Talk:Double-slit experiment

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Contents 1 Double slit in time 2 Shahriar Afshar 3 Interference 4 Arguments against Shahriar Afshar's interpretation 5 silly controversy 6 I've added a link to a pretty good analysis of Afshar's work 7 Afshar experiment page created 8 Afshar Experiment: A Dissenting Opinion 9 Afshar's Response 10 What exactly is changed as light negotiates a single diffracting slit? 11 coherency and pinholes 12 "Thought Experiment" review needed 13 Beams of electrons or atoms? 14 Revert 15 at home 16 A basic question 17 Request for more description of Young's experiment 18 OK some other basic questions 19 Double Slit experiment with only one detector? 20 graphics 21 Does not convey a clear meaning

[edit] Double slit in time

While possibly a little early to include in the main text, the double slit in time experiment http://physicsweb.org/articles/news/9/3/1/1?rss=2.03Cbr20/E is surely experimental evidence that that matter/energy is not just quantised over spatial dimensions (as shown by the classical double slit experiement) but over the time dimension as well. ie: spacetime does have a quantum nature.

http://xenz.stumbleupon.com/

[edit] Shahriar Afshar

I commented out the part in Afshar's result about wavefunction collapse. The experiment uses a stream of photons. All bets are off if the wave function is not for a single photon. Ancheta Wis 08:22, 31 Jul 2004 (UTC)

Afshar claims that the experiment worked for a single photon. -- Gyan 21:56, 1 Aug 2004 (UTC)-

This is incorrect. The experiment only needs to determine whether a particular set of photons passed through slit 1 or slit 2. --ηυωρ 00:02, 8 Nov 2004 (UTC)

Note also that the experiement uses light at 650nm, and the wires are around 1.3mm apart - much too far apart to be acting as a diffraction grating 80.43.203.143 16:49, 11 Aug 2004 (UTC)

The wires do not form a diffraction grating. They are arranged in a way such that they block some light if one slit is closed, but do not interact with light from both slits. --ηυωρ 00:02, 8 Nov 2004 (UTC)

There are other problems with his experiment: http://axion.physics.ubc.ca/rebel.html

Once there is a formal paper or even a preprint out, we can move it back.... Roadrunner 06:43, 22 Aug 2004 (UTC)

This is not a problem; it is an invalid proof. The author presents an experiment which cannot identify the path of a photon. --ηυωρ 00:02, 8 Nov 2004 (UTC)

Shahriar Afshar's rebuttal to W. Unruh can be found in his FAQ. His briefness underscores the fact that Unruh's argument is nonsense. --ηυωρ 05:38, 8 Nov 2004 (UTC)

This Afshar's setup is really silliness. Its correct interpretation, using Afshar's own setup, is here: [1]. --Lumidek 04:22, 27 Nov 2004 (UTC)

• Dear Lumidek, I have responded to your criticism in the same place [2]. For further information please post your questions at my Blog Official Afshar Experiment Blog bfore going "public" with inaccurate remarks on a topic as haughty as this one. Regards. Shahriar S. Afshar

Professor Afshar, hello! Would you be interested in writing a condensed explanation of your experiment for Wikipedia? --ᓛᖁᑐ 12:34, 27 Nov 2004 (UTC)

Sorry, Dr. Afshar, but your remarks about asking questions before "going public with inaccurate remarks on a topic as haughty as this one" is very entertaining from you who published the same nonsense - described as "quantum bombshell" - on 10 different places. Your comments that I should ask questions is sort of arrogant. Let me assure you that I did my QM classes carefully, and it's not necessary for me to ask you any questions. --Lumidek 16:17, 27 Nov 2004 (UTC)

Dear Lumidek, it is certainly not arrogant to ask people to avoid misquoting you, or not to attribute to you what you have not claimed, and it has nothing to do with whether one has done one's QM classes carefully or not, it is simply a matter of due diligence to ensure veracity of one's statements about another individual. I have found through personal experience, that a little patience and reflection before one jumps the gun does wonders when it comes to scientific discourse. The phrase "quantum bombshell," is not my wording, but a title the editorial board of New Scientist chose to use for the graphics. Perhaps the NS editors felt justified in their choice of wording and the tone of the article, due to the fact that they researched the story for 9 months, asking the opinions of at least a dozen international experts in the field, and realizing that at every occasion I had responded to the critic's satisfaction (including some Harvard faculty.) I have no power over cosmetic issues, and I cannot be blamed if other people copy the graphics from the magazine article and post it on their websites! As to the "nonsense" part, that is your opinion, which I respect and hope to address by explaining my views better. I must say though, It would help if you read my paper especially section 3 on the measurement of interference Visibility! Perhaps, time permitting, I should do what Eequor suggests... Regards. Shahriar S. Afshar

Dear Shahiar, it would be very interesting to know the names of a "dozen of international experts" who agreed with the statement that you have falsified the Copenhagen interpretation by this trivial 19th century-style interference experiment. I've read the section 3 about the visibility, and it's not correct. If you want to compute the visibility, you must measure the effect of the wire grid in both situations - if the wires are located at the interference minima, and if they're located elsewhere. But if they're located elsewhere, then it's not true that you can deduce the which way information from the choice of the detector - this IS the same thing that Bill Unruh has tried to explain you. In my approach, if you want to prove the wave character by comparing the "both pinholes open" and "one pinhole closed" situations, the contrast will essentially be the contrast of your second picture on the right, which is going to zero if the thickness is zero. Whatever ensemble of events you pick, you will never violate V^2+K^2 is smaller than one. --Lumidek 17:58, 27 Nov 2004 (UTC)

