Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I What 'Collapses' the Wavefunction of the Universe?

  1. Jan 17, 2017 #1
    Hi all,

    I would like some thoughts on the following quote I read from a book on the history of QM: "Quantum: Einstein, Bohr, And the Great Debate About the Nature of Reality." Please restrict your consideration to the Bohr/Heisenberg 'Copenhagen' interpretation (I realize in other interpretations, the wavefunction is real and/or there is no collapse):

    "The Copenhagen interpretation requires an observer outside the universe to observe it, but since there is none - leaving God aside - the universe should never come into existence, but remain forever in a superposition of many possibilities. This is the long-standing measurement problem writ large. Schrodinger's equation that describes quantum reality as a superposition of possibilities, and attaches a range or probabilities to each possibility, does not include the act of measurement. There are no observers in the mathematics of quantum mechanics." p357


    Is the quote above from the author an accurate implication of the Copenhagen interpretation? Or has the author missed something? (His bio states that he has degrees in physics and philosophy, and the book has very good reviews.)

    Thanks
     
  2. jcsd
  3. Jan 17, 2017 #2

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Sounds like nonsense to me. Who says that the origin of the universe has to be the result of a measurement? I'm not aware that QM has anything to say about the origin of the universe, only how it works now that it's here.
     
  4. Jan 17, 2017 #3

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    I don't think so, since AFAIK the Copenhagen interpretation implicitly assumes that QM cannot be applied to the universe as a whole. Applying QM to the universe as a whole is something that the "no collapse" interpretations like the MWI do.
     
  5. Jan 17, 2017 #4
    I think that is exactly what the author is alluding to, but in the form of a critique of Copenhagen. Because without this "implicit assumption" that you pointed out, the "wavefunction collapse" in Copenhagen does indeed imply that an observer outside the universe is required.... right? That's my understanding at least...

    To provide more context: The book does seem to hold Einstein's side of the debate as sort of heroic... This is why I think the quote I provided from the book is actually a critique of Copenhagen....
     
  6. Jan 17, 2017 #5

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

    Without the implicit assumption, the Copenhagen interpretation isn't coherent, as far as I can tell. The Copenhagen interpretation basically says you can't use QM to describe everything; there must always be some things ("measuring devices"--things that can collapse wavefunctions) that are classical, not quantum. But if you can't use QM to describe everything, then obviously you can't use it to describe the whole universe. So from the standpoint of the Copenhagen interpretation, the author's statement that it "requires an observer outside the universe to observe it" is simply wrong; that's not what Copenhagen says. It says the idea of using QM to describe the universe as a whole is meaningless to begin with.

    It might be, but that doesn't mean it's accurately describing what Copenhagen actually says.
     
  7. Jan 17, 2017 #6
    Understood, thanks for clarifying the issue :)
     
  8. Jan 17, 2017 #7

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Absolutely.

    1. Collapse is not part of the formalism of QM because we have quite a few interpretations where it isn't present eg many worlds.

    2. Why does the universe need collapsing? Now there is a genuine issue here, but collapse isn't it. Its quantum state and preparation procedures are pretty much synonymous there days in modern QM treatments like Ballentine - what prepared the universe?

    Thanks
    Bill
     
  9. Jan 17, 2017 #8

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Yes. That's why we have a number of modern interpretations specifically designed to get around that issue. Everything is obviously quantum - there should be no quantum/classical cut as required by Copenhagen. It doesn't prove Copenhagen wrong or anything like that, but its a blemish best done away with. A number are around, Many Worlds, DBB, GRW and Consistent Histories to name a few.

    To the OP if you want to get on top of this stuff you need to study a modern interpretation. Just purely as a start, and in no way singling it out as any better than any other, you might like to look into Consistent Histories:
    http://quantum.phys.cmu.edu/CHS/histories.html

    But studying any interpretation helps in understanding QM - you should study a few.

