Two-Slit & n-Slit Experiments: Explained

[yasar1967]

I'm trying to be 100% sure if I get this experiment right and it's complications as it obviously is a base to quantum physics.

During the experiment and when electrons are fired one by one, immediately leaving the gun, an electron has an infinite number of choices/paths until it hits the screen.
The electron gun makes all possible choices/paths to collect on one place which is the very straight path up to the screen as the momentum it gives to electrons requires so. Otherwise the electrons would spread out to universe equally, statistically.
Electron is everywhere, not just on the screen where we have more likely to find than in any other places after it leaves the gun.
Electron gun's mere function is to concentrate them on the straight path, again statistically. If we do not use ANY slits at all what we see is the wave collapse function of infinite possibilities but mostly and statistically on the very straight path. What seems as a chunk of electrons displayed on the screen is actually collapsed wave functions of infinite electrons, no patterns. As a matter of fact it's not "no-patterns", it's the collections of "n" wave patterns.
When we use two slits, on the other hand, we eliminate all the other possible wave functions but allow these two to reach to the screen.
That's why it doesn't matter if we fire them one by one or in bulk, their collective wave function is pre-determined, what we see is what we "choose" to see. If we choose to see only TWO of the "n" wave collapses, we use two slits, if three then so on.
It's not that electron leaves the gun as a "single" entity but when it comes near to the slit it splits into two. It leaves the gun as "n" entity all following a different path and the two possible routes we choose to observe and these two paths' overlap we see on the screen.

Am I right or am I right ? :)

 

[RandallB]

NO

The dispersion of the electrons is not an innate part of the beam being generated by the electron gun that “collapses” if we “do not use ANY slits at all” by not even putting up the barrier. The dispersion is created by the barrier being in place with one of two slits, it does not matter the dispersion remains the same. What changes is that within the fixed ‘dispersion’ pattern of the photons we see another ‘interference’ pattern of light and dark spaces appear within the same area of dispersion.

To get a better idea of the concept, suggest you work with the tool at: http://www.physics.northwestern.edu/vpl/optics/diffraction.html"

A description like:

It's not that electron leaves the gun as a "single" entity but when it comes near to the slit it splits into two. It leaves the gun as "n" entity all following a different path and the two possible routes we choose to observe and these two paths' overlap we see on the screen.

Is an attempt to put the issue into commonsense or semi-realistic terms that we can understand classically but cannot be verified by direct observation of what the electron is doing as it goes through the slit(s). Any observational measurement of it near either slit destroys the ‘interference’ pattern. Which is why QM in the “Copenhagen” view considers itself “complete” and attempts to describe what is happening between slit(s) and screen should be abandoned.
Sometimes put as “Shut Up and Calculate” meaning just calculate QM HUP statistics of screen probabilities based on slit configurations as the best explanation that cannot be improved upon by any other means (and has not for 80 years).

Although there are some of us like Einstein that hold out hope that a Local & Realistic description can still be found. Local Realists sometimes considered scientific “nuts”. Einstein was not called “nuts” but often considered as wasting his final decades in a vain attempt to discredit the Non-Local view as in QM as the final answer.

So don’t expect to be 100% sure; the accepted and popular view is you need to “just calculate” without trying to use a “classic sensible” description, even an unrealistic multi pathed one.

 

[ripcurl1016]

In the case of the double slit experiment, we do not know whether the electron passed through the left slit or the right slit, so we assume that it passed through both slits simultaneously. Each possibility being a "state", and because the electron fullfills both possibilities it is said to be in a "superposition of states."


So MWI claims that the electron has two definite choices-either it passes through the left slit or the right slit-at which point the universe divides into two universes, and in one universe the electron goes through the left slit, and in the other universe the electron goes through the right slit. These two universes somehow interfere with each other causes the intereference pattern. So whenever an object has the potential to enter one of several possible states, the universe splits into many universes, so that each potential is fulfilled in a different universe.

 

[cesiumfrog]

we managed to jump WAY past the original post all the way to Afshar

Well, I for one did find the OP difficult to read..

[...] Am I right or am I right ? :)

NO

I think he [the OP] was right.. he basically seemed to be giving the same story that Zee recounted about Feynman, in the first chapter of QFT in a nutshell.

