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I know that the Schwarzschild line-element is:

$$ds^2=c^2\left(1-\frac{2\mu}{r}\right)dt^2-\left(1-\frac{2\mu}{r}\right)^{-1}dr^2-r^2d\theta^2-r^2\sin^2\theta d\phi^2$$

Dividing by $d\lambda^2$, where $\lambda$ is an affine parameter, I get:

$$\left(\frac{ds}{d\lambda}\right)^2=c^2\left(1-\frac{2\mu}{r}\right)\dot{t}^2-\left(1-\frac{2\mu}{r}\right)^{-1}\dot{r}^2-r^2\dot{\theta}^2-r^2\sin^2\theta \dot{\phi}^2$$

where $\dot{X}$ is $\frac{dX}{d\lambda}$.

To my understanding, this is the Lagrangian, so Euler-Lagrange equations hold.

So I must have:

$$\frac{d}{d\lambda} \left( \frac{\partial L}{\partial \dot{\phi}} \right)=\frac{\partial L}{\partial \phi}$$

Evaluating LHS: $-2r^2\sin^2\theta\ddot{\phi}$.

RHS: $\frac{\partial L}{\partial \phi} = 0$.

Ie: $\ddot{\phi}=0$

Geodesic equation for $\phi$:

$$\frac{d\phi^2}{d\lambda^2}+\Gamma^{\phi}_{\mu\nu}\frac{dx^\mu}{d\lambda}\frac{dx^nu}{d\lambda}=0$$

So I conclude that $\Gamma^{\phi}_{\mu\nu}$ for any $\mu, \nu$.

This, however, is in conflict with what I find on the internet. Equation 5.4 from here, Wikipedia, disagrees with this result.

What am I doing wrong?

zabop
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1 Answers1

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Perhaps should be moved to physics stackexchange?

Commend you for trying to re-derive the equation of motion directly from the length geodesic for the Schwarzchild metric, but you applied the EL equations incorrectly. The "Lagrangian" in this case is a function of more than just $\phi$. It is also a function of $t$, $r$, and $\theta$, since each of these are functions of $\lambda$, so you would need to set up reciprocal equations of motion for these coordinates.