Dear Lumidek, here's what you say at my Blog: "after half a day of research and reading, I would agree with you that Prof. Bill Unruh's setup is not equivalent, and there is a different problem with your interpretation. The problem with your interpretation is the point where you say that your V (contrast/visibility) is close to one, and the you see the wave character "very sharply"." So you seem to agree with me that Bill's setup is not related to mine, yet you talk about him in your last comment here. The "trivial 19th century-style experiment" comment is just irrelevant. It is does not matter what type of technology is used in an experiment to make an important contribution; after all Young's "19th century" double-slit setup is still considered as the most important experiment at the heart of QM by notables like Feynman. As for the NS experts I am not at liberty to disclose the list, but you are more than welcome to inquire from them directly. BTW, if as you say, V=0, then why don’t the wires block some of the light? Remember V=0 means no dark fringes exist. Let's continue this discussion at my Blog... Regards. Shahriar S. Afshar

[edit] Interference

1. In the double-slit experiment, does it matter whether more than one light source is used?
2. Has there been any significant discussion regarding the possibility that photons fluctuate between positive and negative intensity?

--ηυωρ 08:14, 19 Nov 2004 (UTC)

[edit] Arguments against Shahriar Afshar's interpretation

(This was on the article page but Afshar feels it should be here Samboy 01:32, 18 Dec 2004 (UTC))

We should however be prudent concerning this experiment. The interpretation is indeed not trivial and it could be that the experiment made by Afshar is based on a misundertanding of the principle of Bohr. This principle tell us (as explained before) that if one of a pair of complementary properties of a quantum object is know for sure, then information about the second complementary is lost. This complementarity can be then expressed as a kind of duality between different representations of the reality associated with different experimental arrangements. In the case of the experiment made by Afshar, based on a variant of the double slit experiment, the two spots in the image plane of the lens give us a information about the distribution of particles in the aperture plane (see the publications cited before). However in counterpart the information about the fringes is very weak. Indeed, due to their particular spatial locations, the wires used by Afshar only tell us that there are some minimum in the fringes and nothing more. Unfortunately, this is not sufficient to reconstruct completely and simultaneously the distribution of particles in the aperture plane and the interference pattern. Finally, it seems that, as pointed out originally by Bohr, we can not use information associated with a same photon event to reconstruct in a statistical way (i.e. by an accumulation of such events) the two complementary distributions of photons in the aperture plane and in the interference plane.

The response to this gentleman (Aurelien Drezet [sorry but he forced me to give out his name]) is already given at length in my Weblog over a discussion that has taken a few months. Briefly the challenge he has to meet in order for his argument to be verifiable is to do what I have mentioned in the 11/17/04 @ 04:38, and 12/08/04 @ 07:44 posts. Namely that he "writes an explicit wavefucntion (complete with all the bells and whistles) that is capable of producing regions with zero amplitude WHITIN itself, without using at least two interfering coherent wavefunctions", all this within the Unitary Time Evolution of the two original wavefunctions originating from the two pinholes. In section 3 of my paper I have shown this to be impossible. We are talking about physics here, and without equations to back one's argument, there is very little value to statements that go counter to the known quantum mechanical formalism. As soon as I receive a valid equation for the above challenge that agrees with the QM formalism, I would announce Bohr's victory and immediately bow out. Anything short of that is simply a childish game of one-upmanship which I am not interested in. My suggestion to him is to set up a web page like Prof. Unruh in which he delineates his views, and write a rigorous paper challenging my arguments presented in my paper. -- Prof. Afshar 01:50, 18 Dec 2004(UTC)

[edit] silly controversy

Can't we agree that an encyclopedia article about the double-slit experiment is not the place for this individual to promote his own controversial idea? He should write an article specifically about the controversy, and allow others to add their own stuff, and get rid of the zillions of additions he's made to various articles, perhaps furnishing links. Even then, I would doubt the encyclopediaworthiness of this. Wikipedia isn't supposed to be a forum. It's supposed to be an encyclopedia. Until his ideas become well-known enough for people other than (I presume) himself to want to write articles about them, he should stick to scientific journals or other forums. I'm not going to get rid of his stuff, because I can't be bothered to get into an edit war. Misterbailey

Dear Sir, (whoever you may be!) I have not written the discussions of my work in Wikipedia, others have (although I have occasionally corrected or added links to my preprint). My experiment and its theoretical aspects are currently being discussed in college courses on Quantum Mechanics and have entered the popular public domain. I highly doubt that an "edit war" would be necessary. As per ηυωρ and Lumidek's suggestion, I will shortly write a brief article about my experiment for Wikipedia. Afshar

[edit] I've added a link to a pretty good analysis of Afshar's work

I've added a link to a pretty good analysis of Afshar's work to the page. It is written in layman's terms. Basically, since the wires are thin, the light is mostly particles as it hits the lens and the wires don't make the light very "wavy". Interesting discussion there; I think it's worth having here just as Afshar's experiment is worth having here. Samboy 08:16, 21 Dec 2004 (UTC)

Another criticism, which directly addresses Afshar's experiment, is available here. (Link provided by Samboy).

