Talk:Many-worlds interpretation/Archive 7

General overhaul necessary
called this article to my attention after doing some tagging and cleanup work. I found the tags (possible original research, sections lacking sources, material being unduly weighted, etc.) to be appropriate. observed that the article displays Original research, undue weight, misrepresentation of sources, and un-encyclopedic style, and I'm afraid that I have to agree. (The "common objections" section seems particularly bizarre, almost like it was copied out of some other FAQ-type web page.) Quantum foundations is a notoriously hard subject to write clearly about, and Wikipedia naturally has to deal with contributors who are more enthusiastic than they are informed. An article on an important topic can too easily die a slow death by a thousand good faith additions. Fairly drastic action will be necessary, I think, to make a useful document. XOR&#39;easter (talk) 19:22, 23 February 2020 (UTC)
 * Some of my complaints referred to the section "Communication between worlds", which you simply nuked. That section claimed that mere interference between isolated systems (which are emphatically not worlds) constituted communication between worlds, and misrepresented Deutsch's famous paper as supporting this assertion. Maybe it would be worth mentioning, though, that some non-linear extension of quantum mechanics would enable communication between worlds, if somebody wrote a paper about this. Another relevant point that the article should contain is the test of Many-Worlds proposed in Deutsch's paper above, the Wigner's friend gendankenexperiment. There are tons of reliable sources about this. Tercer (talk) 19:45, 23 February 2020 (UTC)
 * I wasn't the one to nuke "Communication between worlds", though I do think that was the simplest course of action. I recall nonlinear modifications of QM being discussed in the context of thermodynamics and in beyond-quantum computation; offhand, I don't know of a paper about nonlinearity and communication between worlds/branches, but I can look for one. Deutsch's version of Wigner's friend is probably more important to include, though. XOR&#39;easter (talk) 20:10, 23 February 2020 (UTC)
 * I'm not much for a hack and slash approach, except for when it seems it's the best route. And rarely do I support WP:TNT. I always consider what our WP:Preserve policy states. In part, it states, "Fix problems if you can, flag or remove them if you can't. Preserve appropriate content. As long as any facts or ideas would belong in an encyclopedia, they should be retained in Wikipedia. Wikipedia is an encyclopedia. Likewise, as long as any of the facts or ideas added to an article would belong in the 'finished' article, they should be retained if they meet the three article content retention policies: Neutral point of view (which does not mean no point of view), Verifiability and No original research. Instead of removing article content that is poorly presented, consider cleaning up the writing, formatting or sourcing on the spot, or tagging it as necessary."


 * That stated, I do see that things are being removed with consideration, not carelessly. Flyer22 Frozen (talk) 01:55, 24 February 2020 (UTC)
 * I think the most problematic part of the article is the "Common objections" section. Much of it is copied from the Everett FAQ, a link to which used to be here but was removed by somebody. That might be even a copyright violation. In any case, the style is very un-encyclopedic, and some of the objections are completely inane (e.g. cannot explain the colour of gold, violates conservation of energy). We should retain the serious objections, that were raised by actual scientists in peer-reviewed papers; but that definitely shouldn't be in the format of a FAQ. For now I'll just remove the most egregious stuff. Tercer (talk) 09:28, 24 February 2020 (UTC)
 * Yeah, that section seemed to be more about "common objections made in arguments on physics subreddits" than "common arguments debated by physicists and philosophers". Thanks for trimming it. XOR&#39;easter (talk) 13:56, 24 February 2020 (UTC)

I like the new version, but is there some reason you didn't mention Gleason's theorem? It shows that if one wants to assign probabilities (or, more generally, weights) then the Born rule is the only reasonable choice (conserving the sum over time). Wallace et al discuss this, e.g. here (ref 35 at the moment). --mfb (talk) 06:25, 25 February 2020 (UTC)
 * Please don't WP:HOUND me. I stopped editing spaceflight articles specifically to avoid contact with you (and Insertcleverphrasehere). Tercer (talk) 07:17, 25 February 2020 (UTC)
 * My first edit on this article was over three years ago, and I had it on my watchlist for even longer than that. I don't see a reason to avoid working together on articles just because we had some differences in spaceflight discussions. If you think this is necessary then that's your choice - but don't expect me to abandon every article on my watchlist if you happen to edit it. --mfb (talk) 08:29, 25 February 2020 (UTC)
 * Anyway, to come back to the topic: I think it's useful to mention it, I could add it myself, but I'm not an expert. That's why I asked you. --mfb (talk) 08:32, 25 February 2020 (UTC)
 * This section is about the probability problem and derivations of the Born rule in the Many-Worlds framework. Gleason's theorem belongs to neither category. If you read Wallace carefully, you'll see that he considered the possibility of adapting Gleason's theorem to the Many-Worlds framework, failed to do it, and argued that it was in any case a bad idea. Tercer (talk) 09:19, 25 February 2020 (UTC)

I believe I managed to solve the problems with the "Common objections" and "Probability and the Born rule sections". There still much work to be done, though. The section "Overview of the interpretation", which is arguably the most important of the article, is an incoherent mess of random facts and questionable statements. Particularly worrisome is the complete absence of mathematics. Also, the "Reception" section is quite lame. We can do better than that. Tercer (talk) 10:39, 2 March 2020 (UTC)
 * It's rather unsurprising that Wallace takes the position regarding Gleason's theorem that he does. Roughly speaking, Gleason assumes measurements and derives the set of states, while an Everettian would assume the state vector and want to derive measurements. I don't think Gleason's theorem belongs in the section under discussion (and what the article said before about it was completely confusing). XOR&#39;easter (talk) 15:06, 2 March 2020 (UTC)
 * What the article said before was not only confusing, but also completely wrong. I'm glad somebody else got rid of it. (you seem to have misplaced your message, though). Tercer (talk) 17:27, 2 March 2020 (UTC)
 * I think the article is well headed in the right direction and has much improved, and I very much appreciate the work done so far. Glad to see that FAQ section is no more.
 * I agree the "reception" section is quite lame, so I don't really think this tag necessarily needed to be removed yet. I still think it needs more substantive coverage of the various views, including criticisms. Also, I believe the preferred basis and Born rule issues are considered criticisms of the MWI, but the article as yet does not directly say so to my knowledge.
 * The bit about Occam's Razor needs some work I believe; isn't there a respectable argument made that the extra universes fall afoul of it? The MWI view is that MWI is actually preferred under Occam's Razor, but we should probably represent both sides of this. I also tagged a nearby sentence for possible POV so it can be looked at.
 * Thanks again for working on this. Crossroads -talk- 05:42, 3 March 2020 (UTC)
 * It's my pleasure. I don't think there's a balance problem with the "reception" section (it quotes Deutsch as a supporter, Peres as a critic, makes some random comments, and quotes polls), but as you say a lack of content. It should also cover prominent supporters such as DeWitt, Wallace, Carroll, and prominent critics such as Maudlin and Kent. This is the proper place to quote people's opinions. Factual matters should be covered in the other sections (for example the comments about testability should be move to the "testability" section).
 * I did give ample coverage to the critics in the sections about the preferred basis and probability problems, in fact both sections state in the first paragraph that they are problems, I don't think there's anything lacking. Again, if somebody is of the opinion that Many-Worlds should be rejected because of these problems, I think this belongs in the "reception" section.
 * I've heard this argument about Occam's razor being made several times in informal settings, but not in print. We need a reliable source to add it here. What the article had before was an unsourced he-said-she statement in the FAQ, which I don't find acceptable. Tercer (talk) 09:25, 3 March 2020 (UTC)
 * I see what you're going for now. Some articles cover all criticisms under the reception section, but this method looks good because otherwise such a section could be too long. Regarding if somebody is of the opinion that Many-Worlds should be rejected because of these problems, this belongs in the "reception" section, I agree with that as well, and that the section needs more content. With Occam's Razor, agreed that we need a good source for it and that the old coverage of it was bad. Crossroads -talk- 15:20, 3 March 2020 (UTC)
 * I searched a bit more about somebody defending the Occam razor's argument against Many-Worlds, but I couldn't. I did find Skyrms' paper (which I added to the article for another reason), who mentions the argument only to say it is nonsense. I'm afraid we're falling in the "both sides" fallacy here, violating WP:DUE. The Occam razor's argument is clearly wrong, and the reliable sources say so. Why should we try to find a source saying the opposite and give it coverage here? As an analogy, there are countless people claiming that Bell's theorem is wrong because of this and that reason, and that local hidden variables can in fact reproduce quantum mechanics. The article on Bell's theorem doesn't even mention them, and rightly so. Tercer (talk) 18:19, 7 March 2020 (UTC)
 * I had actually just opened the Skyrms article when you added it here. Spooky! :-) XOR&#39;easter (talk) 18:27, 7 March 2020 (UTC)
 * Perhaps we are both cheating by using Vaidman's article on the Stanford Encyclopedia of Philosophy? ;) I had actually opened that article for a completely different reason, I was wondering if perhaps somebody had tried to go from Lewis' possible worlds in modal logic to a many-worlds theory of probability by applying a measure, not entirely unlike what Everett did. It turns out that Skyrms did not, but his article is an interesting read anyway. Tercer (talk) 18:40, 7 March 2020 (UTC)
 * I found my way to it by looking for citations to Graham's chapter in the 1973 book, to see if there were any secondary references on the earlier claims and counterclaims about deriving probability measures. XOR&#39;easter (talk) 19:08, 7 March 2020 (UTC)

