- #1
Alexrey
- 35
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Homework Statement
I'm following the derivation of finding the energy flux of a gravitational wave propagating along the z-axis where they use the energy-momentum pseudotensor to achieve this, but I can't seem to get an answer that matches theirs.
Homework Equations
We are given a general gravitational plane wave where its wave vector k is given by [tex]k_{a}=(\omega/c,0,0,-\omega/c).[/tex] We then have a stress-energy pseudotensor given by
[tex]t_{ab}=\frac{c^{4}}{64\pi G}k_{a}k_{b}A_{ij}^{TT}A^{ij\, TT}.[/tex] where the amplitude polarization tensor is [tex]A_{ij}^{TT}=\begin{bmatrix}A_{+} & A_{\times} & 0\\
A_{\times} & -A_{+} & 0\\
0 & 0 & 0
\end{bmatrix}.[/tex]
The Attempt at a Solution
It seems to be just some basic algebraic manipulation that is needed but for some reason I can't get the answer of [tex]t_{ab}=\frac{c^{2}\omega^{2}}{32\pi G}(A_{+}^{2}+A_{\times}^{2})\begin{bmatrix}1 & 0 & 0 & -1\\
0 & 0 & 0 & 0\\
0 & 0 & 0 & 0\\
-1 & 0 & 0 & 1
\end{bmatrix}[/tex] which was given in the textbook. I tried some manipulation, but then could see nowhere to go next as all of my matrices ended up being zero. I started out by putting everything in where [tex]t_{ab}=\frac{c^{4}}{64\pi G}(\omega/c,0,0,-\omega/c)(\omega/c,0,0,-\omega/c)\begin{bmatrix}0 & 0 & 0 & 0\\
0 & A_{+} & A_{\times} & 0\\
0 & A_{\times} & -A_{+} & 0\\
0 & 0 & 0 & 0
\end{bmatrix}\begin{bmatrix}0 & 0 & 0 & 0\\
0 & A_{+} & A_{\times} & 0\\
0 & A_{\times} & -A_{+} & 0\\
0 & 0 & 0 & 0
\end{bmatrix}.[/tex] I then simplified to get [tex]t_{ab}=\frac{c^{2}\omega^{2}}{64\pi G}(1,0,0,-1)(1,0,0,-1)\begin{bmatrix}0 & 0 & 0 & 0\\
0 & A_{+} & A_{\times} & 0\\
0 & A_{\times} & -A_{+} & 0\\
0 & 0 & 0 & 0
\end{bmatrix}\begin{bmatrix}0 & 0 & 0 & 0\\
0 & A_{+} & A_{\times} & 0\\
0 & A_{\times} & -A_{+} & 0\\
0 & 0 & 0 & 0
\end{bmatrix}.[/tex] Once this was done though I found out that multiplying [tex](1,0,0,-1)\begin{bmatrix}0 & 0 & 0 & 0\\
0 & A_{+} & A_{\times} & 0\\
0 & A_{\times} & -A_{+} & 0\\
0 & 0 & 0 & 0
\end{bmatrix}[/tex] gave zero and that's essentially where I stopped. I know I must be making a really stupid mistake but I just can't see it. Any help would be appreciated, thanks guys.