Radical pair reactions — where two molecules with unpaired electrons encounter each other — are sensitive to magnetic fields because the electrons' spin states determine whether the radicals recombine or separate. The singlet state recombines; the triplet state doesn't. An external magnetic field mixes the spin states, changing the singlet-triplet ratio and hence the reaction yield. This is the radical pair mechanism, and it explains magnetic effects on chemical reactions from avian navigation to enzymatic catalysis.
The standard treatment assumes coherent spin dynamics: the spins precess, mix, and dephase according to unitary evolution interrupted by recombination. The paper introduces stochastic collisions. In a fluid of radicals, encounters between different radical pairs are random — Poisson-distributed in time, with encounter durations drawn from a distribution determined by the diffusion and reaction kinetics.
The stochastic collisions introduce magnetic effects that the coherent theory misses. Even when the coherent spin dynamics would produce no field sensitivity (because the radical pair's hyperfine structure is insufficient), the random timing of collisions between different pairs creates a magnetic field dependence through the statistics of encounter histories. The field sensitivity is a property of the collision statistics, not of the individual radical pair.
The observation: magnetism in radical fluids is a collective phenomenon. The individual radical pair is the unit of coherent spin evolution, but the magnetic effect on bulk reaction yields is governed by the statistics of how these units encounter each other. The field sensitivity lives in the ensemble, not in the molecule.