"Debate whether the other worlds are real" section
It seems to me that this section should be merged with Reception section, which also concerns debate on the topic. Also, "MWI's initial reception was overwhelmingly negative" should be sourced. Doesn't seem like it's sourced. And given that the lead notes that this interpretation is mainstream and the Polls subsection of the Reception section includes stuff like Sean M. Carroll stating "As crazy as it sounds, most working physicists buy into the many-worlds theory" and Michael Nielsen stating "at a quantum computing conference at Cambridge in 1998, a many-worlder surveyed the audience of approximately 200 people... Many-worlds did just fine, garnering support on a level comparable to, but somewhat below, Copenhagen and decoherence.", it comes across as odd that the reception material above the Polls subsection is mostly negative. And how is Nielsen countering the Carroll statement? Flyer22 Frozen (talk) 20:12, 28 April 2020 (UTC)
 * There are decent sources in print for the initial reception (Byrne's biography of Everett comes to mind as a possibility), so that can be fixed pretty easily, I think. Carroll wasn't citing an actual poll, just making an informal observation in a blog post, to which Nielsen replied in his own blog post. Maybe "counters" is a more adversarial verb than is called for to describe Nielsen saying that Carroll might have had a biased sample. Nielsen also says that his own impression "was that the largest single group of people in the room was people who thought the poll was a waste of time." This is in accord with informal reactions I heard after the Schlosshauer, Kofler and Zeilinger poll came out. XOR&#39;easter (talk) 22:15, 28 April 2020 (UTC)
 * There is a source for "MWI's initial reception was overwhelmingly negative", it is the one cited at the end of the paragraph "The Origin of the Everettian Heresy". I didn't want to repeat the same citation, so I just put it at the end to support the whole paragraph. Note that even Nielsen's statement supports the idea that Many-Worlds is mainstream, as it had a level of support comparable to Copenhagen. Both Nielsen's and Carroll's observations match my personal experience: the level of support of Many-Worlds among people working with quantum gravity is nearly 100%, and within the quantum information community it is a dead heat between Many-Worlds and QBism. Tercer (talk) 07:13, 29 April 2020 (UTC)
 * Thanks. "The Origin of the Everettian Heresy" is a good source. XOR&#39;easter (talk) 14:17, 29 April 2020 (UTC)
 * Yeah, per what sources state and what you have relayed Tercer, I'm stating that the Reception section currently comes across as more negative than it should. Flyer22 Frozen (talk) 18:30, 29 April 2020 (UTC)
 * I agree. We need sources to back that up, though, and they are hard to come by. For example, we could look at the reactions to Carroll's new book on Many-Worlds, "Something Deeply Hidden". But it is pretty much impossible to get an unbiased sample, and the loudest reactions will be from people that hate Many-Worlds. Tercer (talk) 18:57, 29 April 2020 (UTC)
 * I think it's likely that a lot of the more focused statements will be negative, because people write about specifics when they want to get critical (either about MWI as a whole or about somebody else's version of MWI that they disagree with). People who don't think about interpretations a lot might be happy with it, while those who get into the nitty-gritty will have complaints &mdash; and that probably goes for any interpretation that has a significant presence in the community. On top of that, the surveys that have been done are hardly going to give representative pictures themselves, thanks to their own small sizes and biased samples. I don't know what to do about this, really; it's just the kind of trouble we run into when good secondary and tertiary sources are hard to find. XOR&#39;easter (talk) 19:43, 29 April 2020 (UTC)
 * So...any objections to merging the "Debate whether the other worlds are real" section into the Reception section? Flyer22 Frozen (talk) 19:55, 29 April 2020 (UTC)
 * No objections from my side. I never thought that made sense as a separate section anyway. Tercer (talk) 21:10, 29 April 2020 (UTC)
 * No objection from me, either. It existing as a separate section is probably an artifact of the page having grown incrementally without much of an overall plan. XOR&#39;easter (talk) 00:36, 30 April 2020 (UTC)

Does not fit neatly into deterministic dichotomy.
"In contrast to some other interpretations, such as the Copenhagen interpretation, the evolution of reality as a whole in MWI is rigidly deterministic." I couldn't find a precise citation in the source for this. In any case, while it certainly can be argued to be a deterministic theory, it's not the classical deterministic interpretation, which would be Local hidden-variable theory.

In addition, calling a theory deterministic (at least to me) is a compliment, in the same way that calling a theory non-local would be discrediting. Considering the source for this statement is made by its own proponents (possibly primary sources?).

I have added a citation needed, I dare not actually remove it without first graduating, I'd appreciate if a physic, chemist or philosopher could chime in and edit the page if necessary.

--TZubiri (talk) 08:36, 17 October 2020 (UTC)

In line with WP:PRESERVE, we can probably change the line to "Everett considered his theory to be deterministic, as opposed to the Copenhagen interpretation." and move it to somewhere more appropriate without much controversy.--TZubiri (talk) 08:46, 17 October 2020 (UTC)


 * The theory is deterministic, it's not a matter of opinion. If you think this is a compliment or an insult doesn't change anything. Your proposed change is not appropriate, because it makes it seem that this is only Everett's opinion. It's not, nobody disputes it, we shouldn't use the language of a controversy where none exists. Tercer (talk) 09:34, 17 October 2020 (UTC)


 * Yes, it's deterministic (and whether one finds that a benefit or a demerit is a matter of taste). XOR&#39;easter (talk) 18:02, 17 October 2020 (UTC)


 * Then it should not be difficult to find a secondary source that concurs.--TZubiri (talk) 20:20, 17 October 2020 (UTC)


 * It would not be difficult to find dozens, if one bothered; the whole selling point was the idea of replacing "collapse" with unbroken unitary evolution. Determinism is built in by definition. Sources repeating the basic edicts are cheap. The problem is really to decide which among them stand out in any way and are therefore worth pointing to. XOR&#39;easter (talk) 20:46, 17 October 2020 (UTC)


 * Comment in edit summary by XOR easter: " determinism is amply detailed in the body and does not need a specific secondary source in the lede (or, really, a footnote at all, since the lede is really there to summarize the body, but we might as well leave the one that's already there"
 * Most of the mentions to determinism I can find in the body come from Everett directly. Please take a look at the following quotes from WP:PRIMARY
 * "Do not base an entire article on primary sources, and be cautious about basing large passages on them."
 * "Policy: Wikipedia articles usually rely on material from reliable secondary sources. Articles may make an analytic, evaluative, interpretive, or synthetic claim only if that has been published by a reliable secondary source. "


 * Please pick a secondary source that analyzes Everett's proposal, you get to pick one based on your personal judgement and reading history, we don't need dozens, I just would like to read one that you consider worthwile enough to include in the lede. I hear many notable physicists considered MWI to be true, like Feynman and Hawking, perhaps they had something to say on the matter, so hopefully we shouldn't have to rely on self-claims and a tangential mention in an obscure 2016 philosophy paper.
 * --TZubiri (talk) 21:12, 17 October 2020 (UTC)


 * It's hard to find a source that doesn't mention that Many-Worlds is deterministic. This is like asking a source for the claim that Galileo's theory was heliocentric. You're right that secondary sources are needed, though. A good one about Many-Worlds in general is Wallace's book "The Emergent Multiverse" (currently ref 15). Not that I think sources should be in the lead, it's polluted enough already. Tercer (talk) 21:28, 17 October 2020 (UTC)

Schrödinger's position
I'm unhappy about how the history section describes Schrödinger's comment. First of all, the prose is atrocious, the reader must jump back and forth from text to quotation to try and decipher what is going on. Also, the reader gets the impression that there is a disagreement between Barrett and Deutsch. There isn't. They don't even mention each other. Barrett is arguing that Everett and Schrödinger's ideas were similar, and Deutsch is arguing that Schrödinger "had the basic idea of multiple universes". These are perfectly compatible views, we are definitely not in the case where WP:ATTRIBUTEPOV is needed. Another reference that mentions Schrödinger's lecture is Lockwood. He is clear that pursuing Schrödinger's idea results in a many-worlds sort of interpretation, which is compatible with both Deutsch and Barrett. Tercer (talk) 23:45, 1 February 2021 (UTC)
 * I'm not really happy with the history section as a whole. In addition to its placement (wouldn't Wikipedia house style be to have it much later in the article?), it doesn't really give a sense of the actual history of the topic. Whatever we say about Schrödinger, jumping directly from him to Everett is just jarring. Surely Everett's publication in Rev. Mod. Phys., Wheeler's "assessment" of it there, the frosty reception Everett received, etc., deserve more elaboration; and surely we can give a more complete portrayal of people being dissatisfied with "reduction of the wave packet". I mean, sure we ought to talk about Schrödinger because he was Schrödinger and people have talked about him, but starting from a tidbit that probably got thrown into this article because someone read a pop-science book years ago seems the wrong way to go about painting a full picture. For example, the article says nothing about von Neumann having two kinds of time evolution in his textbook, and I'm hard-pressed to find a canonical book by von Neumann to be less significant than a single lecture by Schrödinger. XOR&#39;easter (talk) 00:57, 2 February 2021 (UTC)
 * I think it's important to have it to remark that Everett didn't come out of nowhere, there was a bit of many-worlds-like speculation before him. What we have it's just a tidbit, indeed, but this is an argument to have it as a simple sentence instead of a complex paragraph that tries to divine what exactly Schrödinger had in mind. It is not misleading, though, as Everett was indeed influenced by Schrödinger. Of course not by this lecture, that Everett probably never heard about, but by these papers, that Everett does cite.
 * Of course I agree that we should expand the History section. It's just a matter of getting it done. I did write a bit about it in the first paragraph of the Reception section. Tercer (talk) 09:43, 2 February 2021 (UTC)
 * Maybe we should redo the "history" section from scratch? I'm working at a dual disadvantage, though, as my Internet connection has been unreliable lately and most of my serious physics-history books are in an office that I can't access because of the pandemic. XOR&#39;easter (talk) 14:45, 2 February 2021 (UTC)
 * Sure. It'll be slow going since I've got a lot of work to do lately but I think we'll manage before the WP:DEADLINE. A good guide to the history is, currently Ref 18. I think it overstates the importance of the Deutsch-Wallace theorem but other than that it's comprehensive and balanced. Tercer (talk) 15:28, 2 February 2021 (UTC)

