Talk:Measurement in quantum mechanics/Archive 1

message: Who ever put him, care to explain why? MathKnight 16:37, 10 Sep 2004 (UTC)
replies: text: (William M. Connolley 16:56, 10 Sep 2004 (UTC)

text: Thanks for the input. I will try to work it out a little bit, I hope others in the field will contribute as well. MathKnight 18:14, 10 Sep 2004 (UTC)
thread_title:  Attention message 

message: The following part was added by User:Phys, and was disputed by User:William M. Connolley:
However, in the ensuing decades, after physicists came to terms with quantum entanglement, decoherence etc., the apparent "collapse" turned out to be a phenemological consequence of entanglement coupled with decoherence and is not only consistent with a deterministic Schroedinger's equation but is a consequence of it! Not only that, the insight was that it's was not the process of measurement (which can't even be defined precisely!) which drives the phenemological collapse but a decohering entanglement with the environment. See the relative state interpretation and decoherence for more details. A later reanalysis of the Bohr-Einstein debates in light of our current knowledge of decoherence and entanglement revealed that Bohr's analysis, which were at the heart of the foundations of the Copenhagen interpretation were flawed.
(William M. Connolley 12:17, 26 Sep 2004 (UTC)
thread_title:  Wavefunction collapse 

message: I saw this article listed on Wikipedia:pages needing attention, and boy, does it.  I've tidied up some of the grammar for now; I hope my tidying of some of the clumsier phrasings hasn't introduced some subtle error beyond my understanding.  I've left the example alone.  (Someone who gets MWI should probably expand the "Reject it as a physical process ..." bullet point.) I was going to plough on and try a full rewrite, but it occurs to me that wavefunction collapse and measurement problem both cover similar ground (albeit in the latter's case from a horribly pro-Many Worlds Interpretation POV).  What I suggest is this:
* Merge an NPOVed version of the content from measurement problem into this article.
* Merge the general discussion of how measurement works under Copenhagen currently in wavefunction collapse into this article; simultaneously, move the detailed mathematical discussion from here to wavefunction collapse and include internal links in both directions (ie wavefunction collapse, which is after all a more technical term, becomes the place to go for mathematical formalism; this article describes things in non-mathematical terms and also tries to explain the philosophical aspects, from all angles -- I think an ideal version of this article should give various different viewpoints' answers to at least some of those questions).
I would probably just get on and do this if I wasn't worried that I'm biased towards wavefunction collapse, substantial parts of which are still my turgid prose from a couple of years ago. Bth 18:57, 14 Oct 2004 (UTC)
thread_title: Case for merging?

message: The "wavefunction collapse" section needed serious work. It employed obsolete measurement ideas that predate the development of decoherence theory, and contained many incorrect statements like ``The collapse process has no trace or corresponding mathematical description in the mathematical formulation of quantum mechanics" and claims that the wavefunction collapses "instantly". I cleaned it up in light of decoherence theory.
The section on the von Neumann scheme contained some tendentious remarks at the end, which I have rewritten. References to the Bohm interpretation should include serious disclaimers since only a small minority of physicists accept it.
The "philosophical implications" section was also written without reference to recent work. The questions "Does a measurement depend on the existence of a self-aware observer?" and "What interactions are strong enough to constitute a measurement?" are addressed satisfactorily by decoherence theory. However, the answers to these questions are nontrivial and so I have included them in this section.
The Copenhagen interpretation seems to mean a lot of different things to different people, so I've weakened the statement "According to the Copenhagen interpretation, the answer is an unqualified "yes"." Based on this, I'm guessing the Copenhagen interpretation article needs work. Dave Kielpinski 00:08, 19 December 2005 (UTC)
thread_title:  Wavefunction collapse, von Neumann scheme, and philosophical implications 

message: An anonymous editor has removed my warning to readers on the Afshar experiment external link. No reason was given. I am willing to discuss this point, but for now I have replaced the warning (somewhat toning it down.) Dave Kielpinski 01:29, 19 December 2005 (UTC)
thread_title:  Afshar experiment 

message: I started editing this page; feel free to revert me if you can justify.   I remove this, it's incomprehensible. "The goal of a particular measurement of a particular system, in any experimental trial, is to obtain a characterization of the system in mutual agreement between all members of this system, and therefore by a particular method which is reproducible by all members of the system, at least in principle." If you know what it means, please rewrite. The rest is attempted better exposition. GangofOne 13:52, 30 December 2005 (UTC)
thread_title: changes

message: Should this be merged with Measurement problem? Also, Measurement (quantum mechanics) redirects to Measurement problem and not here. --Apoc2400 12:04, 22 March 2006 (UTC)
replies: text: Yes, definitely merge.  --Michael C. Price talk 00:53, 25 June 2006 (UTC)
thread_title:  Merge with "Measurement problem"? 

