The double slit experiment

[minkwe]

For a change from Bell discussions, I would like to start a thread on a "slightly" different topic, the double slit experiment. Richard Feynman famously said:

"[The double slit experiment] is a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics]."

Feynman also said often that all of quantum mechanics can be gleaned from carefully thinking about the implications of this experiment. And the wikipedia article on the experiment also quotes Anton Zeilinger of (EPRB-experiment fame) as saying:

The observer can decide whether or not to put detectors into the interfering path. That way, by deciding whether or not to determine the path through the two-slit experiment, he can decide which property can become reality. If he chooses not to put the detectors there, then the interference pattern will become reality; if he does put the detectors there, then the beam path will become reality. Yet, most importantly, the observer has no influence on the specific element of the world which becomes reality. Specifically, if he chooses to determine the path, he has no influence whatsoever which of the two paths, the left one or the right one, Nature will tell him is the one where the particle is found. Likewise, if he chooses to observe the interference pattern he has no influence whatsoever where in the observation plane he will observe a specific particle. Both outcomes are completely random.

Feynman and Zeilinger are highly respected physicists but they are just plain wrong. There is no mystery in the double slit experiment.

[jreed]

This is a great topic! I've been thinking about starting a thread with the same subject for some time. However, I have to disagree with the conclusion that physicists are incorrect. If there's no mystery here and you can explain it, you will certainly have my admiration along with the rest of the physics community I'm sure. I hope that you can explain it with topology or something else. I really do!

For a change from Bell discussions, I would like to start a thread on a "slightly" different topic, the double slit experiment. Richard Feynman famously said:

"[The double slit experiment] is a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics]."

Feynman also said often that all of quantum mechanics can be gleaned from carefully thinking about the implications of this experiment. And the wikipedia article on the experiment also quotes Anton Zeilinger of (EPRB-experiment fame) as saying:


Feynman and Zeilinger are highly respected physicists but they are just plain wrong. There is no mystery in the double slit experiment.

[minkwe]

Good jreed,
Where do I start. Let us examine Zeillinger statement:

That way, by deciding whether or not to determine the path through the two-slit experiment, he can decide which property can become reality.

There is nothing mysterious here. By deciding whether or not to go into my office, I can decide whether the reality of the next moment will include my presence in the office or not. If you block or disturb the photon at the slit by attempting to detect it, you should not reasonably expect to have the same result as when you allow the particle undisturbed.

So what is so mysterious about the double slit anyway. Is it a real experimental result we have obtained which is strange, or is it due inaccurate popular-science versions. I believe it is the later. If you know of a specific experiment you would like explained classically, I'll be happy to try.

[jreed]

Good jreed,


So what is so mysterious about the double slit anyway. Is it a real experimental result we have obtained which is strange, or is it due inaccurate popular-science versions. I believe it is the later. If you know of a specific experiment you would like explained classically, I'll be happy to try.

Here's what needs to be explained. Go to this site and watch the short movie clip towards the bottom of the page:

http://physicsworld.com/cws/article/new ... a-makeover

This is a real experiment, done with real electrons, assumed to be point particles and looking like point particles when they hit the target screen and leave a white fluorescent imprint. They have passed through a two slit arrangement before hitting the screen. Now with one slit blocked, you will only get a single smeared out pattern with no diffraction. This part of the experiment isn't shown. With both slits open you get the diffractions shown on the screen. Here's what needs to be explained: How do these particles know that there are two slits open? They can only go through one slit at a time can't they? Opening the second slit actually makes the number of hits less in the diffraction pattern minima. This is Feynman's big mystery.

[minkwe]

Here's what needs to be explained. Go to this site and watch the short movie clip towards the bottom of the page:

http://physicsworld.com/cws/article/new ... a-makeover

This is a real experiment, done with real electrons, assumed to be point particles and looking like point particles when they hit the target screen and leave a white fluorescent imprint. They have passed through a two slit arrangement before hitting the screen. Now with one slit blocked, you will only get a single smeared out pattern with no diffraction. This part of the experiment isn't shown. With both slits open you get the diffractions shown on the screen. Here's what needs to be explained: How do these particles know that there are two slits open? They can only go through one slit at a time can't they? Opening the second slit actually makes the number of hits less in the diffraction pattern minima. This is Feynman's big mystery.