The history section so would be clearer if it was presented in chronological order. Schrödinger first then Everett. FWIW, I am not convinced the the Schrödinger quote is that significant - if you read the whole talk it is clear that he introduces to the idea just to laugh at it. I think Deutsch and Barrett are making too much of it as a progenitor of many worlds. But that's just my intepretation, and we are entitled to quite Deutsch and Barrett, of course. cheers, Michael C. Price talk 06:47, 20 November 2021 (UTC)

Sean Carroll's blog claim
The text inserted here, and re-inserted shortly-thereafter, is not WP:DUE. The source for this is a 2004 post from Sean M. Carroll's blog, which is not a WP:Reliable source about what the physics community thinks. And what one scientist who adheres to MWI claims about what the physics community thinks, and in a post that doesn't even mention any other QM interpretation, is not WP:DUE. This applies all the more so when another physicist on his respective blog rebuts this claim with evidence. This is all cited together under "Polls"; Carroll's own dubious claim does not belong under "Support". Per WP:ONUS, there should be a consensus for it before any reinsertion of this text. Crossroads -talk- 02:37, 20 November 2021 (UTC)

The section is called "support", and the quote is evidence of Carroll's support. And his belief that others share his view - which, as a working professor - is noteworthy. cheers, Michael C. Price talk 06:31, 20 November 2021 (UTC)

The support section is ludicrously short, compared to the rejection section, bearing in mind that this a respected mainstream interpretation.

I intend to add Hawking, Carroll and Brian Cox as supoorters. Any objections? cheers, Michael C. Price talk 06:56, 20 November 2021 (UTC)
 * I'm not sure what benefit that would bring, in terms of meaningful content. Science isn't done by celebrity endorsements. If they have made arguments, that could be useful, but just saying "this is what I believe and other people agree with me" isn't informative enough to bother including here. The current division into "Support" and "Rejection" is somewhat awkward, since some of the material in the latter is about equivocal reactions. XOR&#39;easter (talk) 14:54, 20 November 2021 (UTC)

Celebrity endorsements are noteworthy. Wikipedia is not an academic judging shop. Anyway, something needs to be done, bearing in mind the extortionate lights of the two sections. Merging would be okay with me. cheers, Michael C. Price talk 21:17, 20 November 2021 (UTC)

Disproportionate lengths, I meant. cheers, Michael C. Price talk 21:18, 20 November 2021 (UTC)
 * There's nothing wrong in principle with disproportionate lengths; neutrality in the Wikipedian sense doesn't mean saying one positive thing for every negative thing, but in fairly representing the available sources. Often, people will like an idea without thinking too deeply about it, and the closer you get, the more the disputes come into focus; the more knowledgeable the people are, the less agreement you'll find among them. Some of those disagreements will be petty sniping, while others are more thoughtful. The upshot is that specific negative takes can coexist with more general positivity. And often, it's the back-and-forth of the specific critiques and replies that makes for better encyclopedia material, because it's informative rather than superficial. Consequently, while I'm not really happy with the "Reception" section overall, I think worrying about the relative lengths is mostly a distraction. XOR&#39;easter (talk) 23:37, 20 November 2021 (UTC)

Formulation and Relative State sections
These sections are poorly written. There's a lot of waffle that says nothing and explanations in terms of Bloch spheres and couple subsystems that will not inform the public. I suggest that we retain the last paragraph of the RS section, with an additional lay description of what a relative state is. Something along the lines of, the relative state of the dead (alive) Schrödinger's cat is the vial being (not) broken and the observer seeing the dead (alive) cat. cheers, Michael C. Price talk 11:42, 10 March 2022 (UTC)
 * Yes, they are terrible. I didn't touch them when I reworked the article, except to remove things that were unambiguously wrong. Please do try being WP:BOLD. Tercer (talk) 13:35, 10 March 2022 (UTC)

Thanks, have started the rewrite. Some way to go. cheers, Michael C. Price talk 14:28, 12 March 2022 (UTC)
 * Thanks, it has already improved. Tercer (talk) 09:33, 16 March 2022 (UTC)

That poll
Advocates of MWI often cite a poll of 72 "leading cosmologists and other quantum field theorists" conducted by the American political scientist David Raub in 1995 showing 58% agreement with "Yes, I think MWI is true". The quote in the footnote adds, 58% believed that the Many Worlds Interpretation (MWI) was true, including Stephen Hawking and Nobel Laureates Murray Gell-Mann and Richard Feynman.

Feynman died in 1988.

Feynman's take on MWI in 1982 was "It's possible, but I'm not very happy with it."

