Thursday, May 22, 2014

Schrödinger’s cat

     Schrödinger’s cat is often used to illustrate the absurdity of quantum mechanics. Schrödinger devised the thought experiment to highlight the paradox implicit in the fact of entanglement. We are told that the cat is neither alive or dead (or alternatively, that is both alive and dead) until we open the lid of the box, at which point the wave function describing the cat’s state is said to collapse into one or the other state.
     We are told that opening the box is an “observation”, and that it is the act of observation that causes the wave function to collapse. Opening the box kills the cat, or saves its life. Schrödinger devised this absurd thought experiment in order to clarify the paradoxes that appear to arise from entanglement.
     I understand entanglement as follows: two particles interact. They leave each other’s vicinity. The mathematics of quantum mechanics imply that until one of the particles is “observed” or “measured”, we cannot know which particle is in which state. However, when one of the articles is observed to have State S, the other will be in the complementary state S’. The usual interpretation is that until the measurement is done, the particles are in both states, which are said to be superposed on each other. The measurement forces the “collapse” of the indefinite state of the measured particle into one of the two possible states, and somehow this is commu8nicated to the other particle, which collapses into the other state.
      Experiments have been done that show precisely this state of affairs. The question is whether the interpretation of the model is correct: Are the two particles actually in indefinite states until they are measured? Or is it merely the case that we cannot know which particle is in which state until we measure one of them? Note that measurement is an interaction. So the more accurate question is, Are particles that have interacted with each other in some indefinite state until their next interaction? Or is it the case that we cannot know anything about the state of either particle unless and until we arrange some interaction that results in effects large enough that we can both observe those effects and infer the state(s) that caused them?
     I think that QM is ultimately about the limits of knowledge, about what we can and cannot know about particles. Until we measure the particles, we can’t know what the result will be. More importantly, according to Heisenberg’s principle, the act of measuring the particles changes their states. However, measurement or observation is not a privileged interaction. It’s just the one of many possible interactions, and it will be followed by another one, and then another one, and so on.
     As I understand it, the Copenhagen interpretation argues that the two particles are in superposed states until they are measured, at which point one of two possible states becomes real in some sense, and thus constrains the next interaction. The many-worlds interpretation argues that whenever the wave function collapses, both possible outcomes become real and ontologically separated from each other. I think both interpretations miss a fundamental point: QM, like any other theory, is a model. A model explains the data that have been observed. It can’t explain what isn’t part of the model. Interpreting QM ontologically or metaphysically is absurd.
     Schrodinger’s cat is alive or dead, as the case may, before we open the box. Our observation doesn’t cause cat to live or die: the radioactive atom that did or did not decay before we opened the box caused that. Suppose the wind blows open the lid. Then the wind is the “observer”, and either the cat’s corpse will stay in the box, or else the living cat will jump out and go on its way.
     In short: Human “observation” is not a privileged interaction.           
    WEK 2013-11-18/2014-05-22
Update: New Scientist for 07 May 2014 has an article that expresses similar ideas.

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