Isolated systems provide valuable insights into the workings of a
physical theory by restricting one's attention to small
subsystems . 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
© Max-Planck-Gesellschaft. ISSN 1433-8351