Observation in the double-slit experiment

  • #1
I'm not sure if there's a better category to post this in, and I'm just a casual physics enthusiast, but I'm having trouble understanding this:

"Consider the famous two-slit experiment. When you watch a particle go through the holes, it behaves like a bullet, passing through one slit or the other. But if no one observes the particle, it exhibits the behavior of a wave and can pass through both slits at the same time."

How do I "watch" a particle go through a hole? I don't think I've looked at one going through a slit nor seen it behave like a bullet.

Also if nobody's observing the particle then I don't know how anyone can tell that it behaved like a wave and passed through both slits at the same time.

I know many explanations use these terms, but it just seems nonsensical the way this person is describing it; I'm coming at it from lay-terms, but I'm just trying to make sense of this description mechanically (or physically?).
 

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  • #2
Drakkith
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How do I "watch" a particle go through a hole? I don't think I've looked at one going through a slit nor seen it behave like a bullet.
Since the particles are on the size of molecules or smaller, you have to use specialized detectors. You certainly can't see it with your eye or even with an optical microscope. Note that comparing the impact pattern on the detection screen with one slit open vs with both slits open immediately shows you that particles do not act like 'classical' particles (little bullets) since the pattern on the screen with both slits open is one of a wave, not a series of classical particle impacts.

Also if nobody's observing the particle then I don't know how anyone can tell that it behaved like a wave and passed through both slits at the same time.
What scientists typically do is they set up the experiment and shoot hundreds or thousands of particles through the slits to a screen on the other side, and a detector records the impact position on the screen. If you observe which slit the particles go through (using a separate detector) then the pattern of impacts on the screen are just like classical particles. But if you don't observe which slit the particles go through, then the resulting impact pattern is identical to that of a wave passing through the two slits.
 
  • #3
Nugatory
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Sloppy descriptions of the double-slit experiment are sadly all too easy to find, and you've found one - you're right to feel that something has to be wrong here.

A better description would be something along the lines of: the probability of the particle being detected at any point on the screen includes contributions from all the possible paths between detector and screen (and the contributions aren't all additive, which is how we get interference patterns when the pattern is built up one detection at a time as @Drakkith describes above). If we have any sort of particle detector or any other interaction at one of the slits, that will make it impossible for there to have been paths through one slit or the other slit (for example, there are no paths through the left-hand slit that will trigger a detector at the right-hand slit) so we no longer have contributions from these paths and we don't get an interference pattern. In practice, we usually just close off one slit or the other to eliminate possible paths through it because the interesting quantum mechanical phenomenon is that we do get an interference pattern when paths through both slits are possible, and we do not when only one slit is possible.

The idea that the particle switches between behaving like a particle and like a wave was abandoned almost a century ago. Unfortunately by then it had leaked out into the popular imagination and lives on as a sort of urban legend, something that "everyone knows" even though it isn't true.
 
  • #4
PeroK
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I'm not sure if there's a better category to post this in, and I'm just a casual physics enthusiast, but I'm having trouble understanding this:

"Consider the famous two-slit experiment. When you watch a particle go through the holes, it behaves like a bullet, passing through one slit or the other. But if no one observes the particle, it exhibits the behavior of a wave and can pass through both slits at the same time."

How do I "watch" a particle go through a hole? I don't think I've looked at one going through a slit nor seen it behave like a bullet.

Also if nobody's observing the particle then I don't know how anyone can tell that it behaved like a wave and passed through both slits at the same time.

I know many explanations use these terms, but it just seems nonsensical the way this person is describing it; I'm coming at it from lay-terms, but I'm just trying to make sense of this description mechanically (or physically?).
Try watching this if you have an hour to spare:

http://www.cornell.edu/video/richar...ty-uncertainty-quantum-mechanical-view-nature
 
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  • #5
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Sloppy descriptions of the double-slit experiment are sadly all too easy to find, and you've found one - you're right to feel that something has to be wrong here.

A better description would be something along the lines of: the probability of the particle being detected at any point on the screen includes contributions from all the possible paths between detector and screen (and the contributions aren't all additive, which is how we get interference patterns when the pattern is built up one detection at a time as @Drakkith describes above). If we have any sort of particle detector or any other interaction at one of the slits, that will make it impossible for there to have been paths through one slit or the other slit (for example, there are no paths through the left-hand slit that will trigger a detector at the right-hand slit) so we no longer have contributions from these paths and we don't get an interference pattern. In practice, we usually just close off one slit or the other to eliminate possible paths through it because the interesting quantum mechanical phenomenon is that we do get an interference pattern when paths through both slits are possible, and we do not when only one slit is possible.

The idea that the particle switches between behaving like a particle and like a wave was abandoned almost a century ago. Unfortunately by then it had leaked out into the popular imagination and lives on as a sort of urban legend, something that "everyone knows" even though it isn't true.
I am a curious layman. Would have made a similar post, but hope it is ok to ask here since my confusion is very related in my eyes. How does the setup in any double slit experiment judge between what is a chosen measurement and what is just an interaction? In my understanding interactions happen all the time everywhere, and I am confused how nature knows?
 
  • #6
DrClaude
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I am a curious layman. Would have made a similar post, but hope it is ok to ask here since my confusion is very related in my eyes. How does the setup in any double slit experiment judge between what is a chosen measurement and what is just an interaction? In my understanding interactions happen all the time everywhere, and I am confused how nature knows?
Coupling of the quantum system with any other system that is big enough can lead to decoherence. This was clearly demonstrated in the double-slit top experiment using molecules, where the heating of the molecules lead to a decrease in the fringe contrast. If the molecule emits even a single photon, then the coupling the the EM field is enough for the trajectory to be reduced to a classical path. Another way to see this is that by detecting the photon, one could infer through which slit the molecule traveled, and this removes the interference. And actually detecting the photon is not necessary: simply the fact that the molecule has been coupled to the EM field with one photon is enough to lead to decoherence.
 
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  • #7
Nugatory
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I am a curious layman. Would have made a similar post, but hope it is ok to ask here since my confusion is very related in my eyes. How does the setup in any double slit experiment judge between what is a chosen measurement and what is just an interaction? In my understanding interactions happen all the time everywhere, and I am confused how nature knows?
David Lindley's book "Where does the weirdness go?" is a good layman-friendly explanation of decoherence.
 
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  • #8
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DrClaude & Nurgatory thank you for both your answers.

As I understand DrClaudes answer, some level of threshold will cause a collapse. If I understand that correct, that makes some kind of sense to my still curious layman mind.

I will get the book you recommended Nurgatory, it looks like an interesting read.
 
  • #9
vanhees71
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Have a look at the Feynman lectures vol. III. Feynman starts QM with a careful explanation of the double-slit experiment with quantum particles, and he also gives the right interpretation, i.e., the "shutup-and-calculate interpretation", for an introductory text (SCNR):

http://www.feynmanlectures.caltech.edu/III_01.html#Ch1-S1
 

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