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Double slit in macroscopic world

  1. Jan 23, 2012 #1
    It doesn't have to be a superfluid or silicon oil. Let's suppose a water droplets are falling to a hidrophobic (wd40 sprayed LOL) surface with double slits. It seems probable to me that scenario will happen that droplet will split on surface, pass through both slits (in some random ratio) and then recombine again into single drop completely before leaving the surface towards our water droplet detector.
    Surely recombining different ratios of water mass on departure will have different effects on final droplet momentum resulting in scattering and self interference patterns in our detector after sufficient number of droplets.

    Where am I wrong?
  2. jcsd
  3. Jan 23, 2012 #2

    Simon Bridge

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    The water droplets have effects like surface tension that dominates their behavior.

    To a greater extent you seem to be imagining the QM situation to be that of particles passing through both slits at the same time. You want to see a macroscopic double-slit experiment you have to do it with whole waves.
  4. Jan 24, 2012 #3
    Well, classical waves just being waves don't have quanta so you can't see macroscopic duality.

    Yes, water droplets don't just fall apart, due surface tension. But neither do electrons or photons for some reason obviously.

    Point is, particle doesn't really have to fall apart to go trough two slits. It just squeezes through both (possibly never really decomposing completely). Furthermore, in order for it to leave the surface it really needs to be whole (to acquire critical mass and momentum), otherwise it just sticks to the surface and we can call it absorbed. Effects of two slits being opened instead of only one during that moment a can surely be detected in further droplet voyage. Hence we could say that we have observed both wave and particle properties in single macroscopic object.

    My question is why is this not a good analogy? (Well, better than parallel worlds, new dimensions or other quantum wizardry). Why visualizing a particle that cannot fall apart but can "ooze" through more than one slit is not a good way of thinking about photons and electrons?
  5. Jan 24, 2012 #4

    Simon Bridge

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    Water droplets through slits do not produce a diffraction pattern - electrons do.

    Water droplets have an internal structure that allows them to Ooze - electrons do not.

    The physical size of the droplet has to be bigger than the separation between the slits for it to go through both - the separation between the "slits" in electron diffraction is many (iirc: 5) orders of magnitude the size of the electron.

    When you put a water-detector behind each slit - they show that the water passes through both slits, but when you try the same with QM interference, the detector shows one-particle-one-slit.

    See what I mean?
    Feynman has a very good series of lectures on wave-particle duality which does not do any of the multi-universe stuff and has only 4 dimensions.
    They are on youTube - go look.
  6. Jan 24, 2012 #5
    Thanks for quick reply.

    - My guesstimate was that it would show diffraction since two parts of droplet would interact with each other on way out. You mean it would not show diffraction pattern anyway? Wasn't one of the early attempts to explain diffraction a hypothesis that more than one electron passes through slits at same time and influencing the results?

    - Electron is point like (zero in size), right? But it interacts at distances bigger than zero, right? So in a sense it does have "something" (comparing to zero even infinitely) larger "around" it (quantum field? - i don't know anything about it :), which has to reach and "ooze" through both slits (even entire Universe allegedly).

    - Like said, droplet couldn't really leave the surface before recombining again. So either detector could go off but not both (or none if it fails leave the surface or detectors fail to catch it). Resulting trajectory would depend on number of opened slits and asymmetry of initial trajectory towards the slits and later droplet ratios in them.

    But come to think of it, I can also imagine diffraction not really being shown. I can't suppose why would some part of screen would have less chance of ending with droplet than the other. While it would "self-interfere" but it would always "add" bits of momentum, not "super-position" them. Macroscopic particles don't seem to do super-positioning after all, huh? :)
  7. Jan 24, 2012 #6
    About lectures, I have seen them and read books and Feynman is just about the greatest character ever. I don't understand why he of all people thought us not to ask ourselves "But how it can be like that?" but just to "Shut up and calculate". He also seems to want us to stop trying to visualize it as it is "nothing like we ever seen". I don't agree that people should not try to visualize things, even if they never seen anything like that.
    I can't let go the belief that one day someone will come up with more intuitive and easier description of inner workings of quantum world and rest of the world will go: "Aha! That explains everything! Brilliant!" ;)
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