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It is my understanding of the double-slit experiment that when a measuring device is activated, to measure which of the two slits a particle is travelling through, this measuring is responsible for a different outcome than when the device is left off. When the device is left off particles leave an interference pattern, suggesting a single particle (or its wave) interferes with itself; when the device is activated no interference pattern is left, and particles are observed to travel through a single slit.

Now, does all this imply that this experiment is typically carried out in a vacuum? If not, how is it that interaction with other particles in the environment of the experiment's set-up are not already influencing the outcome, regardless of whether the measuring device is on or left off?

This question may very well be a very naive one, due to my very limited understanding of particle interaction, vacuums and/or physics in general. If so, I would appreciate a brief, layman's, overview of where my understanding of the different subjects at play here is likely to be lacking.

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  • $\begingroup$ No. Well, not for photons. Rather obviously, any experiment is done in an environment which is perfectly transparent to the particles in use. $\endgroup$ – Carl Witthoft Mar 21 '14 at 1:03
  • $\begingroup$ @CarlWitthoft That makes sense; I already doubted that these experiments would overlook such a triviality, but I wouldn't otherwise have known where to look, to verify this. Could you recommend an introduction to understanding what it takes for an environment (or substance, etc.) to be transparent for differing kinds of particles? Or does that subject already require a more than basic understanding of physics? And what would be the name of the topic that I should search for? Is it particle interaction, perhaps? $\endgroup$ – Decent Dabbler Mar 21 '14 at 1:17
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    $\begingroup$ @fireeyedboy: So electrons are about 20,000 times lighter than atoms, so if you were running an electron diffraction experiment in air, for the electrons it would basically be the equivalent of a grasshopper colliding with a pickup truck; according to the Wikipedia page on Paschen's law, the mean free path of electrons in air is 0.5$\mu$m. For an intro for what makes an environment transparent to large particles, maybe some books on nuclear radiation or something (or maybe the Wikipedia page for starters) could be helpful. $\endgroup$ – DumpsterDoofus Mar 21 '14 at 1:43
  • $\begingroup$ @DumpsterDoofus Thank you for your comment. Unfortunately it is a bit confusing for me. Are you suggesting that when electrons are the subject of a double-slit experiment, it should indeed be carried out in a vacuum, because otherwise the surrounding air would interfere with the outcome? Or am I completely misinterpreting what you are trying to convey? $\endgroup$ – Decent Dabbler Mar 21 '14 at 20:27
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    $\begingroup$ @fireeyedboy: Yeah, air would mess it up because the electrons would collide with the molecules. $\endgroup$ – DumpsterDoofus Mar 21 '14 at 20:38
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No. As all ready Stated in the comments by Carl Witthoft; "Any experiment is done in an environment which is perfectly transparent to the particles in use."

This means that you will get usable results in air. But it also means, that your results are not accurate. As air is certainly not "perfectly transparent" to photons or electrons; and single photons/electrons do collide to an single atoms in air. The idea of the scale of this error can be found from the refractive index which quite well describes the transparency of a material.

For Vacuum it's 1 And for Air it's 1.000293 This basically means if 0.03% error is not acceptable, then you need to do this test at Vacuum.

Your question;

does all this imply that this experiment is typically carried out in a vacuum?

Can thus be answered that it's not typically carried out in a vacuum, but if you are expecting to reach a really high accuracy then you have to do it in vacuum. It should be noted though, that the air causes a random error to the measured photons, which easily destroys the results if there is only few units measured. Due to the statistical nature of this experiment, this is mostly not the case.

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Of course, the double-slit experiment has been performed with cold atoms in a vacuum. So in this case there wasn't an environment.

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Before you turn on a measuring devise you either have a pattern to measure or you don't with or without a vacuum it doesn't matter. If you shine light on the experiment you will interfere with the particles or saturate the results depending on what your testing.

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