  Heisenberg's uncertainty relation means that one observer cannot know an
exact position and velocity for another (finite mass) observer. By contrast,
the Poincare transformation of classical special relativity assumes that one
observer knows the other's position and velocity exactly. The present paper
describes a simple-minded way to consider the issue using a semiclassical
discussion of spacetime diagrams, and draws out some possible implications.
Uncertainties arise in transformations. A consideration is raised regarding the
use of light-cone coordinates.
