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Copenhagen - What qualifies as "measurement" and "observer"?

  1. Jul 20, 2015 #1
    Hey guys and gals, clearly I'm new here, not only new here but new to QM (relatively speaking). I've read a few threads here pertaining to a thought I've been pondering on but the answer doesn't seem clear.

    In the Copenhagen interpretation, what qualifies as a "measurement", and/or what qualifies as an "observer"?

    I ask this because of the alleged seemingly common misunderstanding that the one doing the measurement needn't be sentient because otherwise it would stand to reason that prior to being "measured" by an "observer" the most basic building blocks of matter wouldn't be able to combine and interact due to being in an unstable state of possibilities and never collapsing into a physical point in the universe and well, we wouldn't have a universe. In other words, we'd have no objects in the universe as their components would still be in an unstable state of flux from not being measured because of a lack of sentient beings to measure it.

    Is it simply the act of an outside "agent", sentient or not, that can collapse a particle's wave function?

    So pardon the "noob" question, just looking for some clarification.

    Thanks!
     
  2. jcsd
  3. Jul 20, 2015 #2

    atyy

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    Ultimately, the one doing the measurement must be sentient or at least have "common sense" in Copenhagen. One can delegate the measurement to a non-sentient device. However, the important point is that the device is not considered quantum. The device is "macroscopic" or "classical". It is this subjective division of the world into classical and quantum parts that requires the observer that stands apart from the quantum system. Only the classical part is "real", while the wave function describing the quantum part may be just a tool to calculate the probability of measurement outcomes.

    This is of course problematic, and is known as the measurement problem.
    http://www.informationphilosopher.com/solutions/scientists/bell/Against_Measurement.pdf
    http://www.tau.ac.il/~tsirel/download/nonaxio.html
    http://arxiv.org/abs/quant-ph/0209123
     
    Last edited: Jul 20, 2015
  4. Jul 20, 2015 #3
    Can we assume that bigger units of matter can not form from particles whose wave functions remain "uncollapsed"? If we can, how could the now observable celestial bodies to come into existence when there are no sentient beings actively observing or at least none available to produce classical measuring devices?
     
  5. Jul 20, 2015 #4

    atyy

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    It is possible to do so, but not within Copenhagen. One needs an interpretation like Bohmian Mechanics. BM seems to work for non-relativistic quantum mechanics and some relativistic quantum phenomena, but it is unclear if it can be describe all relativistic quantum phenomena.

    Another possibility is the Many-Worlds approach. However, I am not sure whether this approach is correct (some versions are, but those versions are variants of Bohmian Mechanics).
     
  6. Jul 20, 2015 #5
    Is there any discussion of the formation of larger units of matter when it's base building blocks are still in a state of superposition?
     
  7. Jul 20, 2015 #6
    Curious. I'm sure this conundrum has been presented before. I wonder how it was handled. I'll delve more into Bohmian Mechanics. Thanks for the input!
     
  8. Jul 20, 2015 #7

    atyy

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    In a sense this is true in Copenhagen, Bohmian Mechanics and Many-Worlds. However, in Copenhagen, without the observer, the larger units and the smaller units are all not necessarily real. Reality has to be recognized by the observer in Copenhagen, or instantiated with hidden variables as in BM, or quite radically rethought as in Many-Worlds.
     
  9. Jul 20, 2015 #8
    I mean, if one supposes CI to be true and that larger particles can't form from uncollapsed quantum building blocks, the only other possible explanation is, well, something divine in nature is observing and measuring from outside our universe or we're encased in some amazing simulation. This is a disturbing notion. One I know Einstein viewed in much the same light.

    Off to do some reading.
     
  10. Jul 20, 2015 #9

    Nugatory

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    There's something not to like in every interpretation of quantum mechanics, and you've identified what's not to like in Copenhagen. It's the problem that Schrodinger pointed out with his famous thought experiment about the cat in the box: there's nothing in the theory that tells us which interactions count as observations causing collapse to a definite outcome and which lead to an uncollapsed superposition of the possible outcomes.
     
