# Talk:Quantum mechanics

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 Definition:  An important branch of physics dealing with the behavior of matter and energy at very small scales. [d] [e]
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 Workgroup categories:  Physics and Mathematics [Categories OK] Talk Archive:  none English language variant:  American English

I categorized this as status "Developing" because, although there have been a couple of major revisions or additions (the "Effects" and "History" sections), the vast majority of the text is still a near-verbatim copy of the Wikipedia article. Bruce M.Tindall 16:25, 5 April 2007 (CDT)

## Math?

There is a question how do we manage quantum mechanics articles, i.e. whether we put it under Math Workgroup or not. User:Jitse Niesen has just removed the math category. Consider however this post by User:Greg Woodhouse on forum.[1] I'm leaning to Greg's point of view. See also AMS classification on CZ:Mathematics Workgroup, quantum theory is a notable node. Any thoughts? --AlekStos 09:25, 16 April 2007 (CDT)

There are formulations of quantum mechanics that are most definitely on the border between physics and mathematics. The tradition starts with Weyl's book on Group Theory and Quantum Mechanics (1928), then von Neumann's famous book (1932) that contained lots of new math, and more recently Reed and Simon (1972), and Thirring (1979). Beyond my horizon is string theory which is almost only mathematics, although as I understand it, string theory is more than quantum theory (I'm told that it includes general relativity). I would say that quantum mechanics has enough mathematical content to put it in the math category as well. Further, Jitse will appreciate that people who apply quantum mechanics are heavy users of numerical mathematics, so also from that angle there is contact.
Another thing: who is mainly responsible for this article? Is it a CZ author, or is it straight from WP? In other words, if I would change things, on whose toes would I step? --Paul Wormer 09:18, 13 September 2007 (CDT)
Okay, I put maths back in.
The article is basically straight from WP. If you look at the history, you see that the first version has all the signs of a Wikipedia article (for instance, interwiki links to different languages) and that the difference between the version copied from Wikipedia and the current version is fairly small. I think nobody here will complain if you start afresh. -- Jitse Niesen 21:54, 13 September 2007 (CDT)

## Mention on BBC blog

BBC journalist Rory Cellan-Jones admits he knows nothing about quantum mechanics, but it might be useful to know that as a layperson he finds the Wikipedia version of this article easier to read. This is another article identifying CZ's number of articles as a problem. John Stephenson 02:47, 1 April 2008 (CDT)

Next sections moved here from Talk:Introduction to quantum mechanics.

## Proposal

I propose that people that are knowledgeable about quantum mechanics list here topics that should be included in an article on quantum mechanics for the layperson. (Maybe that would be a better title of this article?). I hope that skilled (possibly, or even preferably, non-science) writers pick this up and make a readable, consistent, article out of it. If they slip or misunderstand something don't worry, that has happened to every learner of QM. --Paul Wormer 12:19, 1 April 2008 (CDT)

Recall we have a Student Level subpage that has not been utilized much yet. Such an article would live at Quantum mechanics/Student Level. One problem is I'm not sure we have really defined the role of this subpage, although I would think aiming it at the layperson should be the target. Chris Day 12:31, 1 April 2008 (CDT)
I was not thinking of students, but of journalists and of other interested adults that want to widen their horizon, but it could be useful to (non-physics) students as well. --Paul Wormer 12:38, 1 April 2008 (CDT)
Maybe we need to rename that subpage. It might be used more if it was more of an outreach article rather than aimed at students. After all what does student mean, it could refer to many different levels of expertise. Chris Day 12:43, 1 April 2008 (CDT)
Yes, I agree, "student" reminds one of college and exams. I was thinking of general education (more along the the ideals of Enlightenment). --Paul Wormer 12:51, 1 April 2008 (CDT)
Maybe it should be called something like, "Introductory" or "Prequisite". "Fundamentals"? Just tossing out ideas here. --Robert W King 12:54, 1 April 2008 (CDT)
"Introductory" is OK, "prequisite" smacks too much of school. --Paul Wormer 13:02, 1 April 2008 (CDT)