Until removed it today, this article cited a book that reported the survey results ... by quoting what Wikipedia said at the time. The online archives of Science et Avenir don't appear to go back to 1998 (only to 2006), so I can't pursue this further at the moment. XOR&#39;easter (talk) 15:05, 3 March 2020 (UTC)
 * Yeah, I should have been candid about why I removed that reference, it was clearly a steaming pile of shit. The remaining reference (that is not kosher either) simply says that this was Feynman's opinion, without further comment. Searching a bit more, I found that this poll is also cited in Frank Tipler's 1994 book "The Physics of Immortality". There the poll is credited to "Raub 1991 (unpublished)". I think this solves the mystery, the poll was simply not performed in 1995. Note that Tipler's book comes from before this unreachable issue of Sciences et Avenir, so we might as well regard it as the actual source. Now, Tipler's book is also not halal, but I think we can trust it for this empirical matter. Tercer (talk) 16:11, 3 March 2020 (UTC)
 * (Note: Was writing this at the same time as Tercer's comment.) Okay, this is quite a journey. Here is its addition to Wikipedia in July 2004. I also looked up the French magazine, and it appears to be a popular science magazine, not a true academic journal.
 * The book it is currently sourced to appears to be self-published and promoting fringe theories. Google lists the book's publisher as the book's title; the book itself lists "Alternative Theories Press", which I could not find any evidence for as a real publisher (and sounds fringey as can be anyway). The book states the poll was done in 1995, but says nothing about Sciences et Avenir. The bit about 1995 may be citogenesis from Wikipedia; see below.
 * That book cites Michael Clive Price's Everett FAQ, which is a non-expert WP:SPS and thus not a reliable source. He never gives a year or mentions the French magazine. He cites "Frank J Tipler The Physics of Immortality 170-171".
 * That book, while having an okay (but possibly religious?) publisher and written by a physicist, certainly seems fringey (it purports to prove the existence of God and the physical resurrection of the dead using physics). It only cites "Raub 1991 (unpublished)" for this. Note the year given. No further details exist in the bibliography.
 * I also could not find any evidence that this person is actually a political scientist. The only references to him I found (via searching "L. David Raub" in quotes refer to him merely as a "physics enthusiast" whom Everett wrote a letter to in 1980. We also know it comes from 1991, so the Wikipedia addition was wrong about 1995 at the very least. The claim it was published in that French magazine could be true, but there is no evidence so far. Crossroads -talk- 16:27, 3 March 2020 (UTC)
 * Perhaps the person who first added this to Wikipedia had this French magazine, it would explain why they gave 1995 as the publication date. Unless somebody manages to get this mythical magazine (I also searched through legal and illegal sources, but all I could find was people selling it on ebay), Tipler's book is the best we have. I added it to the article instead of the self-published book (I agree that it's a clear case of Wikipedia citogenesis). Tercer (talk) 16:47, 3 March 2020 (UTC)
 * Tipler's The Physics of Immortality says In his latest book, Dreams of a Final Theory, Steven Weinberg also comes out in favor of the Many Worlds Interpretation. But if you look up Dreams of a Final Theory, you find that Weinberg writes about the difficulty of obtaining probabilities in an MWI, and then he says, I am not convinced that this program has been entirely successful yet, but I think in the end it may be. Then he goes on into calling the philosophy of quantum mechanics irrelevant to its practical use, speculates that all the deep questions about the meaning of measurement [might be] really empty, forced on us by our language, and describes how disappointed he felt that his proposal for a nonlinear modification to QM didn't work out. Hardly the ringing endorsement that Tipler sells it as. Tipler quotes Gell-Mann as saying, Apart from the unfortunate language, Everett's physics is okay, although somewhat incomplete. But when you look that up, you find it follows a passage that reads, I believe Everett's language is highly misleading. Many distinguished physicists have been confused by the reference to "many worlds" instead of "many histories" and to the "reality" of the "many worlds" instead of to the fact that quantum mechanics does not discriminate among them a priori except by their probabilities. Also, Everett denies the importance of probability while discussing "measure" that can really be used in the same way. Gell-Mann calls himself a post-Everett investigator and claims to have arrived at his interpretation independently, without having heard of Everett, in 1963 (though he mentions conversations with Feynman, who knew of Everett in 1957). So, of the four physicists that Tipler names, at least three of them had more nuanced, partly negative or equivocal views than he presents them as having. This is getting into OR territory, but I think it's within the spirit of the law to say it gives me further reason to doubt the reliability of Tipler's book, in addition to the reasons alluded to above. XOR&#39;easter (talk) 18:20, 3 March 2020 (UTC)
 * You should keep in mind that the current decoherent version of Many-Worlds, as defended most prominently by Saunders and Wallace, is Gell-Mann's version, so I find it rather bizarre to deny that Gell-Mann was a many-worlder. Of course, for Gell-Mann himself the distinction between his decoherent histories and Many-Worlds was very important, but as he remarks himself it is a matter of language. As for Weinberg, I don't think "comes out in favor" sounds like a ringing endorsement, but perhaps Tipler is indeed guilty of a bit of embellishment. This is still very far from falsifying somebody else's data. Tipler may be insane, but I don't think he is dishonest. Who is the third physicist you mention?
 * If Sciences et Avenir indeed has an independent publication of this poll that might clear things up. Maybe we could ask at the French Wikipedia if somebody has the relevant issue? EDIT: I posted a request at the Resource Exchange. Tercer (talk) 20:42, 3 March 2020 (UTC)
 * If we could turn up the Sciences et Avenir story, that might be very helpful indeed. Otherwise, we have a pseudoscientific book reporting a survey conducted at an unknown time with an unknown methodology, which seems less than suitable for an article with pretensions of encyclopedic quality. I doubt that Tipler falsified whatever percentages were told to him, but I'm uncomfortable with pointing to his book on general principles, and percentages without a methodology don't mean much in the first place.
 * Gell-Mann seemed insistently, persistently evasive on the "are the other worlds real" question. For example, in The Quark and the Jaguar, he wrote that Everett's choice of vocabulary and that of subsequent commentators on his work have created confusion. [...] the many worlds are described as being "all equally real," whereas we believe it is less confusing to speak of "many histories, all treated alike by the theory except for their different probabilities." To use the language we recommend is to address the familiar notion that a given system can have different possible histories, each with its own probability; it is not necessary to become queasy trying to conceive of many "parallel universes," all equally real. (One distinguished physicist, well versed in quantum mechanics, inferred from certain commentaries on Everett's interpretation that anyone who accepts it should want to play Russian roulette for high stakes, because in some of the "equally real" worlds the player would survive and be rich.) And a bit later, Even the interpreters of Hugh Everett's work who speak of many worlds, equally real, do not claim to have observed more than one of those branching worlds. He seems to be putting a little distance between himself and other "post-Everettian investigators". There's a downplaying where other writers might offer a glorification. (Later, he went on to write that PRA with Hartle which went on about reformulating decoherent histories with "one real fine-grained history" plus negative probabilities, which left me with more questions than answers when I last tried reading it, ~8 years ago.) This is probably a matter of tone that is below the threshold of interest for a survey article.
 * Weinberg's summary statement in his Lectures on Quantum Mechanics (2015) says, My own conclusion is that today there is no interpretation of quantum mechanics that does not have serious flaws. This view is not universally shared. Indeed, many physicists are satisfied with their own interpretation of quantum mechanics. But different physicists are satisfied with different interpretations. In my view, we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is only a good approximation.
 * The other physicist that Tipler mentions is Hawking. To the best of my knowledge, he was an Everettian of some stripe at the time, though he may have drifted in a "there might not be one single mathematical structure for reality" kind of direction later in life; I never had much will to dive into the industry of books by and about him to look for statements about quantum interpretations. XOR&#39;easter (talk) 04:58, 7 March 2020 (UTC)
 * I'm leaning similarly to XOR'easter about this poll. I think it lacks any real WP:RS for it, and may be WP:Undue. Even if the French magazine turns up, it may just reference Tipler's book anyway. I doubt they got information from L. David Raub that no one else did. And its origin is after all a fringe theory pushing book that needed MWI to be true for its claims to work. Crossroads -talk- 05:30, 7 March 2020 (UTC)
 * The methodology of the poll is very clear: The political scientist L. David Raub polled seventy-two leading quantum cosmologists and other quantum field theorists on their opinion about the truth of the Many-Worlds Interpretation. The possible answers were, (1) "Yes, I think the MWI is true"; (2) "No, I don't accept the MWI"; (3) "Maybe it's true, but I'm not yet convinced"; and (4) "I have no opinion one way or the other." The results of the poll were: 58% said yes, 18% said no, 13% said maybe, and 11% said no opinion. The only thing we don't know is who was polled.
 * It's not as if the other polls in the article are any better, they were all done with a small, unrepresentative sample, in dubious circumstances. One of them is even a show of hands! Perhaps you're taking them to see seriously? I think they should be in the article, but more as interesting anecdotes.
 * XOR&#39;easter, I don't see where you're trying to get. Of course all of them would have considered, nuanced positions about the question of interpretations (well, except for Hawking), but I don't see any evidence that would indicate they would have answered anything other than (1) in the poll above. Mind you, if you're accusing someone of falsifying data, you need very good evidence indeed. It seems that Gell-Mann would have liked to answer something like "Yes MWI, but the other worlds are not necessarily real", but it is in the very nature of a poll to erase such subtleties. Tercer (talk) 09:27, 7 March 2020 (UTC)
 * Oh, all the polls are miserable in their own way. I'd never argue that the 2013 "snapshot" was representative, but at least we know where the survey was done and who was included (and the report did get through peer review). In the case of the Raub poll, knowing the question that was asked does hardly any good, because we don't know who it was asked of (or what the response rate was). I find that a serious problem. The "only thing we don't know" is exactly what we need to know. Who did this "political scientist" think were "leading quantum cosmologists"? How many quantum field theorists bothered to write back? Well, I also have my doubts that Feynman would have answered 1 instead of 3, based on what he said about MWI in "Simulating Physics with Computers" and the Lectures on Gravitation, which are much more in line with 3 than with 1. (I'm not even imagining that anything was fraudulent, really; it's easy to picture Feynman irritatedly scrawling something on a postcard and dropping it in the outgoing mail before moving on with his day. He could even have written something back that would sound like 3 to a physicist but read like 1 to Raub.) But mostly I'm concerned by our leading off the section by effectively promoting an unreliable book to provide a third-hand account of physicists' views, for the sake of, well, an anecdote. That seems to me like bad form. XOR&#39;easter (talk) 14:27, 7 March 2020 (UTC)
 * I think the information in that poll is both true and relevant for the article. Removing it makes the article worse. It is an unfortunate side effect that it gives more visibility to Tipler's book, but our main concern here should be making a better article, not erasing insanity from the internet. That said, if we can find this damn magazine, it could solve both problems: providing a reliable source and avoiding mentioning Tipler's book. Now, I searched a bit online, and it turns out that all major libraries in France have the archives of this magazine. I'll ask a couple of friends there to scan the article for me. Tercer (talk) 16:01, 7 March 2020 (UTC)
 * My friend delivered, I have the Sciences et Avenir (I can send it if anyone wants). It is rather disappointing, their source is Michael Price's FAQ, who got it from Tipler's book (the article is otherwise fine, it's just a simple popular science description of MW). Also, they are the ones that made the incorrect claim that the poll was performed in 1995, the error that showed up in Wikipedia and propagated to the whole internet. I think we should keep the poll, in any case. We have no reason to believe that Tipler (or David Raub) falsified data. On the contrary, their claims about the famous physicists are consistent with what else we know about them. In this article, for example, we see that both DeWitt and Martin Gardner agree that Gell-Mann, Weinberg, and Hawking are many-worlders. Tercer (talk) 19:03, 10 March 2020 (UTC)


 * I think the poll should be removed from the article. It’s clearly very messy, questionable, and misleading about a controversial matter. TheScotch (talk) 23:24, 4 June 2021 (UTC)

The poll is not misleading in the sense that the MWI was and is taken seriously by leading experts in the field. DeWitt was one obvious example. Hawking was another example, Weinberg for awhile, although he seems to have recanted at the end. Sean Carroll today, along with Brian Cox. And also (outside the field), Elon Musk and Peter Thiel. cheers, Michael C. Price talk 11:31, 10 March 2022 (UTC)


 * But there is no reference to the actual Poll. The paper that Tipler cites relating to the political scientists whom apparently takes the poll also does not seem to exist. Don't get me wrong I am on board with higher dimensional theories like the MWI but this fact cannot be substantiated and should be removed. Feynman launched campaigns against MWI (see Peter Byrne 2010) so seeing him turn at the last second seems inconsistent and possibly needs some more information. 115.64.94.152 (talk) 04:43, 8 April 2022 (UTC)
 * Apparently Raub's poll was never published-just checked with professor Tipler. Not sure how emails count for evidence though. 115.64.94.152 (talk) 14:20, 11 April 2022 (UTC)

Quantum Field Theory
Everett's original article states that the MWI (then known as RSI) covers quantum field theories, but there is no mention of this in the article. cheers, Michael C. Price talk 08:10, 8 December 2022 (UTC)

Thanks for showing, in the first paragraph, who coined the term "many worlds." .... However,
... however, it would be helpful if you could include in the first paragraph the name Everett gave to his theory. I read it somewhere and I'm looking for it again 2600:8801:BE31:D300:487F:9A92:9BAB:92D2 (talk) 17:55, 19 July 2022 (UTC)

That is included in the History section. It's not relevant enough for the lead, as hardly anyone uses that name. Tercer (talk) 06:19, 20 July 2022 (UTC)


 * Unfortunately the term "Many worlds interpretation" has multiple meanings. Everett's thesis is one complete version but it is named "relative states".  Deutsch "Fabric of Reality" version is way off this track. Johnjbarton (talk) 17:14, 9 September 2023 (UTC)

Incorrect citation
The article says under Properties:

"MWI removes the observer-dependent role in the quantum measurement process by replacing wavefunction collapse with quantum decoherence."

