On the origin of the notion of GW et cetera
  The notion of gravitational wave (GW) came forth originally as a by-product of the linear approximation of general relativity (GR). Now, it can be proved that this approximation is quite inadequate to a proper study of the hypothetic GW's. The significant role of the approximations beyond the linear stage is emphasized. 
A. Loinger (Dipartimento di Fisica, Universita' di Milano, Italy)