I'm going to disagree with you all and claim that since we are an encyclopaedia aimed at a general readership, Quantum mechanics itself should be the introductory article, and more specialist material should be elsewhere. J. Noel Chiappa 12:58, 1 April 2008 (CDT)

I disagree strongly, there is space for all of us under the sun, graduate level articles can be very useful, not only for grad students but also for undergrads, if only to show them that there is more to know. --Paul Wormer 13:02, 1 April 2008 (CDT)
I didn't say we shouldn't have a grad-level article ("more specialist material should be elsewhere"). J. Noel Chiappa 13:07, 1 April 2008 (CDT)
See the forum discussion here. I made a similar point. Chris Day 13:16, 1 April 2008 (CDT)
Perhaps we're doing it backwards? Maybe we need a Quantum mechanics and an Advanced Quantum mechanics? --Robert W King 12:59, 1 April 2008 (CDT)

Well, please consider that there are no other "introduction to" type articles in CZ, as far as I know, at least as main namespace articles. Also consider that CZ articles are supposed to be as accessible as a competent article on a topic can be made to our audience, i.e., university undergraduates. Therefore, our basic rules dictate that the article that lives at quantum mechanics should be an introductory-type article, if such an article is possible (I have no opinion on that).

Variant versions of articles, and other reference material, are supposed to be grouped together in clusters; that is the whole purpose of the Subpages system. Now, it's true that we have a tutorial type article planned for subpages; but an introductory article would not, I assume, be a "tutorial," it would be an introductory article. Moreover, it is unacceptable to me as Editor-in-Chief that we make an advanced article as the main article on this one topic, if a simpler, more undergrad-accessible article is possible.

Given all this, the solution seems simple to me: we invent a new subpage type: "Advanced" pages. These give an advanced or graduate-level (or postgrad-level) treatment of particularly complex topics. Then, if I understand correctly, the current quantum mechanics article would live at Quantum mechanics/Advanced, and the planned (but misnamed) "Introduction to quantum mechanics" article would live at quantum mechanics. Then, if anyone wants to introduce QM with problems, "tutorial"-style, that could live at Quantum mechanics/Tutorial. The latter is not necessary, of course. --Larry Sanger 13:14, 1 April 2008 (CDT)

See forums for similar point. Chris Day 13:19, 1 April 2008 (CDT)
Let us follow our EiC and rename "Introduction to quantum mechanics" to "Quantum mechanics" and the present "Quantum mechanics" to "Quantum mechanics/Advanced". Does one have to be constable to do that? --Paul Wormer 13:32, 1 April 2008 (CDT)

## Content

Here we go again, talk, talk, talk. You guys, give rather your opinion about CONTENT --Paul Wormer 13:05, 1 April 2008 (CDT)

The problem is we have not solved the audience and home problem. This is a recurring discussion that has flared up again. What you are doing is right. Start and the edits will follow. I can't start as I'm clueless on the topic. I can tell you if you have it at the right level though. Chris Day 13:21, 1 April 2008 (CDT)

## Historical approach?

An approach could be historical (I write this from memory, I'm without books):

1. Around 1890 physics is "finished", minor detail: UV catastrophe.
2. 1900 Planck solves UV catastrophe by quantizing the energy of harmonic oscillator (Insert here: explanation harmonic oscillator and its use in physical models).
3. 1905 Einstein proposes that light consists of virtual quantized oscillators to explain photoelectric effect
4. Around 1910 quantization lattice vibrations (Einstein Debye crystal)
5. 1913 Bohr atom
6. 1915 Old Bohr-Sommerfeld quantum theory
7. 1924 de Broglie: (very very light) particle shows wave properties (insert about waves)
8. 1925 Heisenberg, Born, Jordan: matrix mechanics (insert about matrices)
9. 1926 Schroedinger wave equations (time-independent, time-dependent), relation with matrix mechanics
10. 1926 Spin
11. 1927 Pauli exclusion principle, Heisenberg uncertainty relation. Non-relativistic QM essentially finished at this point in time.

