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Schwarzschild Radius — When Gravity Traps Light - Leejohnston - 01-08-2026
Schwarzschild Radius — When Gravity Traps Light
The Schwarzschild radius defines the size of a black hole’s event horizon — the point beyond which nothing, not even light, can escape.
Equation:
rₛ = 2GM / c²
Where:
rₛ = Schwarzschild radius
G = gravitational constant
M = mass of the object
c = speed of light
What this means:
If an object’s mass is compressed inside its Schwarzschild radius, gravity becomes so strong that escape is impossible.
Key insight:
• Earth has a Schwarzschild radius of about 9 mm
• The Sun’s is about 3 km
• Black holes are not “dense objects” — they are regions where spacetime is curved beyond escape
Why this matters:
This equation tells us when gravity wins completely, creating an event horizon and defining what a black hole actually is.