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Homework Help: Transformation of a vector operator

  1. Sep 25, 2013 #1
    1. The problem statement, all variables and given/known data

    Calculate the result of the transformation of the vector operator [tex] \hat{V_{y}} [/tex] by rotation [tex] \hat{R_{x}} [/tex] around an angle [tex] \alpha [/tex].

    2. Relevant equations

    I believe that [tex] \hat{R_{x}} = \begin{pmatrix} 1& 0& 0\\ 0& cos\alpha & -sin\alpha \\ 0 & sin\alpha & cos\alpha \end{pmatrix} [/tex]

    Not sure if the fact that it is an operator makes any difference here...

    3. The attempt at a solution

    So at first glance it seems that the solution should be something like the calculation of [tex] \hat{V_y} \hat{R_x} [/tex], however I am not sure since they are operators. If this is correct then would the solution be calculated by using arbitrary components of [tex] \hat{V_y} [/tex]? If this is completely wrong, what is a better way to look at this problem?

    Thank you for any help.
  2. jcsd
  3. Sep 26, 2013 #2

    George Jones

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    Staff Emeritus
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    Gold Member

    Let's start with a more basic question: given a rotation

    [tex]R_{x} = \begin{pmatrix} 1& 0& 0\\ 0& cos\alpha & -sin\alpha \\ 0 & sin\alpha & cos\alpha \end{pmatrix},[/tex]

    what is the corresponding operator [itex]\hat{R_{x}}[/itex] that acts on states (kets or wave functions)?
  4. Sep 26, 2013 #3
    I guess that is what I am most confused about here. Is how does [tex] R_x [/tex] come correspond to [tex] \hat{R_x} [/tex].

    So some thing like this: [tex] V_i \to R_{ij} V_j [/tex]

    If I had to make an educated guess here I would say it would transform on the ket [tex] \mid \psi \rangle [/tex] in this way: [tex] \langle \psi \mid V_i \mid \psi \rangle \to R_{ij} \langle \psi \mid V_j \mid \psi \rangle [/tex].
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