Double-slit experiment, determining which slit an electron passed thru

In summary: Again, you're not thinking quantitatively. Probabilities drop off to negligible numbers very quickly.
  • #1
tade
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I was reading Feynman's lecture on the double-slit experiment, the attempts to determine which slit an electron passes through.
https://www.feynmanlectures.caltech.edu/III_01.html#Ch1-S6

And the key part is when Feynman says, "Then a terrible thing happens.", about the low optical resolutions of long wavelengths.

However, if the separation distance between the two slits is long enough, would it be possible to determine which slit an electron passes through while still having the set-up produce an interference pattern?
 
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  • #2
tade said:
However, if the separation distance between the two slits is long enough, would it be possible to determine which slit an electron passes through while still having the set-up produce an interference pattern?
Why would that happen?
 
  • #3
PeroK said:
Why would that happen?
to be cautious, if i am allowed to explain my thinking in order to answer your question, because the long distance would make up for the low optical resolution, and then, because Feynman said that:
And it is just with light of this color that we find that the jolts given to the electron are small enough so that P′12 begins to look like P12—that we begin to get some interference effect.
 
  • #4
tade said:
to be cautious, if i am allowed to explain my thinking in order to answer your question, because the long distance would make up for the low optical resolution, and then, because Feynman said that:
If the slits are far enough apart there is no interference in the first place. No electrons get through unless you point the electron gun at one particular slit, whereupon it becomes effectively a single-slit experiment.
 
  • #5
PeroK said:
If the slits are far enough apart there is no interference in the first place. No electrons get through unless you point the electron gun at one particular slit, whereupon it becomes effectively a single-slit experiment.
what if the electron beam is wide enough to cover both slits
 
  • #6
tade said:
what if the electron beam is wide enough to cover both slits
You'll get two separate single-slit interference patterns.
 
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  • #7
... I'm not sure what this has to do with detecting the electron?
 
  • #8
PeroK said:
... I'm not sure what this has to do with detecting the electron?
making up for the low optical resolutions of long wavelengths
 
  • #9
tade said:
making up for the low optical resolutions of long wavelengths
To get an significant interference pattern the slits must be close together. That's fundamental.
 
  • #10
PeroK said:
You'll get two separate single-slit interference patterns.
if the two separate diffractions overlap, will they produce a double-slit interference pattern
 
  • #11
tade said:
if the two separate diffractions overlap, will they produce a double-slit interference pattern
Not really. You may get simply two single-slit patterns with a weak interference on the small overlap region, where few electrons are detected in any case.

There is a quantitative point about these experiments that must be addressed if you are intent on subverting the UP. It is always possible to get a bit of indeterminate interference, coupled with significant which-way detection.

It's not a black and white, one thing or the other.
 
  • #12
PeroK said:
There is a quantitative point about these experiments that must be addressed if you are intent on subverting the UP. It is always possible to get a bit of indeterminate interference, coupled with significant which-way detection.

It's not a black and white, one thing or the other.
oh i see, that seems to be quite different from Feynman's striking declarations in the text.

PeroK said:
Not really. You may get simply two single-slit patterns with a weak interference on the small overlap region, where few electrons are detected in any case.
by the way how does the slit spacing affect the strength of the interference pattern

because in those diagrams demonstrating double-slit interference, they don't define the slit-spacing in terms of absolute magnitudes

1666894989035.png
 
  • #13
tade said:
oh i see, that seems to be quite different from Feynman's striking declarations in the text.
Please quote some specific statements in context.
tade said:
by the way how does the slit spacing affect the strength of the interference pattern

because in those diagrams demonstrating double-slit interference, they don't define the slit-spacing in terms of absolute magnitudes

View attachment 316224
This diagram may be purely illustrative. If those two slits were further apart then eventually all the beam would be absorbed or reflected by the barrier. And nothing or almost nothing would get through.
 
  • #14
PeroK said:
If those two slits were further apart then eventually all the beam would be absorbed or reflected by the barrier. And nothing or almost nothing would get through.
could this be compensated by increasing the rate of electrons of the electron beamor, on the other hand, say if its an experiment of the cumulative interference pattern of one electron at a time
 
  • #15
tade said:
could this be compensated by increasing the rate of electrons of the electron beam
Again, you're not thinking quantitatively. Probabilities drop off to negligible numbers very quickly. The slits must be relatively close together. That's a basic part of the experiment. If you have slits 100m apart and fire a beam of electrons at the midway point, then noting gets through. The numbers matter.
 
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  • #16
PeroK said:
Again, you're not thinking quantitatively. Probabilities drop off to negligible numbers very quickly. The slits must be relatively close together. That's a basic part of the experiment. If you have slits 100m apart and fire a beam of electrons at the midway point, then noting gets through. The numbers matter.
oh, cos i had said, "what if the electron beam is wide enough to cover both slits"
 
  • #17
tade said:
oh, cos i had said, "what if the electron beam is wide enough to cover both slits"
Let me describe what I think you are saying. We have two slits, let's say 10m apart. We have a beam of electrons that covers both slits. And, let's have a very weak electron detection behind both slits that fairly reliably detects which slit, without significantly affecting the electron.

Now, you say, we have which-way information and an interference pattern. And the UP disappears in a puff of qualitative logic.

Now, let's describe what would happen. The slits are so far apart that there is no interfence pattern to begin with. There are effectively two single-slit patterns.

So, you say, let's make the slits extremely narrow, so that the single-slit patterns overlap. Now, we have a very faint beam on the far side of the barrier, because the slits are so narrow that very few electrons make it through. Moreover, not many of these electrons are diffracted by the required 5m in order to overlap. So, we have an extraordinarily faint interference pattern directly between the slits (and mostly single-slit patterns on either side). And, that central interference pattern is now destroyed by any detection of the electron - because it's so faint and delicate, and hangs by a thread (as it were).

