Foundations of Electromagnetism, Equivalence Principles and Cosmic
  Interactions
  We look at the foundations of electromagnetism in this chapter of the
"eletromagnetism" book. For doing this, after some review (constraints on
photon mass etc.), we use two approaches. The first one is to formulate a
Parametrized Post-Maxwellian framework to include QED corrections and a
pseudoscalar photon interaction. We discuss various vacuum birefringence
experiments - ongoing and proposed - to measure these parameters. The second
approach is to look at electromagnetism in gravity and various experiments and
observations to determine its empirical foundation. We found that the
foundation is solid with the only exception of a potentially possible
pseudoscalar-photon interaction. We discussed its experimental constraints and
look forward to more future experiments.
