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Wave/particle duality: what exactly is a particle?

  1. Mar 16, 2008 #1
    One of the biggest conundrums is the wave/particle duality of quantum systems as it is observed in two-slit experiments.

    The interference pattern seen in a two-slit experiment suggests that the quantum system is a wave. But what exactly makes us say that the quantum system also behaves like a particle? My partial answer is:
    1. On the screen behind the two slits we see individual, point-like marks very much like we would expect from a particle (imagine a little ball) hitting the screen, and certainly not from a wave front hitting the screen.
    2. With sufficiently capable detectors it can be shown that the energy transmitted/released while creating the mark is quantized, i.e. it cannot continuously made arbitrarily small, like could be expected from a wave amplitude.
    My question: Are there other points worth noting that suggest to say that the quantum system behaves like a particle? Is it correct to say that we don't know that it is a particle, we only make observations and interpret them? The question would then be: apart from the two observations above, are there other, different observations suggesting particle?
     
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  3. Mar 16, 2008 #2

    ZapperZ

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    I think I've tried to stay away from more of these "wave-particle duality" issue lately. However, I think we need to be clear on something here that we do know for sure. The 2-slit experiment really is an illustration of the superposition principle in quantum mechanics. We need to be very clear of that, because this superposition principle permeates throughout QM, and it isn't just restricted to the two slit experiment. It just happens that the two-slit experiment illustrates it very well AND one of the results or consequences is that we can see the wave-like and particle-like behavior. When we apply such superposition principle in chemistry, we get the bonding-antibonding bonds, and when we apply it to the Delft/Stony Brook experiment, we get the coherence gap.

    This clearly indicates that "top of the food chain" here is the superposition principle. The "wave-particle duality" (and I would argue that there really is no "duality" in QM - read the FAQ) is merely one of the many consequences of this principle. So focusing on it simply means that we are focusing on one of the consequences, rather than looking at the bigger picture. What this means is that if you come up with an "explanation" for the 2-slit experiment, while ignoring other superposition experiments, then your explanation is not universal and may not be valid. Only something that can be consistent with all of those experiments would be something that will be taken seriously, because the same principle applies to all of them.

    Zz.
     
    Last edited: Mar 16, 2008
  4. Mar 16, 2008 #3

    JesseM

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    Another central element of what is meant by "wave/particle duality" is that if you measure which slit it went through, then you no longer see an two-slit interference pattern on the screen, instead you get the sort of pattern you'd see if a wave was coming exclusively out of the single slit where you made the detection. So it's as if each position measurement "collapses" the wave to that single position, and then it spreads out again from there.
     
  5. Mar 16, 2008 #4
    Neither of these effects are really tied to the two-slit experiment. You see the same things you've listed above whether you have slits or apertures or just a light shining on a photographic plate.

    With regard to your second item, we understand enough about the atomic properties of matter to know that chemical transitions cannot be driven to completion unless there is enough enough energy, whether that energy is supplied by a "particle" or a "wave". The minimum detection threshold therefore is consistent with ordinary wave energy incident on a detector. It doesn't let you draw any clear conclusions about whether the light energy is quantised.

    Your first point is a bit different. The point-like nature of the marks on the screen is often compared to the diffuse nature of wave energy; people conclude that it is not possible for something spread out as thinly as a wave to concentrate its energy on something as small as an atom. This thinking is based on a misunderstanding of electromagnetic theory. In fact, a classical antenna can have an effective cross section much bigger than its physical dimensions. The ability of a tiny receiving element to "suck in" energy from a wide cross-sectional area is a consequence of classical antenna theory. So you can't infer from the dots on the screen that light is quantised.

    Marty
     
  6. Mar 16, 2008 #5

    jtbell

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    Greenstein and Zajac's book "The Quantum Challenge" argues that the key property of "particles" is that they cannot be detected in two locations at the same time. Imagine sending a light beam through a beamsplitter that divides it into two separate paths, with separate detectors at the ends. If you prepare the light so as to ensure that it consists of single-photon states (which is not easy!) you find that the detectors never register in coincidence. That is, each photon always goes one way or the other, never both.
     
  7. Mar 16, 2008 #6
    This is really not a conundrum once one adopts the point of view that "particles" are simply quanta of fields (as one does in quantum field theory.)
     
  8. Mar 16, 2008 #7

    DrChinese

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    ZapperZ has pointed out previously (I lost the reference but it is probably in the FAQ) that the Heisenberg Uncertainty Principle leads to the double slit results. In other words, "wave-particle duality" is just another face of the HUP.
     
