A major issue in general relativity, from its earliest days to the
present, is how to extract physical information from any solution or
class of solutions to the Einstein equations. Though certain
information can be obtained for arbitrary solutions, e.g., via geodesic
deviation, in general, because of the coordinate freedom, it is often
hard or impossible to do. Most of the time information is found from
special conditions, e.g., degenerate principle null vectors, weak
fields close to Minkowski space (using coordinates close to Minkowski
coordinates) or from solutions that have symmetries or approximate
symmetries. In the present work we will be concerned with
asymptotically flat space times where the approximate symmetry is the
Bondi-Metzner-Sachs (BMS) group. For these spaces the Bondi
four-momentum vector and its evolution, found from the Weyl tensor at
infinity, describes the total energy-momentum of the interior source
and the energy-momentum radiated. By generalizing certain structures
from algebraically special metrics, by generalizing the Kerr and the
charged-Kerr metric and finally by defining (at null infinity) the
complex center of mass (the real center of mass plus 'i' times the
angular momentum) with its transformation properties, a large variety
of physical identifications can be made. These include an auxiliary
Minkowski space viewed from infinity, kinematic meaning to the Bondi
momentum, dynamical equations of motion for the center of mass, a
geometrically defined spin angular momentum and a conservation law with
flux for total angular momentum.