Your idea is a common attempt to circumvent the UP. If you don't think too carefully or think quantitaively, then it appears the UP must fail. But, once yiou subject your thought experiment to a careful and perhaps quantitative analysis, you find that the UP re-asserts itself.
 
  • #18
PS Reading the Feynman lecture, he covers this point clearly and more coherently than I have in this thread. There is no implication that we get interference or not as an all-or nothing phenomenon. He gives examples where a mixture of interfence and not is the result of the experiment.
 
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  • #19
PeroK said:
The slits are so far apart that there is no interfence pattern to begin with. There are effectively two single-slit patterns.
I see, I think that this takes us back to #12.

PeroK said:
Now, we have a very faint beam on the far side of the barrier, because the slits are so narrow that very few electrons make it through. Moreover, not many of these electrons are diffracted by the required 5m in order to overlap. So, we have an extraordinarily faint interference pattern directly between the slits (and mostly single-slit patterns on either side). And, that central interference pattern is now destroyed by any detection of the electron - because it's so faint and delicate, and hangs by a thread (as it were).
So, say the pattern is faint due to the number of electrons being small. However, are the mechanics of the detection a quantized interaction of a single photon with a single electron?
 
  • #20
tade said:
would it be possible to determine which slit an electron passes through while still having the set-up produce an interference pattern?
No. This is never possible. The two things are mutually exclusive; you can only have one or the other.

Notice that I didn't even quote the qualifier you gave, because it is irrelevant to the question. The answer to the question, exactly as quoted above, is always no, regardless of any other aspects of the scenario. So the same answer applies to all the other versions of the same question that you keep asking with different qualifiers. None of the qualifiers matter.
 
  • #21
PeterDonis said:
No. This is never possible. The two things are mutually exclusive; you can only have one or the other.

Notice that I didn't even quote the qualifier you gave, because it is irrelevant to the question. The answer to the question, exactly as quoted above, is always no, regardless of any other aspects of the scenario. So the same answer applies to all the other versions of the same question that you keep asking with different qualifiers. None of the qualifiers matter.
so if it is always no, is this because there is a mathematical proof which covers all possible circumstances and scenarios
 
  • #22
tade said:
if it is always no, is this because there is a mathematical proof which covers all possible circumstances and scenarios
The "mathematical proof" is called quantum mechanics. Once you understand how the math of QM works for the double slit experiment, the general statement I made is obvious.
 
  • #23
PeterDonis said:
The "mathematical proof" is called quantum mechanics. Once you understand how the math of QM works for the double slit experiment, the general statement I made is obvious.
And so say an experiment with all of the qualifiers is attempted, what do you predict will occur, how will it all "play out", and, what will "go wrong"
 
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  • #24
tade said:
And so say an experiment with all of the qualifiers is attempted, what do you predict will occur
You seem to be relitigating a question which you've already received an answer to - multiple times. You marked this thread as 'I', so we're assuming you have some background (undergraduate) knowledge already.
 
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  • #25
StevieTNZ said:
You seem to be relitigating a question which you've already received an answer to - multiple times. You marked this thread as 'I', so we're assuming you have some background (undergraduate) knowledge already.
uhh, i think i haven't actually received a particular answer, which is connected to #19 as well
tade said:
So, say the pattern is faint due to the number of electrons being small. However, are the mechanics of the detection a quantized interaction of a single photon with a single electron?
as I'm interested in the details of the experimental setup, how it will all "play out"
 
  • #26
PeroK said:
You'll get two separate single-slit interference patterns.
Only if the coherence length of the beam is not large enough. Otherwise there should be two-slit interference patterns (of course superimposed by the single-slit patterns as well).
 
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  • #27
tade said:
say an experiment with all of the qualifiers is attempted, what do you predict will occur, how will it all "play out", and, what will "go wrong"
This is unanswerable because there are many different possible experiments.

tade said:
I'm interested in the details of the experimental setup
Then you need to describe the details of the experimental setup you are interested in. And you would be much better served by first learning the math so that you can figure out for yourself "how it plays out" in whatever experiment you dream up.

Basically it seems like you want answers, but you don't want to do any work yourself to get them. That's not how it works.

Thread closed.
 
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FAQ: Double-slit experiment, determining which slit an electron passed thru

1. How does the double-slit experiment work?

The double-slit experiment involves passing a beam of particles, such as electrons, through two closely spaced slits in a barrier and observing the resulting interference pattern. This experiment demonstrates the wave-like behavior of particles and the concept of superposition.

2. How does the double-slit experiment determine which slit an electron passed through?

In order to determine which slit an electron passed through, a detector is placed at one of the slits. If the electron is detected at that slit, it is assumed to have passed through that slit. If the electron is not detected, it is assumed to have passed through the other slit.

3. Why is it important to know which slit an electron passed through in the double-slit experiment?

Knowing which slit an electron passed through allows us to determine whether the electron behaves as a particle or a wave. If the electron is detected at one slit, it behaves as a particle. If it is not detected, it behaves as a wave and exhibits interference patterns.

4. What happens if we try to determine which slit an electron passed through without using a detector?

If we try to determine which slit an electron passed through without using a detector, we will not see an interference pattern. This is because the act of observation or measurement affects the behavior of the electron, causing it to behave as a particle rather than a wave.

5. Can the double-slit experiment be applied to other particles besides electrons?

Yes, the double-slit experiment can be applied to other particles such as photons, atoms, and even molecules. The interference patterns observed in these experiments support the wave-particle duality concept, which states that particles can exhibit both wave-like and particle-like behavior.

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