# I Christoffel symbols of Schwarzschild metric with Lagrangian

1. Sep 4, 2016

### Stella.Physics

So the Schwarzschild metric is given by

ds2= -(1-2M/r)dt2 + (1-2M/r)-1dr2+r22+r2sin2θ dφ2

and the Lagragian is ${\frac{d}{dσ}}[{\frac{1}{L}}{\frac{dx^α}{dσ}}] + {\frac{∂L}{∂x^α}}=0$

with L = dτ/dσ. So for each α=0,1,2,3 we have

${\frac{d^2 x^1}{dτ^2}}=0$ for Minkowski spacetime

also the geadesic equation is ${\frac{d^2 x^a}{dτ^2}}+Γ^a_{bc}{\frac{dx^b}{dτ}}{\frac{dx^c}{dτ}}=0$

So what happens in the Schwarschild metric and how can I find the Christoffel symbols from the Schwarschild metric and the geodesic equations?

Last edited: Sep 4, 2016
2. Sep 4, 2016

### stevendaryl

Staff Emeritus
For slower-than-light geodesics, you can use an effective Lagrangian (which is basically the square of the one you use):

$\mathcal{L} = \frac{1}{2} g_{\mu \nu} U^\mu U^\nu$

where $g_{\mu \nu}$ is a component of the metric tensor, and $U^\mu = \frac{dx^\mu}{d\sigma}$. This is slightly easier to work with than the one you are using (although it's a little work to see that they lead to the same equations of motion). The Lagrangian equations of motion for this lagrangian are:

$\frac{d}{ds} \frac{\partial \mathcal{L}}{\partial U^\mu} - \frac{\partial \mathcal{L}}{\partial x^\mu} = 0$

This gives the following form of the geodesic equation:

$g_{\mu \nu} \frac{dU^\nu}{d\sigma} + [\frac{\partial g_{\mu \nu}}{\partial x^\lambda} - \frac{1}{2} \frac{\partial g_{\lambda \nu}}{\partial x^\mu}] U^\lambda U^\nu = 0$

Unfortunately, the quantity in square brackets (call it $Q_{\mu \nu \lambda}$) is not exactly equal to the Christoffel symbol $\Gamma_{\mu \nu \lambda} \equiv g_{\mu \alpha} \Gamma^\alpha_{\nu \lambda} = \frac{1}{2}(\frac{\partial g_{\mu\nu}}{\partial x^\lambda} + \frac{\partial g_{\mu\lambda}}{\partial x^\nu}- \frac{\partial g_{\lambda \nu}}{\partial x^\mu})$. However, if you compute $Q_{\mu \nu \lambda}$, then you can find $\Gamma_{\mu \nu \lambda}$ by symmetrizing:

$\Gamma_{\mu \nu \lambda} = \frac{1}{2} [Q_{\mu \nu \lambda} + Q_{\mu \lambda \nu}]$

Last edited: Sep 4, 2016
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