# What happens to light that falls into a black hole?

When light enters a black hole, what happens to it?

I imagine the photons will either fall into the singularity, or the light will orbit just inside the event horizon indefinitely.

(Some background to my question...)

I was reading a description of what someone falling into a black hole would experience, which said they wouldn't realize they had just passed the event horizon. I imagined that there would be an awful lot of star-light that's been shining onto the black hole for billions of years, so the doomed astronaut would see a large flash of light as they cross the event horizon.

I also imagined that when a black hole loses mass and the event horizon shrinks, there would also a flash of light as the captured light managed to escape.

More likely is that I've made an error in my reasoning.

## 1 Answer

Everything that passes the event horizon of a black hole falls into the singularity, including photons. That's why it's a singularity.

There is a particular radius outside the event horizon where a photon will orbit, but the orbit is unstable- if the photon gets perturbed a little closer or the black hole's mass increases at all, it will fall in, and if the black hole's mass decreases due to Hawking radiation or the photon gets perturbed away from the black hole, it will escape.

• Another unstable orbit is, of course, $r = 2M$, ${\dot r} = {\dot \theta} = {\dot \phi} = 0$, of all of the outbound light emitted by things just as they crossed the horizon. This orbit is also unstable, since the horizon will expand once matter enters the horizon. Commented Aug 12, 2015 at 18:35