From maxwell's equations, it occurred to me that photons are stable. Decrease in electric field creates magnetic field and vice versa and somehow there is a harmony that allows photon to exist as long as it travels with c. Therefore I wouldn't think photons would decay as they travel.
However, I know that radiation decays with 1/r from the source and a photon is a form of radiation. Does that mean photons can travel in space with c, but they also have to decay? I kind of imagined them like harmonic sine waves.