Light takes
the only path there is
Every ray here is a null geodesic of the Schwarzschild metric, integrated one step at a time. Nothing pulls the light sideways; the rays run as straight as the geometry allows, and the geometry is bent. Aim close enough and a ray loops the hole before leaving — closer still and it never does.
lawd²u/dφ² + u = 3Mu², with u = 1/r. Photons are captured below an impact parameter of b = 3√3 M ≈ 2.6 rₛ.
tryDrag the beam down until rays start disappearing.
noticeRight at the threshold a ray can orbit the photon sphere several times before escaping — one aim, two completely different fates.
in lifeThe Event Horizon Telescope images are shaped by these paths — the dark centre is the capture region.
found byEinstein predicted the deflection in 1915; Eddington measured it at the 1919 eclipse. Wheeler popularised the name in 1967.