spin contributions add up to the observed value. Otto Stern's 1933 measurement of this anomaly is one of the earliest windows into hadron structure.

Practically, this moment is why hydrogen dominates magnetic resonance. Water is full of protons, each a tiny magnet that precesses in an applied field and can be tipped and listened to — the basis of NMR spectroscopy and of every MRI image ever taken. The neutron's moment, negative and comparable in size despite the neutron's zero net charge, tells the same structural story from the other side.