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Schrödinger's cat - I don't get it

  1. Oct 21, 2008 #1
    Hello everybody :)
    Well, I'm past my first Quantum course examination, with a satisfactory grade. Trouble is, there are some really basic things I still don't get, and somehow, everybody seems to shrug when I ask them about it. My profesor tries to explain but I fail to understand, so I thought I'd give it a shot here:

    I don't get Schrödinger's cat's experiment:
    The whole idea is that the cat's state is a superposition of "alive" and "dead", and only when a measurement of the cat's status is done, will the state function collapse into an eigenfunction of the result, meaning, "alive" or "dead", I guess.
    However, when a state function collapses, it collapses relative to anybody, right? I mean, it's not relative to the one making the measurement, but the state will become the new state for everyone else in the world.
    My question is - how come the cat itself, by being, isn't forming a measurement of his status? I mean, the cat knows if he's alive or not, right? It's like you'd tell me I cannot know if I'm dead or alive until somebody comes to this room and checks me out.
    So, the cat, by being, knows he's alive, for example, and therefore a measurement has been made. The state function has already collapsed. What meaning therefore is there to the opening of the box?
    Moreover - what's the difference between a cat and an electron? This whole issue is very confusing, since the student always gets the feeling the thing that matters is "conciousness" of a living being. It's like quantum physics only has meaning if there are living entities around. But my profesor cleared up the fact that life itself has nothing to do with it. What then, is the difference between a cat and an electron? Could this experiemnt be described as "Schrodinger's electron" instead? (Only less amusing, of course)?

    I'm reading the topic on the net (Wiki for example), but the explenations there are totally unsatisfactory. I'd be really thankful for anyone who could clear this issue up for me.

    Thank you!
  2. jcsd
  3. Oct 21, 2008 #2

    Ken G

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    Gold Member

    I think your objection to the "cat paradox" is entirely justified. There are really two ways of thinking about that paradox. One is that it is a bogus description, because it implies that coupling a macro system to a quantum superposition state somehow causes the quantum superposition state to "imprint itself" on the macro system, when in fact the opposite occurs-- the macro system imprints its properties on the quantum system, in effect measuring it. So that would be my interpretation-- the cat measures the quantum system and the latter loses its superposition status and becomes a "mixed state" until we look in the box. This is more or less the "Copenhagen interpretation".

    The second interpretation is that both the cat and the quantum system together form a new closed quantum system, essentially under the justification that all systems are somehow quantum systems. This requires a lot of faith that things like "wave functions" are still meaningful for systems with all that noise in it (we could never actually specify anything approaching a useful wave function for that actual system). But if one does that, the whole system is in a superposition state even before the cat is coupled to the quantum system, and the coupling can't change that. But when you project onto the cat subsystem, it has to come out alive or dead. But I don't see that as any different to the situation prior to the coupling-- cats can die at any time, so if a cat is in a box, it is already in a kind of superposition state, as is all of reality all the time, if you adopt this particular interpretation (it is called the "many worlds interpretation").
  4. Oct 21, 2008 #3
    You need to remember that Schrödinger was providing an “absurd” example.
    It highlights as obvious that the idea of entanglement is absurd; unless reality makes some division or transition between the microscopic to the macroscopic it cannot be ‘real’.

    But if there is in fact some real physical division or margin where something changes between microscopic reality and observable macroscopic reality (as yet unknown or at least ill-defined to us); QM and the rules for entanglement/super position etc. survive just fine within that constraint.
  5. Oct 22, 2008 #4
    Thank you for the responses. I'd read them more thoroughly when I have a little more time :)
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