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Who is puzzled by the delayed choice?

  1. May 11, 2010 #1

    Demystifier

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    Many physicists (especially experimentalists) seem puzzled by the delayed choice experiments (DCE), because, as they argue, such experiments seem to change the past.
    Here I discuss DCE from the point of view of (not 1, not 2, not 3, but) 7 major interpretations of quantum mechanics: 4 variants of the Copenhagen interpretation (see https://www.physicsforums.com/showthread.php?t=332269 ), statistical ensemble interpretation, many-world interpretation and Bohmian interpretation. None of these interpretations suggests that (DCE) change the past. So why so many people are puzzled by DCE? Obviously, because none of these 7 major interpretations seems acceptable to them. Or more generally, because NONE of the existing consistent interpretations of QM seems acceptable to them.

    1. Shut up and calculate: You can consistently calculate the probabilities for different measurement outcomes without asking anything about deeper meaning of the numbers you obtain. Clearly, without asking such questions there is no reason to suspect that experiments might change the past.

    2. Positivistic interpretation: There is no reality (or it does not make sense to talk about reality) except the measured reality. So, if you don't measure the past (which you don't in DCE), there is no reason to say that experiments may change the past.

    3. Collapse interpretation: The collapse of the wave function takes place at the instant of time at which you make the measurement. Thus, in DCE, the wave ALLWAYS goes through both slits, no matter what you do after that. But, depending on your actions after the wave passing through the slits, at the instant of measurement the wave may collapse to a new wave function which differs significantly from the the wave function before the collapse. It is a radical change of the wave function, but it does not affect the past of it.

    4. Information interpretation: Wave function represents your knowledge about the system. This is knowledge that allows you to predict (with non-perfect certainty) the outcomes of FUTURE experiments. At the instant of measurement your knowledge of course changes (updates), which serves to predict the outcomes of new future experiments. This knowledge is never used to predict the outcomes of the PAST experiments, unless the past experiments have actually been done and their results are recorded. In this case, these predictions on past experiments are allways compatible with the recorded data, so there is no reason to say that experiments change the past.

    5. Statistical ensemble interpretation: Quantum mechanics says nothing about individual particles, but only about statistical properties of large ensembles of particles of equally prepared systems. More precisely, it says something about statistical properties for the case when they are MEASURED. So, if you don't measure the properties in the past, then QM says nothing about them. Since it says nothing about them, then, in particular, it does not say that the past properties have changed.

    6. Many-world interpretation: The wave allways goes through both slits. However, due to decoherence, at the instant of measurement (and at the position of the measurement apparatus) the wave splits into many non-communicating branches, which makes the illusion of collapse as in 3. This branching does not modify the wave function in the past.

    7. Bohmian interpretation: The (pilot) wave splits as in 6. The particle takes one definite trajectory ending-up in one of the branches. During its motion, the particle does not change its past.

    To conclude, I claim that anyone who argues that DCE affects the past must first explain why neither of the existing consistent interpretations of QM is not acceptable to him/her.
     
  2. jcsd
  3. May 11, 2010 #2

    DrChinese

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    What, you didn't want to mention Relational Blockworld? That handles this case nicely too. I guess we could let RUTA give it in his words.
     
  4. May 11, 2010 #3



    Aren't these types of questions the very reason why time is thought to be not fundamental, but emergent in M-Theory?

    http://discovermagazine.com/2007/jun/in-no-time



    "The purpose of this talk is to review the case for the idea that space and
    time will end up being emergent concepts; i.e. they will not be present in
    the fundamental formulation of the theory and will appear as approximate
    semiclassical notions in the macroscopic world. This point of view is widely
    held in the string community and many of the points which we will stress
    are well known.
    Before we motivate the idea that spacetime should be emergent, we should
    discuss the nature of space in string theory."


    http://arxiv.org/PS_cache/hep-th/pdf/0601/0601234v1.pdf

    Emergent Spacetime
    Nathan Seiberg, Institute for Advanced Study, Princeton, NJ 08540 USA



    Is anyone convinced that a reasonable conclusion on the DCE can be reached within the framework of the existing interpretations?
     
  5. May 11, 2010 #4

    Cthugha

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    In my opinion most of these people are puzzled because they do not understand the difference between single-photon interference and two-photon interference. Mixing up the properties of these processes will produce strange interpretations of DCQE experiments - regardless of which interpretation of QM you prefer.
     
  6. May 11, 2010 #5
    Whatever happened to "shut up and calculate"? When did this turn into "everyone gets their own ontology?"
     
  7. May 12, 2010 #6

    zonde

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    I would like to add something to this explanation.
    First, ensemble is really traveling all the paths in experimental setup. There can be no doubt about that even from classical perspective as long as we remember that result of experiment still depends from all subensembles of whole ensemble if we try to do some splitting.
    Second, from perspective of ensemble there is hardly any past because what is past for some part of ensemble is present for other part and future for yet another part.
     
