Talk:Double-slit experiment

Copenhagen non-interpretation

The section "Copenhagen interpretation" contains a bunch of blather about the plurality of Copenhagens, but nothing about the interpretation of the double slit.

I propose to remove this section completely as redundant with the full page on the topic and to replace it with a section "Conventional interpretation" which simply gives an interpretation of the double slit experiments discussed here. Johnjbarton (talk) 17:41, 7 September 2023 (UTC)

I disagree. That would be your interpretation of things, and not encyclopedic. Ldm1954 (talk) 20:37, 7 September 2023 (UTC)

I'm confused by your reply. Any replacement would of course have to be reliably referenced.

My claim is that the current section is off topic. It says nothing about double slit experiments. Controversy about Copenhagen belongs elsewhere. Johnjbarton (talk) 21:54, 7 September 2023 (UTC)

I think it could be shortened and brought more in line with how the intro of the Copenhagen interpretation article currently goes, and then we could add more about what Copenhagen-type interpretations have said about the double-slit experiment specifically. XOR'easter (talk) 21:52, 7 September 2023 (UTC)

OK, I've taken a stab at doing that. XOR'easter (talk) 22:12, 7 September 2023 (UTC)

Awesome, thanks. Johnjbarton (talk) 22:25, 7 September 2023 (UTC)

Sorry, but I have the same objection to what you have written as to the edit of the many-worlds interpretation. You have changed that section from general to one specific interpretation in terms of complimentarity. Where has probability gone? Why are you limiting it? Why have you only given one connection?

What was impressive about this article a week or so ago when I made a redirect suggestion was that it tried to fairly represent all interpretations. Now it seems to be losing this, going downhill. Sorry for being blunt. Ldm1954 (talk) 05:24, 8 September 2023 (UTC)

Previously, the article said nothing about how any version/variant of Copenhagen applied to the double-slit experiment specifically. It just had a lengthy passage about how "the Copenhagen interpretation" is hard to pin down. Now there's at least a little about how to make the connection. I stuck to the Copenhagen-oriented books that I had close at hand and what they had to say specifically about the double-slit experiment. XOR'easter (talk) 06:31, 8 September 2023 (UTC)

I have rewritten it so, IMHO, it is balanced. It now includes the 1) complementarity interpretation, 2) the detection-collapse interpretation, and 3) the statistical probability interpretation. Both 2) and 3) are of course "Mermin-type", i.e. they follow directly from the math, they are not philosophical. Feel free to add a 4) and 5) if you feel they are relevant. Inclusivity please. Ldm1954 (talk) 07:29, 8 September 2023 (UTC)

I just didn't want to go beyond what the sources I had in front of me actually said. XOR'easter (talk) 15:25, 8 September 2023 (UTC)

2 and 3 are not referenced.

2 mixes up decoherence and collapse.

These 3 are not different things. #1 Complementarity refers to one aspect: the disappearance of the interference with path distinction. #2 decoherence provides a microscopic explanation for the disappearance within conventional QM. #3 The Born rule is very much a part of every Copenhagen interpretation surely.

I want to change this section to a fully referenced, clear conventional explanation, including a paragraph on decoherence. But I don't want to be in an edit war for this. So I think the better solution is to delete 2 and 3 until they are referenced. The references should include evidence that they differ from 1. Johnjbarton (talk) 15:50, 8 September 2023 (UTC)

Sorry, not true. They are all in the previous section. 2. is an undergraduate QM problem -- when you detect you change the state. It does not mix up decoherence and collapse, it is standard inelastic scattering that you can find in any QM book. Unless the detector is entangled with the incoming wave, there is a statistically random phase shift.

3. Is the Born rule.

Do you insist that complimentarity is the only explanation of life, liberty and the pursuit of happiness? I consider that inappropriate. Ldm1954 (talk) 16:01, 8 September 2023 (UTC)

Please do not misquote me.

Complementarity is simply one of several aspects of the conventional interpretation. These are not 3 different things, but three aspects of the same thing.

If the idea that these three things differ is so elementary, adding a reference should be easy. Johnjbarton (talk) 16:29, 8 September 2023 (UTC)

I'd be happy with what I wrote being expanded. I just think that in this area, it's very easy to wander into synthesis and also very easy to use words in a way that is misinterpreted. XOR'easter (talk) 16:29, 8 September 2023 (UTC)

I do not understand this revert rationale. The old version says A particular experiment can demonstrate particle behavior (passing through a definite slit) or wave behavior (interference), but not both at the same time. So does the new version. The old version invokes the Born rule; so does the new. What "one interpretation" is being insisted upon? XOR'easter (talk) 19:42, 8 September 2023 (UTC)

The edit removed everything except a complimentarity interpretation. That is not WP:NPOV. The Born & collapse/incoherence approaches are not the same. The Born probability is "Shut up and calculate" -- i.e. it is math and philosophy is not needed. The incoherence is standard Fermi golden rule for non-stimulated inelastic scattering. (Contrast with stimulated in a laser where coherence is preserved. Technically you have an incoherent zero-point term.) To detect you change,so long as you are not entangled. Ldm1954 (talk) 19:53, 8 September 2023 (UTC)

I undid that revert. The content removed was referenced and on topic: directly about double slit.

