# Fundamental formulation of light-matter interactions revisited

## TL;DR

This paper revisits fundamental light-matter interactions, highlighting how gauge transformations can alter physical symmetries and emphasizing the importance of potentials over fields, especially in strong-field physics.

## Contribution

It demonstrates that gauge transformations can change the physical meaning of problems, challenging common assumptions and emphasizing the primacy of potentials in strong electromagnetic fields.

## Key findings

- Gauge transformations can alter fundamental symmetries.
- Potentials are more basic than fields in physics.
- Implications are critical for strong-field interactions.

## Abstract

The basic physics disciplines of Maxwell's electrodynamics and Newton's mechanics have been thoroughly tested in the laboratory, but they can nevertheless also support nonphysical solutions. The unphysical nature of some dynamical predictions is demonstrated by the violation of symmetry principles. Symmetries are fundamental in physics since they establish conservation principles. The procedures explored here involve gauge transformations that alter basic symmetries, and these alterations are possible because gauge transformations are not necessarily unitary despite the widespread assumption that they are. That gauge transformations can change the fundamental physical meaning of a problem despite the preservation of electric and magnetic fields is a universal proof that potentials are more basic than fields. These conclusions go to the heart of physics. Problems are not evident when fields are perturbatively weak, but the properties demonstrated here can be critical in strong-field physics where the electromagnetic potential becomes the dominant influence in interactions with matter.