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Quantum physics: proving wave packet is normalized

  1. Jan 28, 2015 #1
    1. The problem statement, all variables and given/known data
    Following gaussian wave packet: ## \psi (x)= \frac{1}{\sqrt{\sqrt{\pi a^2}}} e^{-\frac{x^2}{2a^2}}##
    Prove that this function is normalized.

    2. Relevant equations
    ## \int_{- \infty}^{\infty} |\psi (x)|^2 dx = 1##

    3. The attempt at a solution
    Is ## \frac{1}{\sqrt{\sqrt{\pi a^2}}} \int_{- \infty}^{\infty} e^{-\frac{x^2}{a^2}} dx## equal to 1?

    I have a solution given, but I don't really get how they even got that solution:

    ## \frac{1}{\sqrt{\pi a^2}} \sqrt{\pi a^2}## = 1
    But that solution totally ignores the double root in the first term, and I'm mond boggled how they managed to integrate the exponent to ending up as ##\sqrt{\pi a^2}##? Actually I don't manage to clearly and unambiuously integrate that exponent myself.
     
  2. jcsd
  3. Jan 28, 2015 #2

    DEvens

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    First, note that the equation in 2 has the square of the wave function. That is why the exponent is exp(-x^2 / a^2) instead of exp(-x^2 / [2*a^2] ). Right? So why didn't you square the multiplier out front as well?

    As to how to do the integral: The problem is you have this integral of exp(-x^2/a^2) with respect to x. And that's hard. But note that it's an integral from -infinity to +infinity. And the hint is that the answer seems to have a square root in it. So what about doing the square of the integral? You can convert to an integral over r and theta in 2-D, and you should be able to do that integral. Then take the square root, and violins.
     
  4. Jan 28, 2015 #3
    You are correct. I forgot to square the constant before the exponent. My eye fell on the fact that it's a Gaussian wave packet, therefore it's a Gaussian integral, which is basically what you're epxlaining (I think). ##\int_{-\infty}^{\infty} e^{-x^2}## would be ## \sqrt{\pi}## and the ##a^2## would just be multiplied under the squareroot following the features of a Gaussion integral. So, yes the solution I had from the course is what I was so puzzled about.
     
  5. Jan 28, 2015 #4

    BvU

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    Did you also learn how to actually do this integral $$
    \int_{-\infty}^{\infty} e^{-x^2}\ \ {\rm ?} $$
     
  6. Jan 29, 2015 #5
    There is mention of it in later chapters, with regards to Dirac and Fourrier.
     
  7. Jan 29, 2015 #6

    BvU

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    Nah, don't need all that complicated stuff for this one. it's a simple but very nifty trick, good to remember (I did:) ):

    They change from the 1D integral to its square, a 2D integral. In polar coordinates the ##dx\; dy## transform to ##r \;dr\; d\phi## and then it's easy (using ##r\;dr = {1\over 2}d(r^2)\ ## ). See Wiki
     
  8. Jan 29, 2015 #7
    Thank you very much. Yes, with the polar coordinates it's not that hard. :)
     
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