Talk:Wave–particle duality

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[edit] Wave-particle duality as MYTH

I added the link to paper Quantum mechanics: Myths and facts in the "External links". In this paper, the Wave-particle duality is taken as Myth. For more information read Quantum mechanics: Myths and facts. Lseixas 04:06, 24 December 2006 (UTC)

Yeah it's annoying how much this particular "meme" is propagated. Quantum entities are just that - entities that follow a particular set of rules based on quantized energy, the principle of least action, and various conservation laws. They aren't waves or particles, however it sometimes does help to think of them as one or the other in certain limiting cases. IMHO it's not myserious at all - it's just objects that obey math that we don't find intuitive. I agree very strongly with the school of "shut up and calculate" :) - JustinWick 19:21, 22 January 2007 (UTC)
I, too, think of duality as somewhat mythical. Yet I use it all the time, since in optics all the progagation part acts like waves and all the absorption/detection part acts like particles. So it's a useful myth. Dicklyon 21:47, 22 January 2007 (UTC)

[edit] For the record

This may not belong in the article, but I just wanted to mention for the record that the whole issue basically goes away in modern quantum mechanics. The wave model was particular to early quantum mechanics. This is sometimes called "Wave Mechanics" today, to distinguish it from modern QM. Typically, undergraduate courses in quantum mechanics never get beyond this. Modern QM deals with states of quantum systems as abstract objects—points in Hilbert space, a complex vector space. These can be manipulated with various kinds of mathematical operators, some of which correspond to physical measurements that can be performed on the system. Like an ordinary 3D vector, the states can be expanded in terms of a set of basis vectors. One can extract the "wavefunction" corresponding to a particular state by expanding it in terms of the basis set of position vectors if you like, but you don't have to do this in general.

None of that probably belongs in this article, but my point is that in modern QM an object is neither a wave nor a particle. It is what it is. If you choose to describe the system in a position-space representation you can describe it as having wave-like properties. You're perfectly free to choose another basis set, however, in which case the description may be quite different even though the results are the same. There are an infinite (IIRC) number of basis sets to choose from, so there is nothing particularly special about "waves" or "particles", except that they are kindof intuitive.--Srleffler 01:57, 13 December 2005 (UTC)

  • Actually, the section called "Theoretical Review", near the bottom of the article, was meant to contain such statements. I didn't feel like writing a grand review of what QM is, so I left it vague, but it is clear from previous edits to this article that many readers are thirsting for a technical rather than a historical presentation. Feel free to add or re-write that section; however, please don't oversimplify analogies, I'm sick of deleting "particles are like guitar strings" type content. You're scaring me with the "IIRC" comment.linas 15:23, 13 December 2005 (UTC)
  • I agree with Srleffler's above setiments, however the statement about infinite basises - yes there are an infinite number to choose from, however the most useful ones tend to be eigenfunctions of important operators. The "wave" function space in free space is important because it is the set of eigenfunction (at least in nonrelativistic QM) of the momentum operator, which is of course very important. Same with the dirac delta being the engenfunction for the position operator. These choices are not merely intuitive, but mathematically necessary to study the results of these operators. I do agree, however, that the whole wave/particle concept is now pretty irrelevant, however to understand the history of physics, it is absolutely vial. I agree with Linas that the final section should be improved, but we do have to be sure to stay away from oversimplification. - JustinWick 15:43, 13 December 2005 (UTC)

[edit] Picture

I complied with your "Good Article" request, but it would be great if this article had a picture. I'm not sure what you could add a picture of, but it looks like this needs one. joturner 17:35, 25 December 2005 (UTC)

