# Einstein equation point mass solution

1. Dec 11, 2009

### espen180

I'm just getting a taste of computational GR, and I have a question regarding the metric for the single point mass solution for the einstein equation.

The metric in spherical coordinates for a point mass at $$r=0$$ is

$$\eta=\left(\begin{matrix}-\left(1-\frac{2GM}{c^2r}\right) & 0 & 0 & 0 \\ 0 & \frac{1}{1-\frac{2GM}{c^2r}} & 0 & 0 \\ 0 & 0 & r^2 & 0 \\ 0 & 0 & 0 & r^2\sin^2\theta \end{matrix}\right)$$

As expected, it is symmetrical wrt angle, and I recognize $$\eta_{00}$$ (time entry) as the negativa square of the gravitational time dilation constant.

The spatial $$\eta_{11}$$ entry for radius goes to infinity as r approaches the swartzschild radius. At this radius, photons can orbit the point mass.

Is the a way to compute the curvature of space and of space-time a radius r away from the sphere without computing the entire Riemann tensor?

2. Dec 11, 2009

### Nabeshin

Not really, though. At r=2M, photons directed radially outward stay at constant radial coordinate, but I wouldn't call this an orbit. The photon sphere, where photons truly do move in circular orbits, is at r=3M.