Event horizon

Definition

An event horizon is the boundary surrounding a black hole beyond which no light or other information can escape. It marks the point at which gravitational pull becomes so strong that escape velocity exceeds the speed of light.

5 Must Know Facts For Your Next Test

1. The event horizon is defined by the Schwarzschild radius, calculated as $R_s = \frac{2GM}{c^2}$ where $G$ is the gravitational constant, $M$ is the mass of the black hole, and $c$ is the speed of light.
2. Within the event horizon, all paths lead to the singularity at the center of the black hole.
3. The event horizon appears as a perfect sphere in non-rotating black holes but can be distorted in rotating (Kerr) black holes.
4. Hawking radiation theorizes that particle-antiparticle pairs near the event horizon can result in one particle escaping, leading to gradual black hole evaporation.
5. Information paradox: The fate of information that crosses an event horizon remains an unresolved issue in theoretical physics.

Review Questions

• What determines the size of an event horizon?
• Explain why nothing can escape from within an event horizon.
• How does Hawking radiation affect a black hole's mass over time?

Related terms

Schwarzschild Radius: The radius at which an object’s escape velocity equals the speed of light; defines the size of a non-rotating black hole’s event horizon.

Singularity: A point at which density becomes infinite and space-time curvature becomes infinitely large, typically found at the center of a black hole.

Hawking Radiation: Hypothetical radiation predicted by Stephen Hawking that results from quantum effects near a black hole’s event horizon, leading to slow loss of mass and energy from the black hole.