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Position measuring device

  1. Nov 8, 2008 #1
    Can anybody provide an example of a device capable of "directly" measuring position relative to some
    origin on a microscopic scale? By microscopic I mean on the scale that QM is important.
     
  2. jcsd
  3. Nov 9, 2008 #2
    What is a "direct" measure? Scattering? electron microscope?
     
  4. Nov 9, 2008 #3
    By direct I mean that the result of the measurement should be independent of any physical theory.
     
  5. Nov 10, 2008 #4

    Fredrik

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    That requirement means that you have to use your knowledge of QM to design the device, so I'm not sure it makes sense to also require "independent of any physical theory".
     
  6. Nov 10, 2008 #5

    Vanadium 50

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    I don't think this is possible even with everyday measurements. How do you know a ruler doesn't undergo thermal expansion by a factor of 2? Physical theory.
     
  7. Nov 10, 2008 #6
    If it's independent of any physical theory, how will you know what units to measure? what units you used? what the units size is? I don't get it.
     
  8. Nov 10, 2008 #7
    Wikipedia doesn't have anything useful in the first twenty listings under DIRECT MEASUREMENT.
    At http://en.wikipedia.org/wiki/Quantum_measurement

    you won't like the implication of "observables" which is perhaps what you had in mind...lots of theory dependence...
     
  9. Nov 10, 2008 #8

    mgb_phys

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    Can you make direct measuremnts of somethigns position on the order of an atom - yes atomic force microscopes and laser interferometers can measure on that scale.

    Are these measurements independant of quantum theory?
    I think what the OP is asking is can you make independant measurements of quantum effects without having to first use the same effect in the measurement - leading to a circular argument.

    But can you make lots of direct measuremnts of the properties of a particle and show that they are random because of QM - no because QM says that when you make the measuremnt you effect that randomness.
    There is no way to 'step outside' QM and make a measurement of a quantum effect without affecting that effect - being 'an observer' in quantum terminology.
     
  10. Nov 10, 2008 #9
    To make my OP a bit clearer and remove any reference to theory let me state it as:

    Can anybody provide an example of a device capable of "directly" measuring position relative to some origin on a nanometer scale? By "directly" I mean that the result of the measurement should be independent of any physical theory.
     
  11. Nov 10, 2008 #10
    So are you saying that one can never measure anything because something about the measuring device might change? I think you will get some resistance to that notion.
     
  12. Nov 10, 2008 #11

    Vanadium 50

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    I'm confused. It doesn't look like you are removing this reference at all.


    What I am saying is that this requirement that you are imposing on quantum mechanical measurements is so severe that classical measurements will fail as well.
     
  13. Nov 10, 2008 #12
    I removed the reference to QM.
    Note that what I am talking about here isn't the measurement of a particles position but the notion of an underlying field of position variables. Both QM and CM assume such an underlying field.
     
  14. Nov 10, 2008 #13

    Fredrik

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    You removed a reference that no one had a problem with and kept the one that everyone was objecting to.

    CM: six numbers, QM: a wavefunction. Is that what you mean by "underlying field"? The wavefunction isn't a measurable quantity.
     
  15. Nov 10, 2008 #14
    I removed the one that makes the question that I am asking more clear. What did I keep that you object to?

    No. Both assume that there is an underlying coordinate system of spatial variables. This underlying coordinate system does not change when going from CM to QM to QFT.
     
  16. Nov 10, 2008 #15

    Fredrik

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    "independent of any physical theory". Three different people objected to that in #4, #5 and #6. No one objected to "on the scale that QM is important".

    OK, that's true.
     
  17. Nov 10, 2008 #16
    Good. So what do you think of the OP then (or the restated version)?
     
  18. Nov 10, 2008 #17
    The coordinate system is not the problem.
    To make the measurement you want in the OP, you need to define what you wish to measure (A buckyball, an individual atom) and what you want to measure it with say electrons (electron microscope) or gamma particles or something “quantum small” that can cast a shadow you can measure with detectors. With a few tricks you can make the measures even more accurate.
    BUT the very first thing you will need to do is defining clearly what these things are in some detail (the thing you are measuring and what you are measuring it with).

    Off the top I’m betting you do not have any special knowledge to tell us exactly what these things are, accept by using details to describe them fundamentally based on some theory of physics.
    And at the scale you are addressing I’m confident you cannot describe these things without a theory that includes some form of HUP equivalent included.

    So, to what I believe the intent of your OP is; the simple answer is:
    NO
     
  19. Nov 10, 2008 #18
    Well I think the answer is no as well. So doesn't that bother anybody?
     
  20. Nov 11, 2008 #19

    Fredrik

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    What Randall said. I can't really improve on that.

    It's annoying, but a lot of things in physics are. The most annoying thing is that experiments can't really reveal "the truth about reality". The only thing they can tell us is how accurately a theory predicts the results of experiments.
     
  21. Nov 11, 2008 #20
    Well it's not the coordinate system per se that troubles me but rather the inability
    to directly measure position at the nanometer scale. All of our physical theories presume
    an underlying space and some metric associated with position in that space. Well if that
    metric can not be operationally defined in a "direct" manner at microscopic spatial scales, and
    therefore measured, then what does that say about our theories that are intended to describe
    physics at those scales? Are macroscopic measurements the only direct measurements we have
    access to? Is it that all predictions based on our physical theories only pertain to that which
    we can measure on a macroscopic scale despite the fact that some are meant to describe
    the microscopic?.
     
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