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What is an observer, defined by QM?

  1. Mar 20, 2012 #1
    Couple noob questions.

    1.What is quantum mechanical definition of an observer?

    2.Why is looking at the double slit experiment with your eyes while the electron goes through any different than a camera looking? Each one is looking, one can just see better. Is it the fact that the outcome can be known with the camera, but not with your eyes that collapses the wave function? So does the electron makes a distinction between resolution capabilities?

    I read some were that you have to accept the fact that qm is all about probabilities. So the electron leaves as a particle, becomes a probability wave and goes through the slits interfering with itself. Now is the probability wave actually manifesting? What ACTUALLY happens to the mass of the electron. Since matter cannot be created or destroyed, and if the particle doesn't actually manifest as a wave then were does the mass/energy from the electron go while its a "probability" while going through the slits.
     
  2. jcsd
  3. Mar 20, 2012 #2
    2) IMO you're being too literal, and may be helped by not putting so much stock in the particle viewpoint. It's only very special situations where it's best to describe the position of a particle by a delta function in space such that you can really call it a particle. Consider it as a wave and you'll almost never be far off and will be much less confused.
     
  4. Mar 20, 2012 #3
    You might find my post from another thread interesting:
     
  5. Mar 20, 2012 #4
    there is no difference between a camera and the (human/life) eye.

    Both collapse the wave function.
     
  6. Mar 20, 2012 #5
    As I said above, that's not the only possible interpretation. You can instead construct the wavefunction of the electron-and-camera system, which remains in a superposition of states until some third object collapses this wave function. Or you can construct a wavefunction of the system that includes electron, the camera, and all other objects of the universe, so that you can consider the whole universe to be in a superposition of states which never collapses. Or you can take the view that it's the human mind that collapses the wavefunction. The thing is, it makes absolutely no experimental difference at what stage you assume the wave function collapsed at.
     
  7. Mar 21, 2012 #6
    it does make a difference -- if you assume a different stage of collapse of the wave function.

    the observations, the maths, the logic/explanation, the interference pattern etc won't match up....
     
  8. Mar 21, 2012 #7
    Yes they will. If you don't believe me, read Von Neumann's proof in his Foundations of Quantum Mechanics.
     
  9. Mar 21, 2012 #8
    if you assume, for example/one, that the wave function collapses well after detection....

    then the whole concept of wave function falls apart.....
     
    Last edited: Mar 21, 2012
  10. Mar 21, 2012 #9
    No, because as I said, you can construct the wavefunction of the particle-and-detector system, and then you'll need a third object to collapse that wave function. Anyway, you can read Von Neumann's proof for yourself.
     
  11. Mar 21, 2012 #10
    i see your logic about entanglement....will go through Neumann's literature
     
  12. Mar 22, 2012 #11
    What Lugita has been saying is, I believe, known as the Many-Worlds interpretation of quantum mechanics.

    Truth is, there is no collapse, there's just the appearance of collapse due to decoherence, which is caused by the fact that when talking about Quantum Mechanics you can no longer separate the measuring apparatus from the measurement. There is no distinction, everything in the end is made of particles (or, if you will be more accurate, amplitude distributions), and physics doesn't care that humans call different things electron, camera, eye or mind. It's all the same thing for physics, and any measuring apparatus used to detect the "actual" state of a quantum system will simply join the superposed state, and then you have a camera that's superposed too.

    And since however you put it physics doesn't seem to care what you think, or even for the fact that you think, once you look into the camera, you become superposed, too, and then there's more than one of you, and each of those versions of you has seen a different result, all as real as you, not a single one of them being "youer" than the other.
     
  13. Mar 22, 2012 #12
    Wow. Good post.
     
  14. Mar 22, 2012 #13
    No, I was talking about the Copenhagen interpretation, in which we can arbitrarily specify the stage in which wave function collapse occurs. See post #3.
     
  15. Mar 22, 2012 #14
    Um... you did mention that if you specify no stage you have a superposed Universe, and that's pretty much all Many-Worlds is about.

    Besides, the Copenhagen assumes the collapse occurs somewhere during measuring, it never says anything about the measuring apparatus and/or measurer becoming superposed too. The ideas of superposed apparatus and observer are the ideas that sort of define Many-Worlds.
     
  16. Mar 22, 2012 #15
    Yes, if you do not cut the Von Neumann Chain anywhere, i.e. you make the whole universe the system under observation, then you have no wave function collapse at all and thus you're in the Many World interpretation.
    In the Copenhagen interpretation, you're allowed to cut the Von Neumann chain anywhere. In other words, you're allowed to divide the world into observer and observed any way you like, and thus you can include measuring devices and whatever else you want in the observed system. So for instance, if a photon interacts with an atom and the atom interacts with a Geiger counter, Copenhagen says that you can treat the photon-and-atom-and-Geiger-counter system as being in a superposition of states, and that the wave function of this system is only collapsed when some other object, say a human, goes and measure the state of the Geiger counter.
     
  17. Mar 22, 2012 #16
    Huh. Huh, huh, huh. I had been under the impression that the Copenhagen Interpretation, or at least the one that is most common, assumes collapse as a fact of nature which happens somewhen/somewhere during observation.
     
  18. Mar 22, 2012 #17
    Yes, but in the Copenhagen interpretation it's up to you what you consider to be a mere interaction (described by the Schrodinger equation) and an actual observation that collapses the wave function.
     
  19. Mar 22, 2012 #18
    So one could say that, basically, Many-Worlds is the objective description of the Universe (the "bird perspective") and Copenhagen is the subjective one (the "frog perspective"). That is very interesting.
     
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