Talk:Measurement problem

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This article is biased as hell. Less cynicism, more objectivity.

Yes, this article is disgustingly POV. I happen to agree with its POV, but still. I will work on it tonight if I get some time. Glenn Willen (Talk) [[]] 21:43, 4 Aug 2004 (UTC)

The reference I added some time ago should give a better overview, even if not only focussing on the measurement problem. By the way, whould this article receive an NPOV-tag?

This stinks of NPOV. Please see Talk:Measurement in quantum mechanics for a suggestion to solve both the many problems of that article, and the POViness of this one. Bth 18:58, 14 Oct 2004 (UTC)

Contents

[edit] Revolution for the article

Dear ladies and Gentlemen,

I revised the article and the reference at the end is the only thing that was not changed because I believe that it looks OK. My guess is that you will agree that this is a much better starting point for a serious article than the previous one.

Sincerely, Lubos Motl, Harvard University --Lumidek 01:16, 23 Dec 2004 (UTC)

Waaaay better. The general problem of this entry is, though, IMHO, that the measurement problem is often the reason and ultimately the test for every interpretation of quantum theory, so that any discussion of the measurement problem quickly becomes a discussion of interpretations of quantum theory. Maybe the discussion of any interpretation should be relocated to the interpretation page and instead the measurement problem should be spelled out in detail?

[edit] Measurement problem?

I disagree that this is a title appropriate to the 2000s.

I have done a lot of quantum mechanical measurements and have never been aware of any general problem. Wouldn't "process" be a more interesting title? I don't see any article with that name.

The concept of "interpretation of quantum mechanics" is also out of date. I have tried to balance with "philosophical interpretation of classical physics" and had much help doing so, but we should work toward understanding physics before interpreting it.

As a step in that direction, maybe we could have one article explaining the interpolation between quantum and classical descriptions necessary in an experiment from the classical point of view and another from the quantum point of view. The problem with the former is that it is mainly historical, while the problem with the latter is that it so far exceeds possible detailed calculation. David R. Ingham 04:29, 21 October 2005 (UTC)

The short section on philosophy in the Feynman Lectures quantum mechanics volume does not appear to indicate that any change from the conventional scientific method is indicated or that there are consequences for other fields of study.David R. Ingham 05:09, 21 October 2005 (UTC)

I think this whole thing should be merged into quantum measurements. Karol 16:26, 22 October 2005 (UTC)
My impression of the measurement problem was explaining why the waveform collapses when it is measured, and what conditions are required for it to be considered a measurement at all. Is this accurate? Remy B 15:40, 18 May 2006 (UTC)

[edit] A source of ambiguity

The current text has:

quantum probabilities (that are able to interfere) to the ordinary classical probabilities.

This formulation involves creating a trap for the non-expert by calling two very different things by the same name, "probabilities." I believe some texts speak of "probability densities" when discussing ψ functions to indicate that what is present is not probably one thing in one case and probably something else in another case, i.e., that there is no randomness or indeterminancy about the ψ function. Instead, in a very absolute way it determines the likelihood with which a macro scale phenomenon will be observed. That the probability that any photon will arrive at point x on the detection screen is the same is better expressed by saying that each photon carries the same probability density of showing up at point x. I don't recall any other term being used to distinguish these two related by very different kinds of things. P0M 17:57, 29 October 2005 (UTC)

[edit] Every interpretation must answer?

"The measurement problem is the key set of questions that every interpretation of quantum mechanics must answer."

This so called "problem" is only a problem for *some* interpretations of quantum mechanics. So it is only in need of an answer within the context of those interpretations. I suggest the following:

"The measurement problem is a key set of questions that *some* interpretations of quantum mechanics *try* to answer."

Otherwise, those interpretations which do not see a problem will be prejudged as failing in some respect.

The real problem are those very questions which propose the measurement "problem" as a problem. By way of analogy consider the following question:

 "Where is London to be found, outside the UK?"

Notwithstanding the possible presence of towns called "London" that do exist outside of the UK, the answer to this question is "nowhere". That doesn't mean London is nowhere. Nor does it mean that the answer is a failure to solve the problem.