 

[jtbell]

I've split off a discussion about Afshar's experiment into this thread.

 

[RandallB]

I think he [the OP] was right.. he basically seemed to be giving the same story that Zee recounted about Feynman, in the first chapter of QFT in a nutshell.

I disagree, the Feynman questions about adding new holes and new screens are meaningless unless you have dispersion that spreads the likely locations of electrons across those holes. Thus the source of electrons Feynman refers to is from ether of the two slits or both. However, not the original beam exiting from the electron gun.

So the answer to the OP is still NO.

(jtbell - Good choice to split this thread)

 

[lightarrow]

In the case of the double slit experiment, we do not know whether the electron passed through the left slit or the right slit, so we assume that it passed through both slits simultaneously

I have a problem with that statement.

In the book "Introduction to the Quantum Theory" - David Park, Chapter 10, paragraph 10.1 he analyzes the double slit experiment and in the subsequent problem 10.1 he asks to consider what would be the pattern on the final screen if we detected, with "magical detectors" behind the slits, the simultaneous passage of the wave in both slits; with "magical detectors" he means detectors which would detect the passing of a particle without changing its wavefunction at all (he computes the case of one only "magical" detector in the paragraph).

I have made that computations and my result is that the interference pattern vanishes.
So, maybe we can't even say that the electron passes through both slits.

 

[ripcurl1016]

The act of detecting or observing which slit the electron goes through forces the electron to go through one or the other slit, which is why the interference pattern vanishes.

 

[lightarrow]

The act of detecting or observing which slit the electron goes through forces the electron to go through one or the other slit, which is why the interference pattern vanishes.

Ok. Now my question is: if you don't detect the electron in a slith or both, can you say that, however, the electron has passed through both slits? How would you prove your assertion?

 

[ripcurl1016]

The only proof is by examining the interference pattern that is created and comparing it to quantum theory.

 

[lightarrow]

The only proof is by examining the interference pattern that is created and comparing it to quantum theory.

Ok, but this proves that a wave has passed through both slits, not a particle.

 

[ripcurl1016]

So when unobserved it is a wave and when observed it is a particle

 

[lightarrow]

So when unobserved it is a wave and when observed it is a particle

Yes, but we should be more precise, since you "observe" it even in the final screen when there is the interference pattern: you have detectors and "clicks" even there. So, where exactly is the difference? It's the fact that in the first case you put detectors so near the slits that you can establish which slit the particle has passed through, while in the second case you can't. So, it's not the fact that we have "observed" the electron, but that we have the information on its position near the slits. It's only this information that creates the particle behaviour, isnt'it?

 

[ripcurl1016]

Exactly, until we go to detect which slit it has gone through, it is not even a matter of its location and which slit it goes through. There are four logical possibilities for the electron to go through: A, B, BOTH, NEITHER. You can systematically go through each route and eliminate each one. Design an experiment to test each route, put a total of nothing box in route A, and find that is has an effect on the particle, bout total of nothing boxes don't have an effect on things that pass through them so that eliminates that route. You can eliminate route B for the same reason. To see if it went through BOTH routes stop the experiment in the middle and will find that it is either on one side or the other but not both. Block both routes and nothing gets through so that eliminates that possibility too. You can eliminate each possible route it went through one by one, but that is assuming that it even makes sense to ask which route it has gone through. The question of asking which route it went through is just an inappropriate question to even ask.

 

[Wallin]

I have a slightly different question: What exactly is involved when we say the electron (or photon for that matter) is detected at one slit or another? How exactly does the apparatus interact with the electron or photon? Is there a kind of absorption and reemission of the detected wave/particle at the slit? Can anyone describe, in laymans terms? Careful, I'm a student of the arts, not of science!

 

[RandallB]

I have a slightly different question: What exactly is involved when we say the electron (or photon for that matter) is detected at one slit or another? How exactly does the apparatus interact with the electron or photon? Is there a kind of absorbtion and reemission of the detected wave/particle at the slit? Can anyone describe, in laymans terms? Careful, I'm a student of the arts, not of science!

