  It is believed that 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 $\vec{B}$ 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 not investigated. In this work we
propose a test of the quality of the classical approximation. We calculate an
observable quantity using the classical magnetic field and also using photons
as input. If the results of both approaches coincide, this will be an
indication that the classical approximation is valid. More precisely, we focus
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 \to \Delta \to N' \pi$.
In ultra-peripheral relativistic heavy ion collisions this conversion can be
induced by the classical magnetic field of one 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 calculate the
cross sections in these two independent ways and find that they differ from
each other by $\simeq 10$ \% 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.