Giving the reference Zurek, Wojciech (March 2009). "Quantum Darwinism". Nature Physics. 5 (3): 181–188. arXiv:0903.5082. Bibcode:2009NatPh...5..181Z. doi:10.1038/nphys1202. S2CID 119205282.

But the Zurek article explicitly disavows any connection to Everett theory. In a paragraph citing Everett's PhD thesis Zurek says: "Indeed, “reality” or “existence” of universal state vector seems problematic." "And there is only one copy of the Universe. Treating its state as if it really existed [26, 27, 28] seems unwarranted and “classical”." Johnjbarton (talk) 17:21, 9 September 2023 (UTC)


 * I also think this sentence has cause and effect backwards. Quantum decoherence theory was developed by Zeh after reading Everett. So this sentence is an invention.
 * Since interpretations are not theories, their role is to spark new ideas which then become testable theories. Johnjbarton (talk) 17:39, 9 September 2023 (UTC)
 * Zeh read Everett after developing his ideas but before publishing; he came to the idea of a universal wavefunction independently . Perhaps we could tweak the article to say "Modern versions of MWI...". XOR&#39;easter (talk) 21:14, 9 September 2023 (UTC)
 * Thanks for the refs, very interesting.
 * I would prefer not to maintain the fiction that there is a MWI. That's how we get into these long-winded discussions about what MWI/Copenhagen/Duality/etc means. Rather I think we should report what the sources say. I guess you would agree that we shouldn't say Zurek's work is about MWI when it clearly is not. Similarly I think it synthetic to group Zeh/Zurek under MWI. MWI should be DeWitt's and I guess Deutsch or whomever avows it.  It's fine to say the DeWitt builds on Everett and that's how we come to discuss Everett here, but his work is "Relative state" and Zurek's is "Quantum Darwinism" (or several other possible titles, depending on the decade). Johnjbarton (talk) 21:40, 9 September 2023 (UTC)
 * The point is that modern Many-Worlds does incorporate decoherence. See e.g. the book by Wallace "The Emergent Multiverse". We do not present the ideas in chronological order here, or pretend that Many-Worlds started and ended with Everett's paper. Tercer (talk) 02:20, 10 September 2023 (UTC)
 * I'm not saying that the sentence would be removed; I didn't remove it because I think it is correct. However the reference is not correct.
 * If your claim is correct, then the reference in the sentence should be to Wallace, not Zurek.
 * (We should also not pretend that Many-Worlds started with Everett's paper because he never said that.) Johnjbarton (talk) 15:10, 10 September 2023 (UTC)
 * I agree that the reference should probably be replaced. A paper that is skeptical about the existence of a universal wavefunction isn't advancing the kind of interpretation that the text here implies it is. Nor is the Zurek paper a systematic review of what other people have proposed. XOR&#39;easter (talk) 17:32, 10 September 2023 (UTC)

Distortion of Everett's thesis?
The article cites Everett's PhD thesis when it says: "MWI was developed with the explicit goal of allowing quantum mechanics to be applied to the universe as a whole, making quantum cosmology possible."

Here is what Everett actually says about his motivation:

''Nevertheless, we have a strong desire to construct a single a11-embracing theory which would be applicable to the entire universe. From what stems this desire? The answer lies in the second type of prediction - the discovery of new phenomena - and involves the consideration of inductive inference and the factors which influence our confidence in a given theory (to be applicable outside of the field of its formulation).''

I changed the text to make this clearer. To be sure it's still not correct, as the sources on quantum cosmology point to multiple origins for the MWI-like ideas. Johnjbarton (talk) 22:27, 10 September 2023 (UTC)


 * The article says:
 * Everett noted that such entangled systems can be expressed as the sum of products of states, where the two or more subsystems are each in a state relative to each other.
 * But Everett never uses the word "entanglement".
 * Am I being pedantic? Shouldn't we report what people said or at least make it clear that we are reinterpreting things they said? Johnjbarton (talk) 22:39, 10 September 2023 (UTC)
 * Yes, you are being pedantic. Physics is already complicated enough without insisting on "original" terminology. There's no doubt that Everett was talking about entangled states. Tercer (talk) 07:34, 11 September 2023 (UTC)
 * If there is no doubt then we should easily be able to find a source to back that up. (I agree that is what he means, but IMO it is wrong to say the people said things they did not in fact said). Johnjbarton (talk) 14:30, 11 September 2023 (UTC)

Observers
(I want to be clear: my criticisms of this article are aimed at making it clearer, more correct, and more verifiable.)

In one part of the article we read "MWI removes the observer-dependent role in the quantum measurement process ..." Later we read a lot of discussion that contradicts this claim, eg "what the observer sees and the state of the object have become correlated by the act of measurement or observation. "

These both can't be true. I believe the first one is not correct. I think the sentence is motivated by some fringe interpretations of QM involving human thought processes. But that is not an orthodox interpretation and thus the meaning of "observer-dependent role" needs referencing. Johnjbarton (talk) 15:18, 11 September 2023 (UTC)

MWI is a realist, deterministic and local theory?
Realist? There is only one state for whole universe, how can the observer's mind not be involved? Deterministic? Only in the same sense that all QM is deterministic. As soon as we exam the results we discover randomness of outcomes. Local? The way we get determinism is by having a whole-universe state.

There is a ref in the next sentence: let's look!

"There is a consistent Lorentz covariant model of quantum phenomena which violates local causality but is local in Bell’s 1964 sense: the Everett picture."

and

"In our discussion of locality in the Everett interpretation we have sought to provide a constructive example illustrating precisely how a theory can be dynamically local, whilst violating local causality; "

So, is it local? Deterministic?

I assert that the given reference provides ample evidence that no simple sentence like the one in the article is sensible. Johnjbarton (talk) 02:26, 11 September 2023 (UTC)
 * What part exactly do you want to change? The quotes support the interpretation being local. MWI is deterministic in contrast to interpretations that propose one outcome is chosen randomly and the others don't happen. Having only one state for the whole universe makes it a realist interpretation. No minds involved. --mfb (talk) 02:35, 11 September 2023 (UTC)
 * I want to change the paragraph to fairly reflect the content of the source. That is clearly not the case now.
 * My preference would be to delete the paragraphs. These "properties" are not physics or science. They are not measurable or falsifiable. They are philosophical categories whose meaning and value have been debated for centuries. That debate continues as the source cited -- in a philosophy/history journal -- makes abundantly clear.
 * Any claims made beyond this source need references. I have some to contradict the paragraph that I can add as well. Johnjbarton (talk) 14:27, 11 September 2023 (UTC)
 * Just to give some examples of the challenges of these categories, Leggett describes "macroscopic realism" Leggett, Anthony J., and Anupam Garg. "Quantum mechanics versus macroscopic realism: Is the flux there when nobody looks?." Physical Review Letters 54.9 (1985): 857. And of  'nonlocal realistic' theories:  A J Leggett 2008 Rep. Prog. Phys. 71 022001. There are articles in philosophy journals like Lambare, J.P. "On the Meaning of Local Realism". Found Phys 52, 98 (2022). https://doi.org/10.1007/s10701-022-00618-1 and so on.  I can fill pages with sources for these kinds of issues.
 * So rather than attempt to fit MWI into categories that are themselves debatable, we would be better off starting with what MWI says directly or what sources say about MWI in regards to these characteristics. I also think this is more interesting for readers. Johnjbarton (talk) 23:18, 11 September 2023 (UTC)
 * There's zero controversy about Many-Worlds being realist and deterministic. About locality, one might need to distinguish between dynamical locality (about which there's no controversy) and state locality (about which there is controversy). Tercer (talk) 07:40, 11 September 2023 (UTC)
 * Great so it won't be difficult to add sources. Johnjbarton (talk) 14:28, 11 September 2023 (UTC)

