The Extended Equivalence Principle and the Kramers-Kronig Relations
  A seemingly obvious extension of the weak equivalence principle, in which all matter must respond to Post-Newtonian gravitational fields, such as Lense-Thirring and radiation fields, in a composition-independent way, is considered in light of the Kramers-Kronig dispersion relations for the linear response of any material medium to these fields. It is argued that known observational facts lead to violations of this extended form of the equivalence principle. (PACS numbers: 04.80.Cc, 04.80.Nn, 03.65.Ud, 67.40.Bz) 
Raymond Y. Chiao