as a photon might do interference with itself on a double slit as long as we detect where it NOT has gone through, in wich case the interference is not observed ?
I have an old broken wrist watch that can do that. . .twice everyday.
"as a photon might do interference with itself on a double slit as long as we detect where it NOT has gone through, in wich case the interference is not observed ?"
That's correct. You are understanding the theory---as long as you don't think about it!