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Experiments without Interpretations!

  1. Jan 9, 2014 #1
    I'm trying to just find the bare bones of the actual experiments done in quantum mechanics without any interpretations of what the results are.

    For instance, in the double slit experiment I'm told that observation is what causes the wave function to collapse. How many different ways and exactly what methods are being used to observe our electron in question? What's the mechanical device, so that I myself can decide if the "observation" or some part of the measurement device itself is what's causing it.

    I know I'm probably asking for something that's almost entirely unreasonable. But I suppose I'm just a little frustrated with the subject and want to plainly know: What is the device, and how do people think these devices work, what is the uninterpreted data (1's and 0's depending on if a light is switched?) without an interpretation of that data.
     
  2. jcsd
  3. Jan 9, 2014 #2

    bhobba

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    There is a problem to start with.

    If even wavefunction collapse occurs is interpretation dependent eg it doesn't happen in MWI. And in interpretations where is does occur if it actually means anything is debatable. Throw a dice and before trowing it its state is represented by 1/6th attached to each face. After the throw its state has collapsed to a 1 (ie a dead cert) on some face. Nothing but this conceptual thing called probabilities has changed.

    I have posted this in another thread, but I find this the best way to understand exactly what the formalism of QM is saying - not what it means - that is another issue - but what its saying. However before trying to figure out what it means, its wise to understand what it says

    My view, purely on the basis of the formalism, without entering into the quagmire of these interpretational issues, is its simply what's required to allow us to predict the probabilities of outcomes of observations, similar to what probabilities themselves are. Indeed the differences between some interpretations is simply a variant of the different ways you can interpret probabilities ie frequentest or Bayesian, but I wont go into that right now.

    I will base the following on the two axioms in Ballentine - QM - A Modern development, which is a very well respected textbook on QM - many people like myself think of it as THE textbook - its that good. Its a very interesting fact that QM really rests on just two axioms. There is a bit more to it, but they are more or less along the lines of reasonableness assumptions such as the probability of outcomes should not dependent on coordinate systems ie symmetry.

    Imagine we have a system and some observational apparatus that has n possible outcomes associated with values yi. This immediately suggests a vector and to bring this out I will write it as Ʃ yi |bi>. Now we have a problem - the |bi> are freely chosen - they are simply man made things that follow from a theorem on vector spaces - fundamental physics can not depend on that. To get around it QM replaces the |bi> by |bi><bi| to give the operator Ʃ yi |bi><bi| - which is basis independent. In this way observations are associated with Hermitian operators. This is the first axiom in Ballentine, and heuristically why its reasonable.

    Next we have this wonderful theorem, Gleason's theorem, which, basically, follows from the above axiom:
    http://kof.physto.se/theses/helena-master.pdf [Broken]

    This is the second axioms in Ballentine's treatment.

    This means a state is simply a mathematical requirement to allow us to calculate expected values in QM. It may or may not be real - there is no way to tell. Its very similar to the role probabilities play in probability theory. In fact in QM you can also calculate probabilities. Most people would say probabilities don't exist in a real sense and why I personally don't think the state is real - but that's just my view - as far as we can tell today its an open question.

    Basically QM is a theory about the probabilities of outcomes of observation, if we were to observe it. The sole purpose of a state is, when combined with an observable, is to allow us to predict the probabilities of the outcomes of observations. And exactly like probabilities its very existence is to change to some outcome when you conduct the experiment, observation or whatever.

    Again, this has nothing to do with what it MEANS. Different interpretations have different takes on that. But purely from the formalism that's what it's about.

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  4. Jan 9, 2014 #3

    atyy

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    Collapse itself is a bit nebulous, and one cannot say "exactly" when it happens. There are interpretations without collapse, such as many-worlds, but the equivalent question there is to ask "exactly" when branching occurs. One way to try to study the measurement process has been to split it into (1) how the measurement apparatus chooses the basis in which collapse occurs, and (2) collapse itself. While the second is still an additional postulate, the first part can be studied by treating the system, the apparatus and the environment as one quantum system that evolves without collapse. This successful programme is called decoherence and is described in the article by Zurek below. Decoherence occurs in all interpretations of quantum mechanics. Experiments probing decoherence are described section IV of the review by Cronin, Schmiedmayer, and Pritchard.

    http://arxiv.org/abs/quant-ph/0306072
    Decoherence and the transition from quantum to classical -- REVISITED
    Wojciech H. Zurek

    http://arxiv.org/abs/0712.3703
    Atom Interferometers
    Alexander D. Cronin, Joerg Schmiedmayer, David E. Pritchard
     
  5. Jan 9, 2014 #4

    ZapperZ

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    Look at my avatar. That is a raw data that comes out of an angled-resolved photoemission spectroscopy on a highly-overdoped Bi2212, a high-Tc superconductor.

    Since you do not want to be "told" on how to analyze and interpret such observation, have a go at it!

    Zz.
     
  6. Jan 9, 2014 #5
    That was about on par with what I needed to see, thanks Bill.

    Awesome, can you enlarge it? I've only ever worked with spectrofluorometers. Also, in my original post I asked, "how do people think these devices work"?
     
  7. Jan 9, 2014 #6

    bhobba

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    You are welcome. I just wish someone had told me that sort of stuff when I first started out.

    Actually QM is silent about that. In my 'explanation' of the axioms of QM and what the formalism says I mentioned an observational apparatus observing a system that has some outcomes to which values have been assigned. From that alone, we get an observable. The particular |bi><bi| it gets mapped to depends on how these devices work and what they do eg do they measure position, spin, momentum etc - but all QM assumes is such a mapping can be assigned.

    At a technical level the following explains the modern take of this 'measurement' issue and what is called 'decoherence':
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    I personally hold to the ignorance ensemble interpretation in that article.

    From that perspective your question would be exactly how does a given device decohere a superposition into an improper mixed state.

    Often that is NOT an easy question to answer.

    Thanks
    Bill
     
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