Quantum Mechanics demystified, a try
One of the great mysteries in quantum mechanics is called ‘Measurement Problem’: a quantum system looses it’s superposition if it is observed in any sense. Let’s have a closer look on this.
A quantum object, like a photon, is normally in a state called superposition in respect to any of it’s properties, e.g. location.
While in superposition, when the object interacts with external world, superposition disappears or collapses as we also say. Interaction being e.g. photon exciting an electron of an atom. If the interacted atom happens to be one of a film or a cathode ray tube, object’s state (position) after the collapse surely becomes perceptible for us humans. So it’s really true observation causes superposition’s collapse.
However, the observation is only a kind of coincidence. The collapse was caused by the interaction itself, whether or not there was somebody or something to make observation. It is just the human egocentricity which makes us to feel it is us observing this interaction making the collapse to take place.
E.g. in double slit experiment, if we replace the measuring device(s) besides the slits with anything at all, not capable of doing any measurement, the interference pattern would be lost anyway.
My understanding is that there can’t possibly be an observation method or device that wouldn’t interact with the photon.
And once the interaction to record which way the photon traveled has taken place, it is not any more the same photon which was initially fired. Having phase, direction and other related characteristics most likely very different from the original photon’s ones. Like traveling to a whole different direction from the initial photon. Understandably interference from thereon possibly isn’t likely to take place.
See photon bouncing
Doesn’t matter whether the observing device is before or after the slits. Or if there is a meter at both of the slits or only at the other one. Always the initial photon’s components, traveling through either (or both) of the slits, get absorbed and a new one(s) emitted.
Obervation methods like tracking devices, calorimeters or particle-identification devices, all interfere with the particle being measured. The same goes on if one tries to tag the photons e.g. with polarity. Measuring the polarity will interfere with the photon to be metered. See article on polarization.