Talk:Double-slit experiment

Contents

1 Shahriar Afshar 2 Interference 3 Arguments against Shahriar Afshar's interpretation 4 silly controversy 5 I've added a link to a pretty good analysis of Afshar's work 6 Afshar experiment page created 7 Afshar Experiment: A Dissenting Opinion 8 Afshar's Response 9 What exactly is changed as light negotiates a single diffracting slit? 10 coherency and pinholes

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 (http://www.kathryncramer.com/wblog/archives/000687.html) 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 (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) 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 (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) 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 (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) 00:02, 8 Nov 2004 (UTC)

Shahriar Afshar's rebuttal to W. Unruh can be found in his FAQ (http://users.rowan.edu/~afshar/FAQ.htm). His briefness underscores the fact that Unruh's argument is nonsense. --ηυωρ (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) 05:38, 8 Nov 2004 (UTC)

This Afshar's setup is really silliness. Its correct interpretation, using Afshar's own setup, is here: [1] (http://motls.blogspot.com/2004/11/violation-of-complementarity.html). --Lumidek 04:22, 27 Nov 2004 (UTC)

• Dear Lumidek, I have responded to your criticism in the same place [2] (http://motls.blogspot.com/2004/11/violation-of-complementarity.html). For further information please post your questions at my Blog Official Afshar Experiment Blog (http://irims.org/blog/index.php/2004/09/25/questions_welcome) 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 (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) 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 (http://irims.org/blog/index.php/2004/09/25/questions_welcome): "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

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 (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) 08:14, 19 Nov 2004 (UTC)

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 (http://irims.org/blog/index.php/2004/09/25/questions_welcome) 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 (http://www.irims.org/quant-ph/030503/) 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 (http://www.irims.org/quant-ph/030503/). -- Prof. Afshar 01:50, 18 Dec 2004(UTC)

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 (https://academickids.com:443/encyclopedia/index.php/User_talk:Eequor) and Lumidek's suggestion, I will shortly write a brief article about my experiment for Wikipedia. Afshar

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 (http://motls.blogspot.com/2004/11/violation-of-complementarity.html). (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] (http://www.thecrimson.com/article.aspx?ref=349210). 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] (http://physics.harvard.edu/faculty.htm). 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 (http://motls.blogspot.com/2004/11/violation-of-complementarity.html). 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 (http://my.harvard.edu/cgi-bin/webevent/webevent.cgi?cmd=showevent&ncmd=calmonth&cal=9719&y=2004&m=3&d=23&id=10416384&token=G6409379:1&sb=0&cf=cal&lc=calmonth&swe=1&set=0&sa=0&sort=e,m,t&ws=0&sib=0&de=0&tf=0) while I was a Research Scholar (http://64.233.161.104/search?q=cache:Xc2bTcl2dFoJ:www.physics.harvard.edu/s/ListAction%3Fcat%3Dresearch+Shahriar+Afshar+harvard+physics+people&hl=en&start=9). 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] (http://physics.harvard.edu/s/directory/research), 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 (http://motls.blogspot.com/2004/11/violation-of-complementarity.html), 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] (http://motls.blogspot.com/2004/11/after-sunset-spoilers.html), 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)

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)

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.

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 (http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#Wavefunction_collapse), 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 (http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#Eigenstates_and_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 (http://en.wikipedia.org/w/index.php?title=Talk%3ADouble-slit_experiment&diff=8737635&oldid=8734633) 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)

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)

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 (http://scienceweek.com/2004/sa040827-5.htm) 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 (http://www.holo.com/holo/book/book5.html) 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? (http://www.msu.edu/user/boswort9/attempt1/cep817web/amasci/scimis.htm)

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)