  11. Jul 20, 2015 #10
    I'm hoping new, recent experiments pertaining to Pilot Wave Theory give it some traction - I'm a bigger fan of a deterministic reality. The fact that some behaviors are being exhibited in a classical environment that many thought were exclusive to the quantum realm are giving me some hope.
     
  12. Jul 20, 2015 #11

    atyy

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    I'm a fan of BM and other hidden variable interpretations too - but assuming you are reading https://www.quantamagazine.org/20140624-fluid-tests-hint-at-concrete-quantum-reality/ - then no, these do not lend the pilot wave theory any support.

    Again, I would like to stress that it is unknown if the pilot wave theory can describe the full range of relativistic quantum phenomena.
     
  13. Jul 20, 2015 #12
    Different source - same information. Sigh. Here's to hoping. Copenhagen is just too creepy in what it infers.
     
  14. Jul 20, 2015 #13

    atyy

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    Let's say BM is true. If that were the case, the CI is considered true but incomplete. CI can be derived from BM.

    Dirac was a famous proponent of the view that CI is true but incomplete.
     
  15. Jul 20, 2015 #14
    Can't comment further until I gain a better understanding of BM. I'll keep that in mind though as I study.
     
  16. Jul 20, 2015 #15

    atyy

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    I should add BM and the pilot wave theory are essentially synonymous.
     
  17. Jul 20, 2015 #16
    It would appear so.
     
  18. Jul 20, 2015 #17
    I know that BM is lacking in certain regards but it just rubs me the right way. CI just doesn't work for instance with my example of a developing universe. Either CI or BM is missing something HUGE.
     
  19. Jul 22, 2015 #18

    bhobba

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    All interpretations have issues - every single one.

    You often hear QM has a measurement problem etc etc. That's not its real issue. The issue is it doesn't matter what problem in QM worries you, the measurement problem or any other, you can find an interpretation that fixes it. But fixing all the issues at once - that's the trick that no one has achieved. Further progress IMHO will need to wait until its possible to figure out (if its possible at all) how to experimentally distinguish them.

    Another big issue is what worries some, others couldn't care less. For me QM is a theory about observations. Observations are the primitive of the theory like points and lines are the primitives of geometry. In modern times decoherence has morphed the measurement problem to why we get any outcomes at all. For me its a non issue - that observations exist is a primitive of the theory - worrying about it is like worrying about why we have points and lines. But for others its a deep concern.

    First though you need to understand what CI says:
    http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

    The modern way to fix its blemish alluded to in the above is Consistent Histories:
    http://quantum.phys.cmu.edu/CHS/histories.html

    Its a nice interpretation, basically many worlds without the many worlds, and actually shares many of the key theorems with the modern version of many worlds. Its issue IMHO is it gets around the measurements problem by not having measurements - in that interpretation QM is a stochastic theory about histories. It looks like defining your way out of difficulties rather than face them head on.

    Another interesting thing is many QM interpretations are simply an argument about what probability is:
    http://math.ucr.edu/home/baez/bayes.html

    Thanks
    Bill
     
    Last edited: Jul 22, 2015
  20. Jul 22, 2015 #19

    stevendaryl

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    I don't understand how it makes sense for observations to be primitive, when you have to have a lot of machinery--measurement devices--to make an observation. The fact that such and such a procedure constitutes a measurement/observation of such and such a property seems like a nontrivial deduction, rather than something primitive. I guess in practice you can do the Copenhagen split between a classical measuring device and a quantum system being measured, then use classical reasoning to figure out what is being measured, and then treat the measurement as a primitive when it comes to applying quantum mechanics. But the whole thing seems very much nonprimitive to me.
     
  21. Jul 22, 2015 #20

    bhobba

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    Its an intuitive idea like a point. Try and tie down a point exactly and you have exactly the same problem - we must rely on our intuition. The same for event in probability theory - in fact that seems quite similar to observation.

    Thanks
    Bill
     
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