Twisting Lightlike Solutions of the Kerr-Schild Class
  Using a complex representation of the Debney-Kerr-Schild (DKS) solutions and the Kerr theorem we give a method to construct boosted Kerr geometries. In the ultrarelativistic case this method yelds twisting solutions having, contrary to the known pp-wave limiting solutions, a non-zero value of the total angular momentum. The solutions show that twist plays a crucial role in removing singularity and smoothing shock wave in the ultrarelativistic limit. Two different physical situations are discussed. 
Alexander Burinskii (Russian Academy of Sciences, Moscow) and Giulio   Magli (Dipartimento di Matematica del Politecnico di ...