Before I give you my explanation, let me start by removing some misconceptions which you might have about the experiment. The actual paper can be found here http://iopscience.iop.org/1367-2630/15/ ... 033018.pdf and is open-access.

You say they closed one slit and opened the other. But this is not really true, they had a mask which they moved from one side to the other. See the images in the above paper. They say in the paper that they are able to completely block all the particles leaving one of the slits. But this is wrong because according to their own diagram, the probability distributions of P1 and P2 overlap. It is impossible to completely block the particles from one slit, without also blocking the particles from another.

You say opening the second slit actually makes the pattern brighter. But you forget a crucial detail mentioned in the paper. The patterns are normalized, a process which eliminates absolute values. You can therefore no-longer compare the intensity in one image with another image, although you can still compare intensities within a single image.

You ask how the particles know that there are two slits open or not. First of all, two slits are always open, the mask is placed after the slits, not touching it. Maybe you mean, how do they know the position of the mask. They know because they bump on it, get absorbed and fail to reach their destination at the detector, and thus fail to influence the pattern produced. As the video shows, single particle does not produce a diffraction pattern, it is a collection of the results of many particles. If you selectively block some of them, you change the pattern. Figure 2 is very revealing.

Another thing, In case you did not know, single slits also produce diffraction patterns, just different from double slits and again different from triple slits etc. Yet in their experiment they do not mention or analyze it but you can see that there is some structure in the so-called "single" slit patterns, just not as clear. But more importantly, the structure is more similar to a double-slit pattern than a single-slit pattern. Look at the tails particularly.

Do you have any problems with my analysis so far? We are just getting started.

[FrediFizzx]

Hestenes' "The Zitterbewegung Interpretation of Quantum Mechanics". Ya put that together with Joy's discovery of the spinor properties of space and you will have solved the double slit "mystery".

[gill1109]

Great topic.

The two-slit experiment can be given a local realist explanation by resort to memory, and by resort to conspiracy, and by resort to post-selection (detection loophole), and by resort to locality (things are so close that the speed of light does not prevent information from getting to wherever it is needed, in time). All these things have been reported in the literature, and confirmed by event-based simulation models, many times over.

So there may be some analogy to Bell-EPR experiments. I will show you that the analogy is actually very strong. It might even be that the experiments are actually equivalent, in a strong sense. ie a loophole free double slit experiment is a loophole free Bell-EPR experiment. Neither of which has, to date, actually ever been performed! But hell ... Gedankenexperiments are also a great way to develop science.

In order to neutralize the previously mentioned four possible "loopholes" one should consider N two slit experiments each done in different labs and each done with only one particle, so only one dot hits the screen. Morevoer, in each experiment, random coin tosses made after the particle is in flight but before it reaches either slit of the screen should rapidly determine whether each slit is open or closed. And every particle does actually arrive at the screen, unless of course both slits are closed.

So just like Bell-EPR we would have four possible setting combinations. As to the outcome, it can be made binary: does the particle hit the screen in one of the regions which normally would be the peaks of the interference pattern or one of the regions which would be the valleys (normally means: if both slits were open). Quantum mechanics tells us four different probabilities for the particle to hit the screen in a positive interference area, depending on which slits are open. Obviously if both slits are closed the probability is zero, and also the probability of the other region. So we have a minor departure from Bell-EPR - under one of the four setting combinations, there is a third possible outcome, which always happens; this "missing outcome" never happens with the other three setting combinations.

We must rule out detection loophole, so the experiment is so perfect that each particle does hit the screen and its position is measured and allocated to one of the two regions, except when both slits are closed, when no particle hits the screen.

