Talk:Copenhagen interpretation
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The flamewar is going out of control, so let's just stop. See Wikipedia:Wikipetiquette, particularly the section "How to avoid abuse of /Talk pages".
In summary, it has been suggested that the current article does not serve the layman well. An overhaul of the article was thought to have removed too much information, and was reverted. The best thing to do now w.r.t. the article is probably to merge the overhaul into the current article, avoiding any unnecessary deletion. The consensus seems to be that deleting the entire article is unnecessary deletion. To avoid controversy, it would be good to be extra meticulous. Before making a claim in the article, quote reputable sources, and document the references in a "references" section. Thanks. -- CYD
I lay that accusation against all the physics articles. You people need to find a layman who'll tell you what's comprehensible, what isn't, what's of no interest, what's irrelevant and what gratuitously creates confusion. And when you find that person, you should declare them Tyrant over all of the physics section. And if you're really smart, you'll also find a historian, a historian of physics if you're extremely lucky, to do the same job. Then you need to distinguish ontology, mathematics and history strictly.
So sayonara suckers and may a place be reserved for you in Gehenna.
--Ark, Friday, June 7, 2002
I don't have any objection to rewriting the physics pages to be comprehensible to the layman. My objection is that in the process of doing so, one absolutely, positively must not sacrifice correctness, and that the changes that Ark suggested did so.
Nobody understands quantum mechanics and no one is not confused by quantum mechanics. Much of the confusion arises from experiments that don't make any sense at all. The goal as I see it is to "present the confusion clearly" and explain clearly why it doesn't make any sense. -- RoadRunner
I removed the sentence:
- The results of this experiment are particularly preplexing when one considers that this is not a thought experiment, but an actual real experiment that can be quite easily performed with photons, electrons, atoms, molecules, and even small viruses.
Firstly, regardless of whether or not this experiment in the form noted is "quite easily performed", it wasn't in fact performed until 1989 (using electrons; see double-slit experiment). Thus, for the greatest part of its life, it was indeed a thought experiment; just as Schrodinger's cat experiment was (although the latter has now finally been performed, I think in 2000 - no cats were injured in the process of the experiment :) ).
Secondly, to perform the experiment with "... atoms, molecules, and even small viruses" would require a coherent source of these objects - something none too easily acheived. Certainly we can imagine that the results would be the same; but that again is the domain of a thought experiment. -- Chas 18 Oct 2001 19:10 UTC
Contents |
[edit] Criticisms
I dont see how quantum entanglement or quantum superposition in any way lends any support to Many Worlds. In fact, Many Worlds has problems with EPR (quantum entanglement) Nor do I see how Bells Inequality is a problem for Copenhagen - it is a problem for hidden variable theories.
The choice between interpretations "is not as a matter of personal taste" - it should be based on a judgement of the difficulties associated with each interpretation.
The article gives the impression that Many Worlds in the alternative to Copenhagen - it is not. Oz 19:56, 13 Sep 2003 (UTC)
If you post a criticism can you please outline that criticism. Just saying XXX says it gibberish is not a criticism. (My mum thinks Copenhagen is just fine). Also, a paradox such as Schrodingers Cat deserves analysis rather than a comment that it is intended to show how "absurd" something is. Oz 20:26, 13 Sep 2003 (UTC)
My Messiah textbook is good but old. I have not found the time to get and read a newer text, but I found a very good and up to date source in Physics Today and have quoted it in this article. Weinberg said there has been recent work, which may help to explain why my material from my older source did not satisfy some. The summary given in the quote is close to what I learned in graduate school, but better worded and supported by more recent physics. David R. Ingham 02:46, 14 May 2006 (UTC)
[edit] Query
Did Bohr use the term "collapse of the wave function"? Oz 23:02, 13 Sep 2003 (UTC)
[edit] "Mainstream"
Luminek, the study you cite provides the following poll results for the preferred interpretation of QM:
- Copenhagen: 13
- Many Worlds: 8
- Bohm: 4
- Consistent Histories: 4
- Modified dynamics (GRW/DRM): 1
- None of the above/undecided: 18
I removed the words "the mainstream" because that implies clear dominance over all other interpretations. While the Copenhagen interpretation is more accepted than MWI or any other specific competitor, it doesn't have a greater-than-50% majority. When more people report "undecided" than "Copenhagen interpretation", that should be a clear sign that it's not the mainstream interpretation. It's a mainstream interpretation, along with MWI, Bohm, and consistent histories. A 1995 study of 72 leading physicists reports that 58% believe MWI is true, so it appears this varies from study to study as well. While Copenhagen may indeed by the most-accepted interpretation, I feel the words "the mainstream interpretation" gave the false impression that all other interpretations where somehow non-mainstream, which is why I'm removing the phrase now.
