Cartesian Kerr–Schild variation on the Newman–Janis trick

Abstract

The Newman–Janis trick is a procedure, (not even really an ansatz), for obtaining the Kerr spacetime from the Schwarzschild spacetime. This 50 years old trick continues to generate heated discussion and debate even to this day. Most of the debate focusses on whether the Newman–Janis procedure can be upgraded to the status of an algorithm, or even an inspired ansatz, or is it just a random trick of no deep physical significance. (That the Newman–Janis procedure very quickly led to the discovery of the Kerr–Newman spacetime is a point very much in its favor.) In the current paper, we will not answer these deeper questions, we shall instead present a much simpler alternative variation on the theme of the Newman–Janis trick that might be easier to work with. We shall present a 2-step version of the Newman–Janis trick that works directly with the Kerr–Schild “Cartesian” metric presentation of the Kerr spacetime. That is, we show how the original 4-step Newman–Janis procedure can, (using the interplay between oblate spheroidal and Cartesian coordinates), be reduced to a considerably cleaner 2-step process.