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A Why the inside of a box is coherent

  1. Mar 1, 2017 #1
    So in isolating a system so that it is in superposition in a volume of space, why is it the inside of the box gains coherence and the outside doesn't?

    It seems to me either the inside and outside have non-symmetric properties, coherence is limited to a maxium volume or density of decoherent particles. The cat inside the box (for instance) does not hypothesize that the outside observers are in superposition, why not? What is different inside and outside the box? Does coherence begin but run into too great of density/volume of stuff and coherence simply can't take hold and fade away outside the box?

  2. jcsd
  3. Mar 2, 2017 #2


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    To me, the actual difference is between Gedanken Cat and Real Cat. Considering a thought ("gedanken") experiment, physicists place into the box a "gedanken cat" which is merely a molecular system. No wonder, it can be in a superposition or in a mixed state. But, considering a real cat, the molecular system is no more than the cat's "physical interface", to interact physically with other things.
    Last edited: Mar 2, 2017
  4. Mar 2, 2017 #3
    If I'm understanding correctly, you're asking why the cat inside the box isn't measured while a cat outside of the box is? you might want to wait for other users as I'm not an expert (understatement of the year...) but the Idea is that measurement is a physical process ,i.e. it's driven by a certain Hamiltonian (complex exponent of the conjugate of the given measurable multiplied by the measurable itself), and it's assume that the box forces a Hamiltonian of an infinite potential well, and as such measurement cannot occur.
  5. Mar 2, 2017 #4


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    The original question is not completely clear, but if (as the replies above are assuming) the question is about how the cat can be in a coherent superposition inside the box while we all agree that the world outside the box cannot be, then the question is based on a mistaken premise. The point of Schrodinger's thought experiment was to point out a problem with then-current (1930ish) interpretation of the mathematical formulation of quantum mechanics: it seemed to predict that the cat would be in such a superposition even though we all know perfectly well that what we really have is a box containing either a dead cat or an alive cat, and there's nothing surprising about not knowing which iuntil we look. It took the best part of the next half-century to understand what the math was really saying; David Lindley's book "Where does the weirdness go?" is a good layman-friendly popularization.

    For the more general question in the original post: The key to maintaining a system in a coherent superposition is adequately isolating it from the environment around it. Measurement and observation only come into the picture because measuring devices are part of the environment, so when we're allowing the quantum system to interact with a measuring device to make an observation we've lost the isolation from the environment.
  6. Mar 3, 2017 #5


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