So, about the excitons - this is a bit like, with semiconductors, people talk about the "holes" as though they were particles moving and interacting:

https://en.wikipedia.org/wiki/Electron_hole

but it's more like a way of describing a system.

Which also means that nobody is making a solid object from photons, it's all "behaves like a solid would if you look at our system a certain way". Right?

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Also - I don't see how the phrases "behaves like a solid" and "presents flow" can agree. Isn't the whole point of a solid that it _doesn't_ flow?