I take it you are new to the forums here, welcome.
Be sure to read the “sticky” posted threads at the top of each forum to be sure you understand what is expected in each forum by the mentors that monitor them.
As to using the posting features, you can fix things like ‘absorbtion’ in #15 by using “edit” on your posting no need to waste a new post. I think you have 24 hours to make such edits. You can then even go to your #16 and edit it be deleting it ( I think my post #17 will turn into #16 I’m not sure) But you do have the chance to clean up typos etc. but only within the first full day.

As to an art student layman description you wanted.
Consider testing a thousand balls thrown at a wall and know that only 50 each will make it though any hole of a given size. With two holes you know a total of 100 go though to hit a target behind the wall and you cannot “see” which hole is traversed by anyone ball. You set up vertical light beams above each hole and detect for a shadow below.
NO absorption and reemission required which would ruin the test but even the impact of the light on the balls might effect the out come – even if small we cannot discard it as it could also ruin the test.
SO we only put the beam across one of the two holes, and carefully record the results for each and every ball that hits the target screen behind the wall.
And we exclude any 50 target hits that happen when a shadow is detected the expected 50 times. Leaving 50 target hits that had to come through the other hole where now light beam test existed to possibly interfere with the test.
You keep track of each of the thousand throws to be sure they are not thrown quickly enough to allow two balls to go through at the same time, that would also ruin the test.
Now you have kept track of 50 balls that could not have been interfered with as they go through that one hole.

I think you already know the rest about how the electron and photon versions of these balls proceed to hit the dark spots in the pattern they used to create before the detection was turned on over the hole they are not going through.

 

[Wallin]

Consider testing a thousand balls thrown at a wall and know that only 50 each will make it though any hole of a given size. With two holes you know a total of 100 go though to hit a target behind the wall and you cannot “see” which hole is traversed by anyone ball. You set up vertical light beams above each hole and detect for a shadow below.
NO absorption and reemission required which would ruin the test but even the impact of the light on the balls might effect the out come – even if small we cannot discard it as it could also ruin the test.
SO we only put the beam across one of the two holes, and carefully record the results for each and every ball that hits the target screen behind the wall.
And we exclude any 50 target hits that happen when a shadow is detected the expected 50 times. Leaving 50 target hits that had to come through the other hole where now light beam test existed to possibly interfere with the test.
You keep track of each of the thousand throws to be sure they are not thrown quickly enough to allow two balls to go through at the same time, that would also ruin the test.
Now you have kept track of 50 balls that could not have been interfered with as they go through that one hole.

I think you already know the rest about how the electron and photon versions of these balls proceed to hit the dark spots in the pattern they used to create before the detection was turned on over the hole they are not going through.

Thanks for your reply. I can see how such a test would work with balls, but photons and electrons are much different (aren't they?). The beam of light detector would definitely ruin a test with quantum phenomena. What's more, if I understand the double slit experiment correctly, if photons and electrons are sent through the slits one at a time, they still interfer with each other. Balls wouldn't do that. I suppose what I need is to understand (perhaps in more technical terms) how the photon or electron detector does its detecting. I have been reading Greene, Feynman, Rosenblum, Kuttner, Smolin and Oerter on the subject of the double slit experiment. All of them describe variations on the classic experiment with some of the variations become quite tortuous. All of this is done to prove the point that the act of observation collapses the probability wave. Since observation seems to be the critical influence in these experiements, I want to understand everything I can about those observations (in this case detection at the slits). So I guess what I'm really asking is how does an electron or photon detector work? I haven't been able to find anyone who could answer this question.

 

[CaptainQuasar]

With a QM understanding of the situation is it consistent to use the description that the electron or photon "passes through" the slits at all? (I'm not asserting that it's not accurate to describe it that way, I'm really asking.)

For example, with a modern understanding can this maybe be seen as the same thing as when a "wave" of light "passes through" the solid matter of a prism?

 

[Wallin]

If what I've been reading corresponds with QM (and what I've read is the limit of my understanding on the subject), QM is not as clear cut as that. When the quantum particle/wave is observed the probability wave collapses, it becomes a detected particle and the interference pattern disappears. When the wave passes throught both slits unobserved, the probability wave persists and the interference pattern is observed. This is true whether quanta are released in the direction of the slits in large quantities or one quantum at a time. I may well have misunderstood Brian Greene, Rosenblum and Kluttner, or perhaps they were dumbing it down sufficiently that I carried away a false impression. Perhaps someone can set me straight.

