Birkhoff’s theorem

A theorem of general relativity, discovered by J. T. Jebsen (1921) and independently discovered, and named after, George D. Birkhoff (1923): Any spherically symmetric spacetime has the same properties as some region from one of a simple family of spacetimes found by Karl Schwarzschild in 1916. More concretely: The spherically symmetric spacetime around any spherically symmetric matter configuration (approximately, in good approximation, the earth) has the same properties as spacetime around a Schwarzschild black hole of the appropriate mass.