I understand that, in the past, the Copenhagen interpretation did in fact have the sort of crushing dominance this article presently describes, and will make note of that. -- Schaefer 02:59, 28 Nov 2004 (UTC)
[edit] Interpretation of classical mechanics
It seems to me that the question of interpreting quantum mechanics was settled in the 20th century and is now historical: It is reality. Any remaining discussions should be called the "interpretation of classical mechanics". --David R. Ingham 22:16, 16 August 2005 (UTC)
- I don't understand your comment David. Are you saying that there is still no controversy over the interpretation of quantum mechanics? If that's what you're saying then I totally disagree. Trious 13:10, 25 September 2005 (UTC)
- Exactly: its interpretation belongs to *metaphysics*. It's therefore also misleading to have the opinion of *physicists* presented as if it represents an expert opinion - physicists are generally poorly educated about metaphysics. The only expert opinion about interpretation that I have read is that of Popper, and he rejected it. Consequently, I'll change the opinion header to "opinion of physicists"; only if a similar poll on people such as him is included, can the specification be scrapped. Harald, Lausanne, 14/10/2005
It seems to me that the trouble with the philosophy of science is that not even most physicists, let alone philosophers know enough physics to attempt to interpret it. I think that when one studies the early chapters in physics texts, instead of just doing the home work, it all becomes clear. David R. Ingham 03:33, 15 October 2005 (UTC)
David, when you say that the interpretation of QM was settled in the 20th century, what exactly do you mean? The word 'settled' suggests that a concensus of sorts was reached. Can you explain what this concensus view is?
[edit] Misinterpretation of the Bohrian view
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.
[edit] Introduction
As Wikipedia becomes more detailed, I suggest that interpretation of quantum mechanics be changed to Philosophical interpretation of classical physics to make this article more up to date. I think that, when the philosophy is settled, we should change that to "interpretation of classical physics" or "scientific interpretation of ordinary language". Language, being imprecise and restricted to common experience, by nature, cannot, without loss, interpret physics, which has become precise and general. David R. Ingham 09:12, 27 February 2006 (UTC)
[edit] Made a few corrections
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.
[edit] Shut up and calculate
About the famous quote "Shut up and calculate", usually attributed to Feynmann and in the Copenhagen interpretation article attributed to Dirac: http://www.aip.org/pt/vol-57/iss-5/p10.html Can someone provide any source proving that this quote is actually Dirac's?
[edit] Outsider View
I'm a mathematician and philosopher, not a physicist. I find Many worlds and Consistent histories both east to understand as they are presented on WP, but after reading this article I can't say I'm any closer to understanding what the Copenhagen interpretation is. OK that's maybe an exaggeration but it certainly took a few reads. What is needed I think is for the first sentence of "The meaning of the wave function", with the two bullet points to be clearly separated from the rest of that section (ideally it should be the first/second sentence of the whole article) and the ambiguity over whether that sentence represents merely the "starting point" for the Copenhagen interpretation or the contents of the interpretation itself, needs to be removed. ("The Copenhagan interpretation merely states that ...") As an outsider it's hard to see that sentence as staking any claim at all really. It is ambiguous over whether it allows the selection of the result of a measurement to be amongst a probability distribution of probability distributions, or whether it always assumes that an observation produces a classical certainty. In the latter case as a non-physicist I would assume it was just wrong (?) and in the former I fail to see why Many-Worlds say would be inconsistent with it (?). If someone like me with a degree in the two subjects closest to but not covering this issue thinks it is unclear, then I can't imagine there is much hope for those with no background even remotely related to the question. --cfp 01:19, 8 August 2006 (UTC)
- Thank you for your comment. The point you make is very good. The trouble with many writers is that they will be satisfied if they predict to themselves that they will be able to make a good defense of the validity of what they have said should anyone challenge them. The very top physicists are also frequently the best writers for the average well-informed reader. They manage to say something in English that will give any reader a clear (and perhaps only qualitative) description of some phenomenon or theory, and yet they will not say anything that their colleagues who are well acquainted with the subject will take amiss. P0M 07:24, 10 August 2006 (UTC)
-
- Let's look at the top of section one:
The Copenhagen interpretation assumes that there are two processes influencing the wavefunction:
While there is no ambiguity about the former...
