I think the difference is about the time dependence of particle.
Both in quantum physics and classical mechanics, time evolution of particle is explained via Hamiltonian of the system. So we can simulate particle or wave packet by the time steps.
This is why do we call quantum physics or classical mechanics an ab initio. We can directly calculate what was happened in the experiments.

But in classical electrodynamics, the Maxwell equation can predict how the filed changes. In order to explain how the particle change in the field, one further introduce Hamiltonian or Newtonian equation of motion. So we need to go back to the Quantum physics or classical mechanics. Hence the classical electrodynamics is not an ab initio, unless one try to understand the filed as quantized wave packet.

• This topic was modified 2 years, 1 month ago by brothernine.