Talk:Quantum mechanics: Difference between revisions

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First discussion

This is a great explanation of quantum mechanics to a lay audience, and the number of good relatively non-technical explanations quantum mechanics is shamefully small. Obviously a lot of work has gone into it, and it shows. I do have a couple of comments.

First, according to the article, "For example, an electron in an unexcited atom is pictured classically as a particle circling the atomic nucleus, whereas in quantum mechanics it is described by a static, spherically symmetric probability cloud surrounding the nucleus." This is true only for the electron whose ang. momentum quantum number is 0. I think you meant by this the electron in the Hydrogen atom...

Secondly, it would be nice if you included the photoelectric effect in the list of observations that could not be explained by classical mechanics, since this led to the idea that light is made of packets called "quanta." Also, it is for this that Albert Einstein won the Nobel prize.

Also, igo I find your comments interesting, particularly since "elementary" quantum mechanics is usually taught using the Copenhagen interpretation. It was taught this way to me, so I wasn't aware that the concept of "wave function collapse" is not intrinsic to the theory, but is rather a part of the Copenhagen interpretation. However, something observable happens to the system which is interpreted as wavefunction colapse, so how else do you refer to this observable change to make it free from any particular interpretation of quantum mechanics?

I also never realized that the Schrodinger equation only holds in a closed system-- well, it's more like I never thought about it, but of course, that makes sense-- the system becomes entangled with another, and so the "measuring" system plus the "measured" system comprise a closed system and so can be modelled with the Schrodinger equation (right?)

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I think that some paragraphs in the section describing the theory are not entirely correct, or at best misleading. I mean the text around "During the process of wavefunction collapse...". These few paragraphs, talk about wavefunction collapse as if it's "really happening". It is well known that there is more than one interpretation of quantum mechanics that tell us what's "really happening". Even though the Copenhagen interpretation is the most commonly cited one, sooner or later it will start a heated debate as to which one should be used in the article.

I suggest rewriting that passage as follows: (a) state that experiments produce measurements that are sometimes discrete in values and the same experiment performed on the same state may result in more than one measured value. (b) explain that these experimental facts are predicted by quantum mechanics (c) offer an interpretation of these experimental facts in terms of wavefunction collapse but stress that this is only an interpretion and acknowledge other ones.

Lastly, I really don't like the sentence "During the process of wavefunction collapse, the wavefunction does not obey the Schrodinger equation." The wavefunction of a system obeys the Schrodinger equation only if it is closed (no external influences). And if a system is closed, its wavefunction always obeys the Scrodinger euqation. To avoid misconceptions, it is better to say that during the process of measurement the system can no longer be considered isolated, and the state of the measurement aparatus must be also taken into account. The analysis of measurement taking the measuring aparatus into account is called the study of decoherence and leads to results that can potentially explain the standard problem of measurement.

I'm going to think about the best way to put this into words and then make changes. I can incorporate comments if any accumulate. --igor

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Quantum mechanics has provoked a strong philosophical debate. The fundamental problem is that causality and determinism is lost: while the probability distributions evolve according to a well established deterministic law, the values of the observables themselves do not. Because of this, Albert Einstein held that quantum mechanics must be incomplete.

It would be helpful to try to give some basic explanation of why Einstein's view is widely held to be incorrect--his view seems like common sense, but common sense is often wrong, as theoretical physicists enjoy pointing out. So, why is it wrong, in this case? By the way, please don't answer this question on the /Talk page--please put the answer on the QM page. Thanks in advance! --LMS

It's not entirely clear that Einstein was wrong on all counts, just wrong on at least one of them. :-) The Bell's-inequality experiments of Aspect prove beyond any doubt that either (1) Observable effects exist that cannot be deterministic results of inherent properties of matter; or (2) The universe is non-local; i.e., physical effects can propogate faster than light. Nobody knows which. --LDC

It proves neither, since neither is the case in the multi-universe interpretation. --JG

I'll put a discussion of these issues on the Copenhagen interpretation page. --AxelBoldt

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Having reviewed more of the literature on this topic, I concede that I was incorrect, so I'm removing the discussion regarding electron clouds and acknowledging that the current description in the article is correct -- Matt Stoker

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Perhaps some mention of the problem that inspired Planck to invent Quantum Mechanics is in order. IIRC, physicists were trying to figure out what electromagnetic waves were in an oven that had a certain amount of heat in it. They knew that an integer multiple of the wavelength of the light in the oven would have to equal one of the dimensions of the oven, but every time they tried to figure it out, they ended up concluding that the oven had infinite energy in it. Planck was able to find the answer by assume that the energy in an electromagnetic wave was quantised such that E ∝ f. This went directly counter to the classical mechanics assumtion that E ∝ Amplitude.

