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I Is quantum collapse an interpretation?

  1. Sep 19, 2017 #1

    zonde

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    A have seen here on forum statements that quantum collapse is interpretation dependent, for example:
    So I would like to ask for explanation that does not relay on quantum collapse for this phenomena:
    We have unpolarized beam of light. It goes trough two orthogonally oriented polarizers. Assuming idealized polarizers we detect no light after second polarizer. Then we insert third polarizer oriented at 45 degrees between the first two. Now we observe 1/8 of the intensity of original beam of light.

    How such experiment is treated in no collapse interpretations?
     
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  3. Sep 19, 2017 #2

    Demystifier

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    In the minimal interpretation, you just change the wording. Instead of saying "collapse" you say "update of information". Everything else remains the same as in collapse interpretation.
     
  4. Sep 19, 2017 #3

    zonde

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    But certainly "update of [experimenter's] information" can not explain this experiment as something different has to happen with beam of light between two setups as we observe different results.
     
  5. Sep 19, 2017 #4

    tom.stoer

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    In Everett's Interpretation we accept that the system including the measuring device, the laboratory, the observer and the environment really is in a state consisting of classically incompatible "branches" which are mutually "invisible". The mathematical description i.e. the state vector with superposition of components (called "branches", "worlds", ...) results from the unitary time evolution w/o collapse; the "invisibility" follows from decoherence. So we simply accept this prediction which fits to our observations - instead of introducing an ad-hoc collapse via an additional postulate. That means we believe that the state vector and its unitary time evolution describe the structure and th dynamics of the "real system out there" and does not only encode information we have access to and which we have to adjust based on measurement outcomes.

    Without this realistic philosophical position there is no need to believe in Everett's Interpretation.

    Mathematically it's standard quantum mechanics w/o ever referring to collaps, projection etc. Born's rule must follow as a theorem from other axioms; we do have indications that this works, but afaik there is no consensus on this issue.
     
    Last edited: Sep 19, 2017
  6. Sep 19, 2017 #5

    Demystifier

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    That's correct, the minimal interpretation does not answer such questions. That's why it is called minimal.

    One of non-minimal interpretations without collapse is the Bohmian interpretation. To see how it explains phenomena like the one you described in the first post, see
    https://arxiv.org/abs/1305.1280
     
  7. Sep 19, 2017 #6

    vanhees71

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    It's indeed very simple. An ideal polarizer is realizing an ideal von Neumann filter measurement, i.e., you prepare photons (or if you experiment with usual light, coherent states of the em. field) with a determined polarization. You just through away the unwanted stuff, not polarized in the direction given by the polarizer. You don't need to assume an instantaneous collapse, it's just a local (!) interaction between the em. radiation and the polarizer. The minimal interpretation just doesn't make the unnecessary assumption of an esoteric mechanism that "collapses" something instantaneous in entire space(time)!
     
  8. Sep 19, 2017 #7

    vanhees71

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    Well, as to be expected it's about non-relativistic particles. I'm not aware that there is a satisfactory BM treatment of photons. Note that photons don't even have a position observable to begin with. So how can BM with its preference for trajectories in position space ever describe photons?
     
  9. Sep 19, 2017 #8

    Demystifier

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    They do, it's just not Lorentz covariant. But that's not a problem for instrumental approaches to QM, because you can always define the position observable with respect to the Lorentz frame in which the detector is at rest. After all, the photon detector determines the photon position, doesn't it?

    There are many ways to do it, if you are ready to postulate a preferred Lorentz frame in a manner which does not contradict existing experiments.
     
  10. Sep 19, 2017 #9

    zonde

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    Thanks, for your explanation.
    I would like to ask, at what point do you place "decoherence"? Does it happens in polarizers or in detector?
    Are you not mixing up collapse with Born's rule? Collapse "happens" at polarizers while Born's rule is applied at the end (at detector). Isn't this so?
     
  11. Sep 19, 2017 #10

    vanhees71

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    The photon detector defines the position of a local interaction between the photon and the detector material. Indeed you can live with a preferred frame as far as it is determined by the physical situation, and the detector-rest frame makes sense.
     
  12. Sep 19, 2017 #11

    zonde

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    So your approach is to ditch particles (photons) up until detection. Well, I suppose it works for my example.
    But if I replace photons with gold atoms and polarizers with SG apparatuses? What about this modified experiment?
     
  13. Sep 19, 2017 #12

    vanhees71

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    I don't understand what you mean. Of course, I have an incoming photon, prepared in its state. Then it's interacting with the polarizer, either being absorbed or let through (with probabilities given by Born's rule for the prepared state). Those photons that come through are polarized accordingly to the polarizer's orientation. That's what a polarizer does after all!

