We prove Birkhoff's theorem in conformal gravity: The most general spherically symmetric, electrovac solution is static, and hence absolutely stable against spherically symmetric perturbations. This result goes beyond the zero-energy theorem since it applies even when the solution is not asymptotically flat. On the basis of the known examples, we conjecture that any field theory lacking linearized spin-0 modes possesses a Birkhoff theorem.

• Received 3 May 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.315