    As you learn more about QM THE book on interpretations in general is by Schlosshauer:
    https://www.amazon.com/Decoherence-Classical-Transition-Frontiers-Collection/dp/3540357734

    Thanks
    Bill
     
  10. Jan 18, 2017 #9
    I'm personally in favor of the deBroglie/Bohm Pilot Wave model. From the research I've done on all the interpretations so far, that's the one that seems most sound to me. I'm planning on studying it in more depth once I get through Griffiths Intro to QM. I definitely also like the decoherence attempts, bridging the QM/Classical boundary... Will get into more depth on the foundations once I get past the introductory textbooks.
     
  11. Jan 20, 2017 #10
    Quantum Cosmology.

    ------
    Hugh Everett III
    "If we try to limit the applicability .... in general systems of macroscopic size, we are faced with the difficulty of sharply defining the region of validity.

    .
     
  12. Jan 20, 2017 #11

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That's a non sequitur if ever there was one!
     
  13. Jan 20, 2017 #12

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Having reread this, it is nonsense for a number of reasons that reveal the author's shaky understanding of QM.

    First, QM does not postulate that things only come into existence when measured. An electron (or a universe) exists whether it gets measured or not. "Existence" itself is not an observable.

    Second, he makes the beginner's mistake about a system being in superposition before a measurement and in a "definite" state after. Systems are always in a superposition of states. After a measurement, they are in an eigenstate of the measured quantity, but still in a superposition of the eigenstates of all incompatible observables.

    Third, something that doesn't exist can't be in a superposition of any states! I think this is another beginner's mistake that while in superposition things are in an "existential limbo".

    It's extraordinary that people who understand so little can write a book about it!
     
  14. Jan 20, 2017 #13
    While I agree with you, there are variants of Copenhagen that are compatible with describing the universe as a whole - if collapse is induced by decoherence so the measuring devices that you mentioned are just systems with many degrees of freedom.
     
  15. Jan 20, 2017 #14
    Well in fairness, it's just a history book, not a technical critique. Most of the remarks he provides are just sourced from letters and papers etc. by the physicists involved at the time. This particular remark came from the author himself which is why I wanted to verify it's accuracy.
     
  16. Jan 21, 2017 #15

    your question is:

    what changes the linearity of the superposition of the wave function of the unverse ?

    and just forget the copenhagen rattle out.
     
  17. Jan 21, 2017 #16
    Not really, that would be assuming a universal wavefunction exists... That isn't the question.

    The question was in the context of the C. Interpretation, and the idea as presented by the author, that if we remove the arbitrary distinction between quantum/classical, then there would be a universal wavefunction, requiring collapse from an outside observer, and if that conclusion is correct or not.

    The answers by members here indicate that this is an invalid question, because that distinction in inherent in the interpretation.... which is why it's just an "interpretation", and that is fair. It still doesn't answer the criticism of the interpretation, but that's another matter entirely.
     
  18. Jan 24, 2017 #17
    Hi Ghost117

    Please read Julian Barbour's The End of Time. After reading it, you will return here and be able to answer your own question.
     
  19. Jan 24, 2017 #18
    The idea that there is a 'wavefunction of the universe' can be questioned. It's a consequence of presupposing unitary-only evolution, and nobody can say for sure that this holds, even though Hugh Everett made it popular.
    On the other hand, if there is genuine physical collapse, as in the transactional interpretation, then an avenue opens to get classical emergence from the collapse process (since the collapse yields a classical probability space, as discussed here (https://arxiv.org/abs/1612.08734). This would explain why the macroscopic world of experience (and continuing on to cosmological levels) does not require a Hilbert Space description. An overview of this is presented in my books.
    For an intro to the basic ideas and current state of research: https://arxiv.org/abs/1608.00660
     
  20. Jan 24, 2017 #19
    Assuming finite periodic boundary conditions for the time axis, the Big Bang would be more than adequate to decisively collapse the wavefunction of the universe once an eon.
     
  21. Jan 25, 2017 #20

    Paul Colby

    User Avatar
    Gold Member

    Is it clear that a given system even has a pure wave function unless it's been prepared? I can see no way within the rules to make that determination prior to measurement.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: What 'Collapses' the Wavefunction of the Universe?
  1. Collapse Wavefunction (Replies: 2)

  2. Wavefunction collapse (Replies: 1)

Loading...