not quite, in classical mechanics it's proportional to both the square of the amplitude and the square of the frequency 63.205.40.243 05:12, 8 Jan 2004 (UTC)

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I would like to request that we start an article "Mathematical content of quantum mechanics" and move most of the math material that is right now in the main article there. Two reasons:

1. This article was meant as a general introduction, accessible without a deep understanding of the math or specifically devised notations like bra-ket.
2. The math treatment right now is incomplete (it doesn't mention that the operators don't have to be defined everywhere, it doesn't mention which operators belong to which observables, it doesn't mention the possibility that an operator may not have eigenvalues, it doesn't mention the importance of the spectral theorem in dealing with operators that don't have a point spectrum.

Correcting the problems in 2 would compound problem 1.

--AxelBoldt

Good idea. How much of the current material do you think should be left in the article, and how much moved to the new page? The old section on "Mathematical Formulism" I found very difficult to read, which was why I expanded it. -- CYD

I can write a very simple version, basically saying that states are elements of Hilbert spaces, observables are operators, and the states evolve according to the Schrodinger equation, and then link to the math article. --AxelBoldt

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I've correct some mistakes here,

1. ``three things that QM explains was misleading. Quantum mechanics explains (literally) thousands of things classical theory cannot. I changed the wording to reflect this.

2. There are classical systems where vairables can only take on discrete values (vibrational modes for example) so I took that part out

Those are all wave's however, whats new is that particles are waves too, and thus this happens for particles; Pretty much all of quantum mechanics predictions occur in classical wave mechanics 63.205.40.243 05:12, 8 Jan 2004 (UTC)

I put it back in, since arguably the explanation of energy quanta is the major achievement of QM and gave it its name. The fact that classical theory can explain some quantizations doesn't take away from the fact that most quantizations are explained only by QM. --AxelBoldt

It's not a matter of ``most. The point is that quantum theory explains the right quantizations (energy levels in atoms for example). But the wording seems better now. --Matthew Nobes

3. The idea that quantum mechanics ``omits quantum field theory is a bit strong. I changed the wording of that paragraph to more accurately reflect things.

4. The history of quantum field theory was off. I corrected the QCD part and added about about the electroweak force.

5. To the primary author, why such an emphasis on the many-worlds interpretation? It's really not widely accepted...

I hear that Hawkings and with him all cosmologists believe in it (since you can't really talk about a measurement apparatus separate from the system if your system is the whole universe) and Feynman also seems to have been a many-worlder. --AxelBoldt

Just because he wrote a pop-sci book does not mean that Hawking speaks for most physicists (or cosmologists). My main point is that there is no particular reason to emphasize many-worlds over (say) Bohmian mechanics. -- Matthew

6. Also, the quotes section seems really silly to me. It gives the impression that QM is somehow wrong, misguided and/or incompherensible. It is none of these things. I would strongly encourge dropping it.

Well, if you are not worried by QM, maybe you don't really understand it... :-) Isn't it remarkable that almost all the major researchers in the field expressed their unease in some way? Why should we suppress these sentiments? Instead, maybe we should add some positive quotes. --AxelBoldt

Umm did you add the new quote? If so you made my point about ``big names versus people who nobody has ever heard of. --Matthew

Rest assured I understand QM just fine :) If read in context the writings of the founders of QM often appear less negative, further by including quotes from before the 1980's you throw out all of the interpretive work that has been done since. Things have been improved by leaps and bounds. Of course problems still remain. A quotes section cannot do justice this vast amount of work. As for positive quotes that doesn't really work becuase the average reader would assume that the Einstein quote is somehow ``better then the (say) Mermin quote on the basis of name recognition, despite the fact the the latter physicist understands *modern* quantum physics. In the end do what you wish, I'm only giving my perspective as a practicing physicist. --Matthew

Could you add some information to the Philosophy part of the article (or maybe create a new Philosophy of quantum mechanics) about the newest improvements in interpretating the theory? Is Copenhagen not the state of the art anymore?

