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Question of Measurement

  1. Aug 26, 2009 #1
    I just had a question about quantum mechanics that has been muddied somewhat by pop culture's understanding of the quantum world. My question is this: Would it be correct to say that the reason that there is inherent uncertainty in measuring the quantum world is because of the methods of measurement (i.e. using photons to hit another photon, which wouldn't produce an accurate result) rather than simply because of the fact that something is being observed (which just seems to be an explanation that people come up with because they don't understand what they're talking about)? In other words, if, theoretically, we had a particle much smaller than any other known particle that we could use to measure those particles, could we then obtain all the information that we needed from them?
  2. jcsd
  3. Aug 26, 2009 #2
    no, the limitations are not instrumental.

    I think about it from the informational point of view. Classical particle has definite position (X,Y,Z) and definite velocity (Vx,Vy,Vz) - 6 numbers.

    QM "particle" contains LESS information then these 6 numbers, that is why you cant obtain them all at the same time. So in some sense it is simpler then the classical particle, that is why it looks so complicated :)
  4. Aug 26, 2009 #3


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    As Dmitry67 says, it is not about precision of the measurement process (although that does obviously exist). In the famous 1935 EPR paper, Einstein et al showed the paradox inherent in Quantum Theory regarding simultaneous measurement of particle properties. As it happens, QM is right: even entangled but separated particles obey the rules of the Heisenberg Uncertainty Principle. That would hardly be expected if the issue was a limitation on the measurement side.
  5. Aug 26, 2009 #4
    As answered above, the heart of the issue is not one of measurement difficulties. It is that the fundamental nature of particles is not like that of a tiny ball, and there was never any reason to believe it would be except that of our bias toward everyday macroscopic behavior. The "particle-wave dual" nature of particles is a way of saying that the fundamental nature of a particle is described by a probability wave, which means the position and momentum (e.g.) are generally *indefinite*...they just plain don't have to have definite numerical values. It is our childhood bias that thinks they should. The wave nature also implies that the more localized (definite) the position, the more delocalized (indefinite) the momentum, and vice versa.
  6. Aug 27, 2009 #5


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    Maybe people understand what they're talking about but this damn thing they are talking about is too vague and don't understand how the proper thing should behave.
    If you, theoretically, could have arbitrary small particle maybe you would arrive at level where constituents of this damn thing are observable and will be able to explain this damn thing with collective behavior of those constituents.
    ... or you would not if no one proves that QM obeys objective reality. ;)
  7. Aug 27, 2009 #6
    Looks like you don't like an answer :)
    But your list of "may be's" is not scietific at all.
    May be on the plank length there are tiny flying unicorns? :)
  8. Aug 27, 2009 #7


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    What a shame to give nonscientific comment about other non scientific comment (I am referring to "which just seems to be an explanation that people come up with because they don't understand what they're talking about")

    And I like this answer, no problem.

    I might not like the other part. And I might conclude that you don't like string theory too much ... but instead like tiny flying unicorns. ;)
  9. Aug 28, 2009 #8
    It must be noted that you can't have an arbitrary small particle, there is a lower limit on length known as the plank length.
  10. Aug 28, 2009 #9


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    Depending on how you look upon quantum mechanics, you can or you can't assign a "true position" to a particle. In Bohmian mechanics, particles DO have "true positions". In more "quantum" versions of quantum theory, they don't, and that's the "real" reason why you cannot do any better. Now, the fact that there are some interpretations of the theory which allow you to think that a particle *doesn't have* a single, precise position indicates that this is not an instrumental problem, but a fundamental property.

    You can think of a particle in a certain quantum state to have "many different positions at the same time". If you now want to build an apparatus to measure "that position" you immediately see the difficulty: the particle has many positions at the same time, so which one "should" you measure ? It's clearly not going to be an instrumental problem in this case, it's fundamental. It is fundamental in a similar way as "what's the frequency of a "click" sound" doesn't have a unique answer (there's a whole spectrum). It is not a problem of the microphone.
  11. Aug 28, 2009 #10


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    vanesch makes a good point. There are interpretations whereby QM isn't about particles (or waves), it's just a theory about the classical objects comprising the experiment, e.g., detectors, sources, beam splitters, mirrors, cavities, etc.
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