[edit] Objects

  • JA: The word "object" does not imply tangibility. If you want to imply tangibility you say "tangible object". An object is an "object of discussion and thought" (OODAT). Some folks would add "object of consistent discussion and thought" (OOCDAT). So, for example, numbers are objects, even imaginary numbers. Jon Awbrey 20:45, 17 February 2006 (UTC)
    • object –n. 1 a material thing that can be seen or touched.
The Concise Oxford Dictionary of Current English, 8th ed., Ed. R.E. Allen, Oxford: Clarendon (1990).
--Srleffler 01:47, 19 February 2006 (UTC)
  • JA: Oh, surely it's not that concise. Jon Awbrey 04:08, 19 February 2006 (UTC)
    • Of course not. There are several more meanings. My point, though, was that the first meaning, the most common one, specifically implies tangibility contrary to what you asserted. In general usage it is not necessary to add "tangible" to distinguish from other meanings. The usage in philosophy might be otherwise, but this article is about physics and the common usage of object is just fine here.--Srleffler 05:07, 19 February 2006 (UTC)
  • JA: (A => 1 or 2 or 3) <=/=> (A => 1). If they are still using logic in physics, and if they are still using numbers in physics, then object does not imply tangible, even in physics. Jon Awbrey 05:14, 19 February 2006 (UTC)
    • Sorry, but that just doesn't make any sense. It's perfectly valid to use a word in its most common sense, notwithstanding the fact that the word may have other senses.--Srleffler 06:19, 19 February 2006 (UTC)
  • JA: Yes, but the word does not "imply" the meaning that its most common use has. It implies only the meaning that all of its uses have in common. Jon Awbrey 06:26, 19 February 2006 (UTC)
    • OK, so then what was your point? Why should we care what the word "implies"? The word is used correctly in the article.--Srleffler 06:30, 19 February 2006 (UTC)
  • JA: That was my point. Someone expressed some doubts, and I was trying to soothe them. That's evidently more difficult than it sounds. Jon Awbrey 06:38, 19 February 2006 (UTC)

[edit] Probability density

The magnitude of the square of the wavefunction is a probability density, not a probability. Probability densities can exceed 1. I believe the article should reflect this mathematical fact - probability density is not that difficult and there is an entire article article on it for those unfamiliar with the term. - JustinWick 05:06, 23 February 2006 (UTC)

I don't believe probability densities can exceed 1. What gave you that idea?--Srleffler 12:26, 23 February 2006 (UTC)
Certainly they cannot exceed 1, but this is merely a result of semantic nitpicking. Probability densities have the dimensions of inverse distance, and so may not even be compared in magnitude with a dimensionless number. However, for all intents and purposes, there is really nothing wrong with saying they can exceed 1, as there is nothing to stop a probability density from being as large as you like, e.g., one can have a probability density of something like "one billion per meter", as long as this density extends over no more than a nanometer. - Expensivehat 04:41, 6 March 2006 (UTC)

[edit] good article?

I've removed this article's good article status. The reason is that the only theoretical explanation that this article provides of wave-particle duality is that quantum systems are solutions to differential equations. That condition is neither necessary (there are quantum systems which exhibit the duality which are not solutions to diff eqs) nor sufficient (there are classical systems which are solutions to diff eqs which do not exhibit the duality) as an explanation of the phenomenon. A historical treatise on the evolution of whether light was modeled by waves or particles is interesting, but certainly not the primary reason purpose for this article. As such, not only is this not a good article, it is a bad article. -lethe talk + 23:01, 7 March 2006 (UTC)

