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Homework Help: Probabilities for orthonormal wavefunctions

  1. Oct 31, 2007 #1
    Ok I have two orthonormal wavefunctions of a system, [tex]\psi[/tex] 1 and [tex]\psi[/tex] 2 and [tex]\widehat{A}[/tex] is an observable such that

    [tex]\widehat{A}[/tex] |[tex]\phi[/tex] [tex]_{n}[/tex] > = a[tex]_{n}[/tex] |[tex]\phi[/tex] [tex]_{n}[/tex] >

    for eigenvalues a sub n

    what are the probabilities p1(a1) and p2(a2) of obtaining the value a sub n in the state |psi1> and |psi2> respectively in terms of only phi sub i and psi sub 1(or 2)
     
  2. jcsd
  3. Oct 31, 2007 #2
    If I understand your question-

    You can't work out the probabilities from knowledge of the eigenvalues of an operator.
     
  4. Nov 1, 2007 #3

    George Jones

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    But the probability that a measurement yields a particular eigenvalue can be expressed in terms of the associated eigenstate and the state of the system, which, if I have interpreted the original post correctly, kac9 has given.

    kac9: I'm having trouble guiding you to the answer without just writing down the answer. This is a basic postulate of (shut up and calculate) quantum mechanics. It must be in your notes and text. If you're using Griffiths, it's equation [3.43].
     
  5. Nov 1, 2007 #4
    George: Ah yes, the state is either in psi1 or psi2- my mistake.
     
  6. Nov 1, 2007 #5
    Well- a hint would be to write down the identity operator in terms of |phi>

    I=sum_i |phi_i><phi_i|

    |psi_1>=I|psi_1>
     
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