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Busting the myth of the observer: the double slit experiment

  1. Aug 9, 2014 #1
    We all remember the animations describing the double slit experiment to the public, laying out the foundations of the mysterious quantum world. Now take the part when we try to determine which slit the electron went through. The narrator will say something like this, in a hushed voice: „And now, the electron, as if it somehow knew we were watching, becomes a particle! It changes just because we observe it!”
    If I’m correct, the notion of the intelligent observer is so serious that it gave rise to the anthropic principle where consciousness interferes with quantum objects. I don’t understand something here because I see an error so glaring, it’s as bright as the Sun.
    How could anyone call a which-way detector an innocent little observer? For a quantum particle, it is a brutal machine, that interacts with it in a physical way. The detector has no choice by the way but to interact, after all, how else would it get any information out of that photon or electron? It places an electromagnetic field in the path of the particle, or is bombarding the path with particles, I don’t know exactly how it does it but there is no choice but to do something like that. And it is perfectly natural for an electron in its wave form to collapse into a particle after you bump it against some other particle for the purpose of measurement.
    Suggesting that all we do is observe gives everyone the false idea that a flying particle in the double slit experiment is bothered by an imaginary line, which we call our line of sight.
    I think the word „observe” should only be used if we know what we are talking about:
    Step one: Brutal interference
    Step two: Drawing conclusions after checking what happened (good luck by the way, after step one)
    So what am I missing here? Surely I can’t be smarter than all those scientists who had good reason to pursue the theory of the intelligent observer.
  2. jcsd
  3. Aug 9, 2014 #2


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    What you are missing is that the narrator of that animation is not a scientist and doesn't know what he's talking about. The notion that observation requires a conscious observer was pretty much abandoned by those who understand QM decades ago, but eradicating it from the popular imagination has proven to be a bit more difficult.

    Your thinking, that a detector cannot be an "innocent little observer", that it must "brutally interact" is much closer to what scientists believe. The words "observation", "interaction", and "measurement" are used, if not interchangeably at least with substantial overlap.
  4. Aug 9, 2014 #3
    Thank you, that's a relief! Let's just hope all those tv shows popularizing science catch up with the latest knowledge, and stop misleading any newbie science fans out there.
  5. Aug 9, 2014 #4
    It seems like you are referring to the clip from "What the Bleep Do We Know!?"

    That's a movie that was made by crackpots, not scientists, and I don't go around calling people crackpots, unless they are really out there (as in, people who claim to be channeling 35,000 year old spirit-warriors). It's a shame because I think it's a pretty nice little illustration in that particular clip, if it weren't for the little bit of their silliness that they had to inject into it.
  6. Aug 9, 2014 #5
    Some Professors of Physics still entertain the notion of consciousness collapsing the wave function.
  7. Aug 9, 2014 #6


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    I admire your optimism but I'm not going to hold my breath. Pop-sci shows don't CARE about actual science, they exist to sell soap.
  8. Aug 9, 2014 #7


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    They are very much in the minority - but yes they still exist.

    It grew out of the Von-Neumann cut issue (the issue was the quantum classical cut can be placed anywhere so you trace it back to the concious observer as the only thing that's different - hence you place it there) in Von Neumann's classic Mathematical Foundations of Quantum Mechanics.

    But since then a lot of water has gone under the bridge and we now understand things a lot better - especially in the area of decoherence:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

    Just after decoherence would seem the logical place to put the cut.

    In fact the other high priest of conciousness causes collapse, Wigner, abandoned it when he heard about some early work on decoherence by Zurek. Unfortunately Von Neumann died young, which was a great tragedy for mathematics, physics, economics, game theory, computer science, all sorts of areas, as he was one of the greatest polymaths that every lived.

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  9. Aug 9, 2014 #8


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    Unfortunately the real issues in QM are rather subtle and not particularly sensationalist.

    Even highly respectable scientists in explaining it to the public lapse into half truths like particles being in two places at once. I sort of give them a bit of a pass because at the beginner level of QM, even in textbooks used for college students, they have a number of half truths (eg the so called wave particle duality) that are only corrected in more advanced texts.

    But that What The Bleep Do We Know Anyway is gutter trash of the first order.

    I shake my head when I see posts of poor high school students made to sit through it. No wonder enrolment in physics/math type majors is declining.

  10. Aug 10, 2014 #9
    Just out of curiosity, where is the quantum cut for the half dead/alive cat, which is unfortunately still being used not for what it was initially intended.
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  11. Aug 10, 2014 #10


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    The cut should be at the detector that opens the vial of poison - it's either triggered or not.

    The "paradox" is the result of placing the cut somewhere above the cat.
  12. Aug 10, 2014 #11


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    As Meatloaf said - you took the words right out of my mouth.

    In modern times its placed just after decoherence, which in that case would just a bit before the actual detector registered - at least that's what I seem to recall from an article I read on how they work.

