Talk:Copenhagen interpretation

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Misinterpretation of the Bohrian view[edit]

I believe that this article incorrectly attributes a positivist slant to Bohrs interpretation. Although this is indeed the view taken by his student Heisenberg, and is often taken as being representative of the copenhagen interpretation, it is not true of Bohrs ideas (regardless of whether they make it into the definition of the cophenhagen interpretation). I myself still struggle to come to terms with Bohrs relationist notions of measurement and reality, but I am certain that it is unfortunately not as simple as a positivism.

Also with regards to whether interpretations of QM is a subject for physics or metaphysics, it is as much physics as Einsteins deriving of the lorentz transformations (ie. special relativity). Both attempt to clarify and extend an existing operationally adequete although conceptually incomplete theory by questioning what relation the theory has to reality.

Nic.

Made a few corrections[edit]

Mainly POV things.

I removed the phrase

"Einstein's Relativity demonstrates that "instantaneous" has meaning only for observers sharing a single reference frame. No universal time reference exists so the "instantaneous wave function collapse" of the Copenhagen Interpretation is left undefined."

since it builds on the misunderstanding that the wave function collapse should be understood as a physical process (it should not). The "instantaneous wave function collapse" is not really a problem in quantum field theory or relativistic quantum mechanics, and in the various "delayed choice" experiments as well as in Einstein and Rosens article on the subject from 1931 we see that the shift involved in the observation/reduction of the wave function is so fundamental it actually stretches backwards in time.

Summary of objections to Copenhagen Interpretation in introduction[edit]

I replaced a somewhat diffuse sentence in the introduction by the sentences:

There have been many objections to the Copenhagen Interpretation over the years. Some have objected to the discontinuous jumps when there is an observation, or the probabilistic element that is introduced upon observation. Others have objected to the subjectiveness of requiring an observer, the difficulty of defining a measuring device, or persistence of classical physics to describe the "laboratory" in which the results are measured.

I hope these 5 kinds of objection are well-supported in the section on "Criticism". I see that most of them are. Note that the objection about "subjectivity" has been more or less met -- already by some of the famous early guys -- by replacing the conscious observer by a "measuring device"; then the weight of the objection shifts to the "difficulty of defining a measuring device". However, the objection about subjectivity is historically important, so I put it in.

There remain two sentences in the introduction about objections to CI. They are:

(1) Others have observed that there is little consensus as to the physical meaning of wave–particle duality.

(2) Yet others have pointed out that some research has called into question whether incompatible conjugate properties can indeed never be defined for the same time.

Both of these statements seem to be irrelevant to the Copenhagen Interpretation; they sound more like objections to quantum mechanics. But I'll leave them alone till I've studied the article some more. 178.38.171.5 (talk) 17:51, 20 April 2015 (UTC)

The idea of wave-particle complementarity was very large in Bohr's mind. Heisenberg saw things perhaps differently. It would misrepresent history to try to leave the wave-particle thing out of an account of 'the Copenhagen interpretation'.
The impossibility of definition of certainly and exactly determined coexisting position and momentum was very big in Copenhagen thinking, a major weapon in the war with Einstein. To try to say it is quantum mechanical as distinct from Copenhagenist would, again I think, misrepresent history.
It would be more useful to study the literature than to study the article!Chjoaygame (talk) 05:34, 24 April 2015 (UTC)

The whole introduction is confusing. Very little of it says what the CI, to a layman. It needs a rewrite. Roger (talk) 22:59, 7 September 2015 (UTC)

I agree. It should be rewritten from scratch. Unfortunately there are not many people with both the necessary expertise and the time to do so, especially given that whatever good work gets done will be eroded later. A big part of the problem in the current article is that some of its editors evidently don't understand quantum mechanics, which makes it impossible to write about an interpretation of it. 74.64.38.121 (talk) 11:40, 8 September 2015 (UTC)
The biggest problem with this article has always been argumentative grad students who think they understand QM . Waleswatcher seems to be the most skilled at both editing and the subject matter to appear in recent years, I hope s/he does more work on the article. JacquesDelaguerre (talk) 12:31, 8 September 2015 (UTC)

My disagreements start with the very first sentence. The CI is not an expression of mathematical formulas in everyday language. It is a way of interpreting those formulas, but not a substitute for those formulas. I also think that the discussion of axioms, configuration space, and objections is way too confusing and dubious for an introductory paragraph. I suggest cutting all that out, and leaving something simple like this:

The Copenhagen Interpretation is a way of understanding the formulas of quantum mechanics. While everyone agrees that there are experiments confirming those formulas, the meaning has generated some controversy. The interpretation is a set of explanations devised by Niels Bohr and Werner Heisenberg in the 1920s, and widely accepted in textbooks.