  8. May 12, 2010 #7

    Demystifier

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    Well, I have mentioned 7 MAJOR interpretations, where "major" means "supported by the largest number of physicists" (and does not imply that they are better than other interpretations).
     
  9. May 12, 2010 #8

    Demystifier

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    Not at all. The motivation for M-theory is entirely different.
     
  10. May 12, 2010 #9

    RUTA

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    The Relational Blockworld interpretation dispatches QM "mysteries," such as DCE and QLE, very simply. Essentially, RBW is a form of ontic structural realism that says QM provides a rule for the acausal, adynamical, 4D description of an experiment. For example, the wave function is neither dynamic nor "real" (has no ontic status), so the measurement problem is a non-starter.

    Once you get past the belief that "explanation" has to entail a "dynamical story" about objects moving in (some) space as a function of time, i.e., accept the blockworld view per relativity, then you open the possibility for spacetime laws, e.g., Einstein's eqns of GR, that aren't even spatiotemporally local (nonseparable). Explanatory power is greatly improved once you decide not to constrain your explanations to "story telling."

    "There is no dynamics within space-time itself: nothing ever moves therein; nothing happens; nothing changes. In particular, one does not think of particles as moving through space-time, or as following along their world-lines. Rather, particles are just in space-time, once and for all, and the world-line represents, all at once, the complete life history of the particle." Geroch, R., General Relativity from A to B, University of Chicago Press, Chicago, 1978, pp. 20-21.
     
  11. May 12, 2010 #10

    Demystifier

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    Yes. Me, for example.
     
  12. May 12, 2010 #11

    Demystifier

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    What IS real (ontic) in RBW?
     
  13. May 12, 2010 #12

    RUTA

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    Any'thing' that can be modeled classically (X and P commute). No size restriction. However, 'things' are not made ultimately of ever smaller 'things', they're made of relations. And, when you model a phenomenon, you must do so in its 4D entirety. That's why the wave function models the experiment as a whole in both space and time (to include outcomes).
     
  14. May 13, 2010 #13

    Demystifier

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    So it is essentially the Rovelli relational interpretation (which is a variant of the information interpretation, number 4. on my list). Is there any important difference between this interpretation and Rovelli interpretation?
     
  15. May 13, 2010 #14


    A virtual world agrees with all the evidence but it seems to imply that all of our assumptions are wrong at the same time - realism, locality, causality, free-will, randomness. Or maybe you're not willing to go that far yet?
     
  16. May 13, 2010 #15

    RUTA

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    We agree with Rovelli (see van Fraassen's characterization of RQM: http://www.princeton.edu/~fraassen/abstract/Rovelli_sWorld-FIN.pdf) that there is no view from nowhere, i.e., there can't be an "external view" since any observer is always defined in the context of everything else. It's hard to say exactly where we differ with RQM because we can't find his axioms whence QM (he admits he doesn't have them in quant-ph/9609002 and I don't find them in van Fraassen). I have the distinct impression, however, that his "real quantum system" exists in addition to the pieces of equipment in the experiment, while there is no additional ontic structure in RBW. Certainly RBW is farther along since we have a way to recover quantum physics relationally (arXiv 0908.4348, we received word today that it is accepted for presentation at PSA 2010).

    As regards quantum information theory, RBW provides an answer to the question "information about what?" When Fuchs says: "The quantum system represents something real and independent of us; the quantum state represents a collection of subjective degrees of belief about something to do with that system (even if only in connection with our experimental kicks to it). The structure called quantum mechanics is about the interplay of these two things -- the subjective and the objective." ( quant-ph/0205039, p. 5.) We would simply say the "quantum system" is the composition of the experimental equipment depicted in Figures 1-4 of the arXiv paper above.
     
  17. May 14, 2010 #16

    Demystifier

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    Thanks, RUTA!
     
  18. May 14, 2010 #17

    RUTA

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    It's semantics I guess, but in RBW I would say "reality" is composed of classical objects, which are spatiotemporal geometric distributions over graphs (which aren't "real"), so "reality" isn't (ultimately) composed of "real" things. Maybe that's along the lines of your "virtual world?" RBW's philosopher of science would probably complain about my posting this, he would have a much better response :-)
     
  19. May 20, 2011 #18

    Demystifier

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    Niels Bohr said:
    "If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet."

    To answer the question posed in the title of this thread, I would add:
    "If delayed choice quantum eraser shocked you more than the rest of quantum mechanics, you haven't understood the rest of quantum mechanics yet."
     
  20. May 21, 2011 #19
    Ok, what would the conclusion be?....:)
     
  21. May 23, 2011 #20

    Demystifier

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    See the first post on this thread.
     
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