In a recent paper (arXiv:1610.02973 [gr-qc]) written by two U.S University Professors (from California State University Fresno and Embry Riddle Aeronautical University), theoretically, is has been shown that massless particles, such as photons, could be produced by the gravitational waves.
In particular they wrote:
"If the production mechanism of electromagnetic radiation fr om the gravitational wave background proposed here occurs and is significant, then we would predict that the gravitational wave signal should also be accompanied by an electromagnetic signal. However, in our process this electromagnetic signal should have roughly the same frequency as that of the gravitational wave. Thus we would predict that the electromagnetic wave coming from the gravitational wave would have extremely long wavelengths , on the order of 100s of kilometers i.e. the associated electromagnetic wave would have very large wavelengths. These wavelengths are of such a length that they could easily have g one undetected up to now."
Two particular questions which we may ask are:
1- How such large wavelength electromagnetic waves could be discovered?
2- Can discovering these sorts of electromagnetic waves be another proof for the existence of gravitational waves?