  Certain remnants of a quantum spacetime foam can be modeled by a distribution
of defects embedded in a flat classical spacetime. The presence of such
spacetime defects affects the propagation of elementary particles. In this
article, we show explicitly that both topology and differential structure of
the defects are important for the particle motion. Specifically, we consider
three types of spacetime defects which are described by the same topological
manifold $\mathbb{R}\times\big(\mathbb{R}P^3-\{\text{point}\}\big)$ but which
are not diffeomorphic to each other. We investigate the propagation of a
massless scalar field over the three different manifolds and find different
solutions of the \mbox{Klein--Gordon} equation.

High Energy Physics - Theory;General Relativity and Quantum Cosmology