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Double Slit Experiment

  1. Dec 7, 2006 #1
    A monochromatic, coherent light source is targetted through a pair of double slits, and produces a series of small dots on a background screen. According to Young, this is caused by the interference of light wave trains, constructively and destructively interfering to produce antinodes and nodes respectively. This is one argument used to establish light to have a wave-like nature.

    Yet when the experiment is repeated in a different manner, this time to slow down the intensity of the photons so that each passes through the slit, one at a time, this also produces the same interference pattern. Yet this is explained by a "probablity function".

    Why doesn't this latter experiment simply disprove this wave-like nature of light suggested by Thomas Young in this particular instance?
  2. jcsd
  3. Dec 7, 2006 #2
    Do you mean in that case "a single, spatially localized particle moves from source to detector"?
  4. Dec 7, 2006 #3


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    Why do you think it might disprove the wavelike nature of light? There's still an interference pattern.

    The fact that very low-intensity appears on the screen as individual pointlike spots suggests that light has particle-like properties. The fact that an interference pattern appears (in a probabilistic way) as more and more spots accumulate on the screen suggests that light has wave-like properties.

    In modern physics, particle vs. wave is not an "either-or" distinction.
  5. Dec 7, 2006 #4
    jtbell, I have the opposite vision: I don't understand where is light's particle-like property here. I mean:

    I have no problem in imagine a very low intensity EM wave hitting a screen which "clicks" in only one point just because the probability that each point "clicks" is very low (low intensity), so that the probability that two different point clicks at the same time is much lower.
  6. Dec 7, 2006 #5
    I don’t follow your logic – or maybe it is better said that you don’t recognize the logic involved in understanding the issues demonstrated here.
    First it is true – as Young said there is NO logical explanation for building the pattern with light through Newton’s’ particle version of light, and as experiment demonstrates, there is no basis to deny the wave like nature of light.
    However, through advancements in producing and detecting light in experiments there is no doubt the light can be detected one particle and a time, therefore photons are real and light IS made of particles.
    What remains is to explain just how, as has been shown by experiment, can individual particles produce a pattern when passed through a double slit!
    There is no logical explanation for it – at least not a “Local Realist” one, which is what you can do by using a "probability function" view or “Non-Local” view. The best of these Non-Local views is QM “Uncertainty” and the formulas or “formalisms”.

    Thus the continuing debate that was immediately sparked from the use of QM, is it logical and correct to abandon a “Local” view of reality? Although Einstein never did, based on experiments it is accepted by most, that yes it is logical to view reality as Non-Local.

    Plus there is also an additional continuing debate as to what is the “BEST” non-local view or theory to explain things like double slits, entanglement, and other non-local observations that cannot be explained in the old classical terms. There are many that claim to be better, BM, MWI, versions of GR, quantum gravity versions, strings, "M", etc.
    Currently if you define “best” as most productive and predictive in the advancement of actually applying science it really has to go to QM, based on almost a century of success. However many of the opposing ideas claim that this is only because QM was first, and the best theory is the correct theory. From this view what is missing is the experiment to provide the logical evidence to show that one ‘non-local’ theory can predict and produce correct results that others cannot. Till that happens it will remain what it is, a debate.

    So is the duality of particle and wave completely understood and explained? Of course not, if it were there wouldn’t be a debate going on as to what theory best explains it.
  7. Dec 7, 2006 #6

    Claude Bile

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    The particle property comes from the fact that the interaction between the photon and detector array is localised. We would not expect this for purely wave-like behaviour.

  8. Dec 8, 2006 #7
    Young conducts the intial double slit experiment and observes the bright fringes on the background screen. His explanation for this phenomenon is that there is wave interference. However, obviously if the experiment is conducted again, this time with the intensity lowered so that only one photon is able to move through the slit per time - there cannot be any interference, otherwise this means that the photon interferes with itself - and this has not been postulated to be the case.

    So how is it that Young's explanation for this particular wave-like phenomena of light is still accepted? His theory requires the wave trains to intersect, causing nodes or antinodes. Yet this is clearly not the case since there is one photon moving through the slits at a time.
  9. Dec 8, 2006 #8

    Doc Al

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    It most certainly has. Dirac himself stated (overstated, actually) in his classic "Principles of Quantum Mechanics" regarding the two-slit experiment: "Each photon then interferes only with itself."

