Can rotating black holes and their matter satisfy a single master equation?

The authors show that Einstein's equations for rotating, axisymmetric spacetimes collapse into a single master equation once the geometric structure is fixed. This dramatically simplifies the problem: instead of solving a system of coupled equations, you solve one. The result applies to Kerr black holes and their generalizations, and imposes strict algebraic constraints on what kinds of matter can exist in these spacetimes.