Point of view is not at issue. These references are about double slit. If you have other references about double slit, add them. No reason to remove the reliable existing content. Johnjbarton (talk) 19:58, 8 September 2023 (UTC)

That is exactly what I am saying. You are continually removing all reliable, standard content except a single, narrow view. I accept complimentarity as a philosophical concepts, but it is not the math. What I included is. Math rules, not philosophy. Please be inclusive -- that is one of the central principles of Wikipedia, and also science (done right). Ldm1954 (talk) 20:12, 8 September 2023 (UTC)

Please see Wikipedia:Balance Your Perspectives Ldm1954 (talk) 20:18, 8 September 2023 (UTC)

You are welcome to add balance. However it must be referenced. Please see Wikipedia:Verifiability.

I have no idea what "narrow view" you are complaining about. I am simply reporting what sources say as clearly as I can. The sources we have describe double slit experiments in terms of complementarity. If you think we have too much content about complementarity, find some source that gives a different perspective. I would welcome that. Johnjbarton (talk) 20:35, 8 September 2023 (UTC)

As would I. XOR'easter (talk) 20:43, 8 September 2023 (UTC)

As I already said, please see the reference on the Born rule, and also what you deleted.

For incoherence of inelastic scattering, please read any text book. If you deny this, you deny all spectroscopies and lasing versus spontaneous. Ldm1954 (talk) 09:09, 9 September 2023 (UTC)

As one simple source, see https://doi.org/10.1016/j.ultramic.2015.03.006 and references therein. You will see that Giulio does not cite complimentarity, nobody who really does research with electrons ever does. I may see him tomorrow, and will ask him. Ldm1954 (talk) 09:18, 9 September 2023 (UTC)

The article you link, "Elastic and inelastic electrons in the double-slit experiment: A variant of Feynman's which-way set-up" is a cool electron-which way experiment. They don't choose to cite complementarity for what ever reason, but their conclusions are the same: "Therefore, from this point of view, these experiments may be considered a very close experimental demonstration of the third part of the Feynman thought experiment, which states that interference phenomena disappear when the experimental conditions are set in a way to determine the slit through which the electron passes."

The role of inelastic scattering in the experiment is technological: it is their mechanism to determine 'which way': "Whereas the images taken by selecting the elastic electrons showed the presence of interference fringes, these phenomena were absent with inelastic electrons." "...we may conclude that the loss of coherence is related to the localization of the inelastic electrons within the slits."

Complementarity is just a name for an almost trivial observation: if you change the experiment to detect paths, interference goes away. It does not explain anything, which is why it is never proven "wrong" or excluded in any example. I do not think anyone can build an "alternative" case to complementarity; it is logically equivalent the lack of observation of particle superposition. You could absolutely say the paper does not mention complementarity, but it does not mention many other things as well. The paper does not make the case against complementarity or "for" and alternative. (Which in my opinion is a strength).

The paper would be a great addition to a section on decoherence in double slit experiments in the main body of this article. (All of this interpretation stuff including complementarity is really pointless in my opinion.) Johnjbarton (talk) 15:24, 9 September 2023 (UTC)

I give up, as you persist in misunderstanding, ignoring math. As you state it above complementarity is philosophy not phyiscs. The uncertainty principle is operator commutation; incoherence is density matrix/statistics; loss of coherence is perturbation theory. When you detect a photon/electron/elephant you add an incoherent phase shift. Giulio does not mention complementarit as he is a strong physicist, and knows elastic/inelastic scattering theory.

You continue to ignore everything else but one view, turning this article from somewhat inclusive to restricted. I will remove this page from my watch list. Ldm1954 (talk) 15:35, 9 September 2023 (UTC)

We're talking about the "Copenhagen" subsection of the "Interpretations" section. It is expressly about philosophy. People who espouse Copenhagen-type views say that the double-slit experiment illustrates complementarity. We attain NPOV not by saying all the things that you or I think are part of explaining the double-slit experiment, but by summarizing what the books written by Copenhagen proponents say about it. If what they emphasize is complementarity, then that's what the subsection should emphasize too. XOR'easter (talk) 19:52, 9 September 2023 (UTC)

I came upon this section again. I have added two small sections, one on "Standard quantum physics" with top-quality sources and "Bohr's complementarity" to reflect that rather more general and abstract concept but directly attributed to Bohr.

