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More Support for The Copenhagen Interp.

  1. Jun 23, 2011 #1
    What do ya'll make of this:


    "It appears that you can't even conceive of a theory where specific observables would have definite values that are independent of the other things you measure," adds Steinberg."

    "Niels Bohr, a giant of quantum physics, was a great proponent of the idea that the nature of quantum reality depends on what we choose to measure, a notion that came to be called the Copenhagen interpretation. "This experiment lends more support to the Copenhagen interpretation," says Zeilinger."

    It does seem to lead support for the Copenhagen Interp.

    Also, seems to be pulling the "conscious" observer a little bit back into the spotlight.
  2. jcsd
  3. Jun 23, 2011 #2


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    From the article: "There is no sense in assuming that what we do not measure about a system has [an independent] reality," Zeilinger concludes.
  4. Jun 23, 2011 #3


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    I believe this is the underlying article:


    Experimental non-classicality of an indivisible quantum system
    Radek Lapkiewicz, Peizhe Li, Christoph Schaeff, Nathan K. Langford, Sven Ramelow, Marcin Wiesniak, Anton Zeilinger

    "Quantum theory demands that, in contrast to classical physics, not all properties can be simultaneously well defined. The Heisenberg Uncertainty Principle is a manifestation of this fact. Another important corollary arises that there can be no joint probability distribution describing the outcomes of all possible measurements, allowing a quantum system to be classically understood. We provide the first experimental evidence that even for a single three-state system, a qutrit, no such classical model can exist that correctly describes the results of a simple set of pairwise compatible measurements. Not only is a single qutrit the simplest system in which such a contradiction is possible, but, even more importantly, the contradiction cannot result from entanglement, because such a system is indivisible, and it does not even allow the concept of entanglement between subsystems. "
  5. Jun 23, 2011 #4
    Well, that is good news to hear. I am a Copenhagenist. All those damn parallel universes just seems all a bit too much.
  6. Jun 23, 2011 #5
    yeah, I was never crazy about MWI either.

    somebody correct me if I'm wrong, but this sounds like a nail in the coffin for MWI?

    Also, thanks for the link to the paper Dr. Chineese. I'll have to read that later 2nite.
  7. Jun 23, 2011 #6


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    Ah, a chance to repeat my favorite line:
    I am not constrained by your lack of imagination. -Hurkyl​

    That said, even if we accept the quote, I don't see the connection to interpretations of quantum mechanics. All of the interpretations I know permit indefinite values. Copenhagen does so between collapses, or when collapses are done in different bases. Bohm's particles don't relate to this sort of state, so it doesn't assign definite values either. MWI, of course, always retains the indefiniteness present in quantum states.

    AFAIK, denying counter-factual definiteness is mainly just a problem for the hypothesis that reality is classical (with hidden variables), and there is simply some new physics (or undiscovered effects of old physics) that result in the observed coincidences.

    Honestly, among the main interpretations of QM, Copenhagen suffers the most from demonstrations of things behaving non-classically, because it is a collapse-based interpretation. Not because the experiment contradicts collapse-based interpretations, but because the experiment puts limits on the usefulness of collapse as an effect on physical state.

    I can't read the paper at the moment so I'm speculating based on the abstract and what's in this thread -- but my impression is that the result is nothing new. The novelty is that someone did the experiment. (or maybe in the derivation of a mathematical criterion to demonstrate the effect. Or maybe that a qutrit is complicated enough to exhibit the effect)

    Right! It's too much work to actually study joint wave-functions and probability distributions. Much easier to just collapse things whenever possible. :wink: (this comment is meant in jest)
  8. Jun 23, 2011 #7
    If the paper clearly demarcated between the various interpretations it would be bigger news than this.

    However, I haven't seen the bohmians convincingly explain GHZ states never mind this.

    But the Copenhagen Interpretation is not going to replaced unless a simpler non-deterministic and non-local alternative appears.
  9. Jun 23, 2011 #8
    Hi Hurkyl,

    Thanks for the reply.

    Doesn't MWI say "everything that can happen, does happen" (albeit in a parallel world)

    Doesn't this experiment seem to not quite jive up with that. It seems like what does happen is now more "limited" than that?
  10. Jun 23, 2011 #9


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    Yes, but the way everything "does happen" is by a wave-function evolving according to the Schrödinger equation and never collapsing.

    A lot of the focus of MWI is on trying to understand large quantum systems, and if/how the observed behaviors of "measurement" and "classical reality" can be an emergent property of a quantum systems. Most of what you will read about MWI is geared towards that application.

    When applied to something this small, pretty much the only thing MWI does is to take the description of what the experiment measures as an assertion of how the photon will decohere towards when it interacts with the experimental setup.
  11. Jun 23, 2011 #10
    ahh, okay, thanks, that was helpful.

    I was probably taking that MWI statement too literally then.

    For example, an electron. I took MWI as saying that when the electron's spin is measured, two Universes/Worlds slpit off, one containing an electron with spin up, and another containing the electron with spin down, and likewise for any other indeterminate state.

    However, it sounds like that means MWI is still saying that when I bought a lottery ticket last night, in some parallel world, I actually won ... too bad it wasn't this one!
  12. Jun 23, 2011 #11


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    Well, yes and no.

    As you know, the measurement results in a decoherence. e.g. the state [itex]a |z+\rangle + b |z-\rangle[/itex], when measured around the z axis, decoheres into the state with density matrix approximately
    [tex]\left( \begin{matrix}|a|^2 & 0 \\ 0 & |b|^2\end{matrix} \right)[/tex]
    (the working hypothesis in MWI is that this is a consequence of unitary evolution in a larger system that includes the measuring device and the environment)

    Such a state is mathematically indistinguishable from the notion of the quantum state being a (classical) probability distribution -- probability [itex]|a|^2[/itex] of being spin up around the z axis,and probability [itex]|b|^2[/itex] of being spin down around the z axis.

    (there are, of course, other ways to describe this state as a probability distribution -- the one described above is merely the most obvious)

    Since the measurement is assumed to be thermodynamically irreversible, this description as a probability distribution should remain stable. So it really is fair to describe this state of the electron as having one "world" with weight [itex]|a|^2[/itex] with a spin up electron, and one "world" with weight [itex]|b|^2[/itex] with a spin down electron.

    (incidentally, I've occasionally seen "world" used in a completely different sense -- if we had a larger quantum system containing the electron, we might say the "world" of the electron is just the relative state of the subsystem consisting only of the electron. Although when I've seen it used this way, it was more geared to the idea of a world being something like "the subsystem of things Alice can observe" or such)
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