Now we have excluded all possibility of memory, conspiracy, post-selection (non detections) we have essentially made the double slit experiment simply another manifestation of the EPR-Bell experiment. (In EPR-Bell we normally think of two binary outcomes, but actually we are only interested in whether or not the two outcomes are equal, so one binary outcome is enough to "carry" the "paradox").

[minkwe]

Highly recommended video: https://www.youtube.com/watch?v=gBxZVF3s4cU for this topic. You can see the single slit pattern around 10:48.

[minkwe]

Jreed,
I'm still waiting for your feedback on my analysis above in case you have any questions so we may proceed.

[jreed]

Jreed,
I'm still waiting for your feedback on my analysis above in case you have any questions so we may proceed.

I don't know where your analysis is going. Are you saying that a single slit gives a diffraction pattern and that means something? That only means that electron scattering gives diffractions also in this case which are a wave phenomena. That's what quantum mechanics or wave mechanics is all about. I said that opening a second slit makes the scattering pattern weaker, not stronger at the diffraction minima.

[minkwe]

I don't know where your analysis is going. Are you saying that a single slit gives a diffraction pattern and that means something? That only means that electron scattering gives diffractions also in this case which are a wave phenomena. That's what quantum mechanics or wave mechanics is all about. I said that opening a second slit makes the scattering pattern weaker, not stronger at the diffraction minima.

We are analyzing the experiment you pointed me to. Do you agree or disagree with the points I made. Say which ones you agree with and which you disagree with and we can make progress. The claim at issue is the one that blocking one of the slits eliminates the diffraction pattern. The results of the experiment does not support this claim. Look at Figure 2.

Also it is well known the single slits produce a diffraction pattern. Look at the video I referenced around 10:48

[FrediFizzx]

What is the difference between an interference pattern and diffraction pattern? Are you mixing the two up?

[minkwe]

What is the difference between an interference pattern and diffraction pattern? Are you mixing the two up?

Fred, there is no difference. The same phenomena, although in "popular science" an unnecessary distinction is usually made. That is why I posted the video, where the presenter does the experiment live, first single slit, then double slit, then triple slit.

[jreed]

I don't know where your analysis is going. Are you saying that a single slit gives a diffraction pattern and that means something? That only means that electron scattering gives diffractions also in this case which are a wave phenomena. That's what quantum mechanics or wave mechanics is all about. I said that opening a second slit makes the scattering pattern weaker, not stronger at the diffraction minima.

We are analyzing the experiment you pointed me to. Do you agree or disagree with the points I made. Say which ones you agree with and which you disagree with and we can make progress. The claim at issue is the one that blocking one of the slits eliminates the diffraction pattern. The results of the experiment does not support this claim. Look at Figure 2.

Also it is well known the single slits produce a diffraction pattern. Look at the video I referenced around 10:48

Ok, let's just say that the diffraction pattern is different for two slits as compared to one slit. Are you happy with that now? How do you explain these diffractions of electrons? You can see the pattern being built up in the video from single particle arrivals. That's the real question behind all this other talk, and that is Feynman's mystery.

[albertjan]

I suppose you are all familiar with the walkers experiments, where a macroscopic equivalent of the double slit is performed. See this movie at Approx 2:58h: http://youtu.be/W9yWv5dqSKk
Its mechanism is somewhat like de Broglie–Bohm.

[minkwe]

This is a real experiment, done with real electrons, assumed to be point particles and looking like point particles when they hit the target screen and leave a white fluorescent imprint. They have passed through a two slit arrangement before hitting the screen.

Now with one slit blocked, you will only get a single smeared out pattern with no diffraction. This part of the experiment isn't shown.

jreed, so in the experiment you referenced, your claim underlined by me above is false. Look at the paper (Figure 2) Do you agree?

Here's what needs to be explained:
[1] How do these particles know that there are two slits open? They can only go through one slit at a time can't they?
[2] Opening the second slit actually makes the number of hits less in the diffraction pattern minima. This is Feynman's big mystery.

