The radius of the event horizon of a black hole, representing the critical distance from the center where the escape velocity equals the speed of light. It is proportional to the black hole's mass.
Named after Karl Schwarzschild, a German physicist and astronomer who found the first exact solution to Einstein's field equations in 1916, just months after general relativity was published. Tragically, Schwarzschild died of an autoimmune disease on the Russian front during World War I, shortly after making this discovery.
The Schwarzschild radius reveals a stunning fact: compress any object - even a feather - into a small enough space and it becomes a black hole! For Earth, this radius is only 9 millimeters, while for the Sun it's about 3 kilometers.
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