"Quantum vortices of strongly interacting photons" (2024) https://www.science.org/doi/10.1126/science.adh5315
Photons affect mass by causing phonons, which affects scattering and re-emission; so thereby photons interact with photons through light-matter photon-phonon interactions.
From the article:
> Absorption of the green laser heats the cube, which changes the phonon population and lattice spacing and, in turn, the opacity.
/? are photons really massless: https://www.google.com/search?q=are+photons+really+massless
Y: because otherwise E=mc^2 is wrong
N: because it's never been proven that photons are massless
N: because photons are affected by black holes' massful gravitational attraction, and/or magnetic fields
N: because "solar wind" and "radiation pressure" cause displacement
If photons are slightly massful, then coherent light photons would draw together over long distances; thus they interact.
"Light and gravitational waves don't arrive simultaneously" https://news.ycombinator.com/item?id=38061551
"Physicists discover that gravity can create light" https://phys.org/news/2023-04-physicists-gravity.html https://news.ycombinator.com/item?id=35633291