What theorem says black holes are characterized only by mass, charge, and angular momentum?

The no-hair theorem says that a stationary black hole is characterized externally by mass, electric charge, and angular momentum.

“Hair” is a metaphor for information about the material that formed or entered the black hole. After a black hole settles down, general relativity predicts that most such details cannot be distinguished from outside; the remaining classical fields are determined by those three parameters.

The result developed through work on black-hole uniqueness. Werner Israel established an early uniqueness result in 1967, and later work by Brandon Carter, Stephen Hawking, and David Robinson extended the analysis to rotating and charged cases. John Archibald Wheeler popularized the phrase “black holes have no hair.”

The theorem is not a universal statement covering every theory or situation. It relies on assumptions such as four-dimensional classical general relativity and suitable stationary solutions. Physicists therefore often call it a no-hair conjecture, and proposed quantum effects such as soft hair continue to complicate the picture.