Ab initio

[Olivier N]

Quantum physics is ab initio when it starts from the Schrödinger equation (or Dirac equation for relativistic systems) and fundamental constants (e.g., electron mass, charge), solving for properties without empirical parameters. Classical mechanics is ab initio when derived from Newton’s laws or Lagrangian/Hamiltonian mechanics, using only mass, force, and initial conditions.
Classical electromagnetism is ab initio when derived from Maxwell’s equations and the Lorentz force law, with charges, currents, and boundary conditions as first principles.This is however only the correct in classical systems, when applied to relativistic systems, it breaks down and we have more fundamental models to explain nature.

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