message: i changed "mathematical formalism of measurement" to "formalism of...". that has been reverted. a small point but article should be honest. a mathematical and rigorous description of the von Neumann measurement scheme would include the PVM associated to an observable, etc. The short paragraph on the case of "continuous spectrum" is kinda funny. Of course it can be rectified using rigged Hilbert spaces but that's not mentioned either. Mct mht 22:46, 14 June 2006 (UTC)
replies: text: That reversion was vandalism by Hyrun.  Reverted.  --Michael C Price 23:41, 14 June 2006 (UTC)
thread_title:  "mathematical" in section title 

message: the subsection giving the e.g. re particle in a box, i'm sry to say, needs to go or a complete rewrite. clean up tag will be added. Mct mht 04:41, 26 June 2006 (UTC)
replies: text: The whole thing needs a rewrite.--CSTAR 19:30, 26 June 2006 (UTC)
thread_title:  subsection need rewrite 

message: the wikilink to resolution needs to point to a more exact page. [[Resolution (logic)] makes sense to me but I could be wrong.  STHayden [ Talk ] 02:35, 22 August 2006 (UTC)
thread_title:  Resolution 

message: Substituted at 15:22, 1 May 2016 (UTC)
thread_title: Assessment comment

message: This talk page hasn't received any comments since May 11, 2007. It's been since June 26 2006 since an actual conversation took place. I've archived the old talk page to start fresh on cleaning up this article. I'll put the appropriate tags on the main page. Here's hoping we can all work together to make this article better. JFlav 02:33, 22 September 2007 (UTC)
thread_title:  Archive 

message: This article needs to be revised or rewritten. It is heavy on derivations and examples and light on any explanation of concepts. In fact, it reads as if it was copied out of someone's quantum mechanics notes. JFlav 02:52, 22 September 2007 (UTC)
replies: text: you wanna volunteer? :-) Mct mht 03:13, 22 September 2007 (UTC)

text: Yeah, I'll do my best. But I don't know how much I qualify as an "expert." JFlav 16:34, 22 September 2007 (UTC)
thread_title:  Cleanup Effort 

message: In the section on observables it mentions the properties of observable operators. Amongst these it says that the operators are Hermitian, and they have real eigenvalues. But ALL Hermitian opertaors have real eigenvalues, so it might be better to put these in the same point. Currently it reads like having real eigenvalues is specific to operators corresponding to observables.  —Preceding unsigned comment added by 82.6.96.22 (talk) 21:44, 24 May 2011 (UTC)
thread_title:  small query 

message: I'll be adding a sentence here and there, or inserting a few words, to hopefully improve comprehension.  I'll start with the lead which is not very informative.  I'm somewhat familiar with the discussion on this topic among experts, but wish to make things more accessible to non-experts.
regards,
Jonathan JonathanD (talk) 03:57, 12 June 2011 (UTC)
replies: text: I've reverted the changes, since the measurement problem is not largely the result of Quantum indeterminacy - the cause is a matter of debate that relates to whatever interpretation of quantum mechanics one holds.  Jonathan, I would advise you not continue making large additions/changes to the lead of articles on this subject (or any subject for that matter).  The lead should only mention material that is discussed in more detail within the body of the main article. -- cheers, Michael C. Price talk 06:19, 12 June 2011 (UTC)
thread_title:  minor changes 

message: AFAIK the best defintion of a measurement is the interaction of a quantum system with a classical one (as mentioned in the Copenhagen interpretation article). Of course this is unsatisfactory as the observer can be described by quantum mechanics too: the observer is nothing more or less than a (hugely complicated quantum system). Therefore a huge problem is why "measurement" causes wavefunction collapse, when the interaction of two quantum systems doesn't. Perhaps this could be addressed in the article?  — Preceding unsigned comment added by 86.26.13.2 (talk) 00:21, 31 October 2012 (UTC)
replies: text: Did you not see that there's a section Measurement in quantum mechanics#What physical interaction constitutes a measurement? Or are you saying that you find that section to be inadequate to answer those questions? --Steve (talk) 13:25, 31 October 2012 (UTC)
thread_title:  Definition of measurement 

message: This and the following sections need major overhaul. The author(s)did not have sufficient understanding to engage in this. (1) The double-slit experiment is primarily about wave-like interference, and sometimes a major player in discussions involving wave vs. particle like behavior (especially of photons). The issue of collapse applies to all measurements. (2) The language in description of von Neumann's measurement scheme needs a bit more clarity. ".. the operator that needs to be measured" needs better language {that has been fixed.} (3) In von Neumann's analysis of measurement, you show 2 different equations (A -> B) where a pure state becomes an entangled state. There is no clue how that happens, either physically or mathematically. (4) There is a system upon which a measurement operator represents a measurement, then there is the quantum representation of a classical device that produces a classical measurement such as a pointer reading—and that needs a different name.  Etc. Most likely some symbols (e.g., M1 & M2 or CMD [classical measurement device], whatever) will need to be used in order to keep things clear. Whoever is doing this, please get someone who is knowledgeable to proof-read what you are writing. von Neumann's analysis of measurement is a serious challenge; it would be really worthwhile to have a real expert contribute to this!
And, from later on:
[This is confusing |psi> is the state prior to interaction with the original measuring device and with von Neumann's apparatus. The next equation shows the transition from the original unmeasured state to an entangled state involving both pieces of apparatus. It is not clear how that happens or how one can, by a unitary transformation, go from an original pure state to an entangled state. Without that, this is all smoke & mirrors.]
I'm not sure exactly what this last comment means in this context, given that the initial state |\psi\rang|\phi\rang is clearly an entangled state (belonging to the direct product of the Hilbert spaces of the system and of the apparatus). It doesn't seem to me that the unitary operator needs to be explicitly constructed for this argument. --Anagogist (talk) 15:20, 3 June 2013 (UTC)
thread_title:  Comment taken from from ''Wavefunction collapse'' section 

