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Homework Help: Basis Vector Rotation

  1. Jul 17, 2015 #1

    I have a 3d space with metric [tex] ds^2= -r^a dt^2 + r^bdr^2 +r^2 dy^2[/tex] and I need to construct an orthonormal frame.

    The first of these three basis vectors is fixed, let's say as [tex]e_0=A \partial_t + B \partial_r + C \partial_y[/tex]

    To find the other two I set [tex]v_1=\partial_t, v_2=\partial_r[/tex] and then apply the Gram-Schmidt procedure. Ultimately I end up with

    [tex]e_1=D \partial_t + E \partial_r + F \partial_y[/tex] and [tex]e_2=G \partial_t + H \partial_r + J \partial_y[/tex]

    Since I have exact (albeit rather lengthy and complicated expressions) for [tex]A,B,C[/tex] in terms of variables such as energy and momenta, the Gram-Schmidt procedure does give me expressions for all the other constants (although they are in terms of energy and momenta as well - importantly, they are not numbers!).

    Now, I am trying to compare my result to the result in the literature where they explicitly construct such a basis with [tex]e_1[/tex] only pointing in the t and r directions. Since [tex]e_0[/tex] is fixed and I can't alter it at all, this is really a rotation of the other two orthonormal basis vectors i.e. I want to rotate [tex]e_1,e_2[/tex] such that [tex]e_1[/tex] no longer has a y component.

    I tried to do this by multiplying as follows:
    [tex]\begin{pvector} e'_1 \\ e'_2 \end{pvector} = \begin{array} \cos{\theta} & \sin{\theta} \\ -\sin{\theta} & \cos{\theta} \end{array} \begin{pvector} e_1 \\ e_2 \end{pvector}[/tex]

    (Does this even hold when we are no longer in Euclidean space?)

    Imposing [tex](e'_1)^y=0[/tex] (I use this notation to denote the y component), I can read off the following equation

    [tex]0=\cos{\theta} (e_1)^y + \sin{\theta} (e_2)^y \Rightarrow \tan{\theta}=-\frac{(e_1)^y}{(e_2)^y}[/tex]

    However, this gives me an expression for [tex]\theta=\tan^{-1}{(-\frac{(e_1)^y}{(e_2)^y})}[/tex]. This will be some ratio of the energy and momenta and not an actual number. So how can I use it to determine [tex]\sin{\theta}[/tex] and [tex]\cos{\theta}[/tex] which I will need in order to work out how the ohther components of [tex]e'_1,e'_2[/tex] look like?

    Thanks very much for your help..
  2. jcsd
  3. Jul 17, 2015 #2


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    One reason to introduce frame fields is that the Euclidean rotations act naturally on them.

    There are elementary trig identities like

    $$ 1 + \tan^2\theta = \frac{1}{\cos^2\theta}$$

    that you can use to express ##\sin## and ##\cos## in terms of ##\tan##.
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