Umm the philosophy/interpretation of QM is really not my field, so I'd be extremely reluctent to write anything about it. Ideas like dechoerence play a large role. It would also be importent to stress the crucial role experiments are now playing in resolving some of the mysteries. --Matthew

Also, what I never understood and maybe you could clear up on mathematical formulation of quantum mechanics: does the theory give a general (hopefully axiomatic) rule about which operator belongs to which observable and which unit vector belongs to which state of the system? In other words, if you encounter a completely new "black box" system with a couple of observables, which Hilbert space and which operators do you use? --AxelBoldt

This seems like two (or more) separate questions. The first is asking how one assigns the various operators to the observable. This is typically done with symmetry areguements a la Wigner. There is a different (practical) problem of mesurments on an unknown state. This is done by applying known measurement tools to the unknown state (i.e. a Stern Gerlach device) --Matthew

I guess the Wigner arguments are the ones I'm looking for. What would be a good reference? --AxelBoldt

Try a graduate level QM textbook. They all treat this stuff, with more or less detail. I learned it from the very clear presentation in L. Ballentine's ``Quantum Mechnics: A Modern Development. Sakurai or Merzbacher might also be good places to look. There's a new book out by Schwinger (posthumously published, entitled Quantum Mechanics) which looks like it treats things very clearly, but I've not had a chance to go through it. Also, the first volume of Wienberg's QFT textbook has a very clear and concise discussion, except he uses the Poincare group instead of the Gallian group. If you can understand his discussion then the application to non relativistic systems will be clear. --Matthew

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I'm not the primary author, but I don't see any special emphasis on the many-worlds interpretation in the article. Could you elaborate?

Also, I believe the many-worlds interpretation has become more widely accepted amongst physicists than the Copenhagen interpretation, especially given the work in the past couple of decades on decoherence. Correct me if I'm wrong, though.

The quotes section is silly, I agree.

The philosophy section is currently rather poor. I ripped out a long description of the Cophenagen interpretation and the many-world interpretation, because those points are duplicated in the respective dedicated articles, and don't explain the philosophical issues. It needs to explain why Einstein felt that a probabilistic theory sucks, the philosophical problems with Copenhagen, etc.

-- CYD

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I took out the following passage:

Quantum mechanics consists of 3 basic principles:

1. 1. All matter and energy is quantized. In other words, everthing comes in packets, or bundles. Therefore, one cannot have any amount of material. Instead, one must have a multiple of the smallest unit.
   2. All matter and energy exhibit has wave-particle duality. Matter and energy exhibit the properties of waves in some instances, and the properties of particles in others, depending on the experiment set up to observe it.
   3. Measurements of physical quantities are probabilistic. This is Heisenberg's Uncertainty Principle, which states that the position and velocity of a particle is not absolutely fixed at any given instant and can only be described in a range of probablities. Furthermore, any object does not have a definite position or velocity until it is measured.

I believe these are very misleading comments, and do not add anything to the article, but I'm willing to discuss it.

In (1), it is unclear what it means for "matter" or "material" to be quantized; less sloppy language is required. Furthermore, many quantum mechanical observables have continuous, not discrete spectra.

(2) had been mentioned earlier in the article, and Wave-Particle duality had already been linked to.

(3) had already been mentioned in the preceding paragraphs, and is much less clear than the prior explanations. Furthermore, it is momentum that is the conjugate observable to position, not velocity. Furthermore, the Heisenberg uncertainty principle works for observable pairs other than position and momentum.

I have not seen these three "principles" in any quantum mechanics textbooks. The postulates of quantum mechanics are described in the article mathematical formulation of quantum mechanics, which admittedly still needs work.

-- CYD

I agree, the first one is simply wrong, and the other two are already covered. And I also agree that we have to start getting serious about the math. forumulation :-) --AxelBoldt

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Can AxelBoldt please justify his recent removal of the statement that QM explains and quantifies the particle nature of light? It seems to me that the particle nature of light is inherent in the definition of the wavefunction - one could dismiss it as a mere "postulate", but the same argument could be applied to the wave nature of matter. AFAIK the relationship between photon energy and frequency is derivable - say from the quantisation of angular momentum.

Actually, in my opinion that whole first section could be replaced by a description of the empirical observations leading to QM (currently relegated to the first paragraph of the "history" section). It seems strange to me to introduce QM with a randomly selected handful of theoretical results. -- Tim Starling

Quantum mechanics doesn't talk about light; Quantum electrodynamics does that. Quantum mechanics is strictly about masses moving around. A photon is a quantum of the electromagnetic field, so you need quantum field theory. AxelBoldt 01:59 Nov 11, 2002 (UTC)

That's all very well, but it leaves us with nowhere to discuss the historical development of QM in a balanced way. Planck's and Einstein's work involving light came well before QED was developed. Perhaps this page should be moved to Quantum physics (which I note is just a redirect to this page at the moment) - that way we can talk about the early development of quantum theory without guilt or confusion. We would also have a place to talk about popular thought experiments such as the single photon double slit interference pattern. -- Tim

Planck's and Einstein's work is mentioned in the History section without guilt or confusion: it preceded and lead to QM, so it belongs there. It is not part of QM however. The particle wave duality page discusses the double slit experiment. AxelBoldt

Okay, I give up. I guess I'm just used to the historical development being used as an intro.