Alternatives to the Copenhagen Interpretation include the many-worlds interpretation, the De Broglie-Bohm (pilot-wave) interpretation, and quantum decoherence theories. Roger (talk) 17:17, 9 September 2015 (UTC)

Well, there's a lot to what you say, Roger. Maybe the difficulty in smithing this lede is, as Bill Clinton might say, "the meaning of meaning". As Bohr pointed out, visualization may not help in the circumstance of apparently being forced mathematically to deny the possibility of reaching any local realistic explanation. So in your suggested lede, you may want a different word or phrase than meaning. Anyway, I've taken another crack at it, at least it's shorter now! JacquesDelaguerre (talk) 18:27, 9 September 2015 (UTC)
Here is a physicist who says that there is no such thing as the Copenhagen interpretation! [1] Actually it is an informed opinion that quotes Bohr and makes some good points. I don't get your objection to the word "meaning". Yes, I am sure people disagree on what ought to be in a meaning. They also disagree over whether quantum mechanics even needs an interpretation. Roger (talk) 18:50, 9 September 2015 (UTC)
Ha, interesting link. As for finding meaning in abstracts, Lewis Carroll covered that nicely:

[S]aid the Mouse. '—I proceed. "Edwin and Morcar, the earls of Mercia and Northumbria, declared for him: and even Stigand, the patriotic archbishop of Canterbury, found it advisable—"'
'Found what?' said the Duck.
'Found it,' the Mouse replied rather crossly: 'of course you know what "it" means.'
'I know what "it" means well enough, when I find a thing,' said the Duck: 'it's generally a frog or a worm. The question is, what did the archbishop find?'
JacquesDelaguerre (talk) 19:06, 9 September 2015 (UTC)

incompatibility[edit]

I here respond to one of the demands for clarification made by this edit. The relevant item is "Incompatible wave functions cannot be superposed.[clarification needed]"

Dirac wrote in the first edition: "This, of course, is true only provided the two states that are superposed refer to the same beam of light, i.e. all that is known about the position and momentum of a photon in either of these states must be the same for each."[1]

Compatibility has much the same meaning as commutability. Wave functions from the same generalized quantum configuration space form a vector space, and can be added. Not from different generalized quantum configuration spaces. For a given particle, for example, one generalized quantum configuration space might be specified by a list of three position coordinates. Another might be a list of two position coordinates and one momentum. Another might be a list of three momentum space coordinates. That's three different vector spaces. They all correspond with one and the same abstract state vector space, but that one abstract vector space lacks a specification of configuration space, and therefore lacks a specified method of preparation. The three vectors could not come "from the same beam". A position coordinate cannot be 'added' to a momentum coordinate.

