What’s a photon sphere?
A hundred years ago, Einstein truly became a household name when his theory of relativity was first tested and confirmed by experiments. Einstein predicted that light passing by a massive object should get deflected by gravity, and that this effect could even be detected by measuring the apparent location of stars near the Sun during a solar eclipse.
For the Sun this deflection is less than a thousandth of a degree.
For a black hole, with its much stronger gravitational field, the light ray can get bent by 360 degrees or more.
Imagine shining a laser pointer at a black hole. If you point it right at the black hole, the light will get swallowed. If you point it to the side, it will get deflected by the black hole’s gravity, but still reach your friend on the other side. But as you point the laser closer to the black hole’s edge, there’s a point where the light just barely doesn’t get captured, but also doesn’t escape. Instead it just orbits in a perfect circle. This is what we call the ‘photon sphere’ of a black hole. For non-spinning black holes the photon sphere radius is 50 per cent bigger than the horizon radius, but for rapidly rotating black holes the photon sphere snuggles right up against the horizon, allowing light to orbit many times without falling in and without falling out.