Afterthought: you say QM doesn't talk about light, but what would you call the theory of atomic radiation based on time-dependent perturbation theory and classical electrodynamics?

I don't know what that is. AxelBoldt 04:14 Nov 11, 2002 (UTC)

Easily fixed - [1] -- Tim Starling

The particle nature of light goes back to Einstein's explanation of the photoelectric effect, which is one of the foundations of quantum mechanics and obviously predates quantum field theory. Quantum mechanics doesn't necessarily deal with masses; you're thinking about the Schrodinger equation for a massive particle. For example, the quantum mechanics of spins doesn't talk about masses. -- CYD

Yes, I think Axel would argue that "quantum mechanics" is different to "quantum physics" - "mechanics" referring only nuts and bolts and the like. After four years of studying the subject, this distinction is new to me, and it sounds like it's new to you too. Obviously mathematicians are better at nitpicking than mere physicists. Nitpicking aside, there's a certainly common usage of the term QM which encompasses QED and the other "extensions". We have to ask ourselves - do we want to use Axel's definition, or everyone else's definition, in this encyclopedia? -- Tim Starling

Well, if you guys agree that QED is a part of QM rather than an extension of QM, then this article definitely needs to be changed. What is the proper name for the theory "observables are self-adjoint operators"? AxelBoldt

I wasn't sure, so I emailled God. Okay, maybe it wasn't God, just a professor of theoretical physics from my university. Anyway, here's what he said:

Yes, I would say that by common usage "quantum mechanics" usually

refers to the (nonrelativistic) quantum mechanics described by the

Schroedinger equation, while the full relativistic theory involving

"second quantization" is referred to as "quantum field theory". But then

an alternative usage of "quantum mechanics" would be to refer to the whole

field of quantum phenomena. As you say, "quantum physics" would be a

better term here; but the second usage certainly does exist.

So what do you think, guys, do we

1. Move the page to quantum physics
2. Change to the popular science definition
3. Stick with the technical definition

I'm going for (1) - it's obviously a less ambiguous term. We can redirect from QM to QP, and put a note in QP about usage of the term QM. -- Tim Starling

In your characterization of the Prof's answer, why did you relabel his "common usage" as "technical definition" and his "alternative usage" as "popular science definition"? This seems to be a distortion to me. AxelBoldt 00:32 Nov 15, 2002 (UTC)

If it's a distortion then I apologise - I only meant to summarise, with labels based on my own experience of the usage of those terms. I take it by your tone that you're voting for number 3? Tim Starling

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Certain pairs of observables, for example the position and momentum of a particle, can never be simultaneously ascertained to arbitrary precision (see Heisenberg's uncertainty principle).

This is not an effect classical physics cannot account for. Rather, this is natural result of quantum theory.

Also, I think it would be better and easier to follow to move the section on the history and development of Quantum theory to the beginning of this article rather than jumping right into the theory itself. But the historical section has a lot of content relating to the philosophy, so it really must be redone. When I get a chance! MattH 10:36 25 Jul 2003 (UTC)

If you mean that the momentum-position uncertainty relation is not directly observable, I tend to agree. I've boldly moved the bullet point to another spot in the article. (Note, by the way, that the energy-time uncertainty relation is observable, in the form of spectral line broadening.) -- CYD

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Very good! But the state is represented by a vector (or rather, by a ray) in a hilbert space only as long as it's not interacting. (A photon from a faraway star is a good approximation of it). A precise description is the density matrix, and that should in my opinion be mentioned.

Daniela

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Regarding the "Schrödinger equation doesn't contain gravity" quote from Feynman:

The quote was deleted because it is, firstly, a fairly inconsequential statement about quantum mechanics, even if spoken by Feynman; and secondly because it leads to some confusion over what is meant by "Schrödinger equation". As explained in the Schrödinger equation article, the term has two slightly different meanings. Modern quantum mechanics textbooks refer to the following equation as the Schrödinger equation:

$H\psi =i\hbar {\frac {\partial \psi }{\partial t}}\qquad \qquad \qquad \qquad \quad [1]$

where the quantum Hamiltonian H is not necessarily specified. Let's call this equation 1. It is obeyed by any quantum mechanical system, provided H is correctly given. In contrast, when older texts refer to the "Schrödinger equation", they also mean a specific form for the Hamiltonian, i.e.

