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Non-technical answer wanted about the double-slit experiment

  1. Sep 27, 2013 #1

    I know that there are numerous threads about this, and I've also read several papers about it on the web. Still, I don't understand how the far-reaching conclusions of QM follow from the setup or results of the double-slit experiment. If anyone here has the patience to explain in non-technical terms why he/she accepts these conclusions, I'd be very grateful!

    So here's how I understand it. You shoot individual photons at a screen with two slits, one at a time. You expect the distribution of hits to be 50/50. Instead, the distribution is what you'd expect from a wave, not a particle.

    For starters, how do you know that you're shooting only one particle? Maybe your particle gun is shooting additional unknown particles, which cause the wave pattern? Why trust the particle gun? Why trust any device in this experiment for that matter, if you're going to be skeptical enough as to accept the ultra-weird premises of QM? If reality is what QM says it is, then that's the way it is on any scale, not just the quantum scale. Every particle in this experiment is "weird" before we shoot the photons. Why do you accept that the gun, the screen etc. are "classical", and stable enough to serve as a background for the weird QM behavior?

    Thanks for your patience, T.
  2. jcsd
  3. Sep 27, 2013 #2


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    You can replace the slits by detectors, and you will always detect exactly one hit at a single detector (in a perfect setup), you never observe something a both.


    Decoherence. In theory, you can treat them as quantum-mechanical objects (and the many-worlds interpretation does that), but you don't have to.
  4. Sep 27, 2013 #3


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    Welcome to PhysicsForums, tulems! These are great questions.

    The answer is that many experiments have been done to control for the things you mention. For example, suppose there are "extra unknown" particles as you suggest. Why then would you get no interference pattern in only the cases where you know which slit? And an interference pattern in other cases? QM explains this, alternative models generally cannot.

    Also, there is no clear divide between the quantum and the classical worlds. Yet you can conduct experiments that clearly demonstrate purely quantum behavior. Such experiments are often quite complicated to follow if you are new to QM. However, here is a reference to one such. Although it is not the double slit, it strictly controls for single photons.

  5. Sep 27, 2013 #4


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    It is the other way round: if you make the assumptions of QM you can explain what happens in the double slit (and in countless other experiments). You cannot explain this by using classical mechanics and up to now, nobody has been able to come up with a less weird theory than QM which has the same predictive power.

    All kinds of things may happen but unless one makes predictions there's no way to decide if a theory is compatible with nature. This is somewhat idealized but in principle, scientists simply use the theory which makes the best predictions yet. For a great scope of phenomena this happens to be QM. This is the same situation as in the past, where everybody used classical mechanics until better theories were made.

    To a certain extent, this is explained by QM. If your system is surrounded by a big environment (like a measurement apparatus), some stable properties emerge from the interaction between the system and this environment. The buzzword here is "decoherence".

    /edit: boy, i am late ;-)
  6. Sep 27, 2013 #5
    edit: I see three other posts appeared while I was composing......have not read those yet.

    There is a good description of particle detection here:


    One can detect [observe] individual particles either on the screen or at a slit. That's believable, not a huge stretch. It's the overall statistical nature of many observations which surprised.

    So far evidence and theory negates hidden variables, 'secret particles'......


    From a practical viewpoint, there are always uncertainties with experimental observations......and theory. And the possibility exists that there are as yet undetected particles, very heavy ones, called supersymmetric particles which we so far lack the power to produce...not likely an issue for double slit results.

    Wikipedia says: [Supersymmetry...]

    It is only recently that some 95% of the matter/energy in the universe was 'discovered' ..dark energy in the 1990's and dark matter maybe early 1970's ....oh and it was not even 'certain' the universe was expanding until about the same time frames.....Einstein sure had no idea in the 1920's....so other surprises likely await.
  7. Sep 27, 2013 #6
    These interference type experiments have been done with microwaves and even ions - particles where it was pretty easy to determine that only one was transmitted at a time. Even with light, the procedures have been done using photomultiplier tubes where the interference pattern could be seen to build up point by point.
    Also, if the interference pattern was dependent on two photons being emitted at the same time - so that they could interact on the fly - then as the laser (or other light source) was dimmed more and more, the clarity of the interference pattern would be expected to degrade.
    As far as "other particles", if you're going to presume that each photon has an entourage of helper particles that allow it to act as a wave, you're not that far from accepted QM. What you are saying is that these helper particles always interact with a specific photon - but cannot be detected by other photons. Wouldn't that be odd?
    Another poster referred to "hidden variables". This was an attempt to avoid some of the QM non-locality ("spooky action at a distance"). The Bell Inequality went a long way in arguing against it. It was published in the '70s in Scientific American and showed a fairly simple set of statistics that were incompatible with any classical description. Unfortunately, its been four decades and the simple experiment described in that article is only now barely being performed well enough to actually demonstrate that classical explanation can't work.
    What I found ultimately convincing were the delayed erasure experiments. You need to read through them carefully to catch what's going on, but once you do, you have to pretty well resign yourself to QM being what rules.
  8. Sep 27, 2013 #7
    There are macroscopic analogues that do capture many (but not all) of the properties of QM:

    The pilot-wave dynamics of walking droplets

  9. Sep 27, 2013 #8
    Thank you mfb, DrChinese, kith, Naty1, .Scott, and bohm2 for the quick yet deep answers! Let me take some time to read through the replies, and then hopefully I'll be able to agree with you, or come up with more native questions.

    All the best, T.
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