- #1
JackDP
- 7
- 0
Homework Statement
Hi all, I'm trying to learn how to manipulate tensors and in particular to differentiate expressions. I was looking at a Lagrangian density and trying to apply the Euler-Lagrange equations to it.
Homework Equations
Lagrangian density:
[tex]\mathcal{L} = -\frac{1}{2} \partial_{\alpha} \phi^{\beta} \partial^{\alpha} \phi_{\beta}
+ \frac{1}{2} \partial_{\alpha} \phi^{\alpha} \partial_{\beta} \phi^{\beta}
+ \frac{1}{2}\mu^2 \phi^{\alpha} \phi_{\alpha}[/tex]
Euler-Lagrange:
[tex]\frac{\partial \mathcal{L}}{\partial \phi^i} = \partial^k \frac{\partial \mathcal{L}}{\partial \phi^{i,k}}[/tex]
The Attempt at a Solution
I have attempted to differentiation the expression several times; I can compute [tex]\frac{\partial \mathcal{L}}{\partial \phi^i}[/tex] with no problems and can compute [tex]\frac{\partial \mathcal{L}}{\partial \phi^{i,k}}[/tex] for the first and third terms.
However, I just cannot figure out how to differentiate the middle term. My attempt:
[tex]\mathcal{L}_2 = \frac{1}{2} \partial_{\alpha} \phi^{\alpha} \partial_{\beta} \phi^{\beta}
= \frac{1}{2} g_{\alpha \lambda} g_{\beta \sigma} \partial^{\lambda} \phi^{\alpha} \partial^{\sigma} \phi^{\beta}[/tex]
Hence
[tex] \frac{\partial \mathcal{L}_2}{\partial \phi^{i,k}} =
\frac{1}{2} g_{\alpha \lambda} g_{\beta \sigma} \left(
\delta_k^{\lambda} \delta_i^{\alpha} \partial^{\sigma} \phi^{\beta} +
\delta_k^{\sigma} \delta_i^{\beta} \partial^{\lambda} \phi^{\alpha}
\right)
= \frac{1}{2} \left(
g_{i k} \partial_{\beta} \phi^{\beta} +
g_{i k} \partial_{\alpha} \phi^{\alpha}
\right)
= g_{i k} \phi_i \phi^i
[/tex]
So as you can see, I have somehow picked up this additional factor of the metric. I'm not sure what to do with it, or where I have gone wrong!
Best wishes,
J