From my understanding photonic (and other) interference effects arise because of a fundamental uncertainty in the path taken by a single photon. So in young's slits the photon could go through either slit and we can't know which one so interference occurs. The moment we do know the wavefunction collapes and two-slit interference is lost. What if the path difference between the two routes is different (which it will be in any physical experiment)?? As two-slit interference is observable in the lab this implies we still have no knowledge of the route a photon takes, despite different path lengths. What if we made the path difference really big, like 1 metre? or a million miles? Let's take a million miles. So we set out to recreate Taylor's beautiful experiment which constructs an interference pattern even though there is only one photon in the apparatus at any time. But we introduce this massive path difference with a couple of mirrors after one of the slits. So it's all set up in a 1/2 million mile lab (we'll need the world to be big and flat for this:) ), we turn on our extremely weak light source, uncover the photographic plate, and then 1 second later cover the plate up and walk away. We do this a million times. Each time we do this we can expect, let's say, one photon to have been emitted. But if we detect a photon we know which route it took because it would be impossible for a photon to have travelled the long route in 1 second. So when we develop each of the million films and map the position of any photons detected what pattern do we get? A Young's slits pattern? I think not but I can't see how taking the film away after 1 second necessitates a collape of wavefunction, as it's clear that simply introducing a path difference does not destroy the diffraction pattern. It's a complete head trip, cos if you left the plate long enough for a photon to have taken either route then diffraction should totally occur, so it's like you have to wait for the photon to check out the whole of either route before it makes its mind up on the probability distribution it will adopt. Crazy. But then in theory 50% of photons in this case would still take the short route AND still create a diffraction pattern, despite NOT having had time to check out the whole situation. What the f**k?! So what do you think? There's no real question here but any enlightening input would be really cool.