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How does wave particle dulity work?

  1. Dec 7, 2014 #1
    I'm having trouble in understanding this concept. How can particle like atom (which is solid) becomes wave like we seen in ocean?
     
  2. jcsd
  3. Dec 7, 2014 #2
    Atoms aren't solid.
     
  4. Dec 7, 2014 #3
    But can you tell me how a particle like an atom behave like a wave?
     
  5. Dec 7, 2014 #4
  6. Dec 7, 2014 #5

    bhobba

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    Th wave particle duality is an outmoded idea that was consigned to the dustbin of history when Dirac came up with his transformation theory in late 1926 which is basically what goes by the name Quantum Mechanics today. Its a by-product of the semi historical approach of most textbooks - but that doesn't make it correct - its basically a crock of the proverbial.

    If you want to see a better basis of what QM is all about check out:
    http://www.scottaaronson.com/democritus/lec9.html

    Without the wave particle duality how is the double slit explained? Check out:
    http://cds.cern.ch/record/1024152/files/0703126.pdf

    If you are just starting out I suggest the following two books:
    https://www.amazon.com/The-Theoretical-Minimum-Start-Physics/dp/0465075681
    https://www.amazon.com/Quantum-Mechanics-The-Theoretical-Minimum/dp/0465036678

    There are also associated video lectures:
    http://theoreticalminimum.com/

    It will require a bit of attention to wade through - but take your time - its not a race.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  7. Dec 7, 2014 #6

    Ken G

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    I think that what bhobba is saying is that there is no need to regard "wave/particle duality" as a metaphysical claim of some kind, that quantum systems "sometimes behave like particles and sometimes like waves", as though they were schizophrenic. I agree with him that this type of schizophrenia is not a good way to characterize the gorgeous consistencies that underpin quantum formalism. But there are other ways to interpret the phrase "wave/particle duality" such that it is not a metaphysical claim, but rather, a very important insight that remains with us today.

    Let's take light as an example of what I mean. Long ago, in the days of Newton, the question was, "is light a particle, or a wave"? Huygens had a theory that light was a wave, Newton that it was a particle. Since Newton had, for the first time, established an excellent theory of particle behavior, it was natural for him to try to apply it to light. The challenge was refraction, which Huygens had a nice wavelike description of, assuming that light slows down in glass, and the resulting wave interferences produced refraction. But Newton's particle theory could work too-- he could get refraction by saying that light particles received a force pointing into the surface of glass that deflected the particles. It was as though glass sometimes made a force that excluded the light, like a tennis ball bouncing off a wall (so that's reflection), and other times made an attractive force that pulled the light into the glass. The key difference in the predictions was that this would mean light speeds up in glass, not slows down. Since Newton was such a star, his model won the day for awhile-- until much later, the technology existed to test the speed of light in glass, and it was found to slow down.

    So that seemed to settle it-- light is a wave. Along comes Einstein and the photoelectric effect, and light is back to being a particle! So that's really the genesis of wave/particle duality, and it happened 20 years before quantum mechanics. The question was, how could light be both a wave and a particle at the same time? So quantum mechanics isn't the genesis of that question, it is the answer of it. The answer is that particles obey wave mechanics, and what we thought of as "particlelike behavior" like precise trajectories just meant the mechanics of short-wavelength waves, and what we thought of as "wavelike behavior" like diffraction just meant the mechanics of long-wavelength waves. So that's what wave/particle duality really means-- it doesn't mean that quanta can't make up their mind if they are waves or particles, it means that waves and particles were always the same thing, we just didn't realize why until quantum mechanics.
     
  8. Dec 7, 2014 #7

    bhobba

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    Its simpler than that. It does indeed sometimes behave like a wave and sometimes like a particle. But most of the time its neither eg when entangled with another particle its a wave that propagates in an abstract mathematical space and nothing like the usual conception of a wave at all. Thinking in terms of the wave-particle duality actually holds back your understanding and should be abandoned.

    The thing is wave-mechanics was also consigned to the dustbin of history when Dirac showed it was part of his more general transformation theory:
    http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

    It wasn't proven wrong, but rather one aspect of a much more general theory.

    Thanks
    Bill
     
    Last edited: Dec 7, 2014
  9. Dec 7, 2014 #8

    vanhees71

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    No, light is not back to "being a particle". It's described by a relativistic quantum field theory, called quantum electrodynamics, QED. It is described as a quantized massles spin-1 field, which is a pretty complicated object, and it cannot be fully explained without some mathematical (group theoretical) background. It turns out, that photons do not even have a position observable as massive particles have. The idea that "light" is a stream of little billard-ball like particles is absolutely flawed.