 

[lightarrow]

With a QM understanding of the situation is it consistent to use the description that the electron or photon "passes through" the slits at all? (I'm not asserting that it's not accurate to describe it that way, I'm really asking.)

For example, with a modern understanding can this maybe be seen as the same thing as when a "wave" of light "passes through" the solid matter of a prism?

In my personal opinion, the 2 slits experiment shows that:

1. The particles producing the interference pattern, be them electrons or photons or what else, cannot have (transverse) dimensions less than the slits distance, because they cannot be spatially localized better than that distance (otherwise the interference pattern would vanish).

2. The very act of detecting them in one slit or another gives them dimensions less than the slits distance, because they can now be spatially localized better than that distance; so you can now say the particle has passed through one only slit, and this makes the interference pattern vanish.

The personal conclusion of this is: quantum objects do have dimensions but these depend on the quantum object and on the experimental setting.

 

[muppet]

Do you PF rules allow you to state stuff that directly contradicts mainstream physics if you're explicit that it's not the accepted viewpoint? if so lightarrow, I think it would still be a good idea if you accompanied it with a clear statement of the widely accepted view. In any case, it's not clear what you mean when you say dimension of the particles here- are you really saying that electrons have sizes comparable to the interatomic spacing in crystal structures??
The orthodox answer to captain quasar's Q would be that prior to the act of measurement, the electron doesn't really exist except as this 'state' (whatever that means...); so it's wrong to think of the electron as a little billiard ball flying through space; but when talking to people who understand this stuff, I think it's a pretty common shorthand to say that "the electron passes through" rather than "the wavefunction associated with the one-electron system varies with time in such a manner that the likelihood of detecting the particle at any given instant is greatest at points in space and time that bear some relation to the trajectory of a moving particle"- I think it's self-evident why
EDIT: Walin, your understanding as far as I can see is correct. Better than that of a great many on here, as you'll probably see.

 

[ZapperZ]

Since observation seems to be the critical influence in these experiements, I want to understand everything I can about those observations (in this case detection at the slits). So I guess what I'm really asking is how does an electron or photon detector work? I haven't been able to find anyone who could answer this question.

Electron detection is relatively easy, mainly because it has an electrical charge. At the simplest case, you put a loop of wire and detect a current going through that loop when the electron moves through it. In more sophisticated detection scheme, one can use a CCD.

Photon detectors can be more involved depending on how accurate and what kind of information you need out of it. A simple photomultiplier is typically what one can use, or even a photodiode. But all of this involves the destruction of that photon being detected. I believe there have been papers on the non-destructive detection scheme, but I don't have those references handy at the moment. Maybe someone else has it.

Zz.

 

[lightarrow]

Do you PF rules allow you to state stuff that directly contradicts mainstream physics if you're explicit that it's not the accepted viewpoint? if so lightarrow, I think it would still be a good idea if you accompanied it with a clear statement of the widely accepted view.

Ok. About the photon, the accepted current view is that it has no size (which does not mean size = 0); about the electron, the accepted current view is that it has size = 0.

In any case, it's not clear what you mean when you say dimension of the particles here- are you really saying that electrons have sizes comparable to the interatomic spacing in crystal structures??

In that setting yes. My opinion is that the only physically meaningful way to attribute size to an object is by measuring it with detectors; then comes the personal conclusion I wrote in my previous post. Anyway, from other threads where he/her has posted, I assumed CaptainQuasar has enough knowledge on the subject to take my conclusion as an invitation to think about things in a different way, rather than to convince him of my opinion.

 

[CaptainQuasar]

So lightarrow... your personal opinion wasn't your personal opinion of the QM interpretation, it was sort of your personal science? Thank you for responding but that did make it a bit confusing. Though it was a good measure on your part to point out that it was personal, that did prevent me from getting too stuck.

 

[Wallin]

Electron detection is relatively easy, mainly because it has an electrical charge. At the simplest case, you put a loop of wire and detect a current going through that loop when the electron moves through it. In more sophisticated detection scheme, one can use a CCD.