Merge Universal wavefunction in to this article?
I just discovered the small page Universal wavefunction. Wouldn't it be better include here? Johnjbarton (talk) 00:08, 12 September 2023 (UTC)
 * I agree with this. Crossroads -talk- 01:02, 12 September 2023 (UTC) struck per below Crossroads -talk- 22:53, 12 September 2023 (UTC)
 * No merge. As the article says it received work from Hartle an Hawking, so is a concept that moved beyond MWI.  cheers, Michael C. Price talk 02:31, 12 September 2023 (UTC)
 * The claimed connection between Hartle&Hawking and Everett is not supported by the sources currently in the article. Please comment at Talk:Universal wavefunction. Jähmefyysikko (talk) 04:13, 12 September 2023 (UTC)
 * Support merge. Universal wavefunction is the central concept of MWI, so it's natural to discuss it here, especially since the current state of that page is not great.
 * Even if the Hawking-Hartle concept is found to be related, there is still some difference in the emphasis, since we are not necessarily interested in the whole universe in MWI. Hartle-Hawking state can be used to discuss the wavefunction of the universe. See a comment by User:Lumidek on Talk:Hartle–Hawking state which indicates the connection. Jähmefyysikko (talk) 06:27, 12 September 2023 (UTC)
 * Argument at Talk:Universal wavefunction convinced me that it should not be merged, since that wavefunction also features in quantum cosmology. Its not completely unrelated to MWI in that context either, but in many cases it might be too surprizing to click a link wavefunction of the universe and end up in MWI. Jähmefyysikko (talk) 20:32, 12 September 2023 (UTC)
 * I also disagree with the merge. The "universal wavefunction" was just an aspirational goal of Everett, and the following literature on Many-Worlds seldom deals with it. I think it's more useful to keep Universal wavefunction about actual proposals for a wavefunction of the universe. Tercer (talk) 14:16, 12 September 2023 (UTC)
 * Ok but that page needs work.
 * Johnjbarton (talk) 21:46, 12 September 2023 (UTC)

That cat again.
Part of the caption of the nice split-film image at the top of the article says:

"in different branches of the multiverse, both of which are equally real, but which do not interact with each other"

In the Tegmark reference also in the top of the article:

What Everett does NOT postulate: At certain magic instances, the the world undergoes some sort of metaphysical “split” into two branches that subsequently never interact.

One of these is not correct or complete.

I removed the last two phrases but @Tercer reverted that with: "obviously they'll interact if you interfere them. the lack of interaction is for the standard version of the thought experiment where there's no interference"

Why does the caption call out the idea that they do not interact? The only important thing for readers to know is the branching. That is what the interpretation requires. The claim "equally real" is dubious, disputable, philosophical add on. Some of us real people believe we live in the real world. I believe most readers will agree and no scientist can disagree since this is not science. "Branches never interact" contradicts Tegmark who calls out this issue prominently. If the caption must call out "never interact" then the details for why Tegmark's claim does not apply should be included. I don't think that is needed: just delete the non-information.

The image should convey the flavor of MWI not delve in to debatable details. Johnjbarton (talk) 15:14, 13 September 2023 (UTC)


 * "Equally real" is fundamental for Many-Worlds. If only one of the branches is real you have a single-world interpretation. The lack of interaction is also fundamental, otherwise you'd hardly have "worlds" or, as Deutsch puts it, "parallel universes". We should definitely include this information at the top of the article, we convey the essential part of the interpretation with it.
 * I haven't read Tegmark's paper, I don't know what is he talking about it, but both these issues are so fundamental for the interpretation that they are covered by several of the references the article already has. Don't make me waste my time looking for quotations. Tercer (talk) 15:46, 13 September 2023 (UTC)
 * "Don't make me waste my time looking for quotations." That's why we are here. I don't believe your claims of "equally real" because "reality" is a philosophical position, not physics.
 * If you don't have time to read the references, then let someone who does edit the article. Johnjbarton (talk) 15:54, 13 September 2023 (UTC)
 * From a footnote in the very lead of this article: each with a different result of measurement being recorded and observed, all coexisting equally. Maybe you should familiarize yourself with the subject before wasting everyone's time. Tercer (talk) 16:40, 13 September 2023 (UTC)

Claim of "objectively real"
The lead sentence says: The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, ... What is the reference for this claim?

It can't be Everett. In Appendix II of his PhD thesis, "REMARKS ON THE ROLE OF THEORETICAL PHYSICS", he outlines a perfectly reasonable stance that theories are just theories: "a physical theory is a logical construct (model),". He uses the word real as a synonym for valid: "It is therefore improper to attribute any less validity or "reality" to any element of a superposition than any other element...".

Both of the references in the section "Debate whether the other worlds are real" related to Everett are second hand accounts by an investigative journalist and a member of a rock band. Many non-scientists don't understand the difference between a theory that models reality and reality (what ever that even means).

I don't think this claim belongs in the lead. Johnjbarton (talk) 22:16, 12 September 2023 (UTC)


 * The link under "objectively real" points to Philosophical realism, which is to say philosophy. In every other way a reader would take this article as a physics article. An MWI article or section of this article clearly labeled as philosophy with references would be fine. But we should not mix up physics and philosophy without calling out the differences. Johnjbarton (talk) 22:30, 12 September 2023 (UTC)
 * The next sentence is:
 * This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe.
 * Tegmark, as far as I can tell, says no such thing.
 * I'm noticing a trend. Every reference I have checked is either off the mark or wildly incorrect :-( Johnjbarton (talk) 14:22, 13 September 2023 (UTC)
 * From the lead: "evolution of reality as a whole in MWI is rigidly deterministic and local."
 * Everett says the wave function changes deterministically. Reality is observed from inside and this perception is not deterministic.
 * The field equations of QM are local so MWI being local in any trivial sense of the word is not notable (Tegmark IIIB) Johnjbarton (talk) 14:40, 13 September 2023 (UTC)
 * John, you are editing with only be superficial knowledge of the subject matter and making howling errors. E.g. Everett does assert the reality of the wavefunction.  cheers, Michael C. Price talk 18:51, 13 September 2023 (UTC)
 * Wonderful! Show me. Here is the dissertation I read: https://www.pbs.org/wgbh/nova/manyworlds/pdf/dissertation.pdf Johnjbarton (talk) 18:53, 13 September 2023 (UTC)

Unhelpful edits
Lots of unhelpful editing from one editor, despite universal opposition on the talk page. Unless anyone else objects I shall revert all the changes in a few days cheers, Michael C. Price talk 20:06, 13 September 2023 (UTC)


 * I'm very puzzled by this reaction and the line of reasoning. I'm doing my best to read the reference cited in this article as well as others I've found. The edits have made are supported by references but the challenges are supported by 'unhelpful' or claims about uncited things happening decades ago. I have opened Talk topics on every issue where there were existing references and even some that were not referenced.
 * I'm sorry you don't agree with the references. I think you formed an opinion which you hold strongly but which is not based on evidence cited in the article. All you need to do is provide references to back up your claims. I did. Both points of view should remain and be fairly represented. Johnjbarton (talk) 21:10, 13 September 2023 (UTC)
 * If you are suggesting to revert the edits in this diff, then I do object. Most of the edits are good: there is good copyediting, small but important additions which make the meaning of the text more precise, sources added, and improvement on the structure of the page. Jähmefyysikko (talk) 21:11, 13 September 2023 (UTC)

Collapse was widely regarded as artificial
The article current says:

Wave function collapse was widely regarded as artificial and ad hoc,

However the Wimmel reference is a fringe QM theory, not in any way representative of any collection of physicists and it makes not claims to be such. It does argue against collapse, but that is not the summary we get in our article. Furthermore the book dismisses MWI as not a serious interpretation. Page 8.

This is in the section "Alternative to wavefunction collapse" which incorrectly describes Everett's approach. He never mentions Occam. Everett's reasoning concerns multiple observers (B observing A observing system) and concerns the consistency of applying a QM to micro and macro systems. His arguments are much better than slamming collapse as "artificial" (duh its a postulate after all). Everett didn't need to take a poll to remove it and neither to we. Johnjbarton (talk) 01:06, 14 September 2023 (UTC)

Leading with physics not philosophy.
As an article in the physics WikiProject I believe the subject should be introduced as physics, not philosophy.

I don't think we should start with

The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse.

The first phrase is debated and philosophy and the second phrase is obscure for non-expert readers.

We should start with physics:

''The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that "all isolated systems evolve according to the Schrodinger equation". Since the universe as a whole is isolated, this implies that the universal wavefunction also obeys the Schrodinger equation and that all possible outcomes of quantum measurements are physically realized in some "world" or universe so the wave function collapse postulate is not required.''

Even non-expert readers will know that the Schrodinger equation is a QM thing and be intrigued to learn more or turn away as choose. But they won't be thinking the article is philosophy.