  9. Mar 16, 2008 #8
    I wholeheartedly agree with that! And I'd add that the HUP does not account for that fact.
     
  10. Mar 16, 2008 #9

    Ken G

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    Personally, I think the main culprit in misunderstandings about "wave/particle duality" is the idea that the concept of particle must come along with the concept of trajectory. I echo jtbell and peter0302 in pointing out that the crucial aspect of a "particle" is that it is a quantum that shows up in one place only, but I'll take it a step further to point out this property has nothing to do with trajectories-- it is not that they have a trajectory that prevents them from being in two places at once, it is that they are particles. As soon as we allow that we can have particles that are not ruled by trajectories (which are nothing but short-wavelength limits of waves), but rather are ruled by wave mechanics, all problems vanish. One can call that a "duality" if they wish, but it is more like a partnership with clearly separate roles.
     
    Last edited: Mar 16, 2008
  11. Mar 17, 2008 #10
    This is the best operative definition of particle-like behaviour I've ever red till now. Thank you, jtbell.
     
  12. Mar 17, 2008 #11
    Hi,

    just a question on translation. In french, we distinguish "particle" from "corpuscule". Is there any french-speaker who knows translation of "corpuscule" ?
     
  13. Mar 17, 2008 #12

    ZapperZ

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    The only caveat here is that you are putting a detector in each path, and therefore will have one or the other being detected, which points to a "particle-like" behavior. If you do not have any detector in the path, then the "particle" definition of Greenstein and Zajac fails, i.e. the photon DOES take both paths simultaneously. So in essence, by using the beamsplitter and checking the which-way path is not the same setup as using the beamsplitter but not checking any.

    I've mentioned this paper before but it bears mentioning again:

    T.L. Dimitrova and A. Weiss, Am. J. Phys. v.76, p.137 (2008).

    They did a very elegant demonstration of the Mach-Zehnder interferometer with single photons and did really neat stuff to it.

    Zz.
     
  14. Mar 17, 2008 #13
    Certainly, if you set up the experimental conditions so as to get the "which path information", then a single photon will behave particle-like and you will surely detect it in one or the other; I didn't notice that the setting was to get such information.
    It's a pity I don't have access to such magazines.
     
  15. Mar 17, 2008 #14

    Ken G

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    I would guess the word you are looking for is "quantum", but I don't know the French usage. I agree that making such a distinction would clear up a lot of semantic confusion, a confusion that I feel is at the heart of awkward "duality" language. Would we have the problem if people referred to "wave/quantum duality"?
     
  16. Mar 17, 2008 #15

    Ken G

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    Right, this is the crucial issue. We define the concept of a particle because it is a vastly useful classical concept. We cannot think "quantum mechanically" because our brain cannot enter into a superposition in order to understand one, so instead we are constantly forcing quantum systems to behave classically when we try to understand them. Then we puzzle over "duality"! We're looking in the mirror and blaming reality for the way we combed our hair.
     
    Last edited: Mar 17, 2008
  17. Mar 17, 2008 #16
    How can you leap to such a conclusion that it "does take both paths simultaneously"? The only thing we can say is that it has an equal probability of taking both paths and its final destination is determined by the superposition of all the possible paths it could have taken, calculated as a wave. Even in a double slit experiment where no attempt is made to register "which slit," the quanta are never detected at more than one location at once.

    There's also no experimental support to assert that particles have no trajectories. The fact that they are always seen at one and only one place at a time, and the fact that nothing has ever been observed to travel faster than light, both support the inference that the particles did travel in one continuous path for their entire journey. All we can say with certainty is that our ability to _predict_ that path that is governed by the wavefunction and superposition.
     
  18. Mar 17, 2008 #17

    reilly

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    I agree, but will go one step more with a two-part question:

    1. How does one compute the probability of a particle trajectory in QM?

    2. How does one compute the probability of a "particle" being at points x and y at the same time t?

    Regards,
    Reilly
     
  19. Mar 17, 2008 #18

    ZapperZ

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    Then go argue with Dirac. He foolishly insisted the the photon "interferes with itself". If you think otherwise, please publish your paper. And while you're at it, please write a rebuttal to the Delft/Stony Brook and Tony Leggett's papers for insisting that the supercurrent in those SQUIDs experiment are flowing in opposite directions simultaneously.

    Zz.
     
  20. Mar 17, 2008 #19

    Ken G

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    Points taken, those are manifestly not the way we construct quantum mechanics.
     
  21. Mar 17, 2008 #20

    Ken G

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    And when you detect them, which slit did their trajectory take them through? We all know this is unknowable if interference is being exhibited, so why hang onto something unknowable simply because we liked it at the classical level?
    To which I would put the same question. We have to stop seeing ourselves in our science, and let nature do the talking as much as possible, or the result is some ugly "duality" idea.
    The source of causality in wave mechanics is manifest to that theory, it does not require "a continuous path". Indeed, it seems likely that if causality is ever violated, it will be due to a wave property not a trajectory-- apologies to FTL enthusiasts.
    All we can say with certainty is that our ability to predict... is all we have.
     
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