Photon detectors can be more involved depending on how accurate and what kind of information you need out of it. A simple photomultiplier is typically what one can use, or even a photodiode. But all of this involves the destruction of that photon being detected. I believe there have been papers on the non-destructive detection scheme, but I don't have those references handy at the moment. Maybe someone else has it.

Zz.

Does the electron actually make contact with the wire loop? If not, does the wire loop influence the electron in any way that can be proven by experiment? Can you spell out CCD for me?

 

[ZapperZ]

Does the electron actually make contact with the wire loop? If not, does the wire loop influence the electron in any way that can be proven by experiment? Can you spell out CCD for me?

When you measure the magnetic field from a current, do you make "contact" with the current itself? If you say no, then the answer is no. If you say yes (via the magnetic field interaction on one to the other and vice versa), then the answer is yes. So it depends on what you mean by 'contact'.

CCD= charged coupled detector. My avatar is a CCD image of a spectrum of photoelectrons emitted from a high-Tc superconductor.

Zz.

 

[Wallin]

Thanks! I guess in the context I'm trying to understand the electron makes contact with the wire. What about the second question. Is the electron influenced in any way by the wire loop that has been proven by experiment (does it alter or change the electron or any of its properties)?

 

[lightarrow]

So lightarrow... your personal opinion wasn't your personal opinion of the QM interpretation, it was sort of your personal science? Thank you for responding but that did make it a bit confusing. Though it was a good measure on your part to point out that it was personal, that did prevent me from getting too stuck.

Sorry, I didn't mean to make you confused because I thought you were more aknoweldged with these things. Forget about that personal idea I have expressed in my post.

 

[reilly]

This thread, of course, is about the Davisson-Germer expt, back in the 1920's. The fact that electrons passing through a crystal lattice created a diffraction pattern when detected was virtual heresy. This expt. completely wrecked the classical notion that a particle always behaved as a particle -- just for the record, a particle is something that is real small. Physicists were scratching their heads and all but deBroglie, were headed towards catatonic breakdowns. Such a thing simply could not happen, but it did. I had the good fortune to hear about this from a physicist who was at Bohr's Institute at that time -- for some the experiment had the same impact as might be today if we found a real UFO hovering over Fenway Park during the last World Series. Big. Huge. Impossible. D&G's experiment drove a great deal of the development of QM.

Why? Who knows? But, then, why the Principle of Least Action, why the equality of inertial and gravitational mass?, why relativity of any kind? At the bottom line physics is about description; what else can we do? Now we can all be jaded, critical of the structure and interpretation of QM, and, oh, for goodness sake, why do all the professional quantum mechanicians, mostly, avoid deep metaphysical arguments and discussions of reality? Physics is physics and philosophy is just that. And the puzzlement over the impossible lead to modern QM, which, with it's flaws, is one of the crowning intellectual achievements in all of history. It's indeed a great shame that today's students have no idea of the climate of the time during which QM was developed -- replaced by the excitement over neural science and practical genetics. That climate tells a lot about QM, and the history makes QM much more understandable. The strange behavior of diffracting electrons simply is. And, this behavior has been of inestimable value in many fields of physics -- the electron microscope immediately comes to mind, we're talking major diffraction here. And, something that is hardly ever mentioned, is the role of scattering experiments, all of which show diffraction patterns of one sort or another. In other words, physics is saturated by particle diffraction phenomena.That's how, for example, the structure of DNA was untangled.

For those who don't like QM -- give an example of some natural phenomena that is completely understood?

Any alternate approach to QM has to go past the slit stuff -- there are probably millions of papers of QM, lot's of luck in getting a new theory to reproduce all the QM stuff. So far, no one has had much luck in replacing QM.

As a retired professor, I applaud the give and take of the posts, and the passion that is so helpful to becoming a good professional anything. It, however, really helps to know the subject under discussion -- read and "listen". Everything discussed here has been discussed in the literature countless times; study is a good thing.

Regards, Reilly Atkinson

 

[CaptainQuasar]

This thread, of course, is about the Davisson-Germer expt, back in the 1920's.

Yes! Thank you reilly, the Davidsson-Germer experiment is exactly the kind of thing I would have hoped to find out about with my question a few comments back!