$\left(-{\frac {\hbar ^{2}}{2m}}\nabla ^{2}+V(r)\right)\psi =i\hbar {\frac {\partial \psi }{\partial t}}\qquad [2]$

This second equation, which we'll call equation 2, does not describe some quantum mechanical systems. For example, the equation for a charged particle in an electromagnetic field is

$\left({\frac {1}{2m}}\left|-i\hbar \nabla -eA(r)\right|^{2}+e\phi (r)\right)\psi =i\hbar {\frac {\partial \psi }{\partial t}}$

Tim Starling's remark on my talk page, that the "Schrödinger equation" (equation 2) doesn't contain electromagnetism, is therefore not incorrect. However, it is slightly misleading, since electromagnetic systems can be described quantum mechanically, i.e. by equation 1. (We can even formulate quantum Hamiltonian for the electromagnetic field, as is done in the theory of quantum electrodynamics.)

As for gravity, equation 2 can be used to describe the motion of a particle in a gravitational field, simply by making V the gravitational potential. It cannot, however, be used to model gravitational phenomena, not even the production of a gravitational field by a particle. Whether gravity can be described quantum mechanically, i.e. by equation 1, remains an open question. -- CYD

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I am suprised to see no mention of Bohmian Mechanics in the entry. I will preface by saying that I myself am no quantum physicist. But, as I understand, Bohmian Mechanics constitutes a coherent interpretation of quantum phenomena without the need for surmising any of the no less than bizarre accounts of reality such as 'there is no fundamental reality' or 'there are infinately many realites', the essense of the Copenhagen Interpretation and Multiple Universes respectively. From the little that I understand, Bohmian Mechanics says that it is a particle 'riding' on a wive (a pilot wave of sorts) that correctly explains experiments like the two-slit experiment (which by the way is also worth mentioning in the entry as it is the simplest and most fundamental experiment that encapsulates the basic paradox of quantum reality - that is, particle/wave duality) I see the earlier attempts at explaining quantum mechanics by way of positing 'new' or 'modified' realities as rather vein attempts to account for the paradox which had every physicist thoroughly befuddled. This is also what gave rise to those all too famous quotes, and they too should be understood in the context of that initial struggle by the scientific community to come to terms with the nature of the paradox they were faced with. But as I mentioned, I was under the impression that BM solved the paradoxical behavior of the wave/particle. - Mike C

There is no paradox in QM except for the paradox of why physicists continually try to confuse the general public by using Copenhagen language to highlight the exact scenarios where the Copenhagen interpretation breaks down, then declaring "that's quantum mechanics for you". Schrödinger's equation:

$H\psi =i\hbar {\frac {\partial \psi }{\partial t}}$

is not ambiguous, so shut up and calculate.

I added links to interpretation of quantum mechanics and Bohm interpretation. A short description of BM was copied from its article. -- Tim Starling 07:36, Dec 4, 2003 (UTC)

--I only meant to say that originally there was a paradox, namely, how in the world can a particle exhibit wave-like properties? This was what had everyone thoroughly baffled by the two-slit experiment. That initial bafflement and wave/particle paradox is an integral part of the history of QM and should be presented in such a context in the entry.

-secondly, Bell's theorum proved that any proposed model of quantum reality must be non-local. I feel that mentioning Bohms interpretation as non-local and as attracting little support from physicists in the same sentence will confuse the reader into the mistake that these two things are somehow connected. In fact, according to Bell, physicists must hold a non-local view of the universe and this should be no reason for them to reject BM. Furthermore, there are at least a number (how many I don't know) of respected mathematicians who now support BM. -- I recomend that we move any further discussion of the Bohm interpretation to the talk page for that entry though.

-Also, as to shut up and calculate, this seems to me to be a grievous prognostication for thwarting all further inquiry into the nature of quantum reality. If you want a perfectly analogous example to this, look at the history of Ludwig Boltzmann and atomic theory at the turn of the 20th century. He was told for decades and vehemntly renounced by the scientific community as the last man standing, holding that atomism was a model that could account for the kinetic theory of gases among other things. He was told, 'don't worry about models, that's a bunch of philosophical rubbish, just do the math' and 'you believe in something that is so small it can't be seen or measured, ludicrous'. Well eventually people went on to prove atoms existed and the microscope soon became powerful enough to study them. The point is that many thought him absurd for even trying to draw a model of reality out of the math much less one that was not observable or even measurable at that time. But if not for his stubborn insistence, it is highly likely that atomic theory would not have fully developed till much later than it did. - Mike C

Transactional Interpretation

quoting removed section:

The transactional interpretation, put forward by John Cramer, describes interactions in terms of standing waves in space-time. These standing waves are formed when advanced (forward in time) and retarded (backward in time) waves interfere. Once formed they look identical to a particle. This interpretation avoids the need for an observer to collapse the wave function. It also makes use of previously discarded backward in time solutions to wave equations, and resolves all of the so-called quantum paradoxes (see above). Despite the elegance of this interpretation, it does not seem to have caught on.

