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Are Wave Functions Real?

  1. May 3, 2010 #1


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    It is sometimes said that wave functions are not real, and simply represent the observer's knowledge of the system. I would like to comment against this point by presenting an experimental setup which would tend to indicate that the wave function is quite real. As far as I know, this setup per se has never been executed (although I am hoping someone might recognize it as something which has been).

    To follow the setup, you should be familiar with the following experiment:

    Bell inequalities and quantum mechanics, J. H. Eberly (2001)

    See Figure 1, the Bell analyzer loop, in which a beam is split into H and V components. Those are then recombined so that the H/V information is erased, leaving a beam with the same properties as it was originally.

    So if you took a pair of entangled particles, Alice and Bob, and ran each through a Bell analyzer loop, the recombined Alice and Bob are still entangled. This is what the above paper is saying.


    Here is my twist:

    Frankenstein photons:

    Split Alice into Alice-H and Alice-V. Split Bob into Bob-H and Bob-V. Now recombine Alice-H with Bob-V (which is identical to Alice-V). Recombine Bob-H with Alice-V (likewise identical to Bob-V). You will now have 2 Frankenstein photons that are polarization entangled!

    Now, if the above is accurate (I don't see how it could be expected to be otherwise), then you would have to admit that you are mixing the wave functions of different photons to obtain an effect that clearly does not occur with either portion of the component wave functions alone.

    So I conclude that the wave function is quite real. Your thoughts?
  2. jcsd
  3. May 3, 2010 #2
    Thanks for bringing up this subject Dr, this is my favorite subject.

    Is this different from other experiments where the wave function is the only explanation?

    I think what is meant by the wave function not being "real" is that it is not a direct observable.
  4. May 3, 2010 #3


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    Great post/question DrC, I’ll be back ASAP!
  5. May 3, 2010 #4


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    Competing interpretations of QM can be divided into wave function realism and non-realism. Bohmian mechanics, for example, assumes the wave function acts as a guide for the quantum particles moving about the experimental equipment. Thus, for BM the wave function is very real. In Relational Blockworld, the wave function is only a description of the experimental equipment, there are no quantum particles moving therein. Thus, for RBW the wave function has no ontic status at all.
  6. May 3, 2010 #5
    Superpositions of mutually exclusive objects are not considered real in ordinary logic. Why should the case be different for QM? In other words, if "Dead cat" and "live cat" are mutually exclusive, the superposition "Both dead and alive" can not be real, even in QM.

    Let x correspond to one wall of your room and y the perpendicular wall, x and y therefore correspond to real material ontological "objects". Now consider the transformation x' = x + y, y'=x -y. Would you then say that x'and y' are material ontological objects? Of course not, they run through the room without any wall. Similarly, for a Hilbert space, there is no implicit association between transformed basis vectors and ontological entities. Correspondence to reality has to be independently established in each case.

    Therefore the wave function is not necessarily real.
  7. May 3, 2010 #6


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    As an experimentalist you orient pieces of equipment so that geometrically you can trace a path from source to detector(s) defined by beam splitters, crystals, mirrors, polarizers, etc. To obtain a wave function for the outcome(s) at the detector(s) of your particular experimental arrangement, you simply start (computationally) with the relationship between source and detector(s) without any intermediary equipment, then modify it step by step according to the arrangement of each piece of equipment defining the path from source to the detector(s).

    Typically, wave function realists assume the wave function moves sequentially from the source through each piece of equipment and impinges on the detector(s) to produce the outcome(s), per the computational sequence of the wave function's constuction. "Weirdness" can result if one alters the arrangement of equipment so as to produce new outcomes at odds conceptually with those of the previous arrangement. [Note: QM is not violated, just the consistency of corresponding ontological stories in both cases.]

    Of course, the wave function non-realist avoids these conundrums altogether. They understand the wave function is a description of the spatiotemporal entirety of the experiment -- all the equipment, its spatial arrangment and orientation, from initiation to termination. So, if you change the arrangement, you change the outcomes per the new wave function. There is no complicating factor of having to tell consistent stories about quantum entities moving through the experimental equipment in its differing configurations. It doesn't matter WHAT you arrange experimentally, you don't need to invoke anything other than facts concerning the experimental equipment to construct the corresponding wave function, so there's absolutely no way to empirically discern the existence or non-existence of said wave function.
  8. May 3, 2010 #7
    RUTA, that was a great read.

