Anthony J. Leggett, at the University of Sussex, publishes in 1972 the theoretical explanation of the just-observed superfluidity of helium-3 by the Cornell team, in the same volume of Physical Review Letters. Leggett shows that, unlike helium-4 — whose atoms are bosons that condense collectively with ease — helium-3 atoms are fermions that must pair up analogously to the Cooper pairs of superconductivity, but with spin one instead of spin zero. To explain the unexpected observed magnetic properties, he introduces the concept of spontaneously broken spin-orbit symmetry: the pairs of atoms collectively align as in a ferromagnetic material, generating an effective magnetic field perpendicular to the applied one. The theory reveals that superfluid helium-3 possesses multiple distinct phases (A, A1, B) depending on the relative spin orientation of the pairs, one of the most complex macroscopic quantum-state structures known in condensed-matter physics.