--Paul Wormer 12:38, 1 April 2008 (CDT)

Sounds like a topic for another article to me: history of theory of quantum mechanics or something like that. --Larry Sanger 13:03, 1 April 2008 (CDT)
Could be used in a timeline! --Robert W King 13:04, 1 April 2008 (CDT)
If we had a lot of room and/or reader's time, I might agree with you. I love to read about science in a chronological fashion, because the growth in the minds of scientists, in understanding of the field, over time, is matched nicely (and produces) an equally growing understanding in my own mind.
However, we don't have that luxury - although I think such a time-oriented piece would be wonderful, perhaps at Development of quantum mechanics, or some such location.
I will go off and try to create the text of the intro paragraphs, and you can see if my goal of 'quantum mechanics for the masses in 3 minutes' is reachable. (I'm something of a believer in Vonnegut's aphorism in Cat's Cradle - although I think he's a bit too hard, because some people have a hard time explaining in words things they understand in non-verbal ways.) J. Noel Chiappa 13:06, 1 April 2008 (CDT)

## A different approach

I accept that, how about starting with an explanation of wave and particle behavior in classical physics and the shock it gave when it appeared that electromagnetic waves and very light particles (electrons) share many properties? --Paul Wormer 13:11, 1 April 2008 (CDT)

That's part of what I'm doing. Give me an hour or so... J. Noel Chiappa 13:18, 1 April 2008 (CDT)
Noel, it is of course OK that you write a first draft, but you are a scientist and I wanted to get non-scientists involved (but probably that is hopeless, anyway). I thought that if we could trigger a non-scientist to write an article that (i) (s)he him/herself understood and that (ii) was acceptable to cognoscenti of QM, we would have a piece that could serve as one of CZ's flagships. I cannot judge whether your work will be readable by people without knowledge of QM, only people that lack the knowledge can. But how do we get those people to read and comment on your work? Many people like writing better than reading, so that's why I thought let them write it. --Paul Wormer 13:52, 1 April 2008 (CDT)
Trust me, I'm writing for the high-school art student! (I have a daughter in that category! :-) Maybe I'm in the best 'seat' to write this, as someone who's in the middle, between those two worlds (non-scientist, and physicist). I would certainly love feedback from non-scientists as to how good a job I've done (almost ready to go with the first chunk, which it the intro and "principal discoveries"), of course. J. Noel Chiappa 13:59, 1 April 2008 (CDT)

## OK, how does this look

OK, I threw up a chunk of text for all to look at. This is of course not complete, but I felt I had enough text there to give a good sense of how, and the level, I thought we ought to come at this topic at. If y'all like it, I can crank our as much more as we think the article ought to cover. J. Noel Chiappa 14:11, 1 April 2008 (CDT)

After a few minor changes, and one a bit larger, I agree with Noel's chunk of text. Now the million dollar question is:
How is this perceived by somebody without prior knowledge of QM?
--Paul Wormer 17:11, 1 April 2008 (CDT)
Let me get my daughter, she just came home from school! :-) J. Noel Chiappa 17:20, 1 April 2008 (CDT)
Both of my kids (both high-school) felt it to be fairly comprehendable. They of course had grammer/etc suggestions, and in two places my daughter had issues: one was the "many of the principles that appear to hold at the large scale at which we experience physical reality are not fundamental, i.e., they do not exist as basic attributes of reality" (which I'm going to have to rework), and the other was the Bell's Theorem stuff (where she objected, to some degree correctly, that it was too hardline a statement of impossibility). I have to go to dinner now, we'll work on it in an hour or so to i) fix issues in the existing text, and ii) add some more content (I'm going to start with a short history of the discovery of QM). J. Noel Chiappa 17:48, 1 April 2008 (CDT)