    You seem to think that it's either-or, particle or wave. Why not both?

    I like the way J.S. Bell put it:
  10. Dec 8, 2006 #9


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    But there IS interference in this case. This experiment has been performed, in this fashion, both with light (photons) and with electrons.
  11. Dec 8, 2006 #10
    So, why exactly my explanation doesn't work?
  12. Dec 8, 2006 #11
    How is there interference of wave trains (multiple) if only one photon passes through the gap at a time?

    Young's explanation of this phenomenon suggested multiple wave train's - i.e a coherent light source.

    These series of bright fringes appear on the screen as a light source is directed through the gap. The explanation, as given by Thomas Young at the time, for these fringes was that there was wave interference taking place. This is one experiment in which light is "apparantly" shown to act as a wave.

    Yet how does light act as a wave, according to Young, if what he hypothesized is not actually taking place. I mean, the latter experiment I described, still produces the same pattern on the background screen. Yet with only one photon. So how is Young's explanation at all correct, and why is this particular incident (the double slit experiment) constantly referred to in modern physics as an experiment which is in the favour of light being wave-like in nature?
  13. Dec 9, 2006 #12
    As jtbell said, the interference pattern appears even with single photons, after many photons have hit the screen.
    At this point, you could say: a single photon passes through a slit and hit the screen.

    But this is wrong for this reason: if you put a tiny detector after one of the slits to see through which slit the photon has passed, you see that the photon actually has passed through that slit, or the other,...ecc, randomly. But the interference pattern on the screen now is disappeared! Now you have only two bright diffused spots on the screen, as in the case of bullets passing through slits, OR as in the case of light passing through slits which lower dimensions are much greater than the light's wavelenght.

    So, you are forced to conclude that the photon, when it's not observed after the slits must pass through both interfering with itself and, so, it behaves as a wave, in that case, and behaves as a particle when its particle-like property is observed!

    Now, I ask to you and to all the others: doesn't this mean that the very concept of "photon" *cannot be attributed to the light only*, but *must be attributed to the light AND to the measurement apparatus*?
    Last edited: Dec 9, 2006
  14. Dec 9, 2006 #13
    Your explanation is not that of a wave but of a particle, (or at least a “wave packet” small enough go through a slit and be detected at a localized location) as Claude Bile has already explained this is particle behavior not wave behavior. That particle behavior can be easily be seen as unique to the light as a photon without considering apparatus (or the entire past and future universe) as some theories might require.

    You give the impression you not understanding the experiment. As “only one photon” is not able to produce a pattern, you must use many; it is just that you can require that they pass one at a time and build up a pattern of a period of time.

    U1 You seem to be demanding a Classical Explanation to a paradox and explained in a neat one liner would be nice. BUT, No one has claimed a pure classical explanation to paradoxes of this type. You need to move on from this one paradox and develop an understanding of what scientists mean by Local and Non-Local. To date only Non-Local explanations provide adequate explanations for such paradoxes.

    Note: As you consider “Local vs. Non-Local” (Search these forums, Google, & Wiki) be sure to draw a distinction between Classical (Einstein or Bell) Local and Non-Local versions of “Local” used by BM, MWI etc.
  15. Dec 9, 2006 #14
    Then, if you call as "particle" in this case, just the fact that only one detector at a time "clicks", we have understood what a photons is: the "click" of the detector, and nothing else.
  16. Dec 9, 2006 #15
    No Photons of light is a description of light not the observation of it. Just as the EM waves of light you prefer are a description of light not the patterns it produces.
    Resolving the paradox between the two is not done with philosophical machinations to redefine photon to your liking. If needed please take it to the Philosophy Threads.
  17. Dec 9, 2006 #16
    Ok. I'm not actually interested in this; what interests me is to understand exactly what does "particle-like" property of light in the above experiment mean.
  18. Dec 10, 2006 #17
    The meaning in Particle, Wave or Field terms of an entity like; a photon, electron, quark, proton, neutron, or even a buckyball in any experiment cannot be understood in classical terms, but requires using a Non-Local view. There are several views available to do so; the most highly recommended currently being OQM as it has been the most useful & successful in the past 80 years. I suppose that can be considered a “particle” view, but there are several other views that are just as acceptable in current science; de Broglie wave theories and quantum field theories included. So you do have a wide choice.

    To be honest, none of those are my preference, but until something better comes along they are the best currently available.
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