Check out this phyics.stackexchange answer: https://physics.stackexchange.com/a/627852/386 https://physics.stackexchange.com/a/627852/386 I've been involved in

Check out this phyics.stackexchange answer: https://physics.stackexchange.com/a/627852/386 https://physics.stackexchange.com/a/627852/386 I've been involved in one of the light-by-light theory groups a long time ago. The situation with theoretical prediction is not as clear.