This is an interesting idea, but it is not nearly accepted enough to be mentioned alongside standard interpretations. Also, the idea of four-d support of the wave function in Hilbert Space with an external evolution parameter is hardly new. Many others have suggested it, and to attribute it one person is very misleading. -- Decumanus 21:34, 13 Feb 2004 (UTC)

Thanks for correcting. As many others have thought of it, do you not think it is at least worth mentioning? Not up there with the big ones, but as an interesting note? It seems a very elegant way of resolving a number of nasty problems with the CI. I often wonder how classical theories like general relativity would look if you replaced the notion of particles with the notion of a standing wave in spacetime. My degree level physics isn't up to it though! -- Mike Howells 21:50, 13 Feb 2004 (GMT)

I very much sympathize with your wanting to include mention of such things. It is very difficult in an article on Quantum Mechanics to include it in a way that is worthy of its acceptance (or lack of) among the community of physicists. There is perhaps a way to include such material as the transactional interpretation, but putting it beside more accepted interpretations is not the place. I myself have done much research in this area, but I am extremely cautious about including my own work in physicis article. In fact, I simply don't, as a rule, even though it is published.  :) -- Decumanus

In that case, you may wish to do something with the (now orphaned) article on the transactional interpretation. It does get a mention in popular science books like Schrodinger's Kittens, so it perhaps deserves a mention somewhere on Wikipedia? MH

Good point. The article on transactional interpretation itself is certainly worthy, in my opinion, of an article, since it is published research. I don't know the topology of these article pages well enough to know where it should go exactly, as a link, but I'm sure that other people do. If not, perhaps there should be a page on non-standard interpretations of quantum mechanics. The problem is that such a thing opens the door to what we call "crankery". The TI is certainly not in that category, however. There are many such theories that fall into the category of published-yet-still-speculative, especially in regard to quantum systems. -- Decumanus 22:12, 13 Feb 2004 (UTC)

A few concerns

I'm a relative newcomer to this article (responsible for adding the Willis Lamb quote) and the quantum-physics branch of the Wikipedia, and have a few questions:

• Why do we have separate articles on, say, quantum states?
• Why do we continue to use the obfuscatory and outmoded term "wave function" when we simply mean "state"?
• What is the point of including a section of "gee-whiz" quotations, most of which make it sound as though QM is ridiculous as opposed to merely nonintuitive?

Also, I *strongly* object to the statement that the Heisenberg uncertainty principle (presumably the position-momentum one--the time-energy one is a different and more subtle issue) is a result of "wavefunction collapse". Wavefunction collapse (as has been rehashed time and time again) is not a process, furthermore, the uncertainty principle says nothing about measurements separated in time, so even if there really was such a thing as wavefunction collapse (it's just a $10 word for "measurement"), the uncertainty principle, which is about simultaneous measurement, would not be a consequence of it. Forgive me for being pedantic about it, but position-momentum uncertainty principle is a generalisation of a truth about noncommuting observables. (Most phenomena that can be ascribed to the uncertanty principle, (which is an excellent heuristic) and some that usually are not (such as vacuum fluctuations in the light field), are actually the result of a noncommutativity. None result from the "collapse".)

That one cannot simultaneously diagonalize with respect to noncommuting observables is easy to show mathematically in a few lines to anybody familiar with linear algebra; the general uncertainty principle is a little trickier to obtain but still is an elementary relation between the product of the deviations and the commutator of the associated observables. This is all very simple to a mathematically literate reader, but may be too technical for the target audience of this article. However, I do believe that noncommutativity can be succinctly described in plain English. David Finkelstein's treatment in "What is a Photon?" (OPN Trends, Vol 3 No 1, Oct 2003) would be a good start. (For those interested in simple and direct exposition of quantum mechanics, I actually recommend the entire issue, especially the Loudon, Finkelstein, and Muthukrishnan/Scully/Zubairy articles.) I'm willing to take a crack at it, and also to add some material to the Uncertainty Principle article. I find the current state, with the glaring inaccuracy, to be rather embarassing, especially given that this is supposed to be a featured article.

--[User:bkalafut|bkalafut], 31 March 2004

I'm not sure what you mean about "wavefunction" being obfuscatory and outmoded. I use it all the time. So do all the papers I read, so do my colleagues. The PRB paper I'm in the process of writing will use it.

I completely agree with you on wavefunction collapse, I think it's a horrible concept. But I'm too afraid to say that in a public place, generally speaking. I'm laughed at when I say it's rubbish in real life, and Wikipedia is not the place for personal opinions. It's a horrible, ugly concept, but unfortunately it is very widely used by physicists. It guides their thought and the way they speak about a wide variety of topics. I have to admit it may be conceptually useful at times. It's certainly too important to ignore. What you have to understand is although it's an inelegant way of expressing an elegant mathematical concept, the Copenhagen interpretation is successfully used by many physicists who are fully aware of the mathematical and conceptual issues. They hold several philosophical viewpoints in their head at once, and switch between them freely.