These two sections avoid insisting that complementarity is "standard". There are sources which would claim it is, but I also know that most textbooks don't teach complementarity.

I hope these changes will allow us to reconsider the "Copenhagen interpretation" section. I want to change this section to simply be a sourced Copenhagen interpretation of the double-slit experiment. Specifically I want to delete to first paragraph, which is not about double slit. It's a long distracting paragraph about historical issues in naming "Copenhagen". I would also delete the main wikilink as the section would not longer be a summary (which does not belong here IMO).

Unfortunately the second paragraph is not a sourced Copenhagen interpretation. It is a sourced interpretation which we have asserted is Copenhagen. Is there a quality source that describes the "Copenhagen interpretation" of double slit? Johnjbarton (talk) 16:04, 1 October 2024 (UTC)

The fundamental problem I have with this article is that I feel that too much is old, and does not reflect current thinking particularly in the electron microscopy/diffraction community. A more modern interpretation is nicely described in a recent article by Peter Schattschneider and Stefan Loffler at https://doi.org/10.1016/j.ultramic.2018.04.007; there are other papers as the idea goes back decades. The most common approach is density matrices, or the equivalent as mutual coherence as discussed in Born and Wolf. (I am not a big fan of the WP page mutual coherence (physics).)

Up until perhaps 2000 only a few people in the ED/EM community worried about these issues, mainly on the question of coherence (or incoherence) of inelastic scattering. With the advent of ACTEM they have become much more relevant. For instance, the only proper way to model the probe in a modern STEM is as a density matrix propagating through a sample.

I would like to see a 21st century interpretation along these lines included. Ldm1954 (talk) 16:52, 1 October 2024 (UTC)

I agree that there are many new interesting variations on the double slit experiment. We might be better off tackling these in a separate article first.

But the specific question I am addressing is two paragraphs in the section "Copenhagen interpretation". The long first paragraph muddies the waters and the second paragraph has three refs to Feynman (who I do not believe ever claimed to represent Copenhagen) and two refs to complementarity, at topic you previously objected to. This is the section I want to fix. Johnjbarton (talk) 22:45, 1 October 2024 (UTC)

I think that section does need to spell out the difficulties with the term "Copenhagen interpretation", since it is both common and confusing. The second paragraph, the one with the Feynman references, basically recapitulated the subsection above, so I removed it and made some other modifications. XOR'easter (talk) 22:56, 1 October 2024 (UTC)

Thank you. However the remaining paragraph has no sources related to the double-slit topic.

Peres is about double Stern-Gerlach. Omnes echos "experts disagree on what is meant by Copenhagen", but never discusses double slit. Faye, Camilleri & Schlosshauer, Scheibe, Rosenfeld, and Mermin are great on difficulties with Copenhagen but never talk about double slit.

So the first part is off topic and the last two sentences are synthesis. I'm not trying to be difficult, but I believe that either one of us would revert this type of content if it were about another topic. The article on double slit should reflect the sources on double slit.

I wonder if there is a suitable source: Copenhagen belongs to no one, so authority on the double slit would be unlikely to speak for "Copenhagen". Johnjbarton (talk) 23:38, 1 October 2024 (UTC)

Omnès discusses the double-slit experiment in The Interpretation of Quantum Mechanics under the names "two-slit device" and "Young interference device". XOR'easter (talk) 23:43, 1 October 2024 (UTC)

Thank you for correcting me. I've made the last two sentences more specific. I think we could just use Omnes refs, but the section is much improved, thanks. Johnjbarton (talk) 00:22, 2 October 2024 (UTC)

Sorry John, but you appear to have misunderstood my comments. I am not discussing variants, I am talking about the 21st century interpretation in the ED/EM community. Once you move from a single wavefunction approach to statistical with mutual coherence/density matrices there is no interpretation issue. However, you objected to including this way back which is when I stopped contributing to this article. Ldm1954 (talk) 10:43, 2 October 2024 (UTC)

N.B., just for this discussion, remember that all current ab-initio whether dft or other are statistical; single wavefunctions are for teaching. Ldm1954 (talk) 10:46, 2 October 2024 (UTC)

New articles about interpretations

Here is a new Nature article about interpretations, Can the double-slit experiment distinguish between quantum interpretations? I didn't have time to read it yet, but looks interesting. Artem.G (talk) 06:04, 14 September 2023 (UTC)

This paper proposes turning the double slit detector 90 degrees such that the axis now measures intensity parallel to the main axis between the source and the slits. With atomic matter waves they claim measurements when compared to quantitative calculations could distinguish between some interpretations. Its not a long paper but it has 115 references!

