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In order to recover the well-known interference pattern in the double slit experiment with massive electrons, one has to perform it in a vacuo. This is because, as far as I know, the interaction with the environment (e.g. air particles) performs a measurement on the quantum states of the propagating electrons and the interference pattern gets washed out.

Now, from a physical point of view: as an observer, I have no idea what interactions the electron wavefunction is having the various air particles. So how can this result in an observable effect such as having the interference pattern changed?

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    $\begingroup$ It's not in order to recover, it's in order to protect the electrons from being deflected or disturbed in other ways from the particles in the environment. $\endgroup$ – Sofia Dec 7 '14 at 2:49
  • $\begingroup$ The words "in order to recover" the pattern are not good. Recovering is something else. You do the experiment in vacuum for protecting the electrons from being disturbed. Also, what means "massive electrons"? The electron has a certain mass, they can't be massive or less massive. About the interaction, it is not between the wave-function and air particles, but between the electron and the air-particle. $\endgroup$ – Sofia Dec 7 '14 at 3:27
  • $\begingroup$ sorry I just meant massive particles e.g. electrons $\endgroup$ – SuperCiocia Dec 7 '14 at 13:11
  • $\begingroup$ God Almighty, don't tell again such a thing! Electrons are massive? I really don't know if there exist lighter particle possessing rest-mass. Remember this: the electron mass is 10^{-27}gram. $\endgroup$ – Sofia Dec 8 '14 at 0:06
  • $\begingroup$ Yes, I know, I just wanted to differentiate it from light. I wanted to be clear that I was not talking about the interference pattern due to a light source. $\endgroup$ – SuperCiocia Dec 8 '14 at 0:33
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  1. The interaction with the environment performs no measurement, it only may distort the wave-function.

  2. In order to observe that the pattern was damaged you don't need to have an idea of what interaction caused the damage. You need to have that idea if you study that specific interaction.

  3. An example of disturbing interaction with air particles: the collision with air molecules can slow down some of the electrons, while other electrons that didn't undergo collision keep their linear momenta. Then, if the electrons in the beam have different linear momenta, the wavelength of the beam isn't constant anymore, and the pattern of interference isn't clear anymore.

Good luck!

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  • $\begingroup$ This is basically the right idea - the air molecules are interacting with the electrons and this disturbs their state. You don't have to call it a measurement - any interaction causes a change of state. Measurement is just one special case of interaction. $\endgroup$ – Floris Dec 7 '14 at 4:36
  • $\begingroup$ What is about the idea with a cloud chamber? What is the pressure in such a chamber? $\endgroup$ – HolgerFiedler Dec 7 '14 at 7:10
  • $\begingroup$ OK, thanks. However, I recently attended a lecture given by Nobel laureate Serge Haroche and he definitely used the expression " the environment performs a measurement "... so was this just a mistranslated construction, or is there any meaning behind it? $\endgroup$ – SuperCiocia Dec 7 '14 at 13:15
  • $\begingroup$ The environment performs no measurement, but I can't know what was the context, maybe some association of situations. Don't take this statement "as is". $\endgroup$ – Sofia Dec 8 '14 at 0:11

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