- the unitary evolution according to the Schrödinger equation
- the process of the measurement
-
- This way of discussing matters assumes that the reader has clicked on a previous link to "wavefunction" and now has internalized it well enough to know what the first sentence in this quotation means. In addition to that rather dubious assumption, the sentence hypostatizes or reifies the wavefunction. It implies that we already have an account according to which a mathematical function is such a perfect mapping of some thing (a proton, electron, etc) that we can speak of the one as the other and not get in trouble. The problem is that the Copenhagen interpretation is an explanation (one of many) of what the mathematics really tells us about the outside world. The mathematics tells us that at any given time we are more likely to observe an interaction between the proton, electron, or whatever and some other entity (such as a detection screen) that is placed at the points indicated by the theory -- provided that there is something there for it to interact with. But generally there are several points of high probability for "manifestation" along a continuum of points of lesser probabilities. We cannot fill out gaps in the theory and predict which of the high probability points it will actually "choose." The theory describes something about the "potentiality for manifestation" or "probability of materialization" (like a ghost "materializes" somewhere), but the theory does not take account of a physical barrier. It describes how a photon's probability distribution looks as time progresses, but it doesn't say anything about the presence of a physical barrier to further progress.
-
- So, if we are looking at an electron in a double-split experiment, we do something to cause an electron to be emitted, we wait for a while, and then we see a sign that the electron has impacted somewhere on a capture screen. Why did it hit at point A instead of the center point directly across from the emitter and perpendicular to the screen containing the double slits? What do we make of the fact that x% of the time an electron will manifest itself at point B, y% of the time it will manifest itself at point D, etc.? Where was the electron in the interval between emission and collision? Did some unknown factor force the selection of one point rather an another for the electron to interact with? Did the electron actually show up at all points, but in our universe it ended up at one specific point, and all other impacts created separate universes in which those probabilities were realized. That kind of question involves "interpretation," i.e., saying what all the math says about the world of real things.
Having thought about this some more, I think the easiest way to present the Copenhagen interpretation (if I've understood it) would be by direct contrast to the many worlds interpretation. E.g. something like:
- The equations of quantum mechanics describe the probability of a certain result being obtained from a measurement. The many worlds interpretation postulates that the notion of probability is a broadly frequentist one. However as any measurement may only be performed once (indeed it is arguable that it could only be performed once), instead of counting occurrences across time or space, those who believe in the many worlds interpretation count frequencies across the same measurement in different "parallel" universes, which means that the particular measurement we obtain is just a result of the universe we happen to be in. Thus in the classic single-photon light diffraction experiment, the many worlds theorist believes that photons are taking every possible path, according to the predicted probability distribution, but in different universes. Those who believe in the Copenhagen interpretation on the other hand view the process of measurement itself as a "physically significant" operation. They believe that prior to a measurement the system is in a superposition of states, i.e. in the single photon slip experiment, the photon (in some sense) occupies every position in a triangle between the slit and the card. Measurement (they believe) just collapses the superposition into a single result, randomly, according to the probability distribution that gave the superposition. The chief point of contention between the Copenhagen interpretation and the many worlds one then is whether measurement does play this "selection" role (as in the Copenhagen interpretation), or whether the "selection" is made prior to the measurement (as in the Many Worlds interpretation, our own universe is selected). There is by definition no physical experiment that could decide between these two interpretations, since any attempt to see "if the selection had occurred yet" would involve a measurement, which, in either case, would result in a selection being returned.
What do you think? --cfp 15:28, 10 August 2006 (UTC)
- It seems o.k., except for "frequentist," which doesn't mean anything to me (except that I can guess from the total context of this discussion). Then there is the problem of how to explain "collapse of the wave function." One of the things that is clear from the double-slit experiment is that when there is an interaction between a photon or an electron and something in its path, then that determines one "event," i.e., we know that something was emitted because (1) we have a fairly reliable emitter device and we know how to trigger it, and (2) we saw something happen at the other end. (Something flashed on a screen, an electrical potential was jogged up a notch, etc.) That's what happens, for instance, if we put a detector in a slit in the double-slit apparatus. If a photon or electron triggers the detector and if a photon or electron continues on from that point and is detected on the screen, the wave function appropriate to the first half of the trip is different from this new wave function, and that is why there will be no interference effect. So wherever we say we "make a measurement" what we are really saying is that we get the absorption of one photon or electron, some change in the detector, and the subsequent emission of another one.