Dear Samboy, the link you have added is that of Lubos Motl (a colleague of mine at Harvard), whose Wikipedia user name is Lumidek. Please take a look at the above Nov. 27 discussion on this page where link [1] and [2] are in fact the same link you have provided. I met with Lubos and discussed our differences. In fact it was him who suggested that I write an article for Wikipedia, which I intend to do soon. Briefly, he must meet the same challenge as Aurelien mentioned in the pervious post, as his argument in his link is NOT rigorously worked out. Please take a look at my weblog to see my detailed response. Therefore, I have moved the link here, as this particualr argument was already settled back in November. Afshar 08:34, 21 Dec 2004 (UTC)

I think the way to handle this, at this point, is to place all of the information about your (Afshar's) experiment available in a single page and simply link to it from other pages. I certaintly think your experiment is (to put it mildly) notable enough to merit its own page. I also agree that there is going to be a lot of controversey about this discovery; any new discovery in Science is met with great skepticism. Einstein never accepted Quantum Mechanics, after all. On the other hand, this skepticism is healthy because many seemingly great discoveries are later on sown to be nothing; Cold fusion being a well-known example. The fact that this experiment has generated so much controversey shows how notable it is. I think it is best for all sides to be heard. And I certaintly don't see any retraction from Lumidek about his blog entry! Samboy 09:12, 21 Dec 2004 (UTC)

Dear Samboy, I do not shy away from controversy in the least bit, and like you, believe it is a healthy aspect of the scientific process--as long as the participants are both objective and knowledgeable in the field. I appreciate your prudent act of getting a separate page started. However, I do not think it should turn into a forum. There already are hundreds of fora (including my own) where discussions on the experiment have been raging for months. As for Lobus' retraction, suffice it to say that he has toned down his comments considerably (actually, changed his weblog comments after our meeting) and as you can see in his discussion there, he admits that "Shahriar may have found a statement due to Bohr in the libraries or archives that was proven incorrect by this experiment." The statement he refers to is in fact the very description of Complementarity in Bohr's own words (published in the journal Nature), and personally verified by the eminent historian of physics Prof. Gerald Holton of Harvard. That is good enough for me! Afshar 09:49, 21 Dec 2004 (UTC)

I think the new page will minimize the kinds of conflicts. I have now reinstated this link at the bottom of the new Afshar experiment page, along with three blog entries with a more positive viewpoint on your experiment. I think this is far more NPOV than just a link to Motl's blog entry. And, yes, just as evolutionists have to put up with nonsense about creationism on the evolution page, we have to look at dissenting views here too. Samboy 10:44, 21 Dec 2004 (UTC)

I agree that it is more factual this way. Sounds pretty fair! Afshar 11:00, 21 Dec 2004 (UTC)

Let me add one more comment. Using the name of Harvard University may certainly be appealing, but it can also be used as in [3]. Don't get me wrong, I think that Viktor Kozeny, the founder of the Harvard funds, is a very bright and in some sense nice guy! But on the other hand, Harvard funds is not related to Harvard university. Shahriar, I am not sure whether it is quite an honest and scientific approach if you repeat so many times that you're Harvard faculty, especially if it does not seem to be true. If you want to see the list of Harvard physics faculty, open [4]. Did they forget you? --Lumidek 19:43, 21 Dec 2004 (UTC)

Dear Lumidek, where have I ever said that I am a Harvard Faculty?! You MUST produce evidence for such a claim if you care about your credibility with the public. This is not the first time you have attributed inaccurate statements to me, some of which I have responded to in your Blog. You make rash and irresponsible comments like this all the time, as others (whom I will not mention here) have also complained. I have every legal right to use Harvard's name as the place where the experiment was actually performed and announced while I was a Research Scholar. So, I kindly ask you to stop this kind of ridiculous personal attacks before it gets too ugly. It is really unbecoming of a Harvard "faculty" member. --Afshar 20:34, 21 Dec 2004 (UTC)

Dear Shahriar, your question "where I ever said that I am a Harvard faculty" is unfortunately very easy to answer. If you look in the second paragraph of this section, you will find your text "Dear Samboy, the link you have added is that of Lubos Motl (a colleague of mine at Harvard), whose Wikipedia user name is...". Also, the very first sentence of the article Afshar experiment says "In March 2004, Shahriar S. Afshar announced at Harvard University the..." which undoubtedly creates the (wrong) impression that Harvard University has something to do with your statements. Moreover, you repeatedly call your experiment "the Harvard experiment". No actual scientist from Harvard would use the name "Harvard" even with 10% frequency of yours. If you want to go far enough and think that you have the right to call your experiment "the Harvard experiment", we may try to ask the responsible people at Harvard. Incidentally, the authentic URL of the list of Research Scholars at Harvard is here: [5], not at Google. --Lumidek 20:49, 21 Dec 2004 (UTC)

Dear Lobus, we were both Research Scholars while I was there, that makes us colleagues. You have called me a colleague in the past yourself, and if you have changed your opinion on that it is your choice. By the "Harvard experiment" I have meant to distinguish between the high flux investigation that was performed at Harvard and the single photon version performed at Rowan university. If it makes you feel better, simply replace "the high flux experiment conducted and verified at Harvard" with "Harvard experiment". As for associating my work with Harvard, the "responsible people" have already clarified the legal boundaries in a written document provided to my legal team at IRIMS.-- Afshar 20:59, 21 Dec 2004 (UTC)

Dear Shahriar, if were were both research scholars in 2003, then you can call us "former colleagues", but not "colleagues". On my blog, sorry to tell you, I used the word "colleague" as a kind of joke - I am often using this word for crackpots - various crackpots who want to claim that they falsified theory of relativity, quantum mechanics, or perhaps energy conservation or anything like that - to point out that they definitely consider themselves to be our colleagues which I find entertaining - as long as it remains on the level of jokes. I don't believe that you were allowed to use Harvard's trademarks for your experiments. Incidentally, if you search for the word "colleague" in [6], you will see that I used exactly the same word for Viktor Kozeny - and my guess is that you will understand that it's not quite serious. --Lumidek 21:36, 21 Dec 2004 (UTC)

