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Double-Slit Experiment - Clarification on 'Observation'

  1. Sep 21, 2010 #1


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    In classical physics, measurement does not play a major role, but in quantum physics measurement plays an active role, correct? In the double-slit experiment, whether or not the path of the particle is measured - a detector finds which slit it passes through. I'm not questioning why this happens (well, I do, but that's not what I'm asking here). I want to know what kind of 'observation' or measurement specifically causes the particle's path to be altered, i.e. at what degree of measurement does this phenomenon (begin to) occur?
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  3. Feb 21, 2011 #2


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  4. Feb 21, 2011 #3


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    That is unknown.

    There are ideas in what direction the answers to such questions are to be sought. For instance, you can look up information about 'quantum decoherence'. The current state, as far as I know, is that all ideas for addressing the measurement problem are interpretations.
  5. Feb 22, 2011 #4
    If we ever settle that, we'll put a few dozen philosophers out of work, so maybe we shouldn't touch this problem for fear of making unemployment worse.

    The amusing thing about quantum measurement is that virtually nobody who discusses it backs up his/her statements with experimental evidence. Greenstein and Zajonc have a nice book, The Quantum Challenge, that discusses experimental evidence regarding measurement that isn't difficult.
  6. Feb 22, 2011 #5
    I think the simple answer is all kinds of measurements alter the particle being measured. Also, all degrees of measurements alter the particle being measured. There is no measurement you can make without altering what your measuring.

    If I want to measure the length of my finger, I need to interact with it in some way, with a ruler, or with light perhaps.

    You can't get something for free, to extract information is to extract energy, energy is conserved.
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