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Uncertainty v. Predictability

  1. Aug 28, 2009 #1

    Physics is not my field of expertise, and i've had a few questions keeping me up nights for some time. I'm hoping that someone here with the expertise i lack will help me come to some conclusion regarding free will, as it relates to my understanding of the uncertainty principle.

    Okay. My understanding of the uncertainty principle is essentially that it is impossible to know, say, both the position and the velocity of a quantum particle at the same moment in time with any accuracy. That's more or less the jist of it, right? Now, that said, just because one cannot know these properties with any accuracy does not mean the do not exist, does it? A sub-atomic particle does have a precise position and velocity, the problem is that we cannot know both with any accuracy. Am i correct in my understanding? If these things are true, the only real conclusion one can draw is that, since everything in the universe is built of these particles, everything in the universe has a predetermined path. That would naturally include human action/thought.

    I'm sure most of you in the know have already come to some conclusion about the stated predicament, but i have not. That is why i ask, so please entertain my curiosity if you find the time. My world view is admittedly in shambles, and i'd love to be able to sleep again.

  2. jcsd
  3. Aug 28, 2009 #2


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    Staff: Mentor

    Here's my description of the precise meaning of the HUP, which I posted some time ago:


    Going beyond this description, to a statement about what is "really happening" before the measurements are made, gets you into the realm of interpretations of QM. There is little general agreement on such questions, because (so far) nobody has found a way to distinguish between a large set of interpretations experimentally. It's generally agreed (except for a few persistent dissenters) that some interpretations (the "local realistic" ones) are pretty much ruled out by experiments that test Bell's Theorem.
    Last edited: Aug 30, 2009
  4. Aug 28, 2009 #3
    This is one of the interesting parts of QM in that there is quite the philosophical debate about it's interpretation. Here's how I generally understand it:

    The uncertainty principle (in terms of position and momentum) could potentially mean two things:

    1.) It's a statement that we do not know all the information available in our theory. By that what I mean is that there is some hidden concept that we have not yet figured out which would correct the uncertainty which we currently cannot circumvent.

    2.) It's a statement that not only can we not know the position and momentum of a particle both with absolute accuracy, but it also means that nature works in such a way that there is no way of knowing the position if we absolutely know the momentum, and vice-versa. Not only that but if nature does work in such a way it means that our current definition of momentum is completely non-nonsensical (on a quantum scale) if we absolutely know the position (and vice-versa) and so a sub-atomic particle does not have both an absolute position and momentum.

    Now, personally I tend to think that a good majority seem to accept 2.) as the correct interpretation because of a famous counter-argument to 1.) which is called Bell's Inequalities.
  5. Aug 28, 2009 #4
    I would say you are incorrect. A sub-atomic particle does not in general have a precise position, but is described by a wave function having a non-zero value over a fuzzy volume.

    Quantum superposition of states has been verified experimentally, so I would say this answer is true regardless of which interpretation you choose. If sub-atomic particles always have to have a definite position, then the double slit experiment would never show any interference, but it does.
  6. Aug 29, 2009 #5


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    No. Much work has gone into that very question. The prevailing answer is that the uncertainty is not simply a matter of measurement; it is a real property of the system.
  7. Aug 29, 2009 #6

    I really appreciate all the replies from knowledgeable folks. Glad someone mentioned bell's theorem, i'll spend some time looking into it.

    I'm especially interested in the idea that uncertainty is not a question of measurement, but an actual property of the system. A truly fascinating idea, as far as i'm concerned.

    I'm sure i'll be back soon for input on other quandaries.

    Thanks you.
  8. Aug 29, 2009 #7


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    Welcome to PhysicsForums! Yes, you should look into the EPR Paradox and Bell's Theorem as those will help you to frame things in your mind.
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