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What counts as an observation in QM?

  1. Dec 28, 2012 #1
    Hi I was day dreaming the other day when I realized how incomplete and vague the fundamental postulates of QM are, I mean what counts as an observation?
    For example simply being in the presence of a charged particle can be an observation, since (using a point particle for the sake of argument) then I can feel the electric force and deduce how far the particle is from me and which direction it must be in.
    If this were to be a proper observation, then by that logic every electron and proton observes every other one.
    Another problem I have is that the notion of an observation is not a very physically sound one, I can deduce many things without directly measuring them, for example I can deduce that everybody who will reply to this thread is on earth, however I did not go ahead and measure that. With this I can go ahead and put limitations on how accurately your momentums can be measured.
  2. jcsd
  3. Dec 28, 2012 #2


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  4. Dec 28, 2012 #3
    Why post links to articles that cannot be read online for free ? and the title does not sound like a consensual position.

    The concept of observation in QM indeed first appears as fuzzy and undefined. But it is only so as long as you keep applying the equations to a small isolated system. In fact it turns out to be quasi-unambiguously "defined" as a macroscopic limit of the properties of a non-isolated system exchanging a large quantity of information (entropy) with the outside. This is the process called decoherence.

    In my introduction to quantum physics I describe the mathematical structure of quantum states and measurements in a simple way I did not find so clearly expressed elsewhere, while staying rigorously equivalent to the established things (density matrix).
  5. Dec 29, 2012 #4
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