> A new study published in Nature Physics introduces a theory of electron-phonon coupling that is affected by the quantum geometry of the electronic wavefunctions
In quantum field theory (standard model of particle physics), photons are the quanta of a field (naturally identified with the quantized electromagnetic field, or U(1) gauge field), which evolves in a nonlinear way that is coupled not only to itself, but also the electron field (as photons are the carrier of the electromagnetic interaction). It seems to me that the concept of “photon” (the particle) is one that is useful in some contexts (like modeling the possible interactions between two electrons in a Feynman diagram), but that the concept of photon is not a fundamental constituent of reality. Certainly, wave packets of the field can interact by way of superposition and thereby altering the space-time evolution of the field through its coupling constant.
Superposition is valid in vacuum, well, actually, until the photons have enough energy to colide and form an electron-positron pair.
It's also valid in most transparent material, again, assuming
1) each photon has no enough energy for example to extract an electron form the material like in the photoelectric effect, or creating an electron and a hole in a semiconductor, or ...
2) there are not enough photons, so you can model the effect using linear equations
And there are weird materials where the non linear effects are easy to trigger.
The conclusion is that superposition is only a nice approximation in the easy case.
New findings: Photons interact as phonons in matter.
"Quantum entangled photons react to Earth's spin" (2024) https://news.ycombinator.com/item?id=40720147 :
> Actually, photons do interact with photons; as phonons in matter: "Quantum vortices of strongly interacting photons" (2024) https://www.science.org/doi/10.1126/science.adh5315 https://news.ycombinator.com/item?id=40600762
"New theory links quantum geometry to electron-phonon coupling" (2024) https://news.ycombinator.com/item?id=40663966 https://phys.org/news/2024-06-theory-links-quantum-geometry-... :
> A new study published in Nature Physics introduces a theory of electron-phonon coupling that is affected by the quantum geometry of the electronic wavefunctions