I made this change but @Tercer did not like it and reverted it. Johnjbarton (talk) 15:49, 13 September 2023 (UTC)


 * I have no opinion yet about the "obscure for non-expert readers" part, but I am confused by the "physics, not philosophy". The subject of how to interpret quantum mechanics is necessarily a philosophical one. XOR&#39;easter (talk) 16:21, 13 September 2023 (UTC)
 * Yes, interpretation is not theory, but it is about physics. Everett's dissertation Appendix II discusses this as does Feynman in his QM intro chapter, similarly Schrodinger's "What is an elementary particle?". I'm not objecting to the lack of testability, but to the focus on "reality" and "determinism" with unqualified links to philosophy articles. These words have very interesting consequences in MWI and to sweep that all away with a link is bad for readers. The difference between MWI and other interpretations is in large part bound up in the way we interpret these words. The words and their meaning in MWI should be discussed with references, not summarily asserted. Johnjbarton (talk) 16:46, 13 September 2023 (UTC)
 * I wouldn't be surprised if terms like "reality" and "determinism" (and "locality", etc.) need more explanation. I'm still confused, though. What is wrong with linking a philosophical term to a philosophy article? XOR&#39;easter (talk) 16:56, 13 September 2023 (UTC)
 * The implication of the link is that the words linked have the meaning described in the article linked. The universal wavefunction is not required by all proponents of MWI to have "mind-independent existence"; see Wheeler 1977. Since all minds in MWI are within the universal wavefunction, the very meaning of "mind-independent existence" deserves discussion. The simple sentence with a link makes the reader believe that this is a closed and obvious issue, but rather this is a contentious unresolved "interpretation of an interpretation". There are PhD dissertations on this subject: Lehner, Christoph Albert. Quantum mechanics and reality: An interpretation of Everett's theory. Stanford University, 1997.  There are books with question marks in the the title: Saunders, Simon, et al., eds. Many worlds?: Everett, quantum theory, & reality. OUP Oxford, 2010.
 * This is not "the sky is blue", it is an unsourced assertion.
 * The issue with determinism is similar. Deterministic evolution of a universal wave function does not result in deterministic outcomes in our perception. That is a fascinating concept, not a one line link to a philosophy article.
 * These are subjects that should be covered in the article with adequate references not asserted and not in the lead. Johnjbarton (talk) 17:27, 13 September 2023 (UTC)
 * The book "Many Words?" is a collection of papers supporting and attacking the interpretation. As far as I remember none of the detractors, and certainly none of the supporters, takes issue with determinism and realism.
 * Again, I urge to actually read some references before editing this article. Arguing based on a book's title is frankly ridiculous. Tercer (talk) 18:40, 13 September 2023 (UTC)
 * Sorry, I'm still catching up. What's the issue with the question mark being in the title? As noted just above, it's a collection of essays pro and con. XOR&#39;easter (talk) 16:35, 14 September 2023 (UTC)
 * Perhaps a more palatable, less contentious approach is to first develop the content then reconsider the lead as a summary of it. Johnjbarton (talk) 18:25, 13 September 2023 (UTC)

Wheeler's view of MWI in later years.
I remove this line once and plan to do so again:

''Everett had already left academia in 1956, never to return, and after his death, Wheeler disavowed the theory. ''

The sentence references Osnaghi, Stefano; Freitas, Fabio; Olival Freire, Jr (2009). "The Origin of the Everettian Heresy". Studies in History and Philosophy of Modern Physics. 40 (2): 97–123. Bibcode:2009SHPMP..40...97O. CiteSeerX 10.1.1.397.3933. doi:10.1016/j.shpsb.2008.10.002.

But when you read the reference you see that Wheeler's disavowal was only claimed by Dewitt. And as I quoted in my first removal, the same source in the same paragraph says "As is apparent from this passage, Wheeler’s attitude towards Everett’s work was not as clear-cut as described by DeWitt".

Thus this contentious disavowal is incorrectly reported in the article.

We could go in to all of this, but to me its a silly He said/She said. Wheeler published top physics articles long after Everett's death so he had plenty of time to publish a disavowal.

And Wheeler does some serious disavowing related to MWI. Specifically writes against the universal wavefunction thing as part of his critique of the notion of determinism. I will include that work in the article. In the meantime the incorrect information should be removed. Johnjbarton (talk) 23:32, 11 September 2023 (UTC)


 * In Search of Schrödinger's Cat John Gribbin, page 246
 * Perhaps it is only fair, at this point, to mention that Wheeler himself has recently expressed doubts about the whole business. in response to a questioner at a symposium held to mark the centenary of Einstein's birth, [Wheeler] said of the MWI, "I confess that I have reluctantly had to give up my support of that POV in the end, much as I advocated it in the beginning - because I'm afraid it carries too great a load of metaphysical baggage."
 * cheers, Michael C. Price talk 02:38, 12 September 2023 (UTC)
 * Here's the original source of the statement. It is a discussion with Wheeler, Wigner, Dirac and others in a symposium in Einstein's honor. Jähmefyysikko (talk) 06:06, 12 September 2023 (UTC)
 * See also this from 1976. There was also something relevant (about Wheeler putting the "relative state interpretation" at arms' length while Everett was still alive) in Byrne's biography of Everett ( and some other passages). XOR&#39;easter (talk) 14:56, 12 September 2023 (UTC)
 * Thanks, I will make some edits. (I have to say I'm unsure why Wheeler's "opinion" as opposed to his published work is so notable.) Johnjbarton (talk) 16:00, 12 September 2023 (UTC)
 * Thanks! Sadly, that one comment at the end of a session of questions for Wheeler, is by far the least interesting thing he said that day.
 * L. Cooper (Brown U.): I had not meant to bring this up, but I think that if you take the Everett-Wheeler interpretation of the quantum theory seriously, you do not really have to violate causality in your shutter experiment. I admit I have not had a chance to analyze this particular ex- periment completely, but in other experiments of the same type, with your own interpretation, I do not believe you have to violate causality. Do you have any comment on that?
 * Wheeler: The interpretation of quantum mechanics that Hugh Everett gave us some years ago and that Leon Cooper himself elucidated considerably more is one in which branches, if I can put it that way, in the act of observation are simultaneously present. I confess that I have reluctantly had to give up my support of that point of view in the end- much as I advocated it in the begin- ning because I am afraid it creates too great a load of metaphysical baggage to carry along. But - they say nobody knows sin like a sinner.
 * So what does Wheeler say? That he gave up support for "branches ... in the act of observation are simultaneously present"? That would be my interpretation based on . See the quote in Universal wavefunction. Johnjbarton (talk) 15:56, 12 September 2023 (UTC)
 * Ok I added "Observers and Wheeler's perspective".
 * I think Wheeler's role in MWI is sufficiently notable to warrant a section. And it also gives a way to discuss the observer aspect that Wheeler focused on.
 * I don't think my text is awesome however. Please review. Johnjbarton (talk) 17:45, 12 September 2023 (UTC)
 * It looks like one of the references is broken. I will try to have further comments later. XOR&#39;easter (talk) 21:56, 12 September 2023 (UTC)
 * Fixed ref, thanks. Johnjbarton (talk) 22:18, 12 September 2023 (UTC)
 * This unreferenced, extreme description of Wheeler's viewpoint is back. I have been advised that I should not edit until a consensus on the topic is reached. Johnjbarton (talk) 15:49, 14 September 2023 (UTC)
 * I don't know why the text is stressing what Wheeler said after Everett's death when Wheeler began distancing in the 1970s. XOR&#39;easter (talk) 17:23, 14 September 2023 (UTC)

According to Peter Byrne
Our article says:

According to Peter Byrne, Everett believed in the literal reality of the other quantum worlds.

And yet in Barrett, Jeffrey A. and Byrne, Peter. The Everett Interpretation of Quantum Mechanics: Collected Works 1955-1980 with Commentary, Princeton: Princeton University Press, 2012. https://doi-org.wikipedialibrary.idm.oclc.org/10.1515/9781400842742

Peter Byrne writes in the Biographical Introduction:

In the second appendix of his long thesis, Everett points out that since his theory could not make predictions differentiating it from the standard collapse, hidden variables, or stochastic theories, it was largely a matter of taste how one interprets the quantum mechanical equations.28 For himself, he was convinced of its empirical correctness (as far as explaining the subjective experience of quantum measurement), and he did not believe that any model was capable of fully capturing “reality.”

Just after that sentence Byrne has the same quote from Everett's dissertation as I included in the article.

So either the reference in the article is inconsistent with other things Byrne said or the sentence summarizing Byrne's book is incorrect. Johnjbarton (talk) 23:34, 13 September 2023 (UTC)