I already explained [1]. In the experiment you referenced, the two slits are always open. Do you agree? The particles which are blocked know that a window is blocking their path because they bump into it and fail to reach the screen, thus failing to contribute to the pattern produced. The particles which are not blocked, do not care what may be happening elsewhere. They continue to the screen and influence the pattern. What is mysterious in this?

I already explained [2], look at figure 2 in the paper describing the experiment you referenced. Focus on the tails and notice that the maxima and minima are exactly the same as in the case when the window is not blocking anything. What they call a single slit pattern is clearly not a single slit pattern. It is a smudged double-slit pattern, even when the window appears to be blocking one slit. You might even recognize that in figure two a pattern is produced when both slits appear to be blocked by the window! What is mysterious in this?

[minkwe]

Ok, let's just say that the diffraction pattern is different for two slits as compared to one slit.

OK, this is a general question, not about the specific experiment (because they do not have a single slit pattern in the referenced experiment). It is true though that a single slit pattern is different from a double slit pattern (as you can see in the video I referenced). Are you asking why they are different? Were you expecting them to be the same?

How do you explain these diffractions of electrons? You can see the pattern being built up in the video from single particle arrivals. That's the real question behind all this other talk, and that is Feynman's mystery.

Another general question about the mechanism of diffraction. Yes, a the diffraction pattern is built up particle by particle. My guess is that this continuous to be mysterious to you because of the preconceptions I'm trying to untangle before I give you my explanation. I promise, I will give you the explanation you just have to be patient and follow the reasoning process. I'm just trying to make sure you actually understand what I'm going to say.

Some other misconceptions about diffraction/interference patterns which must be untangled before you will understand my explanation:
- Constructive/destructive interference (a remnant from wave theory).
- A single particle goes through both slits.
- A single particle interferes with itself
- A single particle produces an interference pattern
- Knowing which way the particle went, disrupts the pattern

[minkwe]

A few comments about the misconceptions:
Constructive/destructive interference:
We now know that quanta/electrons are discrete particles of energy/mass, they cannot disappear at one location and appear instantaneously at another. Constructive and destructive interference, as much as it suggests that photons or particles magically disappear from some locations and appear instantaneously at other locations is inconsistent with physical evidence about electrons and photons. It is not that particles disappear from the minima and appear at the maxima, rather it is that, there are more particles going to the maxima to begin with than the minima. Nothing is "constructed" or "destructed". Then you may ask, why do particles prefer to go into the maxima rather than the minima, and that is what my explanation will answer (in fact it is not my explanation, it has been known but ignored since the beginning of quantum theory. I guess it was not mysterious enough for the copenhageners).

A single particle goes through both slits. (Hawking, Feynman, Brian Greene etc have all repeated this falsehood)
Simply nonsense. Quanta and electrons are indivisible. No need to explore this one further, it is clearly nonsense.

A single particle interferes with itself
Just as nonsensical. Why do you need slits if particles can interfere with themselves? We should be seeing diffraction from a single beam without any slits. Besides, a single particle does not produce a diffraction pattern, you need many particles, as the video you referenced clearly shows.

A single particle produces an interference pattern
Nonsense.

Knowing which way the particle went, disrupts the pattern
More nonsense. It is obvious that disturbing the path of the particles, disrupts the pattern. This is commonsense and not mysterious.

One more misconception:
The importance of the slits
Most attempts to explain diffraction patterns, focuses on the particles, and ignores completely the most important component, the slits. As you will see, my explanation will take into account all the components.

Any clarifications of the above needed, before I proceed?

[jreed]

This sounds like Alfred Lande's theory. Correct? Please let us know the answer. I'm waiting with baited breath!

[minkwe]

This sounds like Alfred Lande's theory. Correct? Please let us know the answer. I'm waiting with baited breath!

Who is Alfred Lande? You didn't say whether you agree or disagree with the points I asked you specifically above. It doesn't appear so far that you are really interested in my explanation. You are not engaging.

In any case, I never heard of Lande or his theory.