Which German physicist gave his name to the radius of a non-rotating black hole?

The German physicist who gave his name to the radius of a non-rotating black hole was Karl Schwarzschild.

Schwarzschild derived the first exact solution to Einstein’s field equations for the gravitational field outside a spherical, non-rotating mass. His work, completed in 1915 and published in 1916, produced the quantity now called the Schwarzschild radius: the radius of the event horizon for an idealized, non-rotating, uncharged black hole.

The radius is given by rₛ = 2GM/c², where G is the gravitational constant, M is the mass, and c is the speed of light. It is proportional to mass: doubling the mass doubles the radius. The Sun’s Schwarzschild radius is about 3 kilometers, while Earth’s is roughly 9 millimeters.

Schwarzschild was serving in the German army during World War I when he completed the calculation. He died in 1916 after developing a serious illness. The name applies specifically to the non-rotating case; rotating black holes are described by the Kerr solution and have a more complicated horizon structure.