The dynamical properties of a time-like hypersurface in Minkowski spacetime are described in terms of a membrane model in which electromagnetic self-stresses are balanced with stresses derived from the shape tensor of the immersion. Radial modes are investigated and in the harmonic approximation a tantalising result emerges for the energy of a harmonic quantum. The particular model is put into the perspective of a class of Willmore-type immersions.