A boundary light cannot cross outward
A black hole has an event horizon: a boundary beyond which light cannot escape to a distant observer.
In 30 seconds
- The event horizon is not a solid surface.
- Nearby gas and light can reveal a black hole’s effects.
- Its gravity at a distance depends on mass; it is not a magical vacuum cleaner.
Make it concrete
Imagine replacing an isolated object with a black hole of equal mass. Far away, mass alone has not increased; “black hole” does not mean unlimited pull.
Change a value. Watch the relationship.
Nonrotating, uncharged black hole. Horizon radius follows Rs = 2GM/c². The radius is proportional to the 100 km ruler. Arrows show causal direction schematically, not calculated photon trajectories.
Check your understanding
Is the event horizon a solid crust?
- The event horizon is not a solid surface.
- Nearby gas and light can reveal a black hole’s effects.
- Its gravity at a distance depends on mass; it is not a magical vacuum cleaner.
Go to the source
- NASA · Black holes ↗
- NASA · Black holes and Schwarzschild radius ↗
- NASA JPL · Astrodynamic parameters ↗
Sources reviewed: 13.09.2026