Now consider a question closer to the question at hand:

 "Where is a particle to be found, *before* it is found?"

Similar question. Similar answer. Similar conclusions. Simply put, the particle's discovery (in spacetime), like that of London, is not within the boundarys that the question has imposed on the answer. It's discovery belongs to a future that hasn't yet happened. Prior to discovering a particle one can, however, model it's discovery - but one can't actually discover it. Likewise, outside of the UK, one can, if not discover London, at least model it's location.

Carl Looper.

[edit] No positivism in Copenhagen

The statement that Bohr's Copenhagen Interpretation is a positivist theory is frequently made but probably not correct. See: http://plato.stanford.edu/entries/qm-copenhagen/ Furthermore, the statement that Copenhagen Quantum Mechanics does not explain a wave function collapse is rather misleading. In Copenhagen Quantum Mechanics there is no such thing as a wave functon collapse. The wave function is not part of the ontology. The change of the wave function after a measurement can be calculated using the QM formalism but this change of the wavefunction just has nothing to do with what happens in reality.

[edit] What is the article's subject?

The article is entitled "Measurement problem" as it stands, however, it is a brief (half-baked) description of various interpretations of QM. The actual problem of measurement is only briefly stated. The statement of the problem differs depending on one's approach. E.g. a physicist may wish to know exactly what makes a "measurement" different from the interactions described by the unitary time evolution, a philosopher may wish to examine what is wrong with the question "where is the particle before the measurement?" etc. A basic physics background could greatly enchance the article.

Isn't it true that "the measurement problem" is a kind of shorthand description for some difficulties in settling on an appropriate way to discuss certain things that was actually used by the original group of physicists who tried to come to terms with the new physics they were discovering? If that is true, then it would be worth tracking the locus classicus and citing it in the article. P0M 05:41, 2 May 2007 (UTC)

A word of caution to Carl Looper - the question analogy given above is somewhat along the lines of "if a tree falls and no one hears..." - it is just as valid to ponder this in the case of classical mechanics as it is in the case of QM. As far as I can tell the measurement problem arises in one of its guises in all of the mainstream interpretations of QM - sometimes as endlessly entangled systems, sometimes as inability to interpret "probabilities of the outcomes", most often as vagueness of the term "measurement" (if you know a well-developed, consistent treatement that avoids the difficulty please provide a reference). In Classical mechanics a particle does not need to "have" a position (for example) at all times, however (1) there is no problem with assuming that it does and (2) measurement of the particle's state does not in general alter its time evolution. Classically you can model a an "undetermined" state by a statistical probability distribution, which "can" (but does not have to) be interpreted as a particle being "somewhere - we just don't know quite where". However placing a collection of particles in a "box" and not wondering "where exactly" poduces very different statistical predictions in the Quantum case than in the classical case (e.g. bandgap structure of semiconductors, Bose-Einstein condensation).

How does a theory, developed on the basis of actual measurements, subsequently fail to accomodate the very measurements upon which it is based? Because the measurements get edited out. The sound of a tree falling gets excluded, and is replaced by the concept of a tree falling. The wave function which otherwise characterises the distribution of particle detections is re-assigned to characterising the particles which otherwise Platonically "cause" such detections. The only mystery here is how a theory which excludes particle detections can plead ignorance with respect to how to recuperate such detections. - Carl Looper —The preceding unsigned comment was added by 210.9.52.115 (talk) 03:14, 27 February 2007 (UTC).

[edit] Change the word mixed state

"the cat seems to be in a "mixed" state": I think the word mixed here should be changed to something else because it can be easily confused with the idea of a mixed state as opposed to a pure state.

[edit] Why is an observer needed?

I have read many books on this subject and understood some. Yet no-one has ever explained why an observer is different from any other object in his/her ability to collapse wave-functions. It seems as soon as a wave-function interacts with something else, it will collapse. The "something-else" may be an atom attached to an observer but it could be anything else. Thus the smell coming from the box will tell the observer that the cat died long before he/she observed the outcome. JMcC 10:47, 24 August 2007 (UTC)