Dear Lubos, in that case I am sorry I took you seriously! Only time will tell who is more entertaining...--Afshar 21:39, 21 Dec 2004 (UTC)

Dear Shahriar, I am happy that we can end this discussion with a happy end - the time will tell (if it has not already told). I agree with your whole last comment, except for your spelling of my first name, and I wish you happy holidays. --Lumidek 21:45, 21 Dec 2004 (UTC)

[edit] Afshar experiment page created

I have just created a page dedicated to the Afshar experiment. I know that Afshar himself is here :) and can make this page far better than I could. In particular, I would like to see him release some images from the experiment and make them available under the GFDL so this new page can be more visual--I think images makes entries far more dynamic. Samboy 09:31, 21 Dec 2004 (UTC)

I'm no legal expert, but what if somebody just made an image (for example using XFIG) basically using the new scientist description? CSTAR 03:27, 22 Dec 2004 (UTC)

You're OK because ideas can't be copyrighted. They can be patented (All sorts of nonsense, such as genetic codes, can be patented), but not copyrighted.

I changed a number of other pages referring to Afshar's experiment, since a lot of them had the same information verbatum in each page. Some of the pages I have changed include:

• Wavefunction collapse
• Copenhagen interpretation of quantum mechanics
• Double-slit experiment
• Niels Bohr
• Wave-particle duality
• Bohr-Einstein debates
• Complementarity
• Transactional interpretation (Minor change: Link updated)
• Interpretation of quantum mechanics

Samboy 10:49, 21 Dec 2004 (UTC)

[edit] Afshar Experiment: A Dissenting Opinion

The Afshar experiment demonstrates that a coherent path distribution must be assumed to predict the number of photons incident on measurement spaces where path information is being observed, but it does not demonstrate that the coherent path distribution is actually realized across the surface of measurement spaces where path information is being observed.

All Figure number references in this article are references to Afshar’s Figure numbers in his paper “Sharp complementary wave and particle behaviours in the same welcher weg experiment”: http://www.irims.org/quant-ph/030503/

In Afshar’s experiment, the two holes can be considered emitters in the sense that he pumps photons through these two holes with his laser, and these photons have to go somewhere. Assuming the Copenhagen Interpretation of different path distribution functions (coherent versus decoherent) for different type of measurements (visibility versus path information), one might ask the question how do these photons get distributed in experiments involving complementary measurement spaces (i.e. how many photons get mapped into each measurement space). This question is also relevant in an experiment that involves only a single measurement space where path information is being measured. The answer of course is that a global coherent path distribution function across all measurement spaces must be assumed to predict the photons incident on each measurement space. The number of photons incident on a measurement space is predicted by integration of the global coherent path distribution function across the surface of the measurement space. But the Copenhagen Interpretation indicates the predicted number of photons incident on a measurement space where we are measuring path information don’t actually exhibit a coherent distribution locally.

So when Afshar says that he has visibility of interference on the photons incident on the lens, if he means that a coherent distribution predicts the number of photons incident on the lens surface, then this is correct. But if he is saying that the photons incident on the lens surface actually exhibit a local coherent distribution across the surface of the lens, then this is incorrect.

In Afshar’s experiment there are 13 measurement spaces. There are 6 measurement spaces where visibility information is being observed corresponding to the 6 wires. There are 7 measurement spaces where path information is being observed corresponding to the 7 portions of the lens segmented by the wires. The number of photons incident on each of the wires is very low because the wires are very thin and Afshar has placed them at the point of maximum destructive interference of the coherent path distribution function. The number of photons incident on each of the lens segments can be predicted by integration of the coherent path distribution function across the face of the lens segment. If the results for all of the lens segments are added up, the result is very close to the value for the whole lens when the wires are absent, since only the portions of the total lens where there is maximum deconstructive interference in the coherent path distribution function are omitted in this integration process.

Afshar’s interpretation of his experiment seems to be that since the wires are very thin and he has carefully placed them at the points of maximum destructive interference, he can treat his experiment as a single measurement space. Since he is observing interference in the photons incident at the wires, he concludes that there must also be interference present in the photons incident on the lens, since he believes both sets of photons are part of the same measurement space.

Even though the wires are very thin, the photons incident on the wires must be treated separately from the photons incident on the lens, since different types of measurements are being made in the two cases, and therefore the photons exhibit different behaviors for the two cases. For the photons incident on the wires, visibility information is being observed, and therefore the photons incident on the wires go through both holes with a corresponding coherent path distribution that exhibits interference in the vicinity of the wires. For the photons incident on the surface of the lens, path information is being observed, and therefore the photons incident on the lens only go through one hole with a corresponding decoherent path distribution.

The argument that the local path distribution across each of the 7 lens segments must be decoherent consists of the following four points:

1) The photons only go through one hole and are resolved at the detectors. Therefore any interference pattern in front of the lens cannot be explained by self-interference.

2) The only other way to explain a coherent path distribution in front of the lens is for a single hole to be emitting a coherent path distribution function. Hopefully we would all agree that this is about as likely as hearing the sound of one hand clapping.

3) If a single hole is (somehow magically) emitting a coherent path distribution function, then it would manifest itself in an interference pattern at the detector image.

4) Since there is no interference pattern at the detector image in Figure 8a, a single hole must not (somehow magically) be emitting a coherent path distribution function.

