  This paper begins with a critical analysis of the concept of 'material point
particle'. We argue that this concept is incompatible with the force laws of
action-at-a-distance electrodynamics, and we suggest that the trajectory of a
particle (the world-line) should be looked upon as a string in static
equilibrium. By complementing our model with a time symmetric interaction law,
we are led to a straightforward derivation of the principles underlying the
electromagnetic interaction between two uniformly moving charged particles. An
extension of our theory to the case of particles in arbitrary motion shows that
we have to modify Maxwell's equations of the microscopic electromagnetic field,
in order to accommodate a field-strength tensor which is no longer
antisymmetric. It is further argued that the radiation reaction force gives
another example of a 4-force which is not orthogonal to the 4-velocity, thus
leading to a variable rest mass. The variation of the rest mass is then
discussed from the point of view of the equivalence principle of general
relativity, and we end up with a theoretical derivation of the homogenous
Maxwell's equations.