Hi, well, as I'm repsponsible for the current state of the article, this remark catches my attention. I am rather totally unclear on what you are trying to say here. Is there any sort of definition of wave-partcile duality aside from random handwaving and appeals to history? To the best of my knowledge, there is no such thing as a "theoretical explanation of wave-particle duality", beyond the usual overheated argumentas about the interpretation of quantum mechanics and local realism. What do you have in mind? linas 00:45, 8 March 2006 (UTC)
Observables in quantum mechanics are represented by operators on a Hilbert space. If two observables do not commute, then the spectra of the two operators exhibit complementarity. We understand waves as momentum eigenstates and particles as position eigenstates, then wave particle duality is described by the noncommutativity of X and P. In relativistic field theory, there is no position operator, and instead we will talk about localization operators. None of this is mentioned in the article, instead there is talk of differential equations, the relevance of which I fail to see entirely. I also don't see the relevance of any discussion of local realism or interpretations of quantum mechanics. As far as I know, all formulations of quantum mechanics exhibit wave particle duality, which shows that this is not dependent on the interpretation. -lethe talk + 01:29, 8 March 2006 (UTC)
Complementarity has nothing to do with wave-particle duality. Complementarity just says things with momentum p have a wave number hbar k. But what's a particle? To talk about particles in QM, you have to talk about "wave function collapse", and that's thin ice. There are questions: when I measure a wave, why don't I ever see two half-particles? At its heart, QM is a wave theory, and no more. At least in QFT, you have the idea of creation and anhilation operators, and so finally you get back to something that can be called a particle. Anyway, you are right; the theory section is lacking; and someone should revise it. I have just about zero interest in writing a "theory" section for this article, mostly because I can't think of anything lucid to say about the theory aside from "shut up and calculate" (or rather, shut up and hit the books). linas 01:43, 10 March 2006 (UTC)
Perhaps you should take a look at complementarity. It is not the principle p = h/λ. I don't agree with much of your comments. I suppose the best solution would be for me to undertake to rewrite the article. Perhaps someday I will. -lethe talk + 04:14, 10 March 2006 (UTC)
I agree; I think Linas is referring to the de Broglie hypothesis. I've added a section on the Heisenberg uncertainty principle, explaining that it follows from the application of de Broglie to classical field theory, which may answer some of your earlier concerns about misleading statements about its derivation. --Michael C. Price talk 14:23, 29 November 2006 (UTC)

[edit] Wavelengths of human sized objects.

The article says "One does not observe the wave-like quality of everyday objects because the associated wavelengths of people-sized objects are exceedingly small." But can a human sized object be described by a single wave? There's no object of that kind of mass that is composed of a single fundamental particle. So wouldn't the non-wavelikeness of large objects be due to the fact that they are made up of lots and lots waves with wavelengths much smaller than the physical size of the object?

You don't have to be a fundamental particle to exhibit a wavefunction. For example, protons have wavefunctions, and atoms, and even some molecules exhibit a little bit of quantum interference. Furthermore, for certain systems like Bose-Einstein condensates and superfluids and superconductors, quantum effects are manifest even though there are a macroscopic number of particles. However, you're still right. The presense of a whole bunch of particles tied together greatly suppresses the quantum effects, if the particles are thermalized, meaning they are randomly arranged with no possibility of coherence of their wavefunctions. In this case, the large numbers make the objects behave classically, and this phenomenon is known as decoherence. So macroscopic objects do not exhibit quantum effects for two reasons: 1. h is too small, the wavelengths are completely unobservable, and 2. because of the large number of thermal particles, all quantum effects get averaged out through decoherence. -lethe talk + 05:15, 27 March 2006 (UTC)

Standard Quantum states that there is only *one* “wave function” for an ensemble of particles , not a sum of individual postulated wave functions of each particle.. This immediately dispenses with the daft idea that particles are also “waves”. QM states the probability of finding *particles* in regions of space. End of story. The “wave nature” of particles is simply that fact that particles don’t obey Newtonian Mechanics. Waves, as clearly shown manner times, are just the statistical collection of individual particles. 217.155.191.217 12:50, 8 February 2007 (UTC)

[edit] Mathematically Possible

If at all, how is the dual nature of light mathematically possible?

--203.200.220.71 11:20, 7 August 2006 (UTC) My question is why do we have to introduce photons for explaining the photoelectric effect? would it not be sufficient to say that only light waves of a certain frequency and above can knock off electrons? --203.200.220.71 11:20, 7 August 2006 (UTC)