## Born's interpretation

Hi Noel, I see that you have given a fair amount of attention to EPR + Bell. As you probably know that is a fairly recent development, at least, the interest in this non-local character of QM is fairly recent (say, since Aspect's experiment). Before the 1980s much more emphasis was given to the probability of the outcome of measurements as expressed by Max Born's rule (now seen as postulate):

${\displaystyle {\mathcal {P}}(\alpha _{n})=|\langle \Psi |\phi _{n}\rangle |^{2}\quad {\hbox{with}}\quad \langle \Psi |\Psi \rangle =\langle \phi _{n}|\phi _{n}\rangle =1\quad {\hbox{and}}\quad A\phi _{n}=\alpha _{n}\phi _{n}.}$

Here ${\displaystyle \scriptstyle {\mathcal {P}}(\alpha _{n})}$ is the probability of measuring the eigenvalue αn of observable A of a system in state Ψ. As you know only eigenvalues of observables can be outcomes of measurements.

This Born postulate was the main problem that Einstein had with QM (before 1935 when he discovered the EPR paradox) and was the content of the famous Bohr-Einstein discussions. Einstein supposingly said: God does not throw dice. About this famous citation I added some time ago the following note to the WP article http://en.wikipedia.org/wiki/Introduction_to_quantum_mechanics

1. Letter from A. Einstein to M. Born dated December 12, 1926 [Max Born, Physics in my generation, Springer-Verlag, New York (1969), p. 113]. Because the quotation is often fancifully distorted, we give the two original German sentences: Die Theorie liefert viel, aber dem Geheimnis des Alten bringt sie uns kaum näher. Jedenfalls bin ich überzeugt, daß der nicht würfelt. (Translation by M. Born: The theory yields much, but it hardly brings us nearer to the secret of the Old One. In any case I am convinced that he does not throw dice.)

Anyway, this long story is to explain why I added Max Born to the history. I understand that you don't want to include the mathematical formulation of the Born postulate, but a little history about it and Einstein's citation wouldn't be out of order, don't you think?--Paul Wormer 12:18, 3 April 2008 (CDT)

