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Breaking Down the Uncertainty Principle

  1. Oct 27, 2009 #1
    I'm currently in an intro chemistry class and a lot of the material is review from the multiple physics classes I have taken. Most of it comes back easily, including how to use the uncertainty principle to solve the small array of homework questions thrown at me but when trying to grasp a deeper understanding of the principle there is something I can't seem to figure out and it isn't mentioned in detail in the chemistry book.

    I understand, conceptually, that the closer the uncertainty of x gets to zero, the more the uncertainty of v would approach infinity but algebraically, i'm not sure how he came up with the formula, more specifically where he got the 4pi from.

    I recall from calculus that 4pi would be two rotations around a circle. This lead me to think that perhaps 4(pi) could be representative of the period of the wave exhibited by the particle--assuming the particle to have both particle and wavelike properties. If this were true, putting the period of the wave in the denominator (as it is in the uncertainty principle) would be equivalent to multiplying by the reciprocal of the period (the frequency).

    However the frequency would be unknown, would it not? So the period would have to be unknown as well.

    Here I go confusing myself again, i'll quit rambling now.

    In summary, where does the 4(pi) come from in the denominator of Heisenberg's uncertainty principle?
  2. jcsd
  3. Oct 27, 2009 #2
    The might be a bit hard to explain for an intro chemistry class. Physically it doesn't really mean anything important. In fact physicists typically define a new value:

    [tex]\hbar = \frac{h}{2 \pi}[/tex] which makes the uncertainty [tex]\sigma_x \sigma_p \geq \frac{\hbar}{2} = \frac{h}{4 \pi}[/tex]

    The Heisenberg Uncertainty Principle is simply a mathematical limit on how well we can measure two certain quantities. In fact there are as many uncertainty relations as quantities we can measure.

    The value "h-bar" divided by 2 is simply the relation between position and momentum.
  4. Oct 27, 2009 #3
    I think you can take a look at some books on QM,like Sakurai's.
  5. Oct 27, 2009 #4


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    This is an accurate description of the original version of the uncertainty principle, but the modern version has a very different interpretation. The original uncertainty principle predates quantum mechanics. It provided some of the motivation for the mathematical structure of QM, but it's not actually a part of the theory. The modern uncertainty principle is a mathematical theorem in QM, and as I said, its interpretation is very different. See this thread for more.
  6. Oct 27, 2009 #5
    For that you probably would have to look at one of them mentioned mathematical derivations. The 2pi comes essentially from the Fourier transform, one could say.

    The uncertaintly principle isn't an additional law in quantum mechanics, but rather follows from the structure that you are allowed to use to describe physics. Basically, one can only specify the position distribution of the electron. What velocity it has follows directly from this mathematically, so you are not free to chose its velocity anymore. Playing around with this contraint, one can find that either you have a single position and a mix of momenta OR you have a mix of positions and a single momentum. You cannot make both to be a single value.
  7. Oct 27, 2009 #6
    The more generalized version is exactly the one you have derived. I don't think you read what I was saying very carefully.
  8. Oct 27, 2009 #7


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    I don't think you read what I said very carefully. The two versions of the uncertainty principle that I was talking about isn't a "generalized" and a "specific" result. I'm distinguishing between mathematical theorems derived from the axioms of QM and a heuristic result that was found before the axioms of QM had even been written down for the first time.

    As I tried to tell you before, this is the correct interpretation of the old heuristic result, but if it's supposed to be an interpretation of the theorem I derived, it's just wrong. QM says that a particle doesn't have a well-defined position and a well-defined momentum at the same time, so it's not just about how well we can measure those quantities.
  9. Oct 28, 2009 #8
    It is correct to say "includes the fact".
    You should just be aware of that the reason for these limits is not the measurements, but just that there are physical contraints that these quantities cannot exist together regardless of whether you measure or not.
  10. Oct 28, 2009 #9
    Ahhh, this is more what I was looking for.

    Thanks for all of the input, everyone.
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