    Also the photo effect does not even prove the necessity for quantizing the electromagnetic field. It's well understood in the non-relativistic quantum theory of electrons bound. It's a standard exercise when it comes to the discussion of timedependent perturbations. You describe the electromagnetic wave as a classical field and treat the interaction of the photon with the corresponding electric field (dipole approximation) as a harmonic perturbation. Then the first-order perturbation theory leads to the correct description of the photoelectric effect. No photons are necessary here!
     
  10. Dec 7, 2014 #9

    Ken G

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    I'm not sure telling people that understanding the situation in terms of entanglement increases their power over the concepts! It is natural that old physics theories get embedded into new more sophisticated ones, just as Newton's gravity is now embedded into general relativity. But a significant fraction of people are still exposed to Newton's theory, and only a tiny fraction ever make a significant foray into general relativity. That's not enough to put Newton in the dustbin.

    And there is great value in showing the transformative theory that supercedes wave/particle duality. But that doesn't put wave/particle duality in the dustbin, any more than Newton's gravity-- there are still important insights in both those antiquated theories, when the goal is conceptual accessibility for the largest number of people.
     
  11. Dec 7, 2014 #10

    Ken G

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    Well, now you are mixing in relativity, which distinguishes light quanta from other types of particles. That's a separate issue. And sure, there are better theories for talking about light than Einstein's explanation for the photoelectric effect, that is nothing new in physics. We don't scrap successful explanations when better ones come along, for obvious reasons. Surely you don't think the QED description of light is the final answer either, do you?
    So take Einstein's Nobel away, it was all a mistake....
     
  12. Dec 7, 2014 #11

    bhobba

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    The issue is people think it acts as a wave or a particle - they forget the sometimes in the principle and that causes all sorts of issues. Best to do away with it entirely - its not needed and simply confuses - as the the confusion of the OP attests.

    Thanks
    Bill
     
  13. Dec 7, 2014 #12

    bhobba

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    Its not a mistake - it was superseded.

    Ken the issue is this. The wave particle duality says it sometimes acts like a particle and sometimes as a wave. That is indisputable but people forget the important highlighted bits and think its a wave or a particle - it isn't - its neither. And since you need to analyse the situation from the viewpoint of the correct theory to know if its behaving like a particle or wave, or neither, what is the point? Simply accept the theory as is.

    Thanks
    Bill
     
  14. Dec 7, 2014 #13

    Ken G

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    Sure, but this happens all the time in physics. There's a big difference between saying something was a bad idea that leads to misconceptions, versus saying that it was superceded. String theorists might claim everything we call physics is superceded by string theory, but most physicists just don't see the value in that characterization.
    Actually, you and I think differently about the insights of "wave/particle duality", and that is the source of our different feelings about it. You see it as valid that people refer to certain phenomena as "particlelike phenomena", and others as "wavelike phenomena." I think that is the source of the problem-- what wave/particle duality should tell us above all is that this classification is artificial at best, and misguided at worst. Everything that happens is particlelike phenomena, and everything that happens is wavelike phenomena-- that's the real message of wave/particle duality. So when we see the graceful arc of a baseball, we should not say "ah, particlelike behavior", because waves can do that too. We should say "ah, short-wavelength behavior." That's the insightful way to look at the flight of a baseball, because it survives quantum mechanics and works just fine mathematically. Of course we'd never calculate its path using wave mechanics, but we could, so the trajectory model was never the simple way to model a particle, it was always the simple way to model a short-wavelength well-focused wave.
    But which theory? The physicist always gets to make that choice, nothing that worked before goes in the dustbin unless it was a complete misconception. Wave/particle duality is only like that if it is framed the way it is often framed-- as a metaphysical claim on the existence of waves and particles that phenomena somehow draw on. In my view, the insightful way to frame wave/particle duality is that the phenomena themselves can be unified by the statement "particles obey wave mechanics." That's a statement like "things fall and orbit because of the force of gravity"-- the statement has been superceded, but it is still a valid and valuable insight.
     
  15. Dec 7, 2014 #14

    vanhees71

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    Well, QED might not be the final answer, which statement is true for any theory, but it's the best we have nowadays, and there is no contradiction of any observation to it so far.

    It's indeed ironic that they gave Einstein his Nobel for the only of his theories which is outdated by now. He got it in 1923, and old quantum theory was obsolete only 2 years later. Even more ironic is that they didn't want to give the Nobel for relativity, because this appeared to them be too shaky a theory to be honoured. Nowadays, relativity theory together with quantum theory is in best agreement with all observations (to an accuracy of 12 or even more relevant decimal digits in quantities like the electromagnetic moment of the electron or the Lamb shift). As you see, even the Nobel committee can err from time to time (but that's quite rare concerning the "hard" prizes concerned with natural sciences and medicine). In any case Einstein's Nobel was very well deserved. I not giving a Nobel to him, who then should ever get one?
     
  16. Dec 7, 2014 #15

    bhobba

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    Yes there is a difference. And in this case we know from our modern vantage point it's a bad idea.