I have expanded the explanation at Photoelectric_effect#Einstein:_light_quanta. --Michael C. Price talk 12:08, 7 August 2006 (UTC)
The introduction of photons, or quantized light energy, to explain the photoelectric effect, is historical and conventional. Einstein proposed it, got the Nobel prize for it, and it worked. But it is not the only way. One can explain quantum transitions between electron energy levels (which the photoelectric effect is a special case of) by the quantum electromagnetic interaction of the electrons; the transition involves a mixture of states, so the wavefunctions of the two states, differing in frequency by the frequency that corresponds to the energy difference, will make a "beat" in the electron distribution, which will allow it to couple electromagnetically to other subsystems of the universe that are at zero space-time interval (i.e. with other atoms that can absorb the photon when it gets there, if you want to put it back into photon terms). The details of this view of quantum transitions is explained by Carver Mead in his book Collective Electrodynamics: Quantum Foundations of Electromagnetism. Einstein was unhappy with the "random" results of his proposal, i.e. the Copenhagen interpretation that led only to probabilities; but he did not make the bold step of retracting the quantization of light, putting all the quantization into electron eigenstates instead, and finding the alternate view that the apparent randomness is due to all the other bits of the universe that every photon event is potentially coupled to. Wheeler and Feynman did a bit of that, but backed off; Mead picked it up later, as did Cramer, in his transactional interpretation of QM. Dicklyon 19:51, 7 August 2006 (UTC)

[edit] Revert by afshar

Ashfar removed my comment in the intro about how photons were the first to be seen as both waves and particles. I gave the explanation that they are "both", and that in a double slit experiment, a detector detects points - and over time diffraction patterns show up.

Anyways, I'm not sure what is wrong about it. I skimmed the article on Complementarity (physics), and it doesn't seem to contradict me. Whats specifically wrong with the paragraph? I thought it would help clarify what wave-particle duality means, and especially what it means for something to "sometimes act like a wave and sometimes like a particle. Fresheneesz 10:17, 23 April 2006 (UTC)

  • Dear Fresheneesz, you must be careful not to say that photons are "both" unless you qualify that by saying "but never at the same time." Bohr's Principle of Complementarity posits that both wave and particle natures of quantum systems (including photons) are equally important but are not both present in the same experiment. Although, I have performed an experiment that violates Bohr's assertion, I would not support such declarations without mentioning the controversial nature of my findings. My results are still being critically studied, and until the dust settles one way or another, it would be irresponsible to declare simultaneous presence of wave and particle behaviors for photons or any other particle. It is however allowed to talk about wave-particle duality in the sense presented by the de Broglie relation in which both wave (wavelength) and particle (momentum) properties show up. I hope this explanation helps. Regards.-- Prof. Afshar 03:48, 25 April 2006 (UTC)
Hmm, what experiment have you done that you mentioned? I believe that slit-experiments involving a single particle at a time imply that photons are in fact both waves and particles at the same time. A particle will hit somewhere on a detector, but throughout the length of the experiment, the particles will eventually hit in a pattern that is predicted by wave diffraction. Saying that the photon acts like a wave sometimes, and a particle sometimes - doesn't quite work in this case. The effects wouldn't show up unless the photon is both, at the same time. Fresheneesz 03:31, 26 April 2006 (UTC)

The experiment shows that discrete particles have a characteristic number, that is usually associated with “waves”, conventionally named lambda (wavelength). This lambda number can be calculated by examining the statistics of the locations of the particle spots. That is, an average “wavelength”, with a standard deviation for that wavelength, can be calculated. This wavelength can be nominally assigned to a particle, but it only has real meaning, when discussing an ensemble of particles. Thus, the experiment indeed shows particle and “wave” nature together. There is of course, no real wave in any medium. The wave is an illusion, just as a water wave is an illusion, brought about by the statistics of large number of water molecules. Wave-particle duality is completely bogus. Standard QM is about particulate objects that follow non Newtonian laws of motion. End of story. Kevin aylward

