Time-symmetric action-at-a-distance electrodynamics and the structure of space-time

We argue that the concept of "material point particle" is incompatible with the force laws of action at-a-distance electrodynamics, and that the traje ctory of a particle is analogous to a string in static equilibrium. Geometr ical arguments lead us to consider a time-symmetric interaction law, and a microscopic electromagnetic field-strength tensor which is no longer antisy mmetric. As a consequence the rest mass of a particle, in our theory, is no longer constant; however its averaged variation is still null.