    Say we programmed a very intelligent artificial intelligence and leave it to itself. Now this AI is sitting inside a computer and starts to contemplate why it's bits flip. It can discern a pattern through experimentation and even devise functions with fourier transforms that give the AI information about what happens when it's bits flip.

    Though no matter the experiment, the AI will never understand the mechanism that allows it's bits to flip.
  9. May 3, 2010 #8
    If the wave function is not real, and pilot waves are not real, then how does the double-slit experiment show interference?
  10. May 3, 2010 #9
    From Speakable and unspeakable in quantum mechanics, by J.S. Bell, p. 171. This is from an essay of the same name.

    I would say that, in terms of the types of mathematics that are taught to budding physical theorists, the one that is the most powerful and yet receives the least amount of attention, is the idea of wave theory itself. The theoretical physicists of today, therefore, are highly skilled in terms of discrete, matrix-like mathematical systems, but when it comes to issues such as deriving the specific harmonic solutions to the wavefunction itself, most theoretical physicists tend to find themselves at a loss, and there is consequently a profound lack of spatio-temporal intuition at play in the contemporary literature.
  11. May 3, 2010 #10


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    My colleagues in physical chemistry deal extensively with wave functions to generate pretty pictures of orbitals. The solid state physicists also rely heavily on a conceptual view using the wave function, speaking of current densities flowing through various lattices. I think where one can use this picture effectively, the ontological mysteries are minimal (for many of my colleagues in these areas, I would say the conundrums of QM are non-existent). However, those who specifically study QM conundrums tend to find the matrix formalism better suited for analysing such phenomena. I can't imagine trying to model the quantum liar paradox via Schrodinger's equation. What a mess :-)
  12. May 3, 2010 #11


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    For an explanation whereby there are no quantum entities moving from the source, through the slits and subsequently impinging on the detector to cause a click in the twin-slit experiment, see the material in Section 2 concerning Figure 4 of “Reconciling Spacetime and the Quantum: Relational Blockworld and the Quantum Liar Paradox,” W.M. Stuckey, Michael Silberstein & Michael Cifone, Foundations of Physics 38, No. 4, 348 – 383 (2008), quant-ph/0510090. In this interpretation of QM, the only entities with ontological status are the pieces of the experimental equipment. Thus, the choice is not limited to "wave or particle" or "wave and particle," as Bell argued. There is another option, "neither wave nor particle," i.e., no "quantum/screened off entities" at all.
  13. May 3, 2010 #12
  14. May 3, 2010 #13
  15. May 3, 2010 #14
    Yes, please do so extensively.
  16. May 3, 2010 #15
    How about a new quantum conundrum - the Real-Unreal duality? If a 'particle' can be both a wave and particle, why couldn't it be both real and unreal, depending on the mode of inquiry? How else would you reconcile conservation of mass/energy with tunneling or quantum jumps and real wavefunctions? Both sides to the debate(real vs unreal) seem to have good arguments for their case.
  17. May 3, 2010 #16


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    "epistemic -- of, relating to, or involving knowledge"

    As opposed to

    "ontic -- of, relating to, or having real being"

    So, "psi epistemic" means the wave function is knowledge about ... , it doesn't necessarily "exist" or have "being." In BM, psi definitely exists and acts on the quantum particles, so psi is ontic. Of course, it could be both, but in RBW it's purely epistemic.
  18. May 3, 2010 #17
    So, my thinking is that your chemist and solid-state friends are much more directly involved with "physics proper," whereas the QM guys seem to be much more in tune with issues of a more "game theoretical" nature. (I'm thinking that QM'ers would probably be much tougher bluff against in hold 'em poker.) I guess, along with Bell, I just have a problem with people's desire to construct direct correlations between the Gendanken experiments of QM and the reality, as it exists, "out there." Bell absolutely despised the notion of "quantum logic," in the sense of being a kind of radical upheaval of the system of thought that has been around since ancient times.
  19. May 3, 2010 #18
    Thanks RUTA. I guess that is no different than what we already talked about above.

    It is intuitive that at it's very nature, the make up of things involve a chaotic mix of harmonic oscillations. This is clear in that nature itself is not very blocky, or hard edged as you learn in wave mechanics that hard edges involve a great number of high frequency functions in fourier analysis and high frequencies undergo rapid damping and interference.

    If the very fundamentals of things are made up of such oscillations though how could a detector made of the same fundamentals detect such a thing.
  20. May 3, 2010 #19
    This sounds like String Theory, would that be a fair assessment?
  21. May 3, 2010 #20
    Could be, I know very little about String Theory, only ever read a layman's book about it once or twice.
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