The third point above is illustrated by Figure 8b where Afshar has induced some of the characteristics of a coherent distribution sourced by a single hole by placing the wires in front of the lens at the points of maximum destructive interference. The image in Figure 8b clearly indicates interference at the detector. The interference pattern is also still clearly visible in Figure 8c, but it does seem to be attenuated somewhat from Figure 8b. Afshar’s explanation for this attenuation is that less photons are incident on the wires in Figure 8c than in Figure 8b, which is certainly true. In summary, Figures 8b and 8c indicate an interference pattern, but this interference is the result of placing the wires in front of the lens. Figure 8a illustrates clearly that there is no interference pattern at the detector without the wires present.

So how does Afshar explain his claim of a coherent distribution across the face of the lens? His explanation seems to be a superposition behavior in the neighborhood of the holes (that the photon goes through both holes exhibiting interference and it also goes through only one hole exhibiting path information). There is a superposition of states before the wave function collapses. Before the wave function collapses, there is a possibility that the photon goes through both holes with a corresponding coherent path distribution, and there is the possibility that the photon goes through one hole with a corresponding decoherent path distribution. Afshar seems to be assuming a superposition of two states at the two holes, but only a single potential path distribution. When we make an observation, the wave function will collapse in a way that depends on the nature of the measurement we are making. If we are measuring visibility information, the wave function will collapse such that the photon goes through both holes with a corresponding coherent path distribution. If we are measuring path information, the wave function will collapse such that the photon goes through one hole with a corresponding decoherent path distribution.

Afshar attributes far too much magic to the thickness and placement of his wires. A couple of examples with different thickness and placement are provided below to illustrate this point. Both of these examples assume a peak-to-peak distance of u = 1.4 mm for the consecutive fringes similar to Afshar’s first experiment illustrated in Figure 1.

For the first example, replace the wires with thin strips with a width of 0.7 mm leaving these strips centered about the maximum point of deconstructive interference. These strips will block some of the portions of the lens where the value for a coherent path distribution is less than the value for a decoherent path distribution (i.e. some of the valleys of a coherent distribution). All portions of the lens where the value for a coherent path distribution is greater than the value for a decoherent path distribution will be left exposed (i.e. all peaks of a coherent distribution). Next determine the predicted attenuation of the radiant flux at the image based on a decoherent path distribution either by calculation, simulation, or experimental measurement. Finally open the second hole and measure the actual attenuation in radiant flux at the image and compare with the value predicted in the previous step. The measured results will be much less than the value predicted by assuming a decoherent path distribution.

For the second example, keep these same strips, but move them to a position where they are centered about the peaks of a coherent distribution instead of the valleys of a coherent distribution. These strips will now block some portions of the lens where the value for a coherent path distribution is greater than the value for a decoherent path distribution (i.e. some of the peaks of a coherent distribution). All portions of the lens where the value for a coherent path distribution is less than the value for a decoherent path distribution will be left exposed (i.e. all of the valleys of a coherent distribution). Go through all of the same steps as in the first example. This time the measured results for the reduction in radiant flux at the image will be much greater than the value predicted by assuming a decoherent path distribution.

Both of these examples demonstrate the same results as the Afshar experiment (i.e. a coherent path distribution function must be assumed to prediction the reduction in photons incident on the detectors). Therefore there is no magic in using very thin wires placed at the points of maximum deconstructive interference.

In summary, the Afshar experiment demonstrates that a coherent path distribution function must be assumed to predict the number of photons incident on measurement spaces where path information is being observed. According to the Copenhagen interpretation, the photons predicted by the assumed coherent path distribution function actually exhibit a decoherent path distribution function locally across the surface of measurement spaces where path information is being observed. The Afshar experiment provides no evidence that the Copenhagen interpretation is incorrect.

[edit] Afshar's Response

The bottom line for the above Dissenting Opinion is the following false conclusion (quoted):

"According to the Copenhagen interpretation, the assumed coherent path distribution function actually collapses into a decoherent path distribution function."

Unfortunately, the major error in this argument is a misunderstanding of what the Wavefunction collapse actually means. Without going into mathematical details, a coherent superposition state can only collapse into an observable with a coherent distribution when a measurement is made. There is no "collapse" from a coherent wavefunction to a decoherent state because upon measurement a coherent wavefunation Psi is mapped into an observable wihtin |Psi|^2 via a projection, and not into some other decoherent distibution, say |Psi_1|^2 + |Psi_2|^2 . If the above quotation were true, then we could not observe an interference pattern in a double slit experiment, or immediately after the wires in my experiment, by direct observation, but we do! We cannot change the definition of wavefunction collpase to an arbitrary and mathematically inaccurate one, just to save Complementarity! Afshar 00:06, 23 Dec 2004 (UTC)

Again you make the mistake of treating the wires as if they are part of the lens.

It is quite telling that the anonymous writer of the Dissenting Opinion (whose name is actually Alex), having now realized that his definition of the wavefunction collapse was indeed erroneous, has again attempted to “correct”his conclusion by replacing the pervious version with the following:

The name is actually Steve, not Alex. In the last paragraph I was using the term "collapse" in a more general way, not in the context of wave function collapse. But the wave function collapse is what causes the distribution to be decoherent at the surface of measurement spaces where path information is being observed. My use of the term "wave function collapse" throughout the remainder of the article is completely consistent with the Copenhagen interpretation. You nit-pick at the wording in the last paragraph because you cannot find any flaws in the logic. I changed the wording only to address your nitpicking, but the argument remains the same.

