Discovering the Kerr and Kerr-Schild metrics

The story of this metric begins with a paper by Alexei Zinovievich Petrov (1954) where the simultaneous invariants and canonical forms for the metric and conformal tensor are calculated at a general point in an Einstein space. This paper took a while to be appreciated in the West, probably because the Kazan State University journal was not readily available, but Felix Pirani (1957) used it as the foundation of an article on gravitational radiation theory. He analyzed gravitational shock waves, calculated the possible jumps in the Riemann tensor across the wave fronts, and related these to the Petrov types. I was a graduate student at Cambridge, from 1955 to 1958. In my last year I was invited to attend the relativity seminars at Kings College in London, including one by Felix Pirani on his 1957 paper. At the time I thought that he was stretching when he proposed that radiation was type N, and I said so, a rather stupid thing for a graduate student with no real supervisor to do†. It seemed obvious that a superposition of type N solutions would not itself be type N, and that gravitational waves near a macroscopic body would be of general type, not Type N. Perhaps I did Felix an injustice. His conclusions may have been oversimplified but his paper had some very positive consequences. Andrzej Trautman computed the asymptotic properties of the Weyl tensor for outgoing radiation by generalizing Sommerfeld’s work on electromagnetic radiation, confirming that the far field is Type N. Bondi, M.G.J. van der Burg and Metzner (1962) then introduced appropriate null coor-