    Last edited: Aug 10, 2014
  13. Aug 10, 2014 #12
    that makes absolute pefect sense. Thanks you two.
  14. Aug 10, 2014 #13
    Is this your personal opinion? As far as I know, "the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far;", and "no positive experimental evidence exists for physical state-vector collapse;", at the detector or anywhere else (M. Schlosshauer, Annals of Physics, 321 (2006) 112-149)
  15. Aug 10, 2014 #14


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    That I agree with, and it's the reason this problem was so perplexing for so long. If you put the cut at the detector the quantum weirdness goes away, but there is nothing in the theory that says that you have to put it there. In effect, Schrodinger was asking why we shouldn't put the cut above the cat, and then-contemporary QM had no good answer.

    With the discovery of decoherence we found an answer - we put the cut at the point where decoherence happens. It's worth pointing out that this is not necessarily the line between microscopic and macroscopic, as we have experiments in which macroscopic (small, but still macroscopic) systems have been maintained in superposition by using various heroic measures to stop them from decohering.
  16. Aug 10, 2014 #15


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    I am by no means an expert in quantum decoherence, so I have more of a question than a comment (although it will come out to sound like a comment). Quantum decoherence, as I understand it is simply the evolution of pure states into mixed states. The density matrix no longer has the property Tr(ρ2)=1. Is this correct? The evolution of a density matrix; however, will not turn a pure state into a mixed one because the Von-Neumann equation reduces directly to the Schroedinger equation for a pure state. And we know that the Schroedinger equation will not turn a pure state into a mixed one.

    It seems to me therefore, that in order to obtain evolution from pure to mixed states one may have to appeal to the quantum kinetic equations, where now the Boltzmann collision factors will indeed turn a pure state into a mixed state. But as we all know from classical non-equilibrium stat mech, or from our studies of the Boltzmann transport equations, we are neglecting a ton of interactions and other particles when we make the kinetic theory approximations. It is our inability to solve the full equations that forces us to truncate the series of coupled equations at the first or second order collision terms (otherwise there would be N coupled, partial differo-integral equations where N is of order 10^23 for a macroscopic system). Is this not borne out in the quantum kinetic case? It seems to me that the decoherence really just comes from our inability to keep track of all the cross terms in the density matrix, and so we just simply "trace out" all the "external" (corresponding to the macroscopic systems) degree of freedoms.

    If we were supermen, and we could keep track of every cross term in the 10^23 by 10^23 density matrix, then couldn't we say that no decoherence actually happens? In that case, there is no longer a nice divide between where we make the cut in Schroedinger's cat's problem. Sure you can say "make the cut where decoherence happens", but "decoherence happens" when we are no longer able to keep track of all the phases...which is still arbitrary!

    Am I completely off my knocker here?
  17. Aug 10, 2014 #16
    I'm with Matterwave on this one. Decoherence explains why the issue of collapse is not a problem in practice: due to decoherence it doesn't matter where you put the cut, since even if you put it after the measurement everything works out since from that viewpoint the quantum coherence of the subsystem decoheres into the total system (= subsystem + measurement apparatus) such that the reduced density matrix of the subsystem gives the same result as if you had put "collapse" at the moment of measurement.

    However, decoherence or not: we still choose where to put the collapse, and of course it never really happens since if someone is smart enough one can always undo the decoherence to get back normal superposition behaviour (in principle, of course). So there are two ways out of this:

    (1) We confess we have to put the cut somewhere, arbitrarily, and as long as we put it late enough in principle we avoid any possible issues (like inconsistency). The theory then works as a predictive theory.

    (2) On philosophical/aesthetical/etc grounds one presumes that the laws of QM are not complete: there shouldn't be the arbitrariness of choosing where the collapse happens (with the caveat that one has to take it sufficiently late), so one must look for a deeper theory.

    Pragmatists opt for (1), realists (and others?) opt for (2), which gives rise to proposals such as GRW theory, de Broglie-Bohm theory, etc.
  18. Aug 10, 2014 #17


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    Bohmians can opt for (1), since Copenhagen can be derived from BM. Just as we can have emergent degrees of freedom, one can have emergent ontology.
  19. Aug 10, 2014 #18


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    Yes - you can put the Von-Neumann cut anywhere. What decoherence does however is break the logic of the argument for introducing conciousness into it. There is a place that's different - just after decoherence.

    For simple cases in practice as well - see the delayed choice experiment.

    Put it just after decoherence - dead simple. That amounts to saying an improper mixture is a proper one.

    Aesthetics is very personal and not subject to scientific verification.

    I think if you delve deeply into the 'whaky' world of interpretations you will find all sorts of reasons for adopting all sorts of positions.

  20. Aug 10, 2014 #19
    Whether I agree or disagree with this, as far as I understand, until you define some specific decoherence mechanism, you cannot justify putting the cut at some specific point. So I don't see any clear reason so far to put the cut at the detector.
  21. Aug 10, 2014 #20
    Stephen M Barr, Professor of Physics, notes nicely in his book "Modern Physics and Ancient Faith":

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