message: Since the article uses the phrase "pure state", I suggest it be defined. The article says " A pure state is represented by a state vector in the Hilbert space."  Is that a definition?  Is each vector in the Hilbert space considered to be a "pure state". Tashiro (talk) 22:06, 2 February 2015 (UTC)
replies: text: Pure states are defined and explained in the Quantum state article. I added a link to the notion "pure state" as well, to clarify. Xaggi (talk) 10:44, 3 February 2015 (UTC)
thread_title: Define "Pure state"

message: The article says there is a connection between measurement as a mathematical operator and ordinary measurements in a laboratory.  Ordinary measurements in a laboratory are connected in some way to physical theories that predict them. The predictive aspect clear from article is that if we happen to observe outcome X in the laboratory then we can model this as the action of an operator and use the eigenvectors associated with the eigenvalue X to predict the state of the system after the measurement. (Do the eigenvalues all have the same physical dimensions? , e.g. kg m / sec or some other physical unit, It would be useful to state explicitly how physical units enter the picture.)  
The method of predicting the probability of outcome X is (I think) only gradually explained in the body of the article.  It involves the representing the system being measured as a sum of eigenvectors of the measurement operator. That representation can be used to compute the probability of a given outcome.  That should be explained to some degree in the qualitative introduction.  At least it should be said that a known state of the physical system and a given measurement operator let us compute the probability distribution for the experimental outcomes.  Since the mathematical explanation assumes the state of the system before the measurement is a "given", the qualitative introduction should explain how this assumption relates to setting up a laboratory experiment.  (I think the standard jargon is making a "preparation".)Tashiro (talk) 22:06, 2 February 2015 (UTC)
thread_title: Clarify the connection between measurement and prediction

message: I do not think this reference adds any particular value to the article. Compared to the systematic review article cited just prior, it is far less detailed, being superficial without being more clear. Self-published expositions of decoherence are a dime a dozen. We should not be elevating works on technical and/or philosophical topics that have yet to go through the wringer of peer review.  recently removed it from Quantum decoherence for much the same reasons. (Both the additions were made from Michigan IP addresses, use almost identical phrasing in the edit summaries, and misuse the  template in the same way, so we have to careful about citation spamming here.) XOR'easter (talk) 01:27, 7 January 2023 (UTC)
replies: text: I have to say that I find this rationale for putting it back to be unsatisfying. "Well-known" is not substantiated — it's one uncited preprint among the dozens posted on quant-ph every day — and would not suffice to establish reliability even if it were true. XOR'easter (talk) 14:26, 7 January 2023 (UTC)
replies: text: , I am proposing removing the addition of Stephen Hsu's paper from the article. It was not used a reference for writing the material, just added later to an already sourced sentence. We frequently get editors trying to add recent preprints to Wikipedia articles without adding content. It is not proper to try to publicize unpublished articles this way, or most newly published papers for that matter. Can you explain how a paper uploaded to arXiv on Dec 5, 2022 is well known? And I am sure many university professors have lecture notes that explain this to their modern physics classes. In the early days of requiring articles to have references (there were many problems with science articles before sources in Wikipedia were required), links to professor's lecture notes were often used. Now we require reliably published sources. StarryGrandma (talk) 18:48, 7 January 2023 (UTC)
replies: text: Dear Starry, it was "well known" to me, since I had already read it recently, and was impressed by its clarity (WP should be understandable by any intelligent adult, not just specialists in their field). But if it was added to the article by the authors, then this is in violation of at least two WP policies: no conflict of interest and no posting of own work, and possibly of Reliable Source. I'd like the reference to remain in the article because of its clarity, but I will stand aside if there is significant doubt about whether the circumstances of its addition met WP requirements. Thank you for presenting the issue here. David Spector (talk) 20:20, 7 January 2023 (UTC)

text: I'm torn. On the one hand, this is clearly a case of citation spamming, but on the other hand the preprint is a nice and pedagogical exposition. In fact this is the first time I have ever seen self-promotion that wasn't of crackpot gibberish. But at the end of the day I think Wikipedia would benefit most from a consistent ban on self-promoting unpublished pre-prints. Tercer (talk) 10:24, 8 January 2023 (UTC)
replies: text: I agree on both points. David Spector (talk) 12:26, 8 January 2023 (UTC)
replies: text: If we're lucky, it'll be formally published in some place like Am. J. Phys. (who have done articles on the topic before), and this whole discussion will be largely moot. XOR'easter (talk) 23:24, 9 January 2023 (UTC)
thread_title:  Hsu preprint