Also you have to consider Wikipedia's audience. The fact is that most readers of this article would not understand the statement "one cannot simultaneously diagonalize with respect to noncommuting observables". Perhaps it is better to stick with the conventional way of speaking, and thus build on what readers may have picked up in school, college and popular science. -- Tim Starling 06:20, Apr 25, 2004 (UTC)

Umm, where in the article is it stated that the uncertainty principle is a result of wavefunction collapse? I don't see it. -- CYD

I think I took it out, or somebody else did. Good riddance! Bkalafut 04:36, 13 May 2004 (UTC)

Regarding Wikipedia's audience, one could explain commutativity as a statement about simultaneous measurability. Perhaps that's too specific to be entirely technically accurate, and I ought to take a closer look at the article I cited--I remember what the article was about, but not specifically what I was referring to. I think I said above that talk of simultaneous diagonalization was too technical for this article; I think we're agreeing on this one. Regarding the term wavefunction, I may be wrong about it being outmoded--it does seem to be in use in quite a few subfields and circles. I still think it's obfuscatory. For one, it's ridiculous to say "the electron's wavefunction", as often happens, instead of the "state of the electron." Wavefunction, in the popular literature, is a $10 word for the $.10 concept "state," and sometimes even for the object itself. (That isn't to say that the vector character is a $.10 concept, rather that it's ridiculous to say that "the electron interferes with its own wavefunction" when it is much more clear to say "the electron interferes with itself.") Furthermore, one sees in the technical literature statements about there being no way to represent a photon (in the sense of a pulse that can only produce on detector click, not in the single-mode sense) or a spin-system as a wavefunction. Two different senses of the word "wavefunction", one technical, and the other slang, but the distinction is probably lost on the lay reader. Also, the analogy inherent in the term is practically useless when applied to things such as the electromagnetic field (which I guess can be represented as a continuous-basis "wavefunction" in terms of coherent states) and many-particle wavefunctions, and it leads to thorough confusion when one starts to talk about "entangled wavefunctions". I understand that wavefunction is accepted slang in some (maybe in most) subfields, and it's even a proper technical term in some cases, it is precisely a consideration of the audience which leads me to favor not using it here.

I've stayed out of the "Copenhagen" debate on epistemology and metaphysics, and I'll stay out here. I have no problem with the concept of "collapse"--and I don't see it as not happening in the Everett Many-Worlds interpretation--my objection is to the term. --Bkalafut 04:36, 13 May 2004 (UTC)

quantum physics, quantum theory

I saw that this was discussed before, but nevertheless I was not happy to see that "quantum physics" and "quantum theory" are listed as simply synonyms of quantum mechanics, when often they are used to make a distinction. I added a couple of sentences to this effect to the introduction, telling that someotime they are meant to mean something slightly different...please check and modify, but, please, dont let them again as simply alternative names.--AstroNomer 09:28, Jun 2, 2004 (UTC)

Are you saying that quantum physics is a superset and quantum theory is a subset? Bensaccount 16:59, 13 Jun 2004 (UTC)

I think what is needed is to have all three articles created (Quantum mechanics, quantum physics, & quantum theory) and then note the difference between them.Bensaccount 17:09, 13 Jun 2004 (UTC)

Revaz Dogonadze

The following paragraph has been added by various anons over the last few days.

Revaz Dogonadze was a main author of the quantum-mechanical theory of the elementary act of chemical, electrochemical and biochemical reactions in polar liquids (1970s-1980s) and co-author of the quantum-mechanical model of enzyme catalysis (1970s). He was one of the founders of Quantum Electrochemistry.

I think it does not belongs here. There are thousands of scientists who have done research on quantum mechanics and related topics. The ones that we mention in the article are very, very famous ones: Einstein, Dirac, etc. I do not think that Dogonadze has done research of the same magnitude as Dirac. He is real, he has published papers which are getting cited (53 citations in 2003, according to Science Citation Index) but I suspect he is more like one of hundreds of good physicists whom we do not mention, rather than Einstein, Dirac or one of other Nobel laureates whom we mention here.

I removed it yesterday, because I saw some dubious past contributions from one of IPs that added this paragraph. It got restored by another anon. I would like to hear what other people think, before doing anything again.

