In non-central relativistic heavy-ion collisions a very strong magnetic field is formed. There are
several studies of the effects of this field, where B is calculated with the expressions of classical
electrodynamics. A quantum field may be approximated by a classical one when the number of
field quanta in each field mode is sufficiently high. This may happen if the field sources are intense
enough. In heavy-ion physics, the validity of the classical treatment was never investigated. In
previous work, we proposed a test of the quality of the classical approximation. We calculated
an observable quantity using the classical magnetic field and also using photons as input. We
focused on the process in which a nucleon is converted into a delta resonance, which then decays
into another nucleon and a pion, i.e., N → ∆ → N'+π. In ultraperipheral relativistic heavy-ion
collisions this conversion can be induced by the classical magnetic field of one of the ions acting
on the other ion. Alternatively, we can replace the classical magnetic field by a flux of equivalent
photons, which are absorbed by the target nucleons. We calculated the cross-sections in these
two independent ways and found that they differ from each other by ~10 % in the considered
collision energy range. This suggests that the two formalisms are equivalent and that the classical
approximation for the magnetic field is reasonable.