... 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 the field thatit radiated, then ... usedfor the Wheeler–Feynman theory, either classically or quantum me-chanically. The principal reason was the use of half-advanced and half-retarded interaction. The Hamiltonian method describes and keeps ... leaves the action invariant(for example, the transformation may be a rotation). The trans-formation is to contain a parameter, a, and is to be a continuousfunction of a. Fora equal to zero, the...