  We study the electromagnetic field equations on an arbitrary quantum curved
background in the semiclassical approximation of Loop Quantum Gravity. The
effective interaction hamiltonian for the Maxwell and gravitational fields is
obtained and the corresponding field equations, which can be expressed as a
modified wave equation for the Maxwell potential, are derived. We use these
results to analyze electromagnetic wave propagation on a quantum
Robertson-Walker space time and show that Lorentz Invariance is not preserved.
The formalism developed can be applied to the case where back reaction effects
on the metric due to the electromagnetic field are taken into account, leading
to non covariant field equations.