 * In the same volume of collected works edited by Byrne there is a letter from Everett to DeWitt, pg 253, with:
 * When one is using a theory, one naturally pretends that the constructs of the theory are “real” or “exist.” If the theory is highly successful (i.e. correctly predicts the sense perceptions of the user of the theory) then the confidence in the theory is built up and its constructs tend to be identified with “elements of the real physical world.” This is, however, a purely psychological matter. No mental constructs (and this goes for everyday, prescientific conceptions about the nature of things, objects, etc., as well as elements of formal theories) should ever be regarded as more “real” than any others. We simply have more confidence in some than others.
 * In my opinion it is beyond doubt that the line "Everett believed in the literal reality of the other quantum worlds" is incorrect based the sources available at this time. Should other sources arrive, the doubt would remain but we could include both views. Johnjbarton (talk) 23:48, 13 September 2023 (UTC)
 * The passage from Everett no where refers to the reality or non-reality of the wavefunction. Similarly with the "heresy" point.  You are pushing your own original research, which is explicitly contradicted by other sources.  cheers, Michael C. Price talk 08:55, 14 September 2023 (UTC)
 * Quoting the beginning of section 3 of Everett's Rev. Mod. Phys. paper: This paper...postulates that a wave function that obeys a linear wave equation everywhere and at all times supplies a complete mathematical model for every isolated physical system without exception. It further postulates that every system that is subject to external observation can be regarded as part of a larger isolated system. The wave function is taken as the basic physical entity with no a priori interpretation (emphasis in original). In other words, every isolated system has a wavefunction; the wavefunction is what is (the basic physical entity). And the previous year, he wrote, The physical ‘reality’ is assumed to be the wave function of the whole universe itself. By properly interpreting the internal correlations in this wave function it is possible to explain the appearance of the macroscopic world to us, as well as the apparent probabilistic aspects. (Quoted on p. 140 of the Byrne biography.) On p. 144, Byrne writes, Everett repeatedly wrote that the branches are “equally ‘real.’ ” On the reasonable premise that he considered at least one branch to be “real,” then, for him, all are “real.” On p. 172, Everett is quoted as writing, In theory the universal state function is the realized fact. In superposition after measurement all elements actually realized (comment on a letter from Norbert Wiener). There's more in other places. Any hedging on Everett's own part seems to be more about acknowledging that quantum mechanics might ultimately be wrong. XOR&#39;easter (talk) 16:24, 14 September 2023 (UTC)
 * Thanks for your efforts. Any thoughts on why Everett wrote his Appendix II? Any thoughts on why S. Osnaghi et al. review would devote a section (5.1) to a discussion of Everett's use of quotes around "real"?
 * Everett was a physicist and a sophisticated thinker. At the minimum don't you think the strong assertion about his belief should be qualified? Johnjbarton (talk) 17:06, 14 September 2023 (UTC)
 * It's not really my place to speculate why anyone wrote a particular block of text (not here, anyway). The whole "Debate whether the other worlds are real" section seems a little unclear on what is actually being debated. XOR&#39;easter (talk) 17:30, 14 September 2023 (UTC)
 * I'm okay with the entire "Debate" section being removed - it reflects a debate from before the release of biographical material cleared up the matter of Everett's beliefs. The debate should probably in the Hugh Everett page, anyway.  cheers, Michael C. Price talk 18:20, 14 September 2023 (UTC)

Two sections for deletion
The "Debate" section on the reality of other worlds, and "Observers and Wheeler's perspective" don't add to the article. I propose excision. cheers, Michael C. Price talk 22:39, 14 September 2023 (UTC)


 * My opinion: Yes on Debate. No on Wheeler.
 * Wheeler had a large impact on MWI, certainty notable in this context, almost as important as DeWitt.
 * The relationship between MWI and observation is clearly important. It was Everett's first argument; it dominated Wheeler's Assessment; it is what makes MWI more interesting than decoherence.
 * Mixing these originally was me being lazy. Wheeler's contributions where directly around observation so coming up with separate content did not come to me. Johnjbarton (talk) 02:54, 15 September 2023 (UTC)
 * Wheeler, as Everett's thesis supervisor, had an impact (mostly negative, but some positives) on the presentation of MWI in its early days, but his views on collapse evolved into his 'participatory universe' view and the 'delayed eraser' experiment - nothing to do with MWI. cheers, Michael C. Price talk 05:38, 15 September 2023 (UTC)
 * As Everett's thesis advisor Wheeler allow Everett's ideas to be known to the world. Wheeler did not need to take Everett in to his group (Everett didn't have a physics background and his interests did not match Wheeler's work). Wheeler did not need to spend endless hours helping Everett revise his thesis and to prepare it for publication. Wheeler did not need to put his reputation on the line by writing his Assessment article for publication in the same issue of Reviews of Modern Physics as Everett's paper (which might not have even been accepted without Wheeler). Without Wheeler's active involvement we would not be discussing Everett at all. His work would be some notes in a basement.
 * Wheeler's other work, which was primarily on gravitation and black holes but also included some other areas in QM, does not detract from or matter as far as his role in MWI goes.
 * However, our personal opinions of Wheeler are not so important. How can we improve the article?
 * My proposal is to rename the section to say "Wheeler's perspective", add a little more content to flesh it out, and to move it to between the "relative state" and "renamed to many worlds" sections. What do you think? Johnjbarton (talk) 15:00, 15 September 2023 (UTC)
 * This not an article about Wheeler. It is about Many Worlds.  Who cares what Wheeler thought about things.  Move it to Wheeler's pages.   cheers, Michael C. Price talk 15:43, 15 September 2023 (UTC)
 * Wheeler published on the topic. The Heresy history paper mentions him over 250 times. DeWitt of Many Worlds only rank 80 mentions. Wheeler absolutely belongs here.
 * I cleaned up the Wheeler section. If we only want to include Wheeler's published scientific contributions I'm ok with that. I included the change-of-heart reference only to clarify a previous version. Johnjbarton (talk) 16:11, 15 September 2023 (UTC)
 * Yes to both. The "debate" section is a ridiculous he-said-she-said, devoid of any scientific content. The section on Wheeler is rather personalist. Who cares what Wheeler thought? The article is not about him, it's about Many-Worlds, and the relevance of his work for Many-Worlds is essentially zero. Tercer (talk) 17:14, 15 September 2023 (UTC)
 * Agreed. Wheeler's only relevance is as Everett's thesis advisor, and that is covered elsewhere.  cheers, Michael C. Price talk 18:14, 15 September 2023 (UTC)
 * Wouldn't you say then that Everett's work is also irrelevant? Wheeler published more on the topic than Wheeler did. He talked to more people about the idea than Everett did. In some cases the theory was called Everett-Wheeler.
 * Based on actual sources rather than personal opinion I can't understand where this idea about Wheeler comes from. You claim of his only relevance is completely opposite of the comprehensive history cited in the article, Osnaghi, Stefano; Freitas, Fabio; Olival Freire, Jr (2009). "The Origin of the Everettian Heresy". Studies in History and Philosophy of Modern Physics. 40 (2): 97–123.
 * If the article is about DeWitt's work, then let's split out a new article on Relative States and start here with DeWitt. Johnjbarton (talk) 18:21, 15 September 2023 (UTC)
 * The "heresy" talks about Wheeler's (mostly destructive) role as Everett thesis advisor, and that is already covered in the article.
 * There is no substantive difference between Everett's Relative state formulation and DeWitt's many worlds popularisation. cheers, Michael C. Price talk 18:33, 15 September 2023 (UTC)
 * This article presents Many-Worlds as it is understood today, more than 6 decades after Everett published his work. And the current sources almost never cite Wheeler's work or mention Wheeler's ideas. He is historically relevant as Everett's supervisor, and that's it.
 * Of course the Osnaghi et al. paper mentions Wheeler all the time, that paper is about the history of the interpretation. This implies we should mention Wheeler in the history section, as we indeed do. Tercer (talk) 19:10, 15 September 2023 (UTC)
 * Thanks, this argument makes sense to me.
 * What throws me off is the Overview that has Formulation (Everett), Relative state (Wheeler's name and which Wheeler was heavily involved in) and Renamed many-worlds (Dewitt). So a major portion of the Overview is history that simply omits one of the three key players. Johnjbarton (talk) 19:37, 15 September 2023 (UTC)
 * Yeah, that's a dumpster fire. It belongs in the history section, not in an overview section. "Relative states" is of only historical interest, nobody uses this terminology anymore. It's also hardly relevant who came up with the name "many worlds" and when, the only relevant thing is what the sources call the interpretation nowadays: either "Many-Worlds interpretation" or "Everett interpretation". Tercer (talk) 19:47, 15 September 2023 (UTC)
 * Yes, I'm pretty sure I've only seen "relative state interpretation" used in a historical sense to refer to Everett's original proposal. XOR&#39;easter (talk) 17:04, 16 September 2023 (UTC)
 * So do you think we could consolidate the historical content from Overview into History and refocus the overview on what MWI is? Johnjbarton (talk) 18:21, 16 September 2023 (UTC)
 * Indeed, and I wish I had done that myself. When I worked on this article a couple of years ago it had more serious problems and eventually I ran out of energy. Tercer (talk) 21:07, 16 September 2023 (UTC)

I have removed the two sections, saving just a paragraph which I added to the history section. The article still meanders a bit, back and forth, but at least it is a bit less cluttered now. cheers, Michael C. Price talk 18:39, 15 September 2023 (UTC)

Worked out example
From article:
 * In particular, it [MWI] models a measurement as a unitary transformation, a correlation-inducing interaction, between observer and object, without using a collapse postulate, and models observers as ordinary quantum-mechanical systems.

Is that a metaphysical description or does it describe a mathematical theory? And if it is mathematical, is there a worked out example? I.e. one describing an actual unitary transformation M corresponding to a measurement. It could be something simple like a superposition |0>+|1> being observed yielding 0 or 1, like in the Schroedinger's Cat experiment. How does M make sure that two later observers see the same state of the cat? It would be nice if there was an explanation that the article could include or point to. Thanks. 2601:644:8501:AAF0:0:0:0:4838 (talk) 00:08, 17 September 2023 (UTC)


 * The first part of you sentence, "models a measurement as a unitary transformation, a correlation-inducing interaction, between observer and object, without using a collapse postulate" is not unique to MWI. The whole body of work called quantum decoherence is just like that. Your worked example was done in 1929 by Nevill Mott, see Mott problem.
 * The second part "models observers as ordinary quantum-mechanical systems." is meta-physics if you want "observers" as humans or redundant with the first part of the sentence if "observers" can be excitations of atomic systems that subsequently are amplified. So No or Yes, depending on your choice ;-)
 * I gather from recent posts on the form of this article that MWI insists on humans in the loop so to speak, so then No example can be created. See Testability in the article. Johnjbarton (talk) 00:18, 17 September 2023 (UTC)