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How do we know uncertainty principal isnt ignorance?

  1. Dec 23, 2014 #1
    what if theres a way to measure speed and direction at the same time and we havent found it out yet? does it have to do with information theory?
  2. jcsd
  3. Dec 23, 2014 #2
    It is position and momentum which cannot both be known very accurately simultaneously.
  4. Dec 23, 2014 #3


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    It is not a measurement problem, it's just the way the universe works. The fact that is not a measurement problem is inherent in the HUP

    If you'd like to see more discussion, do a forum search. This canard has been debunked here many dozens of times.
  5. Dec 23, 2014 #4


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  6. Dec 23, 2014 #5


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    There's no problem with measuring speed and direction simultaneously. The uncertainty principle comes into play when we want to measure two things that in the mathematical formalism of quantum mechanics are represented by non-commuting operators. Speed and direction commute, so they're OK; but position and momentum, or angular momentum along different axes, and many others, are not.

    You will find many explanations of the uncertainty principle that say that it's all about measuring one thing having to disturb another. These explanations are based on an erroneous understanding from seventy-five years ago. If you search this forum you will find a number of correct explanations based on what is now understood properly.

    The basic issue is that I cannot set up a quantum system in such way that it has known and definite values for two non-commuting observables, call them A and B. I can set the system up so that it has a definite value for A, and then I can measure both A and B to as much precision as I like. However, if I repeat this experiment many times I will find that although I always get the same value for A, I get different values for B. The states in which A has a known value (called "eigenstates" of A) are all states that are not eigenstates of B, meaning that B can take on a range of values.
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