I think the work is interesting but too early for use in Wikipedia. On interesting tidbit: they say photons do not act the same due to the "relativistic localization-causality problem" and cite several papers related to the Hegerfeldt theorem. It might be worth reading those refs for addition here. Johnjbarton (talk) 13:40, 14 September 2023 (UTC)

Same.--Chetvorno^TALK 03:49, 15 October 2024 (UTC)

Disagreement over lead sentence

There has been a recent disagreement over the lead sentence. I wanted to get in on the discussion so I thought I'd bring it here to the talk page, if that's okay.
Zanahary prefers:

In modern physics, the double-slit experiment demonstrates that light and matter can satisfy the seemingly incongruous classical definitions for both waves and particles.

Johnjbarton prefers:

In modern physics, the double-slit experiment demonstrates that light and matter can exhibit behavior of either classical particles or classical waves.

In the top version I think the phrase "...light and matter can satisfy the seemingly incongruous definitions of..." is needlessly convoluted. I'm not sure particles and waves actually have "definitions".

In the bottom version, I understand the reason for using "or" rather than "and" - matter and light don't exhibit these properties at the same time or the same part of the experiment. But I think "or" will be confusing for nontechnical readers, they will think: "Okay, this means some matter and light exhibits particle properties and others exhibit wave properties."

I'd suggest In modern physics, the double-slit experiment demonstrates that light and matter can exhibit behavior of both classical particles and classical waves. and clarify the circumstances later, or add the phrase but not at the same time.

--Chetvorno^TALK 03:42, 15 October 2024 (UTC)

Particles and waves do have classical definitions, and “classical particles/waves” doesn’t follow any verbiage I’m familiar with. ꧁Zanahary꧂ 03:46, 15 October 2024 (UTC)

I didn't quite understand. You are objecting to the terms "classical particles" and "classical waves" in John's wording? --Chetvorno^TALK 04:50, 15 October 2024 (UTC)

Yes. ꧁Zanahary꧂ 05:51, 15 October 2024 (UTC)

Maybe instead of trying to go straight to interpretation we should start with the facts. Maybe something along this line:

• In physics, the iconic double-slit experiment, with either light or matter, displays an interference pattern built up one event at a time. The individual event are consistent with quanta or 'particles'; the pattern is characteristic of waves. Quantum mechanics predicts these results while classical theories of particles or of waves do not.

Johnjbarton (talk) 15:34, 15 October 2024 (UTC)

Certainly not. That opening sentence could hardly be more opaque. -Jordgette [talk] 15:40, 15 October 2024 (UTC)

Perhaps you would care to propose a clearer alternative? Johnjbarton (talk) 15:56, 15 October 2024 (UTC)

@Johnjbarton: Agree with Jordgette. The introduction should be understandable by general readers. I mostly like your original lead, I can live with it. But what is your objection to:...can exhibit behavior of both particles and waves? I believe a lot of sources use this wording. The same photon that is absorbed at the screen as a particle becomes part of the collective interference pattern.--Chetvorno^TALK 17:33, 15 October 2024 (UTC)

The problem with the "exhibit behavior of both particles and waves" is that the word "particle" has two meanings, a BB-like object or a quanta. Readers assume BBs but most physics articles assume quanta. That is why I want to use "classical particle".

The "exhibit behavior" is the important bit for me, not 'or' vs 'and'. We just need to avoid making claims about what light and matter "are" vs our observations. I'm ok with your full original proposed version. Johnjbarton (talk) 17:49, 15 October 2024 (UTC)

I agree with all that, and using "classical particles" and "classical waves". Just so I understand, what is your preferred wording? --Chetvorno^TALK 01:04, 16 October 2024 (UTC)

My preferred version is the one in place. But your

• In modern physics, the double-slit experiment demonstrates that light and matter can exhibit behavior of both classical particles and classical waves.

is also fine. Johnjbarton (talk) 02:40, 16 October 2024 (UTC)

I agree with the above version. --Chetvorno^TALK 03:27, 16 October 2024 (UTC)

I still don't like "classical particles" etc. Can we do In modern physics, the double-slit experiment demonstrates that light and matter can exhibit behavior of both particles and waves, classically defined.? ꧁Zanahary꧂ 18:55, 16 October 2024 (UTC)

There is a little ambiguity in whether "classically defined" refers to the particles and waves, or "behavior". But I can live with it. --Chetvorno^TALK 19:31, 16 October 2024 (UTC)

You said above that "particles and waves do have classical definitions..." so what's wrong with "classical particles" and "classical waves"? We can link the adjectives to Classical physics to give readers more info. --Chetvorno^TALK 19:45, 16 October 2024 (UTC)

