How does the double slit experimental apparatus actually work?

  • #1
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Summary:

While watching popular lectures from scientists such as Brian Greene, Sean Carroll and so many others the double slit experiment is often explained but exactly how the single electron or photon source gun chooses which slit to go through is not explained because the apparatus is not explained well enough.

Main Question or Discussion Point

So when explaining the results of quantum double slit experiments that have evolved from the classical wave double slit experiment, popular lecturers of quantum mechanics often show an animation of an electron gun or photon source shooting a lot of particles towards a double slit. The effect is like a shotgun blast where a lot of particles are shooting out from a source. Some particles hit the wall and bounce off, some go through one slit and others go through the other slit. The lecturers normally evolve the classical shotgun blast of tennis ball sized particles, then gradually make the particles smaller and smaller until the quantum effects begin to be seen; that is until the particles get to the size of electrons or photons. As I understand it once a particle leaves the source it travels in a straight line because it is in a vacuum and no other forces act on the particle. So how do the experimenters aim the gun towards the slit? Do they have to pick a slit to target? Do they first choose one slit then move the aim to go through the other slit? If there were no slits, would a single electron or photon always hit the exact same area of the blank wall where the slits should be? This question is especially important when I think about the exactly one particle at a time experiment. I know how physicists can generate a single particle at a time and still get the interference effects, but I do not know if they have to choose which slit to go through and if they do not know that, could they remove the slit and have the particle land on the same spot on the wall all the time.
 

Answers and Replies

  • #2
PeterDonis
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popular lecturers
Are not good sources if you actually want to learn the science. As you appear to have found, their descriptions don't actually tell you what the actual experimental apparatus looks like or how it works. Instead they give you cartoon-like descriptions of some idealized process that might or might not bear a useful resemblance to what actually happens in experiments, and are almost certain to leave you with a number of misconceptions.

As I understand it once a particle leaves the source it travels in a straight line
This is one of the misconceptions that these pop science descriptions unfortunately encourage. A quantum object like an electron or photon does not have a definite position or velocity if it is not being measured. The actual mathematical description of the thing that comes out of the source is a plane wave: a somewhat useful visualization is as a series of plane wave fronts, each one parallel to the screen that has the two slits in it, moving from the source towards the slits. However, these wave fronts are not necessarily waves of any actual "thing": they are waves of probability amplitude (complex numbers that you take the squared modulus of to get a probability).

The plane waves then get diffracted by the slits, and interference between the diffracted waves on the far side of the slits is what produces the interference pattern at the detector. However, each individual quantum object (electron or photon or whatever) does not appear as an interference pattern at the detector: it appears as a single dot at some definite location on the detector. Only the pattern formed by a large number of such dots shows the interference.

The reason QM is so counterintuitive is that the behavior I just described doesn't seem to make sense. On the one hand you have something that behaves like a wave--the simple mathematical model of plane waves, diffraction by the slits, and interference does a marvelous job of describing the overall pattern that shows up on the detector--but on the other hand you have the same thing behaving like a particle--each individual quantum object just makes one dot, not a whole pattern. What this is really telling us is that quantum objects like electrons and photons simply don't fit the intuitive categories of "wave" vs. "particle" that we learn from everyday experience. They are just "quantum objects", and we have to learn the rules for how they work from experiments. The mathematical theory of quantum mechanics is the result of decades of that kind of learning.

how do the experimenters aim the gun towards the slit?
They don't. The source is set up so that the direction of the quantum objects is towards the screen with the slits in it (this direction can be thought of like an arrow perpendicular to the plane wave fronts I described above). But because of the uncertainty principle, this means the source cannot be aimed at any particular position on the screen with the slits. The more accurately the direction of the quantum objects is fixed, the less accurately any aim point on the screen is fixed. (In the idealized mathematical description using plane waves, the position along the screen of the source's "aim point" is completely uncertain. No real source produces exact plane waves, but in practice the position uncertainty is much larger than the spacing between the slits and covers essentially the entire finite area of the screen, which is all that is required.)

If there were no slits, would a single electron or photon always hit the exact same area of the blank wall where the slits should be?
No. The position of each hit (single dot on the screen) would be essentially random over the entire area of the screen. This follows from what I said above about the uncertainty principle.
 
  • #3
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First off, the double slit experiment was primarily a thought experiment (for quantum mechanics). It's easy to make the interference pattern. But it's really difficult to do the interesting part, where you have a single photon (or electron) that is either measured at a slit or not. I vaguely recall hearing that the complete experiment wasn't actually done correctly until about 50 years ago.

To do the experiment correctly the slits are narrow and close together. You would use an electron gun whose beam isn't tight enough to "aim" at a slit.
 
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Thank you PeterDonis and DaveE for your replies. You have helped me confirm what has always bothered me in those lectures when they transit from the classical interpretation to the quantum one. Could either of you comment on the possibility of quantum tunneling through the slit barrier without actually going through either slit, or would this be too off topic? I ask knowing full well that I might not be understanding quantum tunneling.
 
  • #5
PeterDonis
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quantum tunneling through the slit barrier
Quantum tunnelling is not really an applicable concept for the double slit experiment.
 
  • #6
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OK thanks.
 
  • #7
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I just wanted to give a link to a video of the single electron double slit pattern. It has probably been a decade since I had my first taste of quantum mechanics and this video never ceases to amaze me.

Double Slit Experiment with single electrons

And for your own sake, NEVER read the Youtube comments on a science video.
 

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