Why do black holes have an event horizon?
Quick Answer
An event horizon forms because gravity becomes so strong that once you cross a boundary, even light cannot escape. For a non-rotating black hole, that boundary is called the Schwarzschild radius.
The Full Story
A black hole’s event horizon is not a solid surface but a point of no return in spacetime. If enough mass is compressed into a small enough region, the escape speed exceeds the speed of light, making escape impossible according to relativity. The simplest description, developed by Karl Schwarzschild in 1916, gives a radius that depends on mass. For example, a black hole with the Sun’s mass would have an event horizon about 3 kilometers wide. The horizon is central to how black holes are studied, especially through radiation from nearby hot gas and the gravitational waves from mergers.
Key Facts
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"Why can’t light escape a black hole?"