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Quantum Communication

  1. Dec 9, 2008 #1
    Hi,

    A thought experiment I am struggling with..

    Imagine you were disembodied and could only communicate with the outside world through the wave function collapse. What would be the best way to try and communicate with people?
    Conversely, if you were looking for such a person what device could you build that would cut through the noise of the surrounding system and help him communicate cleanly?
     
  2. jcsd
  3. Dec 9, 2008 #2
    Wavefunction never collapses!
    Copenhagen interpretation is obsolete!
     
  4. Dec 9, 2008 #3
    Does that mean that there is no statistical part of quantum mechanics?
     
  5. Dec 9, 2008 #4
    you mean, random?
    QM laws are deterministic. But for the observers events appear to be random.
     
  6. Dec 9, 2008 #5
    If we observe it to be random - how do we know it is determenistic?
     
  7. Dec 9, 2008 #6

    JesseM

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    When you say "communicate through wave function collapse", do you effectively mean that you can measure quantum systems but you can't share the results with anyone else? If so then this does not depend particularly on the copenhagen interpretation. Anyway, if this is what you mean, and embodied observers suspected your existence and wanted to communicate, they could perform a "seance" in which they build an ordinary double-slit experiment with no measuring device at the slits, then ask you a question and tell you to measure all the electrons as they go through the slits if your answer is "yes", but not to measure them if your answer is "no". Then if your answer was "no" they'll see the electrons create an interference pattern on the screen, if your answer was "yes" they won't see an interference pattern.
     
  8. Dec 9, 2008 #7

    JesseM

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    Wavefunction evolution is deterministic between measurements, and to calculate the probability of getting an particular outcome when we measure a particular variable, you take the wavefunction's amplitude on that outcome at the moment of measurement, and the square of that amplitude is interpreted to be the probability of getting that outcome. So whether you see QM as a determinstic or probabilistic theory depends on whether you think "measurement" is really a physically distinct process from the normal evolution of the wavefunction (as in some versions of the Copenhagen interpretation), or if you think measurement actually follows the same rules as any other quantum interaction and the appearance of "collapse" is just a kind of illusion (as in the many-worlds interpretation), or if you think the evolving wavefunction is just an approximation for some more complicated dynamic governing quantum systems which can also explain what happens during measurement (as in Bohm's interpretation of QM).
     
    Last edited: Dec 9, 2008
  9. Dec 9, 2008 #8
    JesseM, good explanation.
    I was jsut thinking, shouldnt Copenhagen interpretation be erased from any popular explanations of QM? I think it creates a lot of confusion.
     
  10. Dec 9, 2008 #9

    JesseM

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    It's probably the simplest interpretation to actually use in practice when doing calculations, since in real-world scenarios we know when a measurement has been made. It's more at a philosophical level, when you try to think about "what is really going on", that it leads to confusion.
     
  11. Dec 10, 2008 #10
    Isn't there another interpretation put forward by Hawkings that the when we make a measurement and collapse the wave function we are revealing an initial condition of the universe?

    That's what I am looking for.. If we define an initial condition as a variable that has a degree of freedom from causation. What preceded it might set the range and probability distribution but the actual value is free, from any known, causal constraint. Are there such variables in QM?


    Please excuse my lack of eloquence.
     
  12. Dec 10, 2008 #11
    If I was disembodied then I would not have the option of measuring..
     
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