So do the electrons refract as well as diffract within a crystal? Do they resolve into a spectrum at all? If so what property of each electron determines its place within the spectrum? The speed / energy of the electron?

 

[RandallB]

Wallin; What, your complaining about a lay answer because it is not technical enough?
If you just want to know how “photon detectors” work use google.

Thanks for your reply. I can see how such a test would work with balls, …

NO, the test will not work with balls to produce interference because going though a single hole balls do change direction with the needed dispersion.

What's more, if I understand the double slit experiment correctly, if photons and electrons are sent through the slits one at a time, they still interfer with each other.

I was wrong you do not understand the rest of the issue. The point is because we know they go through the slits one at a time we know it is impossible for them to “interfere with each other” (edit: to produce the pattern we find).

I have been reading … All … done to prove the point that the act of observation collapses the probability wave. ….
So I guess what I'm really asking is how does an electron or photon detector work?

NOT when you assume a “probability wave” is real and has the ability to “collapse”. You are no longer dealing with your original question when you do that. The principle logic of how to select particles going undisturbed through just one slit is the important part of the test, and described in lay terms by the ball example.

Once you assume or open the question of what is happening from event (slits) to observation (detection screen); you are on a new topic with a large selection of views that are open to argument as you can see in the other posts. None of those various speculations are required by the HUP math used in OQM Theory. QM might be matched by other ideas but science has accepted nothing as more complete than QM yet.

 

[Wallin]

Wallin; I was wrong you do not understand the rest of the issue. The point is because we know they go through the slits one at a time we know it is impossible for them to “interfere with each other”.QUOTE]

I'm confused again (easily done!). I thought that if photons or electrons were "shot" singly through double slits with detectors turned off, they do produce an interference pattern. Its only when the detector(s) are turned on, that the interference pattern disappears. We can know the particles/waves are going through the slits one at a time without knowing "which slit." Its when we know "which slit" that the probability wave collapses. And I'm not saying the probability wave or its collapse is real. I'm not even saying the particle/wave is real. Talking about what's real and what's not real in QM seems a precarious exercise.

Sorry I wasted your time with the question about detectors, but Google is not always an ideal or efficient source of information. I'm not trying to buy a detector, I'm trying to understand how it interacts with the things it detects. I've looked on Google and haven't found a good explanation of how the apparatus interacts with its subject. Guess I'll keep looking.

 

[RandallB]

Not a waste of time, but IMO (in my opinion) the real problem you are working on in your mind has more to do with understanding what is meant by the Bohr “completeness” of QM or what Feynman called “shut up and calculate”. (Terms all easy to find with simple searches.) And how QM Theory does not need or even expect a ‘tangible’ description like “collapse” to describe the theory.

Actually, IMO experimental proof that “superposition” and its “collapse” is real would falsify QM completeness as defined by Niels Bohr using HUP. And I doubt a better technical understanding of photon or electron detection will help you in understanding these issues.

Science only uses statistical mathematics (like HUP) to resolve this paradox, NOT what you might call common sense. No scientist claims it does or demands that a description like collapse, multi dimensions or guidewaves are required.

 

[Wallin]

That's fair. If I knew how, I would shut up and calculate! But being a very poor mathematician, I must resort to questions. Perhaps I'm trying to find sense and meaning in a physical phenomenon which by nature is inexplicable. Is that what your trying to tell me?

 

[reilly]

Yes! Thank you reilly, the Davidsson-Germer experiment is exactly the kind of thing I would have hoped to find out about with my question a few comments back!

So do the electrons refract as well as diffract within a crystal? Do they resolve into a spectrum at all? If so what property of each electron determines its place within the spectrum? The speed / energy of the electron?

Captain, sir!

Yes. electrons can refract inasmuch as it is a consequence of scattering, highly dependent upon superposition of momentum states. Within matter, an electron passing through will bounce around a bit, slow down, and can also scatter away from the surface of the matter. Best place to start is to review the propagation of light through materials,and of charged particles through matter. Jackson and other E&M authors do the first, older books on atomic or nuclear physics do the latter --- particularly those with an experimental twist.
Regards,
Reilly Atkinson