I want to raise for discussion a slightly related issue. The article starts with "...experiment demonstrates that light and matter can exhibit behavior of both classical particles and classical waves. This ambiguity is considered evidence for the fundamentally probabilistic nature of quantum mechanics.", which to me reads like "the uncertainty of whether it is a wave of a particle is the uncertainty that is handled by probabilities". I guess in some quantum eraser setup that may be a valid way to describe things, but, afaik, in the classical double slit experiments the "ambiguity between wavelike and particle-like behavior" and "the probabilistic nature of quantum mechanics" refer to two completely separate uncertainties and should not be conflated like that. Maybe something like "Versions of the experiment contribute to the evidence for the fundamentally probabilistic nature of quantum mechanics." would be more accurate? ("versions", because the original Young setup is just classical wave optics, no quantum probabilities) L3erdnik (talk) 17:43, 1 November 2024 (UTC)

Hm, I don’t understand your reading. I understand it as "the uncertainty of whether it is a wave of a particle is evidence that quantum mechanics is probabilistic". Is that different from yours? Is yours an implicit derivation from this reading? ꧁Zanahary꧂ 17:50, 1 November 2024 (UTC)

Yes, that is how I think the sentence reads, which is problematic since the probabilities in quantum mechanics are related to the observables (like "4% particle is in this spot, 5% it's in that spot, etc.") rather than which properties are more pronounced (as in "40% it's a wave, 60% it's a particle"). Not withstanding specific setups tailored to tie the latter to the former. L3erdnik (talk) 18:25, 1 November 2024 (UTC)

I deleted the sentence:

• This ambiguity is considered evidence for the fundamentally probabilistic nature of quantum mechanics.

This topic is not covered in the article. Johnjbarton (talk) 18:02, 1 November 2024 (UTC)

Photon animation is not correct.

The animation in the section "Mach–Zehnder interferometer" shows particles in contradiction to science. The caption is also illogical since it claims the experiment exhibits wave-like interference but the animation persists in showing particles. Ironically the article topic is one of the key bits of experimental evidence that such a diagram is incorrect.

Photons in a Mach–Zehnder interferometer exhibit wave-like interference and particle-like detection at single-photon detectors.

An acceptable diagram would show optical paths and pulses that change when the detector is inserted in the path. Johnjbarton (talk) 17:50, 30 October 2024 (UTC)

I replaced the image with a different one from the main article. A changed the text to match. Johnjbarton (talk) 18:11, 30 October 2024 (UTC)

You put a completely classical and uninteresting picture instead of this great animation. You are really overreacting to someone drawing photons as dots. The caption is perfectly correct, the experiment does exhibit wave-like interference. Tercer (talk) 18:18, 30 October 2024 (UTC)

Both images are of course completely classical as they must be. The old caption contradicts the old image: no wave-like interference is shown in animation the two-path condition. I think the candle interference is much more interesting and it avoids showing little imaginary dots in flight. I agree that the animation is visually compelling and fun to watch, but it acts to reinforce an incorrect view of this article's topic. Johnjbarton (talk) 18:27, 30 October 2024 (UTC)

Of course wave-like interference is shown. We see it clearly as only one detector clicks after the photon paths converge at the second beam-splitter. The illustration is not showing "little imaginary dots in flight", but the region where $|\psi (x)|^{2}$ is larger than some threshold. The dots are solid red when the integral over this region is close to 1, and semi-transparent when it's close to 1/2. Tercer (talk) 18:32, 30 October 2024 (UTC)

I'm wondering whether we should have a section on the Mach–Zehnder interferometer at all, since there is a separate article on it. I doubt the brief explanation is going to be comprehensible to nontechnical readers. Maybe we could just have a sentence with a link saying that Mach–Zehnder is another split beam experiment demonstrating particle-wave duality. --Chetvorno^TALK 19:51, 30 October 2024 (UTC)

It's two paragraphs in the section "Variations of the experiment". I think it fits very well there. Tercer (talk) 22:47, 30 October 2024 (UTC)

I agree that this topic is a big step up for most readers, though most modern experiments are done this way. Since I believe the current content is unverifiable I support this solution even if it alters the structure of the article. Johnjbarton (talk) 23:44, 30 October 2024 (UTC)

I implemented this suggestion. If source for the model in the image is found we can revisit having a section. Johnjbarton (talk) 17:49, 31 October 2024 (UTC)

We need a source for this new interpretation of the double slit experiment. Then the figure caption would be incorrect as of course the dots can't be photons if they are sometimes 1/2 and sometimes not. Johnjbarton (talk) 23:40, 30 October 2024 (UTC)

The meaning of the dots and their shading is not at all clear from the caption or the adjacent article text. I would have guessed at first that the changes from solid to semitransparent were an animation bug. XOR'easter (talk) 22:27, 31 October 2024 (UTC)

Tercer had to give a whole explanation in terms of concepts that are familiar to a physicist, but not the general reader, and with a bit of thinking and figuring out (and foreknowledge of how the Mach–Zehnder interferometer works) I can parse the intent of the diagram. But it is as clear as mud without prior insight or the willingness to just treat it as magic, and even with Tercer's description the critical role of relative phase in the constructive/destructive interference is just missing from the diagram and explanation, for which illustrated waves are better. I don't think that the diagram works well here. It also ignores the quantum eraser possibilities (the detectors end up in a superposition of 'detected' and 'not detected'). Without a clearer simple presentation, I concur with Chetvorno's observation. —Quondum 21:19, 1 November 2024 (UTC)