If wave-duality is such a load of shit then please describe how a radio antenna measures the signal frequency in terms of radio particles. (And please time-date stamp your signature for further replies.) --Michael C. Price talk 18:31, 28 October 2006 (UTC)
See Afshar experiment for Afshar's experiment. He claims that he measured a violation. I agree with Afshar that such a claim doesn't belong at the top of this article, and I appreciate his modesty. -lethe talk + 04:11, 26 April 2006 (UTC)
Wow, very interesting. I'm honored. I still don't understand why a single photon double-slit experiment also doesn't contradict complimentarity. Fresheneesz 06:55, 26 April 2006 (UTC)
Basically, complementary says you can not both see the diffration pattern and determine which slit the particle went through. In the two-slit experiment with both slits open, you get the diffraction pattern, but no idea which slit a particle went through (because it had to go through "both" to interfere); with one slit closed, you know which slit the detected particles when through, but you lose the diffraction pattern. That's all the duality is. Afshar's experiment can be interpreted as saying that the hole through which the photon went is determined, at the same time as a wave pattern is determined (somewhat indirectly); exactly how it relates to the formal principal of complementarity is subject to considerable dispute. Dicklyon 23:41, 30 May 2006 (UTC)
so the short answer, Fresheneesz, is that the single photon double-slit experiment does contradict complementarity as defined by Afshar, since the photon exhibits wave behaviour by going through both slits and particle behaviour when it "collapses" onto a single sliver-bromide emulsion grain. Of course most people do not accept Afshar's definition of complementarity he gave above as implying that wave and particle properties "are not both present in the same experiment". Rather the conventional view is that pure wave and particle behaviour may not be present at the same space-time event, as quantified by the Heisenberg uncertainty constraints. --Michael C. Price talk 18:23, 22 June 2006 (UTC)
Dear Michael, Once again you have misrepresented the facts. I have never said that the single photon interference experiment violates Complementarity. In fact I have specifically mentioned on page 3 of my original paper (if you care to read before making uninformed statements) that: " It is noteworthy to mention that quantum mechanics does not forbid the presence of non-complementary wave and particle behaviours in the same experimental setup. What is forbidden is the presence of sharp complementary wave and particle behaviours in the same experiment. Such complementary observables are those whose projection operators do not commute [1]." Projection operators for sharp which-way information and interference qualify as complementary observables. A simple interference pattern contains no which-way information, thus it is not subject to Complementarity. Please read the paper by [1] Bandyopadhyay, Phys. Lett. A 276 (2000) 233, you may learn a thing or two on the subject.
As regards Bohr's statement concerning Complementarity, here's what he himself has said: "...we are presented with a choice of either tracing the path of the particle, or observing interference effects…we have to do with a typical example of how the complementary phenomena appear under mutually exclusive experimental arrangements."[2] N. Bohr, in: Albert Einstein: Philosopher-Scientist, P. A. Schilpp, Ed. (Library of Living Philosophers, Evanston, Illinois, 1949). There is no mention of the requirement you impose above that the measurements be made "at the same space-time event." In fact, it is impossible to see an interference pattern if the measurement is supposed to be made at the same spacetime event, as the pattern only builds up over time, which means it requires great many space-time events to register as interference pattern. Sorry to burst your bubble, but you really need to read up on the subject before you embarrass yourself any further. You can start with my paper, and references therein. On the other hand, if you are deliberately spreading disinformation on my work, you should expect to see some repercussions. Regards.-- Prof. Afshar 16:11, 28 October 2006 (UTC)
If you're saying that the Schrodinger wave equation and complementarity are incompatible then just come straight out and say it; get the claim published in a peer rewiewed credible journal and I'd be very surprised. What Bohr meant by complementarity is explained over at complementarity; there's no evidence that he doubted the Schrodinger equation as a non-relativistic model. PS please give the full Bohr quote. --Michael C. Price talk 18:31, 28 October 2006 (UTC)

[edit] Apparent paradox?

