We study dynamics of interfaces in solutions of the equation $\epsilon \Box u
- \frac 1 \epsilon f_\epsilon(u)=0,forf_\epsilonoftheformf_\epsilon(u) = (u^2-1)(2u- \epsilon\kappa),for\kappa\in {\mathbb R}$, as
well as more general, but qualitatively similar, nonlinearities. We consider
equations of this form both in (1+n)-dimensional Minkowski space, n≥1,
and on certain more general Lorentzian manifolds, and we prove that for
suitable initial data, solutions exhibit interfaces that sweep out timelike
hypersurfaces of mean curvature proportional to κ. In particular, in 1
dimension these interfaces behave like a relativistic point particle subject to
constant acceleration.