# 表面重力

## 質量、半徑和表面重力

$\frac{0.107}{0.532^2} = 0.38$

$g = {\frac{GM}{r^2}}$

$g = {\frac{4\pi}{3} G \rho r}$

## 黑洞的表面重力

$k^a \nabla_a k^b = \kappa k^b$,

### 克尔-纽曼解

$\kappa = \frac{r_+-r_-}{2(r_+^2+a^2)} = \frac{\sqrt{M^2-Q^2-J^2/M^2}}{2M^2-Q^2+2M\sqrt{M^2-Q^2-J^2/M^2}}$,

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