This is something that has confused me whenever I read about the double-slit experiment.
When the double-slit experiment is performed, I understand that the resulting pattern for the sequentially fired photons will be an interference pattern, unless the path of the photon is observed or measured, in which case the resulting pattern will be that of a particle i.e. like firing tiny marbles through.
Why doesn't the observation of the pattern count as an observation of the photons' paths?
Edit: Wow what a great response. Thanks all you who took the time to contribute an answer, it is certainly appreciated.
What I have taken away from this is that the pattern produced on the screen is an observation / interaction that causes the photon to 'localise' as is the observation of the photon as it passes through the slits.
The differentiation is one of chronology – the observation or inference of exactly which slit the photon passed through causes the photon to localise before passing through and so it behaves like a particle. Observing the photon after passing through the slits in a way that does not reveal which particular slit it passed through, localises the electron according to the wavefunction complete with interference caused by the slits.
The key difference then is at what point the photon is observed / interacted with and therefore localised – either close enough to the slits to reveal which slit it passed through, or far enough such that it remains uncertain. The former will produce the two bar pattern, the latter the interference pattern.
Best Answer
The photon in transit between the light source and the screen is described by a wavefunction. Specifically, the wavefunction describes a photon that is delocalised i.e. it does not have a well defined position. Because the wavefunction is delocalised it encompasses both slits, which is why we say the photon goes through both slits.
Whenever you interact with this wavefunction you change it, and typically you will change it in such a way as to localise it. This is because the interaction happens at a point, and the result is to localise the photon at that point.
Now, when the photon interacts with the screen this does happen at a point, and indeed the interaction creates a spot on the photographic film/CCD/whatever at the point where the interaction happened. We can't say in advance where the interaction will occur, only that the probability of it happening is given by the wavefunction. So any one photon interacts at a point, but when we take many photons the points where they interact with the screen are distributed according to the wavefunction and together they build up the interference pattern.
However, if you interact with the photon before it has reached the slits then your interaction localises the wavefunction so the photon can no longer go through both slits. Because the wavefuntion immediately prior to the slits has changed, the effect the slits has on the wavefuntion changes as well and therefore so does the final pattern at the screen on the other side of the slits.