2 General Background

Isolated systems provide valuable insights into the workings of a physical theory by restricting one’s attention to small subsystems [79]. They serve as models for systems in the real world, allowing us to deduce statements about their behaviour and to attribute to them various physical properties such as mass, momentum, emitted radiation, etc. Therefore, it is desirable that a theory should allow within its mathematical framework the characterization of such systems. In general relativity, this is a difficult problem. The reason is a familiar one: The metric which, in other theories, provides a background structure on which the physical fields act, is itself a dynamical object in general relativity.

In this section we discuss some of the issues which lead us to focus on asymptotically flat space-times as models for isolated systems in relativity and hence as realistic gravitationally radiating systems.