I find your position incomprehensible. Whether you like the animation or not is independent of whether this article should cover the Mach-Zehnder interferometer. I think it clearly should; it is the most prominent variant of the double-slit experiment, and the one is often taught in its stead, for pedagogical reasons: to understand the double-slit experiment you need differential equations, whereas for the Mach-Zehnder interferometer linear algebra suffices. It would be bizarre (if not even WP:UNDUE) to ignore it while covering other more obscure variants. Tercer (talk) 21:58, 1 November 2024 (UTC)

Tercer, you seem to be uncharacteristically combative, and strangely unreceptive to the general sentiment against your position. Given this, I am going to step back and hope you start sounding a little more like your usual self in a while. —Quondum 23:51, 1 November 2024 (UTC)

I am asking for more opinions on Wikipedia_talk:WikiProject_Physics. My stand is:

• The image is not physically correct, it shows a model of unmeasured flying dots.
• No source supports this model of flying dots. The flying dots are not discussed in the article. They are not photons.
• An article like this should be extra careful not to perpetuate mythological models.
• The image should be removed unless a source can be provided and the caption can be corrected
• I tried two alternatives and both were reverted.

Johnjbarton (talk) 17:23, 1 November 2024 (UTC)

I have already explained you what the image represents. It is correct. Stop being disingenuous. Furthermore, images are usually unsourced. If you applied this impossible standard consistently you'd need to removed almost all of the images in this article, and probably half of Wikipedia's. Tercer (talk) 21:43, 1 November 2024 (UTC)

Your explanation conflicts with the caption and physics. The caption is clearly incorrect according to your description. If you included your description in the article it would be original research as it has no source. I'm not challenging every image in this article nor half of those in Wikipedia. I am challenging this image. Johnjbarton (talk) 01:24, 2 November 2024 (UTC)

You are simply repeating that it's incorrect without any elaboration. This is not productive. Tercer (talk) 15:42, 2 November 2024 (UTC)

I am not obligated to prove it is incorrect. I'm not asking you to prove it is correct. I am asking for a source that supports the claims in the caption and concepts visualized in the diagram. Our opinions on the concepts are not relevant.

Nevertheless here is my elaboration:

• The caption says "Photons in a Mach–Zehnder interferometer", but the dots split while photons do not. Saleh, B. E. A. & Teich, M. C. (2007). Fundamentals of Photonics. Wiley. ISBN 978-0-471-35832-9.
• Your own description ("the region where $|\psi (x)|^{2}$ is larger than some threshold.") contradicts the caption.
• The image claims "wave-like interference" but does not illustrate it.
• The standard mainstream quantum description of the Mach-Zehner interferometer does not use time-dependent theory. For example, Vedral, Vlatko, 'Quantum mechanics', Introduction to Quantum Information Science, Oxford Graduate Texts (Oxford, 2006; online edn, Oxford Academic, 1 Jan. 2010) Section 2.10 The Mach–Zehnder interferometer. OR Schneider, M. B., & LaPuma, I. A. (2002). A simple experiment for discussion of quantum interference and which-way measurement. American Journal of Physics, 70(3), 266-271.
• The red circles clearly depict particles. Asserting that they depict something else is disingenuous.
• Claiming that the circles are packets of probability imply the possibility of detection in that region of space. The possibility of detection eliminates the interference. This is a fundamental aspect of QM. (See https://www.feynmanlectures.caltech.edu/III_01.html or any one of dozens of books that discuss "which-way" experiments)
• The visualization of quantum systems has been a dream of physics for a century. (Griffiths, D. J., & Schroeter, D. F. (2018). Introduction to Quantum Mechanics (3rd ed.). Cambridge: Cambridge University Press; 1.2 The Statistical Interpretation). It would be extraordinary if Wikipedia could provide a tidy diagram that succeeds where so many others have failed.
• In my personal opinion many physicists use a model along the lines you outlined previously to make experimental designs and predictions. For the most part these models work, just like for the most part classical electromagnetism works. I think they work because they are similar to first order Feynman path treatments of these problems, not because the little dots "exist". But nothing in this paragraph matters for Wikipedia because these models are not published and do not in fact represent mainstream QM.

