  Given a linear, constant coefficient partial differential equation in
${\mathbb{R}}^{d+1}$, where one independent variable plays the role of `time',
a distributional solution is called a null solution if its past is zero.
Motivated by physical considerations, we consider distributional solutions that
are tempered in the spatial directions alone (and do not impose any restriction
in the time direction). Considering such spatially tempered distributional
solutions, we give an algebraic-geometric characterization, in terms of the
polynomial describing the PDE at hand, for the null solution space to be
trivial (that is, consisting only of the zero distribution).