    There's no fundamental difference between this example and the SG aparatus. The only difference is that the SG apparatus doesn't filter (except you put some blocking material in some of the partial beams) but entangles position with the spin component in direction of the magnetic field. This is even described by unitary time evolution of QT since it's not an "open system" from the point of view of the gold atoms as in the case of the photons, where you have treated the polarizer effectively.
     
  14. Sep 19, 2017 #13

    zonde

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    Hmm, probably I misunderstood you.
    But then the idea of collapse is that polarization of photon "collapses" from it's initial polarization to the new polarization state according to polarizer's orientation if it goes through the polarizer. Isn't this the same what you are saying?
     
  15. Sep 19, 2017 #14

    vanhees71

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    I don't know, what you understand by the term "collapse". Usually, it's understood that the quantum state after a measurement goes instantaneously into a corresponding eigenstate of the operator representing the measured observable. This process is clearly outside of the quantum dynamics based on local interactions (in relativistic QFT), and it violates Einstein causality. However, if you look more closely at it, this assumption is simply not needed and by nothing justified from the formalism and its application to the desription of real-world observations and experiments. That's why I am a proponent of the minimal statistical interpretation, which is practically just the flavor of the class of Copenhagen interpretations without a collapse assumption. I think, it's pretty close to what Bohr meant, although I cannot say with certainty what he really thought, because his papers are pretty enigmatic (too much philosophy for my taste). The same holds for Heisenberg's writing. I prefer Dirac and Pauli, which give a clear picture of quantum theory without philosophical additions that obscure the scientific content of the theory!
     
  16. Sep 19, 2017 #15

    martinbn

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    @vanhees71 : But going from a superposition of vertical and horisontal polarization to let's say vertical, is not unitary. It is a projection. How do you explain things without it.
     
  17. Sep 19, 2017 #16

    vanhees71

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    That's exactly what I said. The reason is that we treat the polarizer effectively, i.e., not as part of the quantum dynamics.
     
  18. Sep 19, 2017 #17

    martinbn

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    That only pushes the projections further down the chain. The question, the way I understood it, was can you have an interpretation that never needs to use a projection (or something equivalent)?
     
  19. Sep 19, 2017 #18

    vanhees71

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    I don't think so, because we "project" all the time by sorting out the states we want.
     
  20. Sep 19, 2017 #19

    zonde

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    Yes, this is my understanding. Did I said something different?
    It's local in my example. Well, if you consider EPR experiment then yes, it's non-local.
    You have my simple example. So show how do you treat it without collapse.
    Basically you are saying that "collapse" is emergent from more complete unitary dynamics. But that does not change the fact that it represents real physical phenomena.
     
  21. Sep 19, 2017 #20

    vanhees71

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    If you have a collapse, it's never local, because it claims that the quantum state changes instantaneously, in the extreme case from a mixed to a pure state. That's outside the quantum dynamics based on local interactions.

    Which real physical phenomena does the collapse represent? I don't know a single example!
     
  22. Sep 19, 2017 #21

    zonde

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    Surely you know that photon beam interacts with polarizer. And I suppose that you consider it real physical phenomena.
    So you don't think that collapse prepresents this phenomena, right?
     
  23. Sep 19, 2017 #22

    Mentz114

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    Surely you can call it what you like ? We know it is a projective measurement between light and polarizer and the physics is well understod. As with SG apparatus.
    You think it should be part of the unitary evolution but there is no reason to expect that.
     
  24. Sep 19, 2017 #23

    vanhees71

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    No, the local interaction between between the em. field and the matter in the polarizer represent this phenomenon, according to QED. I don't need a collapse to describe it.
     
  25. Sep 19, 2017 #24

    vanhees71

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    On a microscopic level, it is part of the unitary evolution, but it is completely sufficient to describe the interaction of the photon with the polarizer effectively with the polarizer being described classically. Then the polarizer is reduced to a projection operator to the polarization state given by its orientation.

    This effective description is of course not given by unitary S-matrix since we don't resolve the microscopic interaction between the photon and the polarizer.
     
  26. Sep 19, 2017 #25

    Mentz114

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    The part I've emphasized is a source of controversy, is it not ?

    If a measurement has to extract /add information from/to a system then it cannot avoid changing the state of some part in a non-unitary (dissipative) way. This is implied by thermodynamics which seems to apply at macroscopic and microscopic scales.
     
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