Now comes your part. Provide a source for your model illustrated in the diagram. Such a source would be very interesting, but I do not believe one exists because, very sadly, no such concept matches experimental all observations. Johnjbarton (talk) 18:22, 2 November 2024 (UTC)

So your objection is to the very idea that one can draw wavefunctions? You need to learn basic quantum mechanics. Drawing wavefunctions is done in pretty much every textbook. Plenty of wavefunctions are drawn at Matter wave, Wave packet, and Quantum tunnelling, for example. Including time-dependent ones. Your claim that is somehow "not mainstream" is just nonsense. And no, just because you plot $|\psi (x)|^{2}$ doesn't mean that you can make a measurement there and suppress interference. This is just gibberish. Tercer (talk) 13:19, 3 November 2024 (UTC)

Please stop asserting that I am making statements that I have not made. At no point have I said one cannot draw wavefunctions. I've drawn quite a number myself when I was working on photoelectron holography.

I have never seen a diagram like this one in a mainstream publication. If you have, just give us the source. Johnjbarton (talk) 17:06, 3 November 2024 (UTC)

In the video abstract of this paper, at around 0:45. Tercer (talk) 17:23, 3 November 2024 (UTC)

It's a ... confusing picture. Lots of moving dots, no indication within the picture of what the dot colors mean, blocks flashing sometimes but not all the time, no sense of what I ought to be paying attention to at each moment. At the very least, it's poorly captioned. XOR'easter (talk) 19:19, 2 November 2024 (UTC)

Agree with Johnjbarton, XOR'easter, Quondum. I don't think anyone who has not already been exposed to this experiment is going to understand the diagram. The representation of photons as particles is going to promote the erroneous idea that the photons split in half at the beam splitter. I think this diagram should not be used. --Chetvorno^TALK 20:05, 2 November 2024 (UTC)

I think that deleting this diagram would be throwing away the baby with the bath water - it is not perfect, but does illustrate the point quite well in my opinion. The problem is that the caption has to be brief and can't elaborate on all the details, but the second paragraph of the text itself does. Instead of removing this illustration (which, in my view, does help reader to comprehend the text) I propose to be more constructive about it, for example adjusting the caption to something like "Photons in a Mach–Zehnder interferometer exhibit interference (wave-like behavior) while being detected at single-photon detectors one by one, in discrete way (particle-like behavior). See the text for the details." L3erdnik (talk) 03:20, 4 November 2024 (UTC)

Your caption would be fine, except that it implies that the red dots are photons. They are not. That is the issue here. The dots have no physical meaning but they imply that particles are traveling along paths and splitting in half at mirrors. This kind of realistic interpretation is what many QM experiments have shown to be impossible.

Mach Zehnder interferometer alternate candle images

The diagram I tried to add, one you see in textbooks on Mach-Zehnder would match the caption you proposed.

The only meaning for the dots which I can come up with is this: the dots are just "fingers" tracing out active paths in the interferometer for different experimental setups. But then the animation is way off because the some half dots are still in the machine when only full dots should be shown and vice versa. The time scale is completely wonky. The animation would be correct if the paths simply highlighted as the detector moved. Johnjbarton (talk) 03:40, 4 November 2024 (UTC)

Isn't the half-dot the most natural way of visualizing a photon in a superposition state (which the text talks about)? Then, when it is measured in the upper path, the superposition collapses either to being detected up there or to continuing through the left path no longer in superposition, a solid dot. L3erdnik (talk) 04:14, 4 November 2024 (UTC)

A half a photon is not natural. The timing you describe is based on particles and not quantum mechanics. These kinds of pseudo-particles makes sense some of the time but fail in general. That's why these kinds of descriptions are not in QM textbooks. Johnjbarton (talk) 06:29, 4 November 2024 (UTC)

As I have already explained, the dots represent the region where the wavefunction of the photons is larger than some threshold. The wavefunction does split at the beam splitter, with half of each going each way. This is not a matter of interpretation, you just compute the wavefunction (as a function of time) and plot it. Tercer (talk) 07:44, 4 November 2024 (UTC)

I specifically said "a photon in a superposition state", not "half a photon". Are you objecting to representation of a superposition state as paler dots, or to the description "photon is in a superposition of states in the upper and the left arm" (then we should be talking about the text of the article first because it uses this phrasing)?

Or maybe to photons shown as being somewhat localized even before detection? Maybe normally it's not too accurate, but not in a way that matters in any way - one can modify the setup slightly to know exactly when new photon is emitted each time and then the photon would be localized in all directions - and how is it inaccurate then to represent that photon by a blob in which it is localized? L3erdnik (talk) 14:11, 4 November 2024 (UTC)

Sorry I should have challenged "a photon in a superposition state" which is what you said. A photon in a superposition state cannot be drawn. It can't be drawn as two less-bright photons. It can't be drawn as sometimes here, sometimes there. That is the core aspect of the debate about QM from Einstein/Bohr onward. You can't draw superposition.

