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I Schrodinger's Cat variant

  1. May 10, 2016 #1
    For the sake of animal lovers, I come up with a variant of Schrodinger's Cat experiment thus:

    a. in place of the poor cat is a Young's slits setup, with one translucent side of the box doubling as the screen,
    b. in place of the lethal injection is a switch,
    c. at a certain fixed time (say 5 minutes after setting up), either one of the slits, depending on the state of the above switch, will be closed.

    The interference pattern is viewed from outside with the box still closed. What can I expect after (c)? If I understand Schrodinger's argument correctly, there should be interference pattern because both slits are in a superimposed state of open and closed and that state has not been observed. Is that correct? Or is the switch counted as an observer and there is no superimposed state after all?
  2. jcsd
  3. May 10, 2016 #2

    Paul Colby

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    Gold Member

    Your experiment is not very clear to me. What activates the switch? What does looking at the interference pattern do exactly? What are you trying to understand?
  4. May 10, 2016 #3
    The radiation detector. In the original Schrodinger's Cat experiment it activates a lethal injection. In my experiment it flips the switch.

    If the slits are in a superimposed state before some observer opens the box, then I expect an interference pattern to be seen as long as the box stays closed. If the slits' state is decided at the moment one slit is closed, then there should be no interference pattern after (c).

    I want to understand what counts as an observer. In my experiment, which or who is the observer?
  5. May 10, 2016 #4
    After 5 minutes the scientist will observe either an interference pattern, or none; and know from that whether the switch was closed or not. You don't have to open the box and look at the switch itself; by observing its effects directly, that counts as an observation of the switch.

    To make this gedanken similar to the cat experiment, you could replace the translucent side with a camera, contained entirely inside the box. After six minutes it takes a picture of the pattern, which may show interference or it may not. Afterwards the scientist opens the box and looks at the picture, deduces from it the switch's state (alternatively he could just look at the switch to see its state). Then the question is, what did that picture show prior to box opening? Was it half interference and half no interference? This is now exactly analogous to cat gedanken. But of course it destroys the whole point of your example.

    So if you provide any observation to deduce the switch's state from the outside, that observation will "collapse" the wave function. It would be similar to measuring, let's say, oxygen going into the box containing the cat. Then if you observe that the air flow stops, you can deduce the cat was killed without opening the box. It would never be the case that only half as much air went in because the cat was half dead / alive! The observation of air flow would immediately "collapse" the state of the cat to one or the other.

    Of course the common sense view is that the cat died (or not), or the switch closed (or not), whether anyone could know the fact (or not). Even if the box is never opened, common sense view says the switch is definitely in one state or the other, we just never find out which. But (as we all know) the point is extremely debatable.
  6. May 10, 2016 #5
    Perhaps my first post was a bit confusing, and so let me clarify. After 5 minutes one of the slits will be closed anyway so that there should not be any interference pattern. The switch only affects whether the left or right slit is closed. Observing the interference pattern won't tell the position of the switch. But if both slits are in an superimposed state, what prevents an interference pattern being formed?
  7. May 10, 2016 #6


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    Staff: Mentor

    They aren't in a superimposed state. When Schrodinger suggested his famous thought experiment, he was not trying to show that the cat would be in a superposition of dead and alive until the box was opened to make an observation- he and everyone else knew perfectly well that that isn't right. Instead, he was pointing out a problem in the then-current understanding of QM, namely that there was nothing in the theory that said that we'd have either a dead cat or an alive cat, but not a superposition. The explanation was not found until some decades later, and it works for your slits as well as the cat.

    Google for "quantum decoherence" (but be warned that the math is somewhat daunting) or try Lindley's more layman-friendly book "Where does the weirdness go?"
  8. May 10, 2016 #7


    Staff: Mentor

    All I can do is reiterate what Nugatory said. Another name for the issue is the Von-Neumann regress. I says you can place the quantum classical cut pretty much anywhere and that is the root cause of the problem. The solution is now we understand decoherence better, is to place the cut there - ie just after decoherence. In fact in modern times observation and decoherence is pretty much synonymous.

  9. May 11, 2016 #8
    Thanks folks. All my questions have been answered. Apparently a few of the layman-friendly books I read contain an incorrect version of the Copenhagen interpretation that I found too weird.
  10. May 11, 2016 #9


    Staff: Mentor

    That's par for the course.

    Come here for the truth.

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