To start with, I know nothing at all of the math of QM, so that's all complete Greek to me, believe it or not! (Although maybe that's actually a plus - since I have developed my understanding in completely non-mathematical terms, I expect it makes it easy for me to explain in non-mathematical terms.)
So, I'm a little lost when you speak of "the probability of measuring the eigenvalue αn of observable A of a system in state Ψ .. only eigenvalues of observables can be outcomes of measurements" (although I vaguely get what's being talked about there) - I'm afraid you'll have to 'translate' that into the sort of language used in the article for me to be able to really comprehend it!
As to EPR/Bell, in part I added text on that because it's one of the wierder (and currently more fashionable, I concede :-) areas of QM - but I think the fact that it forces us to choose between 'Reality' (i.e. attributes have values before they are measured) and 'Locality' (i.e. no instantaneous action-at-distance) is certainly a major blow to our (and Einstein's :-) world-view, and therefore makes it rather important. (Later comment: I've just been re-reading Mermin's paper, and I think I got that wrong; I think 'Reality' goes no matter what - no hidden instruction sets - and so you have to have non-Locality. But let me re-read it again tomorrow morning when my brain is fresh! And consider the same comment repeated below, everywhere I talk about chosing between locality and reality.)
Speaking of the Herr Professor Doktor, I have read (in the Mermin article I ref'd) material which indicates that his real problem with QM was not so much the randomness part of it (as implied by the famous "God/dice" quotation), which I think he came to sort of accept, as stuff like the classic Copehagen interpretation 'an attibute actually has no value before it's measured' (and I think Mermin actually dug up a quote from the Born/Einstein letters to substantiate that; I'll try and find it). Leading from that, of course, one gets to the Reality/Locality choice he (correctly, and perhaps before anyone else did) saw QM as forcing.
Hence the EPR paper, which basically (as I understand it indirectly) boils down to:
i) QM means you have to chose between Reality and Locality
ii) Reality and Locality clearly both exist
iii) Ergo something's wrong with QM
Only, of course, as someone put it, 'the joke was on him'! i) was right, ii) wasn't! Like many others, I don't doubt he would have been stunned to hear of Bell's work and the experiments that resulted from it - and like everyone else, I expect he'd have gone off to think long and hard - and like everyone else, I'd love to know what he would have said when he came back!
Speaking also of the Mermin article, it is absolutely first-rate, BTW - I very highly suggest you read it, if you can't get ahold of it, let me know, and I'll tell you where I have some private scans of it online (since it's still under copyright, although I might try and email them and see if they'll make it publicly available).
I also put in a lot of text about the Cat, because to me, I think in the last couple of decades we have gotten a much better handle on the EPR/Bell part of QM, so now the whole 'collapse of the wave function' part is (to me, and as I said in the article) the biggest remaining area of confusion.
Yes, perhaps in terms of what it enables us to do (technology-wise) it's not so important, but the importance of QM (as I indicated in the article) is not just in what technical tools it gives us, but in what it tells us about the nature of reality (which is why I give a lot of space to Bell/EPR, even though the technolgy output is, so far, minimal - although who knows where we'll be in 40 years - I doubt in the 1930s anyone could have visualized the large-scale-integration semiconductors we saw start in the 1970s).
And perhaps there will be nothing behind that particular curtain anyway, and we've just been applying the theory incorrectly, and the Cat's wave-function decays very quickly on its own anyway, or something.
Anyway, let me know more about what your Born point is, and I will certainly consider it. (I hope my comments here have given you a better feeling for why I have covered the ground that I have in the article.) J. Noel Chiappa 18:06, 3 April 2008 (CDT)
Found that quote about Einstein's real problem with QM, BTW:
"Einstein does not consider the concept of 'determinism' to be as fundamental as it is frequently held to be (as he has told me emphatically many times)" - Pauli to Born
I've got a copy of "Born-Einstein Letters", I can give you a pp. if you're interested. It's also interesting to note that Born perhaps didn't really grasp what was bugging Einstein: at one point, he says:
"your axiom of the 'independence of spatially separated objects ..' is not as convincing as you make out. It does not take into account the fact of coherence; objects far apart in space which have a common origin need not be independent."
If, by this, he's thinking of the possibility of so-called 'instruction sets', this of course misses the whole point of the Bell/Aspect results; there are no, and cannot be instruction sets. J. Noel Chiappa 18:26, 3 April 2008 (CDT)

## Poor little kitty

Okay, in the section about the attempted torturing of a poor defenseless cat, a bit of an addition is needed.

The intro is clear, as is the explanation of the incomprehensible concept.

The only problem is in Paragraph 3, the one that begins with a good sentence: "Most formulations of quantum mechanics operate as if a quantum system does not have a definitive state unless it is somehow measured (in itself, still something of a point of contention)."

Now you throw in the atom (not heretofore mentioned) and the concepts of radioactivity and decay--wha? I mean, an old no nukes baby like moi would get it, but....

You need to add in a little something, like: Most formulations of quantum mechanics operate as if a quantum system does not have a definitive state unless it is somehow measured (in itself, still something of a point of contention). To understand Schrodinger's question, we need to introduce a couple of other concepts: the atom, (the smallest whatever kind of building block it is, I forget), radioactivty, blah blah blah, and radioactive decay, yadda, yadda.

Aleta Curry 22:19, 3 April 2008 (CDT)

Is it permissible original synthesis, or do we need EC approval for research, to restate it as Schrodinger's Politician? Howard C. Berkowitz 17:15, 27 March 2011 (UTC)
We might include a link to this cartoon. Sandy Harris 00:56, 28 March 2011 (UTC)

## How not to change the sense of what someone meant, part one

This sentence: "if one could only see the result of flipping a coin a million times," die you mean, Noel

1. if only as in "if I only had a brain"
2. if only as in "if one only had five dollars, as opposed to the five million the fellow actually has"
3. "if I could just...!"

