The classical field theory of electromagnetism (EM) originates with Maxwell in 1875. A Direct Particle Interaction version of the same theory was presented by Schwarzschild in 1903, with subsequent contributions from Tetrode in 1922, and Fokker in 1929. We will refer to this as STF DPI.
Unlike the Maxwell field theory, STF DPI is built upon direct pair-wise interactions between charges along their mutual light cones, without necessary reference to electric and magnetic fields, though these can be introduced for mathematical expedience. In any case, STF DPI asserts that the dynamics of charges can be calculated from these pair-wise interactions alone.
STF DPI is intrinsically time-symmetric in the sense that all pair-wise interactions between charges everywhere in the universe, considered independently, is time-symmetric. Note this does not mean that the combined influence of all charges on a single, nominally local charge, is time symmetric. This is because the combined influence depends on the velocity and distribution of other charges in time and space.
Of particular importance here is that radiation is entirely anti-symmetric at its source. From the outset a challenge for STF DPI was to explain how the combined influence of pair-wise interactions between a local emitter of radiation and all other charges, each time-symmetric, could conspire so that their summed effect is entirely anti-symmetric in time.
In publications in 1945 and 1949 Wheeler and Feynman argued that such a conspiracy could be achieved in DPI if all radiation is destined to be absorbed. If that was indeed the case, it would turn out that all so-called radiation was not `true` radiation in the commonly understood sense, but rather a long range interaction between local and distant currents. Observationally it would be difficult to distinguish between the two possibilities. This suggestion of Wheeler and Feynman seemed to restore the viability of DPI. For now we will ignore the thermodynamic implications of this suggestion by Wheeler and Feynman.
In 1998 it was discovered that the universe is expanding at an accelerating rate, sufficiently fast that most radiation emitted now is destined never to be absorbed. In other words: the universe is mostly transparent to light. This discovery has since been interpreted (e.g. by Davies) as undermining the conjecture of Wheeler and Feynman, making Direct Particle Interaction untenable once again.
We note in passing that it seems to have been overlooked by those writing on this subject that the Wheeler and Feynman conjecture can be saved if radiation is always directed towards an absorber. It would mean, for example, that radiation from the Sun is not quite isotropic as one would otherwise suppose. It turns out that the departure from isotropy necessary to comply with the Wheeler and Feynman conjecture would be very difficult to detect, meaning that it could be the case and have escaped observation.
In these pages is presented a recently developed modified version of the Direct Particle Interaction theory of Schwarzschild, Tetrode and Fokker. In this theory radiation emerges as a particular mode of motion of distant particles within a Direct Particle Interaction framework. It overcomes the earlier obstacles to the adoption of DPI.