It seems that you need only to observe what is happening for it to change! Note the absurdity: if I don’t look for where the photon passes, it always finishes below. But if I look at where it passes, it can end up above. The astonishing thing is that a photon can end up above even if I haven’t seen it. That is to say, the photon changes trajectory due to the fact that I was waiting for it at the gate, on the side where it hasn’t passed. Even if I haven’t actually seen it!
