Preskill, Thorne, Foreword to Richard Feynman's, "Feynman Lectures on Gravitation“. They report that at the time, Einstein was unenthusiastic about the proposal, because Kraichnan's procedure circumvented Einstein's hard-won geometrical insights about the gravitational field. Preskill and Thorne also compare similar work by Gupta, Feynman, Kraichnan, Deser, Wald, and Weinberg: pdf-file