8.2 Other choices for physical identification
The question of whether our definition of the complex center of mass is the best possible definition, or
even a reasonable one, is not easy to answer. We did try to establish a criteria for choosing such a definition:
(i) it should predict already known physical laws or reasonable new laws, (ii) it should have a clear
geometric foundation and a logical consistency and (iii) it should agree with special cases, mainly the
algebraically-special metrics or analogies with flat-space Maxwell theory. We did try out several other
possible choices [29
] and found them all failing. This clearly does not rule out others that we did not think
of, but at the present our choice appears to be both natural and effective in making contact with physical
phenomena.