Aleta Curry 22:29, 3 April 2008 (CDT)

Pretty much the second. So how about "if the only result one were allowed to see was the outcome of flipping"? Does that express it clearly? ("were" is the proper subjunctive there, right?) J. Noel Chiappa 23:38, 3 April 2008 (CDT)

## Way too complicated

The article as it stands, and most of the suggestions made above, are much too complicated for an introductory article. For a better model, go to http://en.wikipedia.org/wiki/Basic_concepts_of_quantum_mechanics. Sincerely, George Garrigues 04:10, 29 June 2009 (UTC)

Hi, George, a friendly semi-official word here from your local neighborhood Constable -- I've just reviewed a lot of the hoohah at WP about the various forks of the various quantum mechanics articles that you have been involved with and I'd like to caution you in advance that Citizendium doesn't work in quite the same way as WP. Any major editing, and particularly mass deletions and/or link changing *must* be first discussed on the appropriate pages. Every author and editor at CZ has, of course, the right to make changes, corrections, and additions to existing articles, but I think that all reasonable people will agree as to what the limits are without some form of prior discussion. Having said that, we look forward to your participation in the project. Best, Constable Hayford Peirce 17:13, 29 June 2009 (UTC)
I'd suggest that Quantum mechanics is a very big topic and one has to anticipate a lot of articles covering different aspects. Apparently this article is intended as an introduction, so it will have to adapt as these other articles are built so as to guide the reader gently to the articles they will find most interesting. Among the topics needing separate articles will be:
QM and angular momentum
QM and Hilbert space
QM and the band theory of solids
QM and atomic physics
QM and the electromagnetic field
QM and its combination with gravity
I'm sure more can be envisioned. Maybe a way to start is to make a beginning list of such topics and write a paragraph on each of them here in anticipation of a later link to a full article? John R. Brews 15:44, 25 November 2010 (UTC)

## Questions

The "principle findings" section does not mention De Broglie or the idea that particles such as electrons have wave aspects. Surely this is fundamental?

As I heard the story, Einstein showed mathematically that if Heisenberg's relation held for position & velocity of a particle, it must also hold for time & energy of an event. He intended this a a refutation of Heisenberg, but Bohr promptly turned it into an application of quantum mechanics, inventing the notion of quantum tunneling as an explanation for radioactive decay. I'd say this story, perhaps amended by someone who knows the history better, belongs in the article.

Somewhere — not sure if in this article or a separate one — I think we need a fairly detailed but non-mathematical explanation of the sorts of paradoxes that come up. (We may need a mathematical one elsewhere, too.) For example, in the two-slit experiment, do we have a wave (obviously, since it makes interference patterns), and if so, why does it act only at a point and in quantized steps? Or do we have a particle (obviously, ...), and if so, does it go through two slits at once and how do interference patterns get involved? I think Schrodinger's Cat comes in in this context.

Once we've done that, I think we should go on, again non-mathematically, to talk about interpretations of QM. They all avoid the paradoxes, but they all involve weirdness. You can choose which weirdness, since the math & predictions of QM are the same in all cases.

• hidden variable: it only looks non-deterministic; there are unmeasurable things that determine everything
• many worlds: the cat dies in one and lives in another; you don't know which one you are in until you open the box
• Bell & company: there are non-local effects that account for the oddities
• Copenhagen: it is impossible to say anything sensible about whatever happens between observations; don't even try

Probably I missed a few. Anyway, I'd like to see a discussion of the choices and an explanation of why the Copenhagen interpretation is most usual. Sandy Harris 12:19, 27 March 2011 (UTC)

On the last question, Occam's razor? Why assume a real universe if the mathematical formulation turns out more complicated than describing the observations? Peter Jackson 10:06, 28 March 2011 (UTC)