Cloud Chambers and The Double Slit

[Amanita-Virosa]

As I understand this, if the double slit experiment is performed in a cloud chamber the interference pattern of the electrons disappear and tracks are seen within the vapour.

Does the interference pattern disappear because by the process of observation (permitted by the cloud chamber) we have "collapsed" the wavefunction? If so what happens after the first "position measurement" (condensation of the cloud) has been made. Does it behave as a particle (like a billiard ball) or does it "regain" an uncollapsed wavefunction? If it does (and this is what I understand to be happening) why do we see a track and not just random points thoughout the chamber and one might expect from an indeterministic "collapse" process.

What if the vapour in the chamber was replaced by a medium that might interact with the electron in a similar manner by didn't allow us to see the track - say, a very dense gas under pressure maybe? Would the interference pattern reappear or not? Is it interaction or observation (measurement) causing the disappearance of the interference pattern?

Thanks

 

[DrClaude]

The interaction between the cloud particles and the electron consists in a measurement, so this is indeed what makes the interference pattern disappear ("collapse" of the wave function is specific to a particular interpretation of QM, so I'll leave it as "measurement").

The fact that we can observe or not the track is irrelevant. The interaction with the cloud particles is the only ingredient really necessary here.