You can draw the probability density of QM. That is the probability of detection in a specific experiment. If the experiment is "detect localized particle" then you can't get interference. To my way of thinking that means the animation should have no visual indications of interference when the detector is inserted. (This diagram does not have any clear indication of interference)

Photons are never localized before detection and we have no idea when they will be emitted or detected. The entire issue of "photons" is quite interesting and involve complex correlation experiments. But the real issue here is that the sourceable descriptions of the Mach-Zehnder interferometer uses quantum states not photons. Johnjbarton (talk) 17:34, 4 November 2024 (UTC)

The article

• Marshman, E., & Singh, C. (2016). Interactive tutorial to improve student understanding of single photon experiments involving a Mach–Zehnder interferometer. European Journal of Physics, 37(2), 024001.

Provides an extensive discussion of the topics discussed here. Under Student Difficulties there is a section entitled "3.2. Difficulties due to a single photon as a point particle model" that is particularly relevant. Johnjbarton (talk) 18:25, 4 November 2024 (UTC)

I have stared at the animation for a while. I believe I understand its intention. The creator put a lot of intention into detail. I think that I can see the intent. I think it is a not incorrect depiction of some interpretations of what is going on but not consistent with other interpretations. I think that it is probably an undue depiction of one interpretation. I don't want to debate that. The question is whether it belongs in this article. I don't think it does.

• It needs a lot of explanation to be understood. That sort of defeats the purpose of animation. It ought to support the text with an easily understood visual depiction. Instead, we need a lot of text to support the animation. I think it fails its purpose in that sense.
• Content in Wikipedia should be restricted to information paraphrased from reliable sources. It should be free of synthesis. It should not take a lot of own interpretation of the material in reliable sources. Where do you find an animation to paraphrase? How do you paraphrase an animation? In this case, I think that there is no animation in RS that this diagram is paraphrased from. I think it relies on a lot of OR to go from written mathematical descriptions to this animation. That does not mean that there can be no animations in WP. But it does mean that if it is opposed and there is no reliable source animation that it should be removed unless there is a strong consensus to keep it.
• It is too esoteric. The learned editors here in this discussion cannot agree on its correctness. The general reader doesn't have a chance.

Constant314 (talk) 17:07, 4 November 2024 (UTC)

Excellent perspective. I wish now I had challenged the image on grounds of confusion. Johnjbarton (talk) 17:59, 4 November 2024 (UTC)

Removed diagram per consensus of Constant314, Chetvorno, Johnjbarton, XOR'easter, Quondum --Chetvorno^TALK 18:33, 4 November 2024 (UTC)

The learned editors here all agree on its correctness. The only one challenging it is Johnjbarton, who doesn't know quantum mechanics. Tercer (talk) 20:40, 4 November 2024 (UTC)

It is not about correctness. It is about appropriateness. Constant314 (talk) 20:54, 4 November 2024 (UTC)

If an expert has to stare at the picture and then use their expertise to invent a "reading" of the picture that makes it not wrong, it's not a very good illustration. Personally, I think that the half-shaded dots feed into the lazy tropes about quantum mechanics (like "the cat is both dead and alive") that we should actively avoid perpetuating. XOR'easter (talk) 04:42, 5 November 2024 (UTC)

How else would you do an animation of the interferometer? Tercer (talk) 08:39, 5 November 2024 (UTC)

Knowledge of the way and interference pattern

"Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as would a wave)." - the references after this sentance are not for experiments. And it's controversial to real experiments which show that interference pattern still exist if observer knows the way of particle, those experiments are described it another part of the article: "An experiment performed in 1987 produced results that demonstrated that partial information could be obtained regarding which path a particle had taken without destroying the interference altogether." 37.57.176.109 (talk) 19:38, 2 November 2024 (UTC)

The experiment you mention is a statistical one - the experiment doesn't determine which slit a particular photon goes through, but is arranged so there is a greater probability that each photon goes through one slit than the other. As a result there will still be an interference pattern, but the interference pattern will be fainter. There is a mathematical equation, the Englert–Greenberger relation that expresses a tradeoff between the probability of the photon going through one slit rather than the other, and the brightness of the interference pattern.

An example of such an experiment would be if the laser emitting the photons is positioned so one slit, say the left, is illuminated with twice as much light as the other. This gives us partial "which-slit" information: twice as many photons would pass through that slit, so the probability that any photon hitting the screen passed through the left slit is 2/3. The result will be that the interference pattern on the screen will be dimmer than if the two slits are illuminated with the same brightness, so the probability of a photon passing through each slit is 1/2. The greater the percentage of light that goes through one slit, the more which-slit information we have, so the dimmer the interference pattern will be.

An experiment that determines with certainty which slit a photon goes through will produce no interference pattern at all. To determine which slit each photon goes through requires having detectors at the slits. A photon will hit the detector, changing its direction, which will destroy the interference between the slits. --Chetvorno^TALK 21:13, 2 November 2024 (UTC)