## How to see the back of your own head

Stories from Physics for 11-14 14-16

The *Schwarzschild* radius is the distance from the singularity of a black hole at which the escape velocity equals the speed of light. The photon sphere is an imaginary sphere of radius 1.5 times the Schwarzschild radius. Here, a horizontally aimed photon would go into orbit around the black hole and an astronaut, also in orbit at the same altitude, would be able to see the back of their own head by looking forwards.

### References

**How to see the back of your own head**

M. L. Kutner, *Astronomy: A Physical Perspective*, Cambridge, Cambridge University Press, 2003, p. 150