- So what does "collapse of the wave function" mean? It really just means "termination of the wave function." Why does it terminate here rather than there? I believe that many people insist that there must be a reason for its "collapse" (or the manifestation of the particle) at one point rather than another. But if that were the case then it seems to me that we should be able to disturb the probabilities by some simple manipulation of the detector screen. If I am flying a spinnaker from my sailboat and arrange a set of explosive devices that enables me to cut all connections to the boat at the same time, then I could throw the sail forward and it could impact a screen ahead. What part of the spinnaker hit the screen first would be fairly random, and over time I suppose that a fairly consistent pattern of hits would develop. Probably the sail would tend to hit the screen first at the center. But we should be able to change the most likely point of impact by slanting the screen so that the right end was closer to our boat and the right edge of the sail wold be more likely to hit it than the center or the left edge. But as far as I know nobody has ever suggested that the detector screen has to be exactly parallel to the screen containing the two slits. Does a pebble surface of the detection screen make any difference? But every screen is pebble surfaced. P0M 19:14, 10 August 2006 (UTC)
For frequentist see Frequency probability. Should have linked it sorry. That sentence wasn't very clear in any case. And if I was going to mention frequentism for many worlds I should have mentioned the rival theory (Bayesianism) for the Copenhangen interpretation. As this is a specific philosophy of science article I was assuming some general philosophy of science. Maybe I shouldn't have. Anyway as for angling the screen, just think of what happens when you do it with a multi photon beam rather than a single one. You can think of the waves as spreading out from the slits (think of ripples). As you change the angle of the screen you will change the point at which you are cutting the "ripples", so the pattern will change in a predictable way. It's not really an issue I don't think. --cfp 21:36, 10 August 2006 (UTC)
- The pattern changes in a predictable way, and one pattern is a mapping of the other pattern, no? I think we are saying the same thing in different ways. People who are wedded to the idea of hidden causal factors claim that there are real causal reasons for why a particle "chooses" the path it uses in each case. But if that were true it seems to me that people could easily find some way to mess with these hidden factors and change the experimental results. But I'm just speculating. Have you studied http://plato.stanford.edu/entries/qm-copenhagen/? P0M 01:53, 11 August 2006 (UTC)
Nope I hadn't seen that page. Looks like there's plenty to base an article on in there. Like I said I'm not a physicist so I don't know what I'm talking about. All I was saying was that this page needs a lot of work. --cfp 19:20, 11 August 2006 (UTC)
- I started out as a physics major and got seduced to the dark side of philosophy and Chinese language, so I'm not the ideal person to write an article like this even though I've maintained an interest in physics. On the other hand I am less likely than some to assume that I can't understand something because I'm too stupid. If the article does not make sense to you at some point then it is probably either wrong or else the writer knew what s/he was trying to say but it is not coming through right. P0M 20:41, 11 August 2006 (UTC)
[edit] New material on Quantum reality
Here is some new source material:
[edit] Penrose
From The Road to Reality by Roger Penrose, 2004, section 21.6, (top of p. 508 in my copy):
- If we are to believe that any one thing in the quantum formalism is 'actually' real, for a quantum system, then I think that it has to be the wavefunction (or state vector) that describes quantum reality. (I shall be addressing some other possibilities later, in Chapter 29; see also the end of 22.4.) My own viewpoint is that the question of 'reality' must be addressed in quantum mechanics—especially if one takes the view (as many physicists appear to) that the quantum formalism applies universally to the whole of physics—for then, if there is no quantum reality, there can be no reality at any level (all levels being quantum levels, on this view). To me, it makes no sense to deny reality altogether in this way. We need a notion of physical reality, even if only a provisional or approximate one, for without it our objective universe, and thence the whole of science, simply evaporates before our contemplative gaze!
This illustrates my contention that the word "interpretation" in this article is a poor choice. The interpretation is clear: It is reality. This article is about how to make use of this theory, using our classical minds and notebooks and tools that we can only describe classically. David R. Ingham 04:06, 16 August 2006 (UTC)
[edit] Weinberg
I put this into two articles' discussion pages some time back, but it is also relevant here.
Physics Today, April 2006, "Weinberg replies", p. 16,
- ... but the apparatus that we use to measure these variables—and we ourselves—are described by a wave function that evolves deterministically. So there is a missing element in quantum mechanics: a demonstration that the deterministic evolution of the wave function of the apparatus and observer leads to the usual probabilistic rules [Copenhagen interpretation].
So the "Copenhagen interpretation" is not philosophy nor is it basic physics. It is an empirical rule, that has not yet been fully justified theoretically, for using the classical approximation in quantum experiments. David R. Ingham 04:22, 16 August 2006 (UTC)
