As a hobby physicist I don't understand how the double slit experiment is performed in terms of single photons. Is the experiment really conducted by sending and measuring single photons? Is there not always interference of some kind (except for some device like the LHC)?
If yes: what would happen if the experiment is conducted over a prolonged period of time, for instance: 1 photon every n hours. Would they theoretically interfere with "themselves"? (don't know if i'm still making sense here).
If no: why is the interference considered to be originating from the sent photons and not from an external influence of some sort?