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What is so 'counter-intuitive' about the double-slit experiment?

  1. Aug 14, 2012 #1
    One photon produces a point on the screen.

    Many photons produce an interference pattern, whether the photons all arrive at the screen at once, or whether they are fired at the screen one by one.

    What is so 'counter-intuitive' about that? When we know that the electron shell that produces the photons is dynamic, the electrons themselves are constantly moving within a shell described by a probability pattern.

    Again, what is so counter-intuitive about that? It makes perfect, intuitive, sense.

    Now, I should point out that I'm not a physicist, merely a lay person with an interest and understanding slightly above that of pop-science.
    So, feel free to tell me to get lost back to my pop-science, if you want!

    Thanks in advance for any replies,
  2. jcsd
  3. Aug 14, 2012 #2


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    Well, "counter-intuitive" is somewhat in the eye of the beholder (or the mind of the intuiter) so it's OK if nothing here seems counter-intuitive to you.

    But it seems pretty damned weird to me that I can get an interference pattern when I fire the particles one at a time (what exactly is interfering with what, when there's only one particle in flight?) and even weirder that the pattern should disappear if I change nothing except whether I measure which slit my one particle goes through.
  4. Aug 14, 2012 #3


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    It does not seem weird to me that measuring a particle by bouncing photons off of
    it and causing that particle to recoil , would cause the pattern to disappear.
  5. Aug 15, 2012 #4
    What is counter-intuitive is how do photons (being discrete particle) produce interference pattern which can be found in case of interference pattern by waves (e.g . lightwaves).

    Its not strange that they produce a pattern. What is strange is that that pattern matches so well with light-wave interference pattern!
  6. Aug 15, 2012 #5


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    Starting with classical particles the problem is the following:

    Open slit A (slit B is closed) and observe the pattern. Now close slit A, open slit B and observe the pattern. You will find a region where in both cases some particles appear in the pattern.

    Now open both slits A and B and observe the pattern. It is strange that opening both slits will (in some regions) result in less dots compared to a setup with only one single slit opened.
  7. Aug 15, 2012 #6
    It's only counter-intuitive if think of photons as little balls flying through space.

    There is no rule that you must think of it this way. So if you don't, great.
  8. Aug 15, 2012 #7


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    That (recoil) is not the cause. You can prove this by inserting a polarizer at each slit (both open). The relative angle settings (the difference) of the 2 polarizers determines whether there is interference or not. That means that it is the interaction of the waves and not some element of the slit which controls the result.
  9. Mar 30, 2013 #8
    The idea that a particle can go through multiple slits at the same time and interfere with itself is pretty interesting. I mean, after a good education in quantum physics, it makes perfect, simple sense, but it's still counter-intuitive, from my perspective. Also the idea that measuring the system doesn't simply tell us where the particle really was in the probability field, but actually forces the particle to pick a spot and stick to it. Also, the realization upon studying the double slit experiment that we are not made of many solid, well-defined particles, but rather, a facsimile of a solid body made by so many probability waves.

    "Anyone who is not shocked by quantum theory has not understood it"
    - Neils Bohr

    (A favorite of mine, not to imply anything, of course! :wink: )
  10. Mar 30, 2013 #9


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    It's just counter-intuitive in the sense that no intuitive, common-sense model of what's going on works to explain it.

    Let's record the pattern of photons on a black and white photographic plate, where the "picture" is really lots of discrete dots each of which is either black or white. Let's make the intensity of the light through the slits so low that one dot at a time turns black.

    Here are two facts about the pattern of dots as they appear:
    1. One dot at a time turns black.
    2. Averaged over time, the pattern of dots shows the interference pattern.

    Why is fact number 1 weird? It's not weird by itself, but it seems to suggest that light arrives in the form of discrete particles. It takes a certain minimum energy to blacken a dot on the photographic plate. But that energy doesn't accumulate with time; one moment, it's not there, and the next moment it's there. So the best way to understand the blackening of the dots is that at low intensities, only a single photon at a time passes through the slits, and that photon blackens whatever dot it hits.

    This conclusion becomes weird in combination with fact number 2. If only a single photon at a time goes through the slit, then why would there be any interference pattern at all? If the photon goes through one of the slits, then why does the other slit make any difference at all? Why isn't the pattern of blackening just the sum of the pattern for one slit and the pattern for the other slit?