  1. ^ Dirac, P.A.M. (1930), 1st edition, p. 8.
  • Dirac, P.A.M. (1930). The Principles of Quantum Mechanics, 1st edition, Oxford University Press, Oxford UK.Chjoaygame (talk) 08:29, 24 April 2015 (UTC)
I do not have the first edition, but a search doesn't find that language anywhere in two more recent editions, and you did not provide enough context to see what was actually being said there. Anyway, the statement I deleted from the article is false - any two wavefunctions can be superposed. There are no restrictions on that whatsoever. That is one of the most basic facts about quantum mechanics. Dirac says so himself, very clearly: "Conversely, any two or more states may be superposed to give a new state." p.12,, 4th edition. Waleswatcher (talk) 10:04, 6 September 2015 (UTC)
In the 4th edition the more or less corresponding text on page 8 is as follows.
Corresponding to the description that we had in the case of the polarization, we must now describe the photon as going partly into each of the two components into which the incident beam is split. The photon is then, as we may say, in a translational state given by the superposition of the two translational states associated with the two components. We are thus led to generalization of the term 'translational state' applied to a photon. For a photon to be in a definite translational state it need not be associated with one single beam of light, but may be associated with two or more beams of light which are the components into which one original beam has been split. In the accurate mathematical theory each translational state is associated with one of the wave functions of ordinary wave optics, which wave functions may describe either a single beam or two or more beams into which one original beam has been split. Translational states are thus superposable in a similar way to wave functions.
The circumstance that the superposition idea requires us to generalize our original meaning of translational states, but that no corresponding generalization was needed for the states of polarization of the preceding section, is an accidental one with no underlying theoretical significance.
Perhaps this may clarify. I previously cited the 1st edition because it seemed succinct.Chjoaygame (talk) 20:08, 21 September 2015 (UTC)
The meaning of that is perfectly clear: in QM, states where the photon is in different places can be superposed, much like states with different polarization can be superposed in classical optics. A little later he points out that the fact that states where the photon is in different places can be superposed is a specific example of a general principle, namely that any two (or more) states can be superposed. Contrary to what you assert above, there is no restriction on that whatsoever, nor is there any such thing as "incompatible wavefunctions", nor is there any problem superposing a state specified in terms of position with one specified in terms of momentum. Waleswatcher (talk) 23:11, 21 September 2015 (UTC)
I wrote above: "Dirac wrote in the first edition: "This, of course, is true only provided the two states that are superposed refer to the same beam of light, i.e. all that is known about the position and momentum of a photon in either of these states must be the same for each."[1] In reply, Editor Waleswatcher above writes: "I do not have the first edition, but a search doesn't find that language anywhere in two more recent editions, and you did not provide enough context to see what was actually being said there." Likewise, Editor Waleswatcher writes here: "there is really no reason to discuss "beams" at all, especially not in a "concept" section that is supposed to gently introduce the idea of quantum superposition."
  1. ^ Dirac, P.A.M. (1930), 1st edition, p. 8.
These views proposed by Editor Waleswatcher are contrary to how Dirac saw it. Dirac's deliberately gentle and non-mathematical introduction is wholly predicated on the idea of beams, as one may verify by reading the 4th edition,[1] starting for example here. Careful reading of Dirac's text shows that in the later editions, that Editor Waleswatcher says he searched in vain, Dirac does indeed rely on the superposed states coming from the same beam, contrary to Editor Waleswatcher's claim. For example, Dirac writes on page 13: "What will be the result of the observation when made on the system in the superposed state? The answer is that the result will be sometimes a and sometimes b, according to a probability law depending on the relative weights of A and B in the superposition process. It will never be different from both a and b." The superposition is here considered observable as a splitting of a single beam, as implied in context. The word beam doesn't explicitly appear in the sentence, but the common input beam is needed for the result to be "sometimes a and sometimes b". On page 5, for example, Dirac uses the word beam 8 times. On page 6, once. On page 7, 8 times. On page 9, 15 times. On page 10, Dirac writes "a very strange idea has been introduced—the possibility of a photon being partly in each of two states of polarization, or partly in each of two separate beams". As cited by me, the first edition does make this point about the beam explicit, but this is dismissed by Editor Waleswatcher on the grounds that he didn't read it. How much of the first edition does he expect me to transcribe into this talk page to provide him with the "context" he wants?
Dirac is not entirely an easy read, but he is reliable. According to Helge Kragh on his page 77: "When Heisenberg received the fourth edition of Principles in 1958, he gave Dirac the following fine compliment: "I have in the past years repeatedly had the experience that when one has any sort of doubt about difficult fundamental mathematical problems and their formal representation, it is best to consult your book, because these questions are treated most carefully in your book."" Again, Kragh says on his page 78: "Most physicists welcomed Dirac's exposition and praised it for its elegance, directness, and generality. To Einstein it was the most logically perfect presentation of quantum mechanics in existence."[2]
  1. ^ Dirac, P.A.M. (1958). The Principles of Quantum Mechanics, 4th edition, Oxford University Press, Oxford UK.
  2. ^ H. S. Kragh (1990). Dirac: A Scientific Biography, Cambridge University Press, Cambridge UK, ISBN 0-521-38089-8.
To make his own points, it is thus evident that Editor Waleswatcher misreads or misrepresents Dirac's text, which is a reliable source.Chjoaygame (talk) 08:57, 8 January 2016 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── In any vector space, two vectors can be added. From this, it does not follow that the coordinate representation of two different vectors using two different coordinate systems can be added to yield a new vector; the result is, in general, nonsense. In other words, states may be added (unless there is a superselection rule in which case the states can be added mathematically, but the resultant state is not physically realizable), not two wave functions referring to different bases. YohanN7 (talk) 10:33, 8 January 2016 (UTC)

Thank you for this constructive comment. It brings out that a state, unspecified, and a wave function, unspecified, are not necessarily quite the same thing.
In talking about states, corresponding with vectors, one makes assumptions or presuppositions about the correspondence. Your preliminary phrase, "In any vector space" is therefore important and valuable. The correspondence relies on proper respect for it. Physically, in Chapter 1, Dirac deals with this by his first edition comment: "This, of course, is true only provided the two states that are superposed refer to the same beam of light, i.e. all that is known about the position and momentum of a photon in either of these states must be the same for each."[1]
  1. ^ Dirac, P.A.M. (1930), 1st edition, p. 8.
Chjoaygame (talk) 11:31, 8 January 2016 (UTC)

Pseudoscience[edit]

There is a quote, as well as I remember that goes "The worst case of pseudoscience pretending to be science foisted on students today is the Copenhagen interpretation of quantum mechanics." I haven't been able to find any reference to it, though. (I think it was on a radio program.) It isn't easy to find with a web search, as too many unrelated things appear. Maybe someone here knows about it? It does seem that CI is taught as the only interpretation in the usual way QM is taught. Gah4 (talk) 04:57, 16 May 2016 (UTC)

Seems like it should matter whose being taught in this assertion. Is CI the way that QM is most commonly taught to serious physics students or students in HS/Physics 101, etc? It seems to me that popular understanding of QM is at least as much Many-worlds as CI (people love the idea of parallel universes). Actual physicists, on the other hand, have to know the difference between the math and it's interpretation, utility and episteme, etc. The surveys referenced on this page (sources 43-45) seem to suggest that the most common views amongst serious physicists are either wholly or partly compatible with CI. This doesn't seem to paint CI as pseudoscience. Pwoodfor (talk) 13:29, 27 August 2016 (UTC)
They teach Newtonian mechanics in high school and Physics 101, not because it is correct, but because it is easier to teach, and also works most of the time. Seems to me that is also true of CI. Then again, when teaching Newtonian mechanics, it isn't usual to come right out and state that it is wrong. Students only learn that later. Gah4 (talk) 09:44, 7 September 2016 (UTC)