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Difference Between Two Thoughts on Wave Function Collapse?

  1. Apr 9, 2015 #1
    I've recently been reading the book The Dancing Wu Li Masters which is supposed to be a very basic introduction to quantum physics. I have a question on the following quote:

    "Up to now, we have said that the collapse occurs when somebody looks at the observed system. This is only one point of view. Another opinion [...] is that the wave function collapses when I look at the observed system."

    I'm new to quantum physics in general, so I was wondering what the difference was between "somebody" and "I" looking at the observed system in. I'm questioning it since it was so specifically expressed as two opinions on the question of when the wave function collapses.

    Thanks for any help/enlightenment anyone can offer!
  2. jcsd
  3. Apr 9, 2015 #2


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    There are two views on wave function collapse. One is that the wave function is real and really collapses. In this case, presumably the collapse occurs no matter who looks at it. I had never heard this view until recently, so I think it is largely a strawman.

    A second view is that the wave function is not necessarily real, and represents one's state of knowledge. In this case the wave function collapse is an update of one's knowledge, so the wave function collapses when you observe it. Of course, you are free to define who you are, and you can use another person or an instrument as a proxy for your state of knowledge. So the important distinction is that in the second case, the cut between you and the quantum system can be shifted. As far as I understand, this is the orthodox view.
  4. Apr 9, 2015 #3
    Thanks! I hadn't heard of the second view that you've just pointed out until now, but it seems like a very interesting way to consider the wave function.
  5. Apr 9, 2015 #4


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    One reason for the second view is that if we take the wave function of the quantum system as real, and we believe the observer who collapses the wave function should obey the same underlying laws of physics as the quantum system, then we will try to write a wave function that includes quantum system and the observer. However, in that case, one finds that there is no definite result (the cat is dead and alive), contrary to our experience (the cat is dead or alive). To fix this one can try to say that there is no definite result and that all outcomes occur (Many Worlds), or we can introduce new variables (Bohmian Mechanics), or we can say that a notional cut between the observer and the quantum system is always needed, and the wave function is not necessarily real, as the cut can be subjectively shifted (orthodox Copenhagen).
    Last edited: Apr 9, 2015
  6. Apr 9, 2015 #5

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    It's not. It's a mishmash of a caricature of physics with eastern mysticism. Read it, enjoy it, but don't think it's a good explanation of physics.
  7. Apr 9, 2015 #6
    You're right about that, perhaps I should've thought more before saying what the book was about. It's certainly not anywhere near an academic level of quantum physics explanation, and I do hate the lack of mathematical/concrete explanations that it has. However, I do enjoy some of the views it has on physics in general. Just out of curiosity, what do you mean when you say it's not a good explanation of physics? Do you mean the entire book, and its thoughts? Also, (sorry to bug you more) but it sounds like you might be able to recommend some other books that I could use to learn more about physics in general, and more specifically quantum physics. Could you?

    Thanks for your input!
  8. Apr 9, 2015 #7


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    I don't like that book either - it's sort of a cross between new age ideas and sensationalist views of modern physics.

    A much better book is Feynmans - QED:

    And the associated video lectures:

    But if you want the real deal, including the dreaded math, this is the book to get:

    It too has video lectures:

    Regarding your original question decoherene has shed a lot of light on the issue:

    Last edited by a moderator: May 7, 2017
  9. Apr 9, 2015 #8
    Thanks a lot! The Theoretical Minimum looks very interesting, I'm going to have to order a copy of the book soon, and the online lectures are an awesome bonus. Thanks for your input, Bill!
    Last edited by a moderator: May 7, 2017
  10. Apr 9, 2015 #9


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    I've read a lot of Susskind's Quantum Mechanics Theoretical Minimum and I agree with bhobba that it is excellent.
    Last edited: Apr 9, 2015
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