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Suppose that there is a body with a mass $m$. How could I get the geodesic inside the body (in Schwarzschild metric)? Because I have only described the metric outside a body before. We also know the radius $r$ of the body and the density is homogeneous.

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The internal metric is derived in the same way as the external metric, by solving the Einstein equation. In general there won't be an analytical solution, but for the case you give of a homogeneous spherically symmetric body the solution is the Schwarzschild interior metric:

$$ ds^2 = -\left[\frac{3}{2}\sqrt{1-\frac{2M}{R}} - \frac{1}{2}\sqrt{1-\frac{2Mr^2}{R^3}}\right]^2dt^2 + \frac{dr^2}{\left(1-\frac{2Mr^2}{R^3}\right)} + r^2 (d\theta^2 + sin^2\theta d\phi^2) $$

Calculating the geodesics requires solving the geodesic equation for this metric, which rapidly gets messy and horrible. As far as I know there aren't any simple solutions.

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