    As I have mentioned previously the wave particle duality is - quantum objects sometimes act like a particle and sometimes like a wave. To apply the idea you need to know when that sometimes is and exactly what like means. And to figure that out you need to know the full theory. Like a wave means certain equations like Schroedinger's equation sometimes has wavelike solutions - and mostly not BTW - but sometimes it does. But waves of what? Its simply waves of the expansion of this thing called the state in terms of eigenfunctions of position. And exactly what is a state - now there is a rub - that varies with interpretation. But in Copenhagen for example its simply a degree of confidence that resides in the head of a theorist. Waves of degrees of confidence? Yea - that's really like waves in other areas of physics isn't it. And then you have to ask what is its practical value. I know - it explains the double slit experiment. But does it? As the link I gave explains - its really got nothing to do with it - when the electron (I will use electrons to avoid issues with photons which are problematical) reaches the screen with the slits that acts as a position observation so that after going through the slits it has unknown momentum - it is that that causes the interference pattern. Not it acting sometimes like a particle and sometimes like a wave - in that experiment its neither - its simply quantum stuff.

    How you get that from - quantum objects sometimes act like a particle and sometimes like a wave beats me.

    Thanks
    Bill
     
  17. Dec 7, 2014 #16

    bhobba

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    It cant be waves of particles because that will not explain interference. It cant be just waves because that will not explain the photoelectric effect. Its something different entirely and that something is explained by quantum mechanics.

    Take the photoelectric effect for example. Einstein got a Nobel prize (he deserved a Nobel 100% for sure - but as Vanhees explains from our modern vantage the photoelectric effect is seen as maybe a bit political) for explaining it using particles - but its actually a lot more complex than that:
    http://www.mat.univie.ac.at/~neum/physfaq/topics/photodetection.html

    This is just another reason why the so called wave-particle duality is misleading at best and IMHO downright WRONG - but that's just my view - more exactly its simply quite misleading rather than actually wrong. Quantum objects do indeed sometimes act like a particle and sometimes like a wave - but the devil is in the detail.

    Thanks
    Bill
     
    Last edited: Dec 7, 2014
  18. Dec 7, 2014 #17

    Ken G

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    Yes, that's all true, it's true of every theory. What this means is, we don't put theories in the dustbin when they get superceded, we take the lessons from them and move forward. Something that is just a misstep has no lessons, but viewing light as comprised of particles was not such a misstep, so Einstein's Nobel for that was no kind of mistake.
    Yes, given all the things he did, that is an irony indeed.
     
  19. Dec 7, 2014 #18

    Ken G

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    So you would say it is a bad idea to watch the trajectory of a baseball and marvel at the things short-wavelength waves can do? Because in my view, that's quite an insightful thing to do indeed, and not a bad idea at all. Indeed, it could have been done from the early days of Hamilton's principle, we would only have needed to interpret the principle of least action as an example of wave interference-- which we now see, thanks to Feynman, is just what it is.
    And I said, one does not have to frame "wave/particle duality" in those terms! Yes, if you force it to mean that, it's a bad idea. But the "bad idea" is to frame wave/particle duality like that. The good way to frame it is, particlelike nature means just one thing: discrete behavior in the "things that are following wave mechanics". Not the motion, not the mechanics, and not the trajectories, those things are all perfectly wavelike behavior-- that's the lesson we didn't get until the last century, even though it was right before our eyes all along.
    Give me an example of a "non-wavelike solution" and I'll give you an example of a wavelike solution that we simply failed to notice is one of the things waves do. Even entanglement, but we don't normally talk about two-point correlations in waves-- though that's wavelike too.
    Indeed, but that's endemic to quantum mechanics, so is not fundamentally a wave/particle issue.
    Again, that has nothing to do with wave/particle duality, it has to do with interpretation of the quantum state. What does it mean for a system to have a state-- is it a state of knowledge or something the system actually "possesses"? Frame the solutions to the Schroedinger equation in any language you like, and that question will still be staring you right in the face. Banish the issues of wave/particle duality as far as you like-- and that question is still there, staring right back at you.
    That doesn't mean a double slit interference pattern is not a wave effect, it means that wave effects can be thought of on a deeper level. The same could have been done with sound waves long before the Schroedinger equation, had we only had the foresight to do it (just replace "unknown momentum" with "range in k values in the Fourier transform of the wave amplitude"-- imposing spatial restrictions induces Fourier modes not present before the slits were encountered, that's there in sound waves also.) That's not surprising-- what physical phenomenon cannot be thought of on a deeper level? This doesn't mean the old ways of talking about sound waves, like Huygens' principle, goes in the dustbin.
    That isn't the useful way to frame wave/particle duality, the useful way is to interpret the "duality" as meaning "particles and waves were always the two sides of exactly the same coin, and we could have seen that centuries ago had we the foresight to realize it."
     
    Last edited: Dec 7, 2014
  20. Dec 7, 2014 #19

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