"According to the Copenhagen interpretation, the photons predicted by the assumed coherent path distribution function actually exhibit a decoherent path distribution function locally across the surface of measurement spaces where path information is being observed."

Again you nit-pick at the wording because you cannot find any errors in the logic. Remove the word "function" from the above quote if it makes you happy, but the argument remains the same.

I'm afraid, the more he awkwardly tries to avoid self-contradiction, the more his lack knowledge on even the most rudimentary QM formalism and language becomes apparent. There is no such thing as "coherent path distribution function" in QM. I suggest that he formally study QM before making more clumsy statements. Need I say more?! Afshar 07:50, 23 Dec 2004 (UTC)

Actually what is quite telling is all you can do is nit-pick at the wording, but can find no flaws in the logic. There is no self-contradiction in my argument. There is self-contradiction in your argument (i.e. the photon goes through both holes to exhibit self-interference and it goes through one hole to exhibit path information).

Dear Alex, such "nit-pick at the wording" is called rigorous analysis. In physics, words (and any "logical" game play with those words) mean nothing if they are not associated with the mathematical machinery of an accepted theoretical framework such as the QM formalism. As for my arguments showing a violation of Complementarity being self-contradictory, you may benefit from a short course on the history of physics, which will clarify to you that such "paradoxes" have invariably led to new chapters in physics, e.g. Schrodinger's cat, EPR paradox, etc. Does the old reductio ad absurdum ring a bell? Afshar 20:06, 23 Dec 2004

Afshar, this is not the place to include ideas about QM which are still being debated. Wikipedia reflects standard professional opinion and your self-promoting references are inappropriate. I am removing your section from this page and will not allow a reference to your material here either.--DrChinese 20:09, 20 Feb 2005 (UTC)

[edit] What exactly is changed as light negotiates a single diffracting slit?

In the current article it is stated:

The waves interfering must be coherent, i.e., the light has the same frequency and is in the same phase. In Young's experiment, this was achieved by passing the light through the first slit, and thereby diffracting it, producing a coherent wave; this is more typically achieved now by using a laser, and removing the first slit.

Was the above procedure fully explained and justified by classical wave mechanics? According to classical wave mechanics, light consisting of a spectrum of colors is a superposition of electromagnetic waves with different wavelength. Does classical wave mechanics allow superposition of electromagnetic waves that closely match in wavelength, but are not in the same phase? Does classical wave mechanics predict interference patterns when the lightsource is a narrow band of the spectrum, but not perfectly monochromatic?

Quantum electrodynamics allows unlimited superposition, so according to quantum electrodynamics incoherent lightsources remain incoherent after the first (single) slit. What does classical mechanics allow, and should an article on double slit experiment present an interpretaton both in terms of classical wave mechanics and in terms of quantum electrodynamics? -Cleon Teunissen | Talk 09:44, 23 Mar 2005 (UTC)

In the 19th century, interferometry was used to measure the wavelength of spectral lines of elements. Sodium light is dominated by a very narrow band of the spectrum. Light from a Sodium lamp is incoherent, and it will form an interference pattern. The purpose of the first slit is to get a point source of light, that is the necessitiy. I don't think coherency matters at all. --Cleon Teunissen | Talk 10:28, 23 Mar 2005 (UTC)

[edit] coherency and pinholes

Cleon Teunissen | Talk 10:24, 7 Apr 2005 (UTC) In the section on the necessary conditions to produce an interference pattern I write that coherency of the light is not a requirement. I don't know how Newton's rings used to be explained in terms of classical wave dynamics.

In quantum electrodynamics Paul Dirac made the following observations about the quantum of action of the electromagnetic field, the photon "Each photon interferes only with itself. Interference between two different photons never occurs." (source: external link: Science week article on entanglement quoted from: Dirac, P. A. M. The Principles of Quantum Mechanics (Clarendon, Oxford, 1982))

I don't think that Thomas Young needed coherent light for his first slit experiment, and I don't think the light he used was coherent. However, it seems that according to classical wave mechanics inferference will occur only if the waves are coherent.

The first holograms by Dennis Gabor were obtained by using light from a mercury line that was cast on a screen with a pinhole in it. External link: history of producing holograms It seems to me that if the pinhole is small enough the light does not have to be coherent, pathlength will ensure that a hologram is produced.

Many lasers are designed in such a way that the light leaves the laser cavity through a pinhole. (Inside the cavity the light is "bouncing" in all directions.) Coming from a point source, the diverging laser light can be refracted to a parallel beam with the help of a lens. (Holography requires diverging light coming from a point source.)

It is not clear to me whether the concept of coherency has any meaning in Quantum Electodynamics (QED). Due to the way they are produced, the photons of laserlight are in a specific form of quantum entanglement, in a sense they are all "clones" of one starting photon. external link: Site with answers. See section: laser light is "in phase" light?

It seems to me that some interferometric experimental results just cannot be explained by classical wave mechanics. I think that only path length matters, as is illustrated by the sum-over histories approach to QED calculations developed by Feynman. I think coherency is an attempt to explain in terms of classical wave mechanics something that cannot be explained in classical wace mechanics. --Cleon Teunissen | Talk 10:24, 7 Apr 2005 (UTC)

[edit] "Thought Experiment" review needed

Can someone who knows more than I do please review this passage:

"If either slit is covered, the individual photons hitting the screen, over time, create a pattern with a screen. But when the experiment is arranged in this way, the fringes disappear -- for reasons related to the collapse of the wavefunction."

Specifically, "a pattern with a screen" is meaningless to me. A fringe pattern? A single peak?

And "when the experiment is arranged in this way" - presumably meaning still a single electron, but it is one slit still covered? No mention is made of uncovering it.

