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Action a distance, collapse and relativity

  1. Jun 23, 2013 #1
    Relativistic Quantum Mechanics exist. But I don´t understand, it would must be see contradictory. He has the locality postulate, contradicting the collapse in a extensive system objects entangled. Yes, it is and old question: EPR, Bell Iniqualities. But I don´t know what is the final solution to the question.
  2. jcsd
  3. Jun 23, 2013 #2


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    Gold Member

    What question?
  4. Jun 23, 2013 #3
    IMHO, there is no final solution. The measurement problem is widely recognized as such (http://plato.stanford.edu/entries/qt-measurement/ ). As far as I understand, collapse is in contradiction not just with relativistic quantum mechanics, but with any standard unitary evolution of quantum mechanics, as unitary evolution does not allow irreversibility or turning a pure state into a mixture.
  5. Jun 23, 2013 #4
    DevilsAvocado, if you go to answer a question, it is better read it first.

    Hi, akhmeteli, the measurement problem is "semi-solved". The problem is where is the cut between classical and quantum. I don´t know link, but I answer a question about it a little ago. There ir wavefunction more or less semiclassical ( WKB aproxximation), and if and system is more classical than another that it is used like measurement device, then all the apparatus more classical than this are of course measurement device too. There is no problem in praxis. I was more interested with the contradiction between collapse and relativity. I don´t know if there a solution, the problem is not it may be antiintuitive, the problem is we lost any reference to define the position operator. And I don´t know what happens with the GR invariance in collapse moment
  6. Jun 23, 2013 #5
    The measurement problem may be "semi-solved", or "99% solved", or "99.99% solved", but it is not solved, and it can be very relevant to the Bell theorem - see, e.g., my introduction to a very long thread: https://www.physicsforums.com/showthread.php?t=369328 .
  7. Jun 23, 2013 #6
    You've been asserting this for a while, but I still don't understand why you think this. What is wrong with saying that the wavefunction evolves according to the Schrodinger equation, until a measurement is made, at which point it collapses according to the Born rule? Now there are issues with this view, like what constitutes a measurement, and there are philosophical implications of this view which people may find problematic, but I don't see any contradiction.
  8. Jun 23, 2013 #7
    As I said at https://www.physicsforums.com/showpost.php?p=4234411&postcount=813 , you can indeed escape a logical contradiction in this way, but the resulting theory makes no rhyme nor reason, if you ask me, for example, it is not even properly defined for the reason you mention: it is not clear what constitutes a measurement. Furthermore, there is no positive experimental evidence of collapse, according to Schlosshauer (please see my referenced post). If you are fine with that, you are fine with that, but the measurement problem is still widely recognized as such. Again, you may think that unitary evolution stops during measurements, but your opinion has no experimental confirmation.
  9. Jun 25, 2013 #8
    I don´t read the published version, but the arxiv only talk about a particular case of spins, there is no a general alternative to collapse.

    But my problem is how can you deduce Lorentz Invariance and GR Invariance even, in spite of nonlocality, no the measurement problem, that in praxis is totally solved: you know when a measurement apparatus is classical...
    Last edited by a moderator: May 6, 2017
  10. Jul 2, 2013 #9
    I have found a solution to this problems. It is Rovelli´s Relational Quantum Mechanics. It consist in considering that all systems (quantum or classical) are measurement apparatus, and the wavefunction is relative to another system. It has no an absolute character. http://arxiv.org/abs/quantph/9609002
  11. Jul 2, 2013 #10


    Staff: Mentor

    Then you do not understand decoherence. It turns a pure state into an improper mixture. Whether that solves the measurement problem is open to debate and interpretation dependent.

    Its not in contradiction to unitary evolution (it just a different way it can change a state) but, depending on interpretation, may or may not even exist as something beyond the calculations a physicist might perform.

    Last edited: Jul 2, 2013
  12. Jul 2, 2013 #11


    Staff: Mentor

    Errrrr. Come again. Interpretations exist where it doesn't even exist (eg many worlds), or its nothing but a calculational device without physical significance (eg the ensemble interpretation). Its only an issue for some interpretations.

    BTW its solved in the dehoerence ensemble interpretation I hold to - but it raises other disquieting things - but this is not the thread to go deeply into it.

    I think its much 'better' to say all interpretations have issues and the measurement problem is only an issue in some.

  13. Jul 2, 2013 #12


    Staff: Mentor

    There is no contradiction - its just not elegant having both. And interpretations like many worlds exist where it doesn't even exist and in other interpretations like the ensemble interpretation it not an actual physical thing - just something that occurs in calculations.

  14. Jul 3, 2013 #13
    Sorry for the delay - I have only noticed your post recently.

    Yes, they only consider a particular case. You believe there is no "general alternative to collapse." But I don't think this is in contradiction with what I said: "IMHO, there is no final solution." Not right now. Even if there is no "general alternative to collapse", that does not mean we have to accept collapse. For example, I don't accept collapse for the reasons given in my post 3 in this thread. Furthermore, according to Schlosshauer, there is no positive experimental evidence of collapse.

