I'm really not following you. If two photons meet at an angle – any angle –, there will be a rest frame in which they're meeting from exactly opposite (180°) di

I'm really not following you. If two photons meet at an angle – any angle –, there will be a rest frame in which they're meeting from exactly opposite (180°) directions. They can absolutely produce electron positron pairs if their energy is high enough. The process γ γ′ → e+ e− is a classic calculation in QED, and although it's never been observed directly for two single/isolated photons[0], there is no good reason to believe QED should be wrong in this particular case and, indeed, all evidence we have is pointing towards this process being perfectly possible[1]. [0]: https://en.m.wikipedia.org/wiki/Breit%E2%80%93Wheeler_process https://en.m.wikipedia.org/wiki/Breit%E2%80%93Wheeler_proces... [1]: https://www.energy.gov/science/np/articles/making-matter-collisions-light https://www.energy.gov/science/np/articles/making-matter-col...