... Feyn-man soon learned that there was a great obstacle to this delayedaction-at -a- distance theory: namely, if a radiating electron, say inan atom or an antenna, were not acted upon at all by ... causality states that all observable effectstake place at a time later than the cause, Maxwell’s equations forthe electromagnetic field possess a radiative solution other than thatnormally adopted, ... reason that(20) and (21) are equally satisfactory approximations to S(xi+1,xi)whenthere is no vector potential. A term, linear in velocity, however, arising from a vector potential, as A xdt...