  This is one of a number of papers in which the metric for space-time is
defined on the subatomic level by means of the interchange of photons, and
constrained to be consistent with radar. It is shown that the discrete nature
of particle interactions necessitates a small inherent delay in the radar
method, and hence that the resulting geometry is non- Euclidean. Thus the
forces of gravity and electromagnetism can each be seen as an effect of the
same interaction between elementary particles, and we can identify
gravitational and inertial mass.
