The event horizon marks the surface of no return, at the Schwarzschild radius for non-spinning holes. Spinning (Kerr) black holes drag spacetime, creating an ergosphere outside the horizon. Photons passing near the horizon are gravitationally lensed, producing the bright ring seen by the Event Horizon Telescope.
r_s = 2GM / c²
T_Hawking = ħc³ / (8πGMk_B)
r_ISCO ≈ 6GM/c² (non-spinning); shrinks toward r_s as spin → 1
- Mass — scales the horizon size, accretion disk, and lensing ring; smaller holes have hotter Hawking radiation.
- Spin a* — introduces frame-dragging, shrinks the innermost stable orbit, and reveals the ergosphere region.
- Disk temperature — shifts accretion-disk color/brightness from dull red to blue-white, mimicking accretion rate changes.
- Hawking glow toggle — visualizes (exaggerated) thermal particle emission at the horizon; in reality it is far too faint to observe for stellar-mass holes.
The Event Horizon Telescope imaged the shadow of M87* and Sagittarius A* using exactly this photon-ring lensing effect, confirming general relativity's predictions.