The paradox article says "A paradox is an apparently true statement or group of statements that leads to a contradiction or a situation which defies intuition." What then is an apparent paradox? Is the article wrong? Is there a source/reference for Lethe's interpretation "paradoxes are apparently true but lead to contradiction. apparent paradoxes look like they should lead to contradition, but are actually only counterintuitive"? Dicklyon 17:21, 21 June 2006 (UTC)

The second part ("which defies intuition") makes my case weaker, so I won't revert if you remove the addition again. I restored the word "apparent" because the person who removed it a month ago only did so in order that it would support an argument he was having with me, and I don't that's a good reason to change the article. I am having an rfc with him, and so was reviewing his edits. -lethe talk + 19:02, 21 June 2006 (UTC)
OK, I found other definitions that agree (not just copies of the wiki def), so I'll take it out again. Don't want to step in that other mess... Dicklyon 04:16, 22 June 2006 (UTC)

[edit] Particle and its definition

The opening sentence did not have an internal link to the articles known as "elementary particles" or "composite particles" so I added the needed brackets. Be that as it may, my quick read of the article on "elementary particle" is one that is internally cyclic and does not really define what constitutes a "particle". Is anyone ready to tackle this task? Bvcrist 03:57, 14 August 2006 (UTC)

This point concerns me as well. The article doesn't seem to say explicitly what it means to "behave like a particle". I don't know if my thoughts can be worked in somehow, but here they are. To me, "behaving like particles" means the amount of that thing is quantized. You can have one photon in a box as a standing wave, but you can't have a quarter that much energy. I think the confusion arises because a lot of people hear "particle" and they (understandedly) think "localized". It's impossible to reconcile "wave" and "localized", so that leaves people confused. I say emphasize that "particle" refers to the quantization but does not imply the thing is a small dot. A single particle can extend over a large volume. Spiel496 05:20, 14 October 2006 (UTC)

[edit] Modern interpretation

Someone has been putting in and taking out the attempt at a "modern interpretation":

Quantum mechanics was and is still a controversial sicence. It "evolved" throughout the years, but somehow, old interpretations are still used and being taught.
Light and matter seem to both exist as particles. What behaves like a wave is the probability of being at a certain place. The wave function is said to be to be a mere mathematical abstaction.

The big problem with this attempt is that it represents only one POV, and doesn't say whose it is. Phrases like "is controversial" and "is said to be" are non-encyclopedic. If an interpretation is be discussed, we need to know whose it is, and we need to include other ones as well. Dicklyon 16:17, 20 August 2006 (UTC)

Kevin aylward 12:53, 25 October 2006 (UTC) The big problem with your reply Michael, is that this modern interpretation IS the correct modern impetration. The whole idea of wave particle duality is completely bogus. All graduate level academic physics text books state quite clearly that QM is about the probability of, for example, locating a particle in a specific volume. Do a web search on “postulates of quantum mechanics”. This interpretation is not controversial in the slightest, and is universally held by professional physicists.

And more appropriate at interpretation of quantum mechanics. --Michael C. Price talk 18:37, 20 August 2006 (UTC)

[edit] Wave behavior of large objects

I recently deleted the last paragraph/sentence in the "Wave behavior of large objects" section.

"Whether objects heavier than the Planck mass (about the weight of a large bacterium) have a de Broglie wavelength is theoretically unclear and experimentally unreachable; above the Planck mass a particle's Compton wavelength would be smaller than the Planck length and its own Schwarzchild radius, a scale at which current theories of physics may break down or need to be replaced by more general ones."

1. The paragraph before this one in the section is pretty spectacular. Going to the Planck mass just weakens a potentially good argument about decoherence (which should be linked internally to Wikipedia def of decoherence).

2. I believe the deleted sentence to be logically flawed. (The de Broglie wavelength depends inversely on velocity. Where/why does the Compton wavelength enter the sentence? Slowing to pm/s velocities to measure fm de Broglie wavelengths of complex micron-sized objects seems adequate support for unreachability.

3. If it is meant as stated, it seems to be a bogus method of getting to Compton wavelengths and the Swartzchild radius. Why pick the Planck mass for any other reason?

4. The paragraph weakens an otherwise good article. Those who don't know any better won't get anything out of it. Those who do know, see something amiss.

5. I am concerned about the issue since I was planning on referencing the article in a general physics paper.

AQM2241

[edit] FULLERENE AND SCIENTIFIC RIGOR

I would like to know if the infamous fullerene experiment has been reproduced elswhere in the world by more mainstream scientists.

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