The modified electromagnetism and the emergent longitudinal wave
  The classical theory of electromagnetism has been revisited and the possibility of longitudinal photon wave is explored. It is shown that the emergence of longitudinal wave is a consequence of Lorenz gauge (condition) violation. Proca, Vlaenderen & Waser and Arbab theories are investigated. 
Arbab I Arbab and Mudhahir Al-Ajmi
physics.gen-ph