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I'm quite perplexed by the notion of 'observation' in regards to the collapse of a particle's probability wave. Does a particle's wave only collapse when it is involved in a strong interaction (such as a collision with another particle, like bouncing a photon off another particle to determine the other particle's position) or does any interaction with another particle or field also cause this collapse?

For example, I presume traditional detectors such as those mentioned when talking about the double slit experiment are devices that do exactly as I stated above and (in the case of the double slit) have a stream of photons (like a curtain) going from the top of the slit to the bottom which the electron (or particle being shot at the slits) has to pass through and therefore get smashed into by the photons.

So given an environment where there was no other gravitational or electromagnetic influence, or where the effects of such have been taken into account, what would happen if you had a region of space within which a massive neutral particle 'is' (ie, it's probability wave fills this volume), and then shot a beam of photons across the bow of this region (so as to pass nearby, but not intersect), and then have a photo-sensitive plate on the other side to see if the photons passed straight by, or were pulled towards one direction slightly by the warping of spacetime caused by the massive neutral particle (gravitational lensing on a mini scale)? Does this even make sense? And if the photons were being pulled slightly towards the massive neutral particle, would the waveform collapse at this point, since if the mass of the neutral particle was known, the amount by which the photons path had been bent could confirm it's position couldn't it?

Or perhaps take the above and use an electromagnetically positive particle as the target and an electromagnetically negative particle as the beam you shoot by it to see if it is effected by the electromagnetic field caused by the target particle if that makes more sense.

I might be making some silly errors or assumptions here, I'm only a layman with no formal education or in depth knowledge, so be gentle :)

machinemessiah
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    Here's a related question, which might even be a duplicate: http://physics.stackexchange.com/questions/1353/is-there-a-difference-between-observing-a-particle-and-hitting-it-with-another-p. Take a look at that and see if it answers your question. – David Z Dec 26 '11 at 03:04

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You're really asking two questions here. Assuming I understand your question correctly you're interested in what constitutes an "observer", but you're also asking about what the wavefunction collapse is.

I can't comment usefully on what the collapse is because I don't think anyone knows. If you believe the many worlds treatment of QM there isn't any collapse anyway.

I can, I hope, be a bit more helpful with defining an observer. Have a look at my answer to What exactly is the 'observer' in physics and/or quantum mechanics? and see if that helps.

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John Rennie
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