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After reviewing the meaning of various equivalence principles and thestructure of electrodynamics, we give a fairly detailed account of theconstruction of the light cone and a core metric from the equivalence principlefor the photon (no birefringence, no polarization rotation and noamplification/attenuation in propagation) in the framework of linearelectrodynamics using cosmic connections/observations as empirical support. Thecosmic nonbirefringent propagation of photons independent of energy andpolarization verifies the Galileo Equivalence Principle [Universality ofPropagation] for photons/electromagnetic wave packets in spacetime. Thisnonbirefringence constrains the spacetime constitutive tensor to high precisionto a core metric form with an axion degree and a dilaton degree of freedom.Thus comes the metric with axion and dilation. Constraints on axion and dilatonfrom astrophysical/cosmic propagation are reviewed. E\"otv\"os-typeexperiments, Hughes-Drever-type experiments, redshift experiments thenconstrain and tie this core metric to agree with the matter metric, and hence aunique physical metric and universality of metrology. We summarize theseexperiments and review how the Galileo equivalence principle constrains theEinstein Equivalence Principle (EEP) theoretically. In local physics thisphysical metric gives the Lorentz/Poincar\'e covariance. Understanding that themetric and EEP come from the vacuum as a medium of electrodynamics in thelinear regime, efforts to actively look for potential effects beyond thislinear scheme are warranted.

Equivalence Principles, Spacetime Structure and the Cosmic Connection