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Is Uncertainty intrinsic property or a consequence of measurement ?

  1. Jan 2, 2014 #1
    Hello friends,

    Most people that I heard put uncertainty as an intrinsic property of the universe which is evident when we make a measurement. But to me it seems that intrinsic property and making a measurement are two entirely different things.

    If uncertainty were to be just(purely) intrinsic property, then I suppose the uncertainty would stack with the passing of time. But it doesn't.

    At the same time the uncertainty can be an effect of just measurement itself, because of the definition, the uncertainty is only defined for the simultaneous measurement of canonically conjugate variables.

    So the question why do we think/believe that uncertainty is intrinsic to the universe, when it seems its just a measurement aspect for conjugate variables.

    Thanks
     
  2. jcsd
  3. Jan 2, 2014 #2

    phinds

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    The HUP is emphatically NOT an artifact of measurement but an intrinsic property of quantum objects. This is proven conclusively by the single slit experiment, among a number of ways in which it has been verified.

    There are numerous threads on this forum about this.
     
  4. Jan 2, 2014 #3

    ShayanJ

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    This is a very naive reasoning.The formulation of QM answers the question.Its just waiting for you to go and learn it!
    Well,of course if you don't observe a system,you can't see its features!!!
    The point is that obtaining more accuracy than what HUP allows is impossible even in principle.
     
  5. Jan 2, 2014 #4
    I think a good example of stacking of uncertainty can be seen by looking at the random walk problem. The dispersion of the steps(variance) is proportional to the number of steps taken(= N.p.q) since there is intrinsic uncertainty(probability) of taking steps in right or left.

    Is it "even in the principle according to the mathematical formulation of QM",

    This is what I know so far about HUP, I think HUP is nothing but the generalization of the statistical nature of Quantum Mechanics via canonical conjugate variables, which is to say there is nothing that stops you from getting a value for momentum and position simultaneously but these values we get are not going to predict where the particle will be without some uncertainty(No matter how fast you make the subsequent measurement or if we make simultaneous measurements on large number of identical systems).

    From classical mechanics(on which most of the QM formulation relies), value of canonical conjugate variables at one instant are enough to describe a system(Lagrangian, Hamiltonian mechanics) for any other instant, but the values of these same variables in QM are not enough to predict what is going to happen next. Instead if we repeat the measurement of these variables again and again(even on a stationary energy state) we will get a dispersion of values (let's say [itex]\Delta p_x[/itex] and [itex]\Delta x[/itex]) of conjugate variables. And it is the product of the statistical dispersion/variance ([itex]\sqrt{\left\langle p_x^2 \right\rangle - (\left\langle p_x \right\rangle)^2 }[/itex]) and ([itex]\sqrt{\left\langle x^2 \right\rangle - (\left\langle x \right\rangle)^2 }[/itex]) which cannot be lower than the limit.

    Edit: In other words, momentum and position in a particular direction cannot have simultaneous Eigenstates(according to the formulation of QM)
     
    Last edited: Jan 2, 2014
  6. Jan 2, 2014 #5

    phinds

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    This has NOTHING to do with the HUP.
     
  7. Jan 2, 2014 #6

    atyy

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    The commutation relations lead to both types of uncertainties.

    There is intrinsic uncertainty of the state, as well as measurement uncertainty.

    The intrinsic uncertainty of the state says if we have a state, and we perform separate precise measurements of position and separate precise measurements of momentum, the standard deviations of the distributions of the results will obey an uncertainty principle. Here since the measurements are separate, they can be precise. Since the measurements are precise, we say the uncertainty is intrinsic to the state.

    Examples of measurement uncertainty (several different definitions exist, so it depends on which definition one uses) are found in:
    http://physicsworld.com/cws/article...y-reigns-over-heisenbergs-measurement-analogy
    http://arxiv.org/abs/1304.2071
    http://arxiv.org/abs/1306.1565
     
    Last edited: Jan 2, 2014
  8. Jan 2, 2014 #7
    It would have if HUP were to be a purely intrinsic property, that was my point in the later part of the quoted post.
     
  9. Jan 2, 2014 #8

    phinds

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    I don't follow you here at all. The HUP IS an intrinsic property and the random walk is totally unrelated to it.
     
  10. Jan 2, 2014 #9
    Nice reply, thanks.

    But how does one conclude that measurements were precise, just because one can have a value for the measurements does not conclusively validate the measurement is precise. Because one can always assume that the dispersion of the different measurements is a consequence of measurement itself, just like one would get the dispersion in measurement results in classical sense.
     