    I gave the reference to the thread where I argued (using other people's arguments) that locality has not been ruled out yet, so your problem is not my problem:-). And no, I don't "know when a measurement apparatus is classical", I believe it's always quantum.
  15. Jul 3, 2013 #14
    I don't think your statement is in contradiction with what I said: unitary evolution cannot turn a pure state into a mixture. With all due respect, if you want to move the goal posts, introducing a new term "improper mixture" (new relative to my statement), I don't have to modify my statement.

    I agree with this statement, and again, I don't think this is in contradiction with my statement: "IMHO, there is no final solution."

    I respectfully disagree (if you mean, however, that unitary evolution and collapse "take turns", please see my post 7 in this thread). What's more important, and maybe you could even agree with the following statement, there is no generally accepted view on this issue. So again, "IMHO, there is no final solution."
  16. Jul 3, 2013 #15


    Staff: Mentor

    Mixture covers both improper and proper mixtures. And it's not mere semantics - its fundamental and crucial to the issue.

    Now exactly what are you claiming - it can't be turned into a proper mixture or an improper mixture?

    And since an improper mixture can't observationally be distinguished from a proper one exactly where does the problem lie?

  17. Jul 3, 2013 #16
    I believe your opinion is controversial, in the best case. For example, Vaidman believes that "There is a serious difficulty with the concept of probability in the context of the MWI." (http://plato.stanford.edu/entries/qm-manyworlds/#4 ). So do you believe that problems with probability have nothing to do with the measurement problem? (By the way, I think it's a positive aspect of MWI that it rejects collapse, if I am not mistaken) Or, for example, Peter Shor wrote that "adherents of the many-worlds interpretation and the de Broglie-Bohm pilot-wave interpretation claim to have solved the measurement problem, but in each case they have replaced it with something equally abhorrent to our intuitions" (http://physics.stackexchange.com/qu...-problem-be-solved-by-string-theory-other-toe )
    I am not sure about the status of the measurement problem in the ensemble interpretation, but anyway, the interpretation itself is not generally accepted, so I don't believe we have a generally accepted solution of the measurement problem.
    Again, I don't know much about this interpretation, but it is not generally accepted either, so there is no generally accepted solution of the measurement problem.

    What is "better" and what is "worse" is a matter of opinion, but I still think there is no final solution of the measurement problem. Anyway, there is no doubt the measurement problem is widely recognized as such. If you disagree... Well, let us agree to disagree...
  18. Jul 3, 2013 #17
    With all due respect, I don't buy such reasoning: next time I'll tell you that the sum of the angles of a triangle is 180 degrees, and you'll tell me that this is wrong for "improper" triangles? And that triangle covers both improper and proper triangles? You can move the goal posts any way you like, but you cannot make me play your game.

    I claim that unitary evolution cannot turn a pure state into a mixture under a standard definition of mixture.

    The problem is unitary evolution and collapse of standard quantum theory are mutually contradictory. And if you are saying that there is no such problem in some other interpretation, then this other interpretation either reproduces all the results of standard quantum theory, and then it reproduces this contradiction, or it does not reproduce all the results of standard quantum theory, then let them (this interpretation and standard quantum theory) sort it out which one is right and which one is wrong, and until they sort it out there will be no final solution of the measurement problem.
  19. Jul 3, 2013 #18


    Staff: Mentor

    Well both are standard definitions. Since decoherence transforms a pure state into an improper mixture (its done by what is called tracing over the environment) that is false:

    Under any interpretation that is false - simple as that. They are not contradictory - just different. It is like saying red is contradictory to blue - nonsense.

    Last edited: Jul 3, 2013
  20. Jul 3, 2013 #19


    Staff: Mentor

    In the context of the MWI yes - since there is no collapse and no measurement problem. The issue here is one of elegance. One can simply assume in the MWI the experience of each world is that it occurs with a certain probability. That can simply be assumed and indeed Gleason's theorem shows its the only reasonable experience possible. But since the MWI claims to be QM with the state taken literally, and its totally deterministic, that you have to assume a probabilistic experience is a blemish - you would like it to be derivable from that alone - that's where the issue arises. The attempts so far are not to everyone's satisfaction. The key point though is it does not invalidate the MWI which has no collapse or measurement problem. Of course in doing so it has introduced other issues.

    Last edited: Jul 3, 2013
  21. Jul 3, 2013 #20
    That certainly explains why Everettian's have been so busy trying to formulate some kind of derivation of the Born Rule, all these years.

    On the other hand, regarding interpretations of QM and relativity, so far as I know, whilst relativistic MWI's and Everett-style interpretations are extant in the literature, I know of no corresponding relativistic versions of e.g. Collapse theory (emerging models on the way though), Bohm or many minds. Just saying.
  22. Jul 4, 2013 #21
    You say it is false, I respectfully disagree. Please feel free to move the goal posts without me.
  23. Jul 4, 2013 #22


    Staff: Mentor

    They were moved many many moons ago with the discovery of decoherence. Its fully detailed in the paper I linked to.

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