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Uncertainty principle - Nature of observer

  1. May 20, 2014 #1
    Uncertainty principle -- Nature of observer

    About the fact that electrons have a determined position only after having been observed/measured, I don't understand how they make the difference between being watched by an eye/instrument or any non-measuring object just sitting there.
  2. jcsd
  3. May 20, 2014 #2

    Simon Bridge

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    Welcome to PF;
    The "observer" is an interaction.
    If an electron does not interact with anything, then how do you know it exists at all?
  4. May 20, 2014 #3


    Staff: Mentor

    Just to expand on what Simon said, there is a bit of a fine point some treatments don't make clear. In QM an observation is when a 'mark' of some sort is left here in an assumed common sense macro world that exists independant of human observation. There are some issues with this view but that is a whole new thread. So as to not entirely leave it up in the air decoherence has a lot to say about it:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

    Last edited by a moderator: May 6, 2017
  5. May 27, 2014 #4
    I agree with you freeelectron, it doesn't make a difference. Any flotsam in the experiment is an effective "observer".

    I believe you can find this definition at the end of the quantum text book written by Sakurai or Shankar where he discusses the Bohm interpretation.

    Being "watched" means the electron has already interacted with a photon.
    Last edited: May 27, 2014
  6. May 27, 2014 #5


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    Nature got along just fine before observers happened along. The whole observer dependent thing is pure hubris, IMO. Seriously, why would the universe 'care' about 'peeping'? My view: the universe is not observer dependent, observers are universe dependent.
  7. May 28, 2014 #6


    Staff: Mentor

    Mate I think that IMO isn't really required - of course you are correct.

    This whole observer thing came from Von Neumann's classic - Mathematical Foundations of Quantum Mechanics. I actually learnt QM from that book - I now wish I didn't for various reasons, but because of that I know what it says.

    He showed the cut between classical and quantum can be placed anywhere (that's the so called Von Neumann cut) and in tracing it back the only place different was an observers conciousness - so guess where he placed it. Without going into the dubiousness of that line of reasoning, which I believe is hogwash despite my great admiration for Von Neumann, modern developments have shown a place different - just after decoherence - so the argument these days is not valid.

    Last edited: May 28, 2014
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