Any opinions on this? Andris 22:17, Jun 26, 2004 (UTC)

Dear Andris, Professor Revaz Dogonadze (1931-1985) was one of the greatest scientists of the XX century, founder of the well-known scientific school of Quantum Electrochemistry, main author of a well-known Quantum-Mechanical Theory of the Elementary Act of Chemical, Electrochemical and Biochemical Processes in Polar Liquids. He was author of many classical scientific works. With kind regards, Professor Zurab D. Urushadze. July 26, 2004.

Dear Prof. Urushadze, please, substantiate your claim. If you can give references to scientific books/articles/print encyclopedias (either in English or Russia) that recognize Dogonadze as a distinguished scientist and his works as "classical", that will be very helpful. Andris 08:46, Jul 26, 2004 (UTC)

Dear Andris, thank you for your answer. I inform you about some important links and publications: 1) Famous Electrochemists ( http://chem.ch.huji.ac.il/~eugeniik/electrochemists.htm ); 2) Famous Chemists ( http://www.liv.ac.uk/Chemistry/Links/refbiog.html ); 3) People: Quantum Mechanics ( http://physics.designerz.com/physics-quantum-mechanics-people.php ); 4) Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, Vol. 204, 1986, Elsevier Sequoia S.A., Lausanne, Prof. Revaz Dogonadze Memorial Issue (ISSN 00220728); 5) R.R. Dogonadze, E. Kalman, A.A. Kornyshev and J. Ulstrup (Eds), The Chemical Physics of Solvation, Parts A-C, Elsevier, Amsterdam, 1985-1987 (ISBN 0444426744); 6) Alexander M. Kuznetsov and Jens Ulstrup, Electron Transfer in Chemistry and Biology, John Wiley & Sons, 1999 (ISBN 0471967491). I inform you also about contact details of Dr. Jens Ulstrup, a distinguished scientist, Professor of the Technical University of Denmark: Phone (direct): (+45) 4525 2359, E-mail: [email protected] . With best regards, Prof. Zurab Urushadze, July 27, 2004

proposed structure for the article

My last edits reflects my view that this is the best way to present a scientific theory:

• start with the limitations of previous theories (here: electromagnetism): no quantization and no wave-particle duality
• describe the theory, and how it overcomes the limitations of the previous theory (this is the bit that I saw missing, and corrected)
• describe predictions of the new theory (here: entanglement)
• describe limitations of the new theory

Pcarbonn 12:12, 11 Jul 2004 (UTC)

I have now restructured the article along those lines. See Wikipedia:WikiProject Science to further discuss this. Probably needs some more work though. Pcarbonn 19:46, 16 Jul 2004 (UTC)

This is fine, only you have now, perhaps accidentally, deleted the insertions I had made a few weeks ago to soften any implication that Bell test experiments had conclusively confirmed quantum mechanics. They have not. There are still serious loopholes in all the experiments -- they are still trying to design a loophole-free one. Caroline Thompson 21:49, 16 Jul 2004 (UTC)

Sorry, this was accidental. I had to move quite a few things around, so this may have happened. I would suggest your insert it again in the "Quantum effect" sub-section. (you know this subject better than me) Pcarbonn 19:04, 18 Jul 2004 (UTC)

quantum entanglement, Bell's inequality

I have a question about these two sentences:

"This phenomenon is called entanglement and its difference from ordinary correlation is described by Bell's inequality. Experimental violation of Bell's inequality are, despite the presence of loopholes, currently accepted as one of the major verifications of the quantum theory."

If the difference of the quantum phenomenon (entanglement) and the normal correlation is described by the Bell's inequality, then its verification would be the verification of the quantum theory, wouldn't it? Or better the "ordinary correlation described by Bell's inequality" should be written.

Pál 23:51, 14 Jul 2004 (UTC)

Wave-particle duality vs uncertainty principle

I think the subsection "Quantum mechanical effects" has a false suggestion. Say thinking of the momentum-spatial coordinate uncertainty principle it suggests, that of of them is particle like variable, and the other is wave like. There are two basic representations, the Schrodinger (spatial coordinate) and the Heisenberg (momentum) representation of QM, and both have the wave-particle duality. The relativistic theory prefers the momentum representation, but it does not mean, that either the wave or the particle behaviour is lost. The propagation is wave like and the creation and annihilation is particle like. User:Hidaspal 20:30, 15 Jul 2004 (UTC)

Fine. In the section 'Quantum mechanical effects', I would really like to have an explanation on how those effects are explained by the theory, e.g. by referring how the wave function behaves when those effects are observed. Otherwise, the section just explains what wave-particle duality is, but we can find that in the corresponding article. I'm sure many people would find this interesting. Unfortunately, I don't know enough myself to explain it correctly. Could you help ? Pcarbonn 19:26, 16 Jul 2004 (UTC)