  11. Jan 2, 2014 #10

    atyy

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    The idea behind the definition of a precise measurement is one that if the state is ψ(x), then the distribution of measured positions if ψ(x)ψ*(x) (and similarly for other quantities) if the measurement is precise. I don't know if this is quite right for a quantity like position in an infinite dimensional Hilbert space, but that's the basic idea. You can look at the other papers I linked to on joint uncertain measurements for more precise definitions of what a precise measurement is.
     
  12. Jan 2, 2014 #11
    OK, What I'm trying to convey is, random walk problem has intrinsic uncertainty(probability) in it(the step taken at any particular point is independent of every variable i.e. intrinsic).

    And therefore the dispersion(N.p.q) of the steps is proportional to the number of steps taken, whereas in QM momentum dispersion in a particular direction is not proportional to the number of measurements(in other words not proportional to the elapsed time), and for stationary states of the bound systems the dispersion is constant.

    Whereas, HUP is the product of the dispersion of canonical conjugate variables, which one can easily formulate from commutation relation and Schwarz inequality.

    It seems the uncertainty principle only shows the statistical nature of quantum mechanics, and that there is very thin line between the precise measurement and the intrinsic nature of the measured property.
     
  13. Jan 2, 2014 #12

    Nugatory

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    You can assume that, but if you do you're not using quantum mechanics, you're using some other theory. The formalism of QM works as atyy describes ("The commutation relations lead to both types of uncertainties") with no wiggle room in this area.

    Note that this does not mean that QM is "right" or "true", just that if you use it you are committed to intrinsic uncertainty in some pairs of measurements. Of course, if you don't use QM, you have to use something else, and so far no one has found a remotely plausible alternative that matches experimental results with equal success - and a lot of people have been trying for more than a century now.
     
  14. Jan 2, 2014 #13

    Nugatory

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    That's true, but random walks are Markovian and the evolution of the state function according to the Schrodinger equation is not. That's why the uncertainties don't "stack" in the same way.
     
  15. Jan 2, 2014 #14
    I don't get this, I thought Markovian evolution was "memoryless" and therefore uncertainties don't stack, as happens with Schrodinger evolution(they don't stack either).
     
    Last edited: Jan 2, 2014
  16. Jan 2, 2014 #15

    bhobba

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    Markovian assumptions applied to a particle leads to the basic Wiener process. One has to do something utterly unintuitive and physically not particularly clear without a careful analysis, to derive QM from that, and go over to complex numbers.

    Thanks
    Bill
     
  17. Jan 2, 2014 #16

    bhobba

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    The random walk leads to a Wiener process. It is an interesting fact that QM can be derived from that by going over to complex numbers - this is the path integral formulation of Feynman.

    Its physical basis is entirely different though, and sorting out exactly why the introduction of complex numbers accomplishes this feat requires careful analysis. Some say its the central mystery of QM - why complex numbers.

    To understand the uncertainly relations you need to delve into the math - its an intrinsic property of QM associated with the non-commutativeity of observables.

    Thanks
    Bill
     
  18. Jan 2, 2014 #17

    bhobba

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    Its to do with the probabilistic nature of QM. Measurement outcomes are precise, but done under exactly the same conditions different outcomes will result. Why that is is the very essence of QM.

    Here is a modern take since I suspect you know a bit about probability models:
    http://arxiv.org/abs/quant-ph/0101012

    The modern view is QM is one of the two most reasonable generalized probability models that can be used to model physical systems - the other being bog standard probability theory. However, as in the case of the random walk, things do not quite work out if you use that to model say particle position. The deep reason, as the paper above points out, is there is no continuous transformation between pure states in probability theory - that's why you need to over to complex numbers to get it to work and why you in fact get QM.

    Thanks
    Bill
     
    Last edited: Jan 2, 2014
  19. Jan 3, 2014 #18
    It looks like you have different definitions of "intrinsic". The latter is "independent" whereas in the former the uncertainty follows naturally from QM formulation.
     
  20. Jan 3, 2014 #19
    Isn't the latter part of your statement the meaning of "imprecise"? Looks like a contradiction.
     
  21. Jan 3, 2014 #20

    dextercioby

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    That's meant in the sense that the precision is basically getting one and only number/reading shown by an apparatus out of the multitude of possible measurement outcomes.
     
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