What happens when both slits are uncovered and a single photon is used? That's what I came here to find out, and what this thought experiment appears to be addressing, but this explanation is not helpful.

Apparently there used to be more explanation, but it was removed, and I'm not sure if it was done because it was incorrect: http://en.wikipedia.org/w/index.php?title=Double-slit_experiment&diff=9310882&oldid=9119157

That extra explanation is completely correct and I'm uncertain why it was removed. --Laura Scudder | Talk 05:15, 9 August 2005 (UTC)

Looks like it has been put back in --ssam 22 Oct 2005

Why is this bit called a though experiment? I have performed the experiment with single photons. --ssam 22 Oct 2005

[edit] Beams of electrons or atoms?

The article says that the experiment can also be performed with beams of electrons or atoms; it would be nice if some more detail could be added for this (when were the experiments performed, what kinds of atoms were used, etc.) Cwitty 23:54, 4 November 2005 (UTC)

• Gerhard Paulus group

[edit] Revert

I reverted the recently added text, in part because it made incorrect statements, such as:

Since the photons are emitted one at a time, the photons cannot be interfering with each other -- so exactly what is the nature of the "interference"?

Most of the new text wasn't bad, but a few things like this spoil it.linas 15:18, 6 February 2006 (UTC)

Can this change be clarified: the photons cannot vs. the photons are not - i don't understand how these statements were substantially different. And: a few things like this... why not remove/edit/clarify those things which are incorrect? I added two book references from which the added text was drawn, and the added text appears to be accurate according to these references.

A single photon can and does interfere with itself. The word "not" needs to be striken. linas 04:42, 7 February 2006 (UTC)

My apologies; it seems that sentence was there all along. I must have gotten confused. I have removed that sentence in its entirety. linas 04:56, 7 February 2006 (UTC)

The reason i think the added text belongs in here is that:

• the text in it's original form does not illuminate the fact that it is a particular interpretation which treats the wavefunction collapse as a physical reality. It also refers to this as "modern quantum physics", when the Copenhagen interpretation is not strictly "modern" (i.e. there are more recent interpetations being debated alongside it) - so it makes more sense to just link directly to Copenhagen interpretation.

What part of the text implies that its a Copenhagen interpretation? linas 04:42, 7 February 2006 (UTC)

Again, never mind, it looks good now. linas 04:56, 7 February 2006 (UTC)

• the reference to QED adds depth to the description of the experiment which i think was lacking. It'd be nice if someone working in the field went through this section of the double-slit experiment article to include more information. I haven't found any other single source that collates everything about it.

Double-slit can be explained, computed, discussed, manipulated, etc. without requiring QED. Or QFT, for that matter. I think you are trying to say that "path integrals can provide insight." linas 04:42, 7 February 2006 (UTC)

I changed QED to path integral formulation which is what you want. linas 04:56, 7 February 2006 (UTC)

• The title of the section in dispute: The Thought Experiment - seems problematic: It implies that something described in this section is only a thought experiment - but it doesn't say what. One of the book references provided, "Q is for Quantum" by John Gribbin, explicitly states that the modified double-slit experiment with a detector placed at one or both of the slits is, in fact, a real experiment which has been successfully carried out; if this is the case, then naming this section "the thought experiment" is misleading.

Yes, that title does seem out of place. Fix it. Just please be very very careful with your edits. The articles on quantum tend to attract a lot of editors who have never actually studied QM, but think they can explain it anyway. A lot of subtle errors result, making more work for me and for others who try to tend to this article. If you are not absolutely, totally sure of what you are doing, don't do it. linas 04:42, 7 February 2006 (UTC)

Sorry, its possible I shot first, asked questions later. It looks good now, I think. linas 04:56, 7 February 2006 (UTC)

Thank-you for reconsidering and explaining all of that. I do promise to be careful with articles involving specialist knowledge; --Dissembly

--User:Dissembly 7th Feb 2006

[edit] at home

Say if we can do this experiment at home. (anon User:210.201.31.246 on 21 Feb 2006)

Yes you can. linas 00:12, 22 February 2006 (UTC)

very easily in fact in some countries people have thin textile before the windows, so you can look out the window, but people won't be able to look inside. Trough these fine textile look to a sunny object you will see an interference pattern

[edit] A basic question

I have a PhD in QM, but it's some years (20 plus) since I used it, so I'm rusty. Can I ask you guys for feedback on a basic question? One thing that always bugged me about most explanations of electron diffraction in the two slits experiment was that they were based on the single-particle approximation, with the electron being the particle and the slits being modelled as an external potential, so the only wavefunction being considered is that of the single electron. Now, in reality the slits are gaps in some kind of physical device made of, guess what, electrons and nuclei, so the electron that is passing through the slits is actually interacting, under the influence of electromagnetic forces, with all of the other electrons and nuclei that form the boundaries of the slits. That interaction will be mediated by photon exchange, in a way that is linked to the relative speed of the electron being diffracted, and it should be more properly modelled as a many-body wavefunction. Has anyone investigated whether the diffraction effects could reflect the wave-nature of the electromagnetic intereaction between the electron and the screen containing the slits? It seems to me that this could, in principle, allow for diffraction effects even if each electron definitely went through only one slit. (unsigned question from User:81.155.67.43 on 31 March 2006)

Interesting question. Several answers:

-- As long as the simplest theory of twoslit diffraction is sufficient to explain the outcome, there is no incentive to look for more complex explanations. Occam's razor.