    So the two facts together seem to suggest that light travels as a wave, but is absorbed discretely as a particle.
  11. Mar 30, 2013 #10


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    First you must be more careful in talking about 1)! If a point blackens on the screen, it's not said that the light source consisted only of a single photon. It could as well be a coherent state, out of which one (or less likely two or more) photons got absorbed by the screen.

    With this little caveat however, that's indeed what quantum theory (here QED, because photons are involved, which are even more weird than massive particles can be, if you wish to say that nature is weird it all ;-)). Quantum theory teaches us that "light" or more generally "electromagnetic radiation", is neither fully understandable if described as classical wave fields nor if discribed as classical particles (BTW: in the case of photons the latter idea is totally misleading since there is not even a single-photon position observable). The full story, as far as we know, is given by quantum-field theory. As soon as you accept this upshot of more than 100 years of research in quantum theory, all the weirdness is gone. Admittedly, it's a pretty unintuitive picture at the first glance, but math is helping us to get it into a consistent picture of nature at all. Nature isn't caring about to be intuitive for human beings, it just is as it is :-).
  12. Mar 30, 2013 #11


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    I would disagree. The basic weirdness of quantum mechanics is there in QED, as well. It doesn't go away.
  13. Mar 30, 2013 #12
    I searched a bit on this topic and there seems to be a very interesting Google Tech Talk on YouTube: (found also a discussion on it, but haven't read it much yet).

    It practicly states that Quantum Entanglement and Measurment (in double slit experiment) is the same thing.

    The talker concludes that chosing between Many-worlds and Zero-world interpretation (as he calls it) is a mater of taste - I guess he meant to personal intuition... (And that it's One-world interpretation which math tells us is unreal.)

    If I understood him correctly Zero-world existence would mean there is no real physical world in sense we humans think about it, but that consciousness, which emerges out of fundamental physical reality, is more kind of a high quality computer simulation (where Quantum Entangled plays mayor role).


    I'd really like to see your opinions on this talk and talker's idea on Zero-World interpretation of our reality.
    Last edited by a moderator: Sep 25, 2014
  14. Mar 30, 2013 #13


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    Basically true

    The choice between any interpretation is a matter of taste.

    That's where he lost me. This mumbo jumbo he went on about that if you reject many worlds you are left with nothing but consciousnesses - we are our thoughts - when he said it I went - what - come again. There is nothing - zero - zilch in what he said that leads to that conclusion. Here are the facts - measurement and entanglement are the same thing - yes - or to be more specific measurement is a specific type of entanglement and decoherence spells out the details of that entanglement. Specifically what decoherence does is transform a superposition into an improper mixed state. If it was a proper mixed state ie an actual ensemble of systems one of which has been picked out and its what you observe - all issues resolved. But because it is an improper one it merely is observationally indistinguishable from it. Simple solution - merely assume it is - that's one solution and the one I use. Others exist - Many Worlds - nothing is selected - instead each state of the improper ensemble continues to evolve - valid - but a bit too weird for many - I am one of those. The other is Decoherent Histories which is based on a consistency condition decoherence automatically enforces. It's basically an update to Copenhagen that included decoherence and has been described as Copenhagen done right - by which is meant it clarifies some of the more obscure parts of it. There are undoubtedly others as well but this consciousnesses stuff simply leaves me cold - its not and never has been required in QM. You can introduce it - and there are interpretations that do - I cant prove them wrong - and its choice is a matter of taste - by why oh why do you want this consciousnesses created reality - its not required - to me its even worse than Many Worlds. Wigner once held to it but when he found out about decoherence he abandoned it - its like solipsism - I cant prove anyone that holds to it wrong but most reject it as being an unnecessarily weird view of the world - which it is.

    The issue I have with things like interpretation of reality is philosophers have been arguing since time immemorial what reality is. Since we cant even get agreement on that I cant see how you can interpret it. I think its better to view it as physics gives the best description we have of the world around us and what it tells is 'reality'. How you interpret it depends purely how you interpret the physics. Under my interpretation of QM the world out there exists independent of us and it is revealed by our observation.

    Last edited: Mar 31, 2013
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