Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Wavepacket Problem

  1. Jan 16, 2009 #1
    1. The problem statement, all variables and given/known data

    A wavepacket, psi(x,t), can be expressed as a linear combination of eigenstates. Assuming that only 2 eigenstates, phi0(x) and phi1(x), contribute to the linear combination write down the expression for |psi(x,t)|^2.

    2. Relevant equations

    [Boltzmann's constant = 1.38 x10^23 J /K, Planck's constant h = 6.626 x10 ^-34 J s,
    Gas constant R = 8.314 J K /mol]

    3. The attempt at a solution

    I need help with how to start solving this problem.
  2. jcsd
  3. Jan 16, 2009 #2
    This is just an educated guess, but I'd say something like:
    [tex] \psi = a_0 \psi_0 + a_1 \psi_1 [/tex]
    [tex] \int \psi \psi^{*} d^3r = \int (a_0 \psi_0 + a_1 \psi_1)(a^{*}_0 \psi^{*}_0 + a^{*}_1 \psi^{*}_1) d^3r [/tex]
    Now all you need to do is expand the RHS and see what you get (the integral is over all space, so what does that tell you about what the integral must equal?). If I were you I would also assume that the orginal wavefunctions (psi0 and psi1) are normalised, and that the constants a0 and a1 have no imaginary component.

    That's what I would do for this question anyway, but I'm definitely no expert on the subject. lol.
  4. Jan 16, 2009 #3
    The integral must equal 1. But do I need to expand the RHS? It just says to write the expression not to evaluate the expression.
  5. Jan 16, 2009 #4
    oh yeah, my bad.
  6. Jan 16, 2009 #5
    Hey, no problem. Thanks for your help. I'm not sure how to expand this. I can handle numbers but when symbols are used, I start to have trouble. I'm guessing there will be four results, two zeroes and two ones. The ones being psi1 x psi1*, and psi0 x psi0*, but I'm not sure how to expand this and get those results.
  7. Jan 17, 2009 #6
    Unless I made a mistake, you should end up with something like:
    [tex] |\psi|^2 = |a_0 \psi_0|^2 + |a_1 \psi_1|^2 + a_0a^{*}_1\psi_0\psi^{*}_1 + a_1a^{*}_0\psi_1\psi^{*}_0 [/tex]

    Obviously the normalisation constants of the first two terms of that expression are going to be real. You can prove that by writing the normalisation constants as a complex exponential [itex] a_0 = r_0e^{i\theta_0} [/itex]. If you do that with a1 aswell, then you could simplify the other terms involving a1*a0 and a0*a1.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook