is this still a theory that light can behave as a solid
It's a postulate; meaning, the theory was proven by all the experiments that the scientists could have done. Saying something is a solid is a completely different subject. This is a light particle that is tiny. As long as it has mass, it can become 'solid' or tangible given that Kizaru's ability exists, but such ability does not exist yet in our world (at least from my knowledge) so we cannot manipulate photons to make it tangible yet in real world.
@sag:
Until he fights his natural enemy, the mirror mirror fruit. I'm actually expecting a mirror, glass or similar fruit appearing in the new world cuz OP is lacking in that department so far. But only Oda can tell
Edit: Maybe a prism fruit… though it might need a lot of creativity input for it to work because its not a fighting element per se
This mirror or any material that bends the light that we see will be ineffective to Kizaru if One Piece world is just like our universe; he will just melt them away. If this man Kizaru really existed, it would virtually be impossible to beat him without the use of sea or sea stone (but realistically, he will just evaporate or melt them away though).