Center of mass frame, angular momentum and asymptotic structure equations

OSVALDO M. MORESCHI

Durban, Sud África

Conferencia; 16th International Conference on General Relativity and Gravitation; 2001

The difficulty associated with a well behaved definition of angular momentum at future null infinity is related to the complicated structure of the Bondi-Metzner-Sachs (BMS) group of asymptotic symmetries. There is a multiplicity of Poincaré subgroups, one for each supertranslation. Because of this difficulty one can find in the literature several inequivalent definitions of angular momentum which suffer from the so called supertranslation ambiguities. The only definition of angular momentum without supertranslations ambiguities is the one we presented in reference; however our construction can be criticized on the grounds that we select a unique frame based on the properties of future null infinity in the limit for the retarded time going to infinity. A global choice of this nature can hardly be thought to be able to describe the local meaning of intrinsic angular momentum of the sources at an arbitrary retarded time of a radiating system. In order to provide with an unambiguous notion of angular momentum in radiative spacetimes, it is essential to make use of a reference frame system, which embodies the notion of rest frame.