  We propose a readily achievable experimental setting where an external
magnetic field is used to tune the emergent quantum electrodynamics (eQED) of
dipolar-octupolar quantum spin ice (DO-QSI). In $U(1)_\pi$ DO-QSI -- the
proposed ground state of QSI candidates Ce$_2$Zr$_2$O$_7$, Ce$_2$Sn$_2$O$_7$
and Ce$_2$Hf$_2$O$_7$ -- we show that the field can be used to control the
emergent speed of light (and, consequently, the emergent fine structure
constant). Depending on the field's alignment with the crystal, one may induce
different speeds for the two polarizations of the emergent photons, in a
fascinating analogue of the electro-optic Kerr effect. In $U(1)_0$ DO-QSI --
yet to be uncovered experimentally -- we find a number of unusual field-induced
transitions, including a transition between $0$- and $\pi$-flux QSI phases, as
well as phases with frustrated flux configurations. We discuss experimental
signatures of these effects in the spinon excitation spectrum, which can be
readily accessed for instance in inelastic neutron scattering measurements. Our
proposal opens the gate to a plethora of experimentally accessible,
engineerable eQED phenomena in the emergent universes of quantum spin ice.
