Exploits nonlinear optical interactions in a medium (ruby) to cause one beam to cast a shadow from another beam.
Cross-gain modulation (XGM)
XGM has been investigated extensively to design optical logic gates with SOA. It is assumed that there are two input light beams for each SOA. One is the probe light and the other is the much stronger pump light. The probe light cannot pass through the SOA when the pump light saturates it. The probe light can go through the SOA when the pump light is absent.
[1] https://www.oejournal.org/article/doi/10.29026/oes.2022.2200...
"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