-- You might find that the Mott problem to be the many-body treatment that you are looking for, although it is for cloud chambers and not the double-slit. Although it actually solves the opposite problem: it explains the abscence of a "diffraction effect", as it were. You might also find the Renninger experiment intriguing.

--There are quantum effects when the size of the particle being scattered is non-negligable with regard to the size of the slit. These have been experimentally measured and theoretically confirmed for the diffraction of large molecules through very very small slits.

But your suggestion founders: a many-body wavefunction is inherently quantum mechanical; and adds to the confusion of "wheres the electron" the additional confusion of "which electron are you talking about, they're all indistiguishable?" In a certain sense, many-body quantum mechanics aka QFT is even more mired in subtle confusions when it comes to the interpretation of quantum mechanics. linas 23:51, 31 March 2006 (UTC)

Does that mean 'No'? I would agree with your appeal to Occam's razor if the conventional explanation of the two-slit experiment was sufficient- but it is far from sufficient. The conventional explanation relies on the Copenhagen interpretation, and indeed is often cited as an illustration of the counter-intuitive implications of the Copenhagen interpretation. I'm glad that you spotted that a many-body wave-function is inherently quantum-mechanical- it would have worried me if you said it wasn't. To make myself clearer...I am unhappy with the conventional explanation of the two-slits experiment, in relation to electron diffraction, precisely because it depends upon the Copenhagen interpretation. It seems to me that there is another potential explanation of it, to do with the photon-based interaction between the electron and the screen containing the slits, that does not require the electron (or its wavefunction, more precisely) simultaneously to pass through both slits, which is the implication of the conventional explanation. I wondered if anyone had tried a calculation along these lines (my QFT/QED is far too poor for the job).

[edit] Request for more description of Young's experiment

I would like to know how large/small the slits need to be. Actually there could be several articles referenced from here relating to the different variations of this experiment (and well yes, there are many references:-)

According to basic description the slits should be less than a wavelength. I think we were told in College that slits could be made (or were made) by cutting in covered glass.

But I have a recollection that the experiment at that time (1964) was meant to demonstrate that light after the slits would behave differently, as if one beam.

So you can see this is more a request for more explanation.

Thank you for this article, the many references and the interesting inspiring writing.

   Donald Axel

--d-axel 09:20, 12 May 2006 (UTC)

Why doesn't the article give Hugh Everett's Many Worlds resolution to the double-slit experiment? The article seems to favor the Copenhagen Interpretation point of view, which violates Wikipedia's rule that articles must be neutral and not have a point of view.

Michael D. Wolok 13:29, 15 May 2006 (UTC)

[edit] OK some other basic questions

Hopefully this isn't common knowledge.
1) How do the detectors that can be set up at either slit work?
2) What happens if you have 3 slits in a line? in a triangle shape? unevenly spaced? what if they are round pinholes rather than slits? I've never seen these variations mentioned. I assume that a different pattern appears on the screen for each arrangement of slits/holes.
3) Water waves sent through small gaps make a moving pattern on a screen as the waves move. Does the electron/photon pattern vary over time, or is it static? I don't remember from class, it's been a while!
4) If you gradually increase the separation of the slits, does there come a time where the photons/electrons no longer pass through both? Does each particle pass through both anyway? If not, then how, if you send them through 1 at a time, does a particle which passes through 1 slit only know the other slit is there? 5) If detectors on both slits detect 1 photon, I assume they slow it down but don't stop it, which makes it not coherant with say, another photon sent through the other slit at the same time? But the article says that the light doesn't have to be coherent for the diffraction pattern to show up.??

Thanks for any responses!

[edit] Double Slit experiment with only one detector?

What if you were to only place a detector in one of the slits instead of both? If a particle acts like a wave until it is detected, wouldn't the single detector always detect electrons(or whatever particle) since the wave always travels through it?

[edit] graphics

Please review wikimedia-diffraction for some nice graphics suitable for this article.

[edit] Does not convey a clear meaning

One sentence in the article is too vague to be meaningful.

A very easy way to see the interference pattern is to look at light in the background through a slit made by common objects such as two pencils. The interference pattern of lighted bands and obscure bands is evident to anyone.

1. What is "the interference pattern" supposed to refer to? The aforesaid pattern produced by double slits? You don't need additional apparatus to see what is there, so the sentence most refer to some other interference pattern -- but this sentence leaves the reader to depend on total guesswork.

1. "Look at light in the background"??? Nobody can see light. One sees things of a certain magnitude by means of reflecting light off of them. So what is the author really trying to say? My guess is that s/he refers to looking at some lighted surface that stands at some distance from the observer.

1. Now we are instructed to look at this "light" through a single slit made by two pencils. Pencils have become animate, and operate in pairs. They go around carving slits in things? Probably not. Probably the author is trying to say that one should hold two pencils close together so that there is a very narrow gap between the two pencils.

1. If all this guesswork, which should not be necessary, is correct then the author is describing a single slit experiment, and asserting that "the interference pattern... is evident to anyone." According to normal English syntax, "the interference pattern" has to refer to an earlier mention interference pattern. (A little dog came walking over the hill. The dog was a Corgie.) But the earlier interference pattern was produced by interference between light transmitted through two slits.

The existence of a diffraction pattern produced when light is passed through a narrow gap is well known. But that phenomenon is different from the interference pattern produced by two slits illuminated by coherent light.P0M 07:42, 29 August 2006 (UTC)

Agreed, this article almost seems like it was made to confuse interested viewers, coherence